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Earth

Famous “Blue Marble” photograph of Earth, taken from Apollo 17
Designations
Adjective	Terrestrial, Terran, Telluric, Tellurian, Earthly
Orbital characteristics
Epoch J2000
Aphelion	152,097,701 km 1.0167103335 AU
Perihelion	147,098,074 km 0.9832898912 AU
Semi-major axis	149,597,887.5 km 1.0000001124 AU
Eccentricity	0.016710219
Orbital period	365.256366 days 1.0000175 yr
Average orbital speed	29.783 km/s 107,218 km/h
Inclination	Reference (0) 7.25° to Sun‘s equator
Longitude of ascending node	348.73936°
Argument of perihelion	114.20783°
Satellites	1 (the Moon)
Physical characteristics
Mean radius	6,371.0 km[1]
Equatorial radius	6,378.1 km[2]
Polar radius	6356.8 km[3]
Flattening	0.0033528[2]
Circumference	40,075.02 km (equatorial) 40,007.86 km (meridional) 40,041.47 km (mean)
Surface area	510,072,000 km²[4][5][6]148,940,000 km² land  (29.2 %) 361,132,000 km² water (70.8 %)
Volume	1.0832073×1012 km³
Mass	5.9736×1024 kg
Mean density	5.5153 g/cm³
Equatorial surface gravity	9.780327 m/s²[7] 0.99732 g
Escape velocity	11.186 km/s 40,270 km/h
Sidereal rotation period	0.997258 d 23h 56m 04.09054s[7]
Equatorial rotation velocity	465.11 m/s
Axial tilt	23.439281°
Albedo	0.367
Surface temp. Kelvin Celsius	min mean max 184 K 287 K 331 K −89 °C 14 °C 57.7 °C
Atmosphere
Surface pressure	101.3 kPa (MSL)
Composition	78.08% Nitrogen (N2) 20.95% Oxygen (O2) 0.93% Argon 0.038% Carbon dioxide About 1% water vapor (varies with climate)[8]

Earth (pronounced [ˈɝːθ] (help·info))^[9] is the third planet from the Sun. Earth is the largest of the terrestrial planets in the Solar System in diameter, mass and density. It is also referred to as the Earth, Planet Earth, the World, and Terra.^[10]

Home to millions of species,^[11] including humans, Earth is the only place in the universe where life is known to exist. Scientific evidence indicates that the planet formed 4.54 billion years ago,^{[12][13][14][15]} and life appeared on its surface within a billion years. Since then, Earth’s biosphere has significantly altered the atmosphere and other abiotic conditions on the planet, enabling the proliferation of aerobic organisms as well as the formation of the ozone layer which, together with Earth’s magnetic field, blocks harmful radiation, permitting life on land.^[16]

Earth’s outer surface is divided into several rigid segments, or tectonic plates, that gradually migrate across the surface over periods of many millions of years. About 71% of the surface is covered with salt-water oceans, the remainder consisting of continents and islands; liquid water, necessary for all known life, is not known to exist on any other planet’s surface.^[17][18] Earth’s interior remains active, with a thick layer of relatively solid mantle, a liquid outer core that generates a magnetic field, and a solid iron inner core.

Earth interacts with other objects in outer space, including the Sun and the Moon. At present, Earth orbits the Sun once for every roughly 366.26 times it rotates about its axis. This length of time is a sidereal year, which is equal to 365.26 solar days.^[19] The Earth’s axis of rotation is tilted 23.4° away from the perpendicular to its orbital plane,^[20] producing seasonal variations on the planet’s surface with a period of one tropical year (365.24 solar days). Earth’s only known natural satellite, the Moon, which began orbiting it about 4.53 billion years ago, provides ocean tides, stabilizes the axial tilt and gradually slows the planet’s rotation. A cometary bombardment during the early history of the planet played a role in the formation of the oceans.^[21] Later, asteroid impacts caused significant changes to the surface environment.

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History

Main article: History of Earth

See also: Geological history of Earth

Scientists have been able to reconstruct detailed information about the planet’s past. Earth and the other planets in the Solar System formed 4.54 billion years ago^[12] out of the solar nebula, a disk-shaped mass of dust and gas left over from the formation of the Sun. Initially molten, the outer layer of the planet Earth cooled to form a solid crust when water began accumulating in the atmosphere. The Moon formed soon afterwards, possibly as the result of a Mars-sized object (sometimes called Theia) with about 10% of the Earth’s mass^[22] impacting the Earth in a glancing blow.^[23] Some of this object’s mass would have merged with the Earth and a portion would have been ejected into space, but enough material would have been sent into orbit to form the Moon.

Outgassing and volcanic activity produced the primordial atmosphere. Condensing water vapor, augmented by ice and liquid water delivered by asteroids and the larger proto-planets, comets, and trans-Neptunian objects produced the oceans.^[21] The highly energetic chemistry is believed to have produced a self-replicating molecule around 4 billion years ago, and half a billion years later, the last common ancestor of all life existed.^[24]

The development of photosynthesis allowed the Sun’s energy to be harvested directly by life forms; the resultant oxygen accumulated in the atmosphere and resulted in a layer of ozone (a form of molecular oxygen [O₃]) in the upper atmosphere. The incorporation of smaller cells within larger ones resulted in the development of complex cells called eukaryotes.^[25] True multicellular organisms formed as cells within colonies became increasingly specialized. Aided by the absorption of harmful ultraviolet radiation by the ozone layer, life colonized the surface of Earth.^[26]

Beginning with almost no dry land, the total amount of surface lying above the oceans has steadily increased. During the past two billion years, for example, the total size of the continents has doubled.^[27] As the surface continually reshaped itself, over hundreds of millions of years, continents formed and broke up. The continents migrated across the surface, occasionally combining to form a supercontinent. Roughly 750 million years ago (mya), the earliest known supercontinent, Rodinia, began to break apart. The continents later recombined to form Pannotia, 600–540 mya, then finally Pangaea, which broke apart 180 mya.^[28]

Since the 1960s, it has been hypothesized that severe glacial action between 750 and 580 mya, during the Neoproterozoic, covered much of the planet in a sheet of ice. This hypothesis has been termed “Snowball Earth“, and is of particular interest because it preceded the Cambrian explosion, when multicellular life forms began to proliferate.^[29]

Following the Cambrian explosion, about 535 mya, there have been five mass extinctions.^[30] The last extinction event occurred 65 mya, when a meteorite collision probably triggered the extinction of the (non-avian) dinosaurs and other large reptiles, but spared small animals such as mammals, which then resembled shrews. Over the past 65 million years, mammalian life has diversified, and several mya, an African ape-like animal gained the ability to stand upright.^[31] This enabled tool use and encouraged communication that provided the nutrition and stimulation needed for a larger brain. The development of agriculture, and then civilization, allowed humans to influence the Earth in a short time span as no other life form had,^[32] affecting both the nature and quantity of other life forms.

The present pattern of ice ages began about 40 mya, then intensified during the Pleistocene about 3 mya. The polar regions have since undergone repeated cycles of glaciation and thaw, repeating every 40–100,000 years. The last ice age ended 10,000 years ago.^[33]

Composition and structure

Main article: Earth science

Further information: Earth physical characteristics tables

Earth is a terrestrial planet, meaning that it is a rocky body, rather than a gas giant like Jupiter. It is the largest of the four solar terrestrial planets, both in terms of size and mass. Of these four planets, Earth also has the highest density, the highest surface gravity and the strongest magnetic field.^[34]

Shape

Main article: Figure of the Earth

Size comparison of inner planets (left to right): Mercury, Venus, Earth, and Mars

The Earth’s shape is very close to an oblate spheroid—a rounded shape with a bulge around the equator—although the precise shape (the geoid) varies from this by up to 100 meters.^[35] The average diameter of the reference spheroid is about 12,742 km. More approximately the distance is 40,000 km/π because the meter was originally defined as 1/10,000,000 of the distance from the equator to the north pole through Paris, France.^[36]

The rotation of the Earth creates the equatorial bulge so that the equatorial diameter is 43 km larger than the pole to pole diameter.^[37] The largest local deviations in the rocky surface of the Earth are Mount Everest (8,848 m above local sea level) and the Mariana Trench (10,911 m below local sea level). Hence compared to a perfect ellipsoid, the Earth has a tolerance of about one part in about 584, or 0.17%, which is less than the 0.22% tolerance allowed in billiard balls.^[38] Because of the bulge, the feature farthest from the center of the Earth is actually Mount Chimborazo in Ecuador.^[39]

Chemical composition

See also: Abundance of elements on Earth

The mass of the Earth is approximately 5.98×10²⁴ kg. It is composed mostly of iron (32.1%), oxygen (30.1%), silicon (15.1%), magnesium (13.9%), sulfur (2.9%), nickel (1.8%), calcium (1.5%), and aluminium (1.4%); with the remaining 1.2% consisting of trace amounts of other elements. Due to mass segregation, the core region is believed to be primarily composed of iron (88.8%), with smaller amounts of nickel (5.8%), sulfur (4.5%), and less than 1% trace elements.^[40]

The geochemist F. W. Clarke calculated that a little more than 47% of the Earth’s crust consists of oxygen. The more common rock constituents of the Earth’s crust are nearly all oxides; chlorine, sulfur and fluorine are the only important exceptions to this and their total amount in any rock is usually much less than 1%. The principal oxides are silica, alumina, iron oxides, lime, magnesia, potash and soda. The silica functions principally as an acid, forming silicates, and all the commonest minerals of igneous rocks are of this nature. From a computation based on 1,672 analyses of all kinds of rocks, Clarke deduced that 99.22% were composed of 11 oxides (see the table at right.) All the other constituents occur only in very small quantities.^[41]

Internal structure

Main article: Structure of the Earth

The interior of the Earth, like that of the other terrestrial planets, is chemically divided into layers. The Earth has an outer silicate solid crust, a highly viscous mantle, a liquid outer core that is much less viscous than the mantle, and a solid inner core. The crust is separated from the mantle by the Mohorovičić discontinuity, and the thickness of the crust varies: averaging 6 km under the oceans and 30–50 km on the continents.^[42]

Geologic layers of the Earth^[43]

Earth cutaway from core to exosphere. Not to scale.	Depth[44] km	Component Layer	Density g/cm³
	0–60	Lithosphere[45]	—
	0–35	… Crust[46]	2.2–2.9
	35–60	… Upper mantle	3.4–4.4
	35–2890	Mantle	3.4–5.6
	100–700	… Asthenosphere	—
	2890–5100	Outer core	9.9–12.2
	5100–6378	Inner core	12.8–13.1

The internal heat of the planet is probably produced by the radioactive decay of potassium-40, uranium-238 and thorium-232 isotopes. All three have half-life decay periods of more than a billion years.^[47] At the center of the planet, the temperature may be up to 7,000 K and the pressure could reach 360 GPa.^[48] A portion of the core’s thermal energy is transported toward the crust by Mantle plumes; a form of convection consisting of upwellings of higher-temperature rock. These plumes can produce hotspots and flood basalts.^[49]

Tectonic plates

Main article: Plate tectonics

According to plate tectonics theory, the outermost part of the Earth’s interior is made up of two layers: the lithosphere, comprising the crust, and the solidified uppermost part of the mantle. Below the lithosphere lies the asthenosphere, which forms the inner part of the upper mantle. The asthenosphere behaves like a superheated material that is in a semi-fluidic, plastic-like state.^[50]

The lithosphere essentially floats on the asthenosphere and is broken up into what are called tectonic plates. These plates are rigid segments that move in relation to one another at one of three types of plate boundaries: convergent, divergent and transform. The last occurs where two plates move laterally relative to each other, creating a strike-slip fault. Earthquakes, volcanic activity, mountain-building, and oceanic trench formation can occur along these plate boundaries.^[51]

Earth’s main plates^[52]

A map illustrating the Earth’s major plates.	Plate name	Area 106 km²
	African Plate	61.3
	Antarctic Plate	60.9
	Australian Plate	47.2
	Eurasian Plate	67.8
	North American Plate	75.9
	South American Plate	43.6
	Pacific Plate	103.3

Notable minor plates include the Indian Plate, the Arabian Plate, the Caribbean Plate, the Nazca Plate off the west coast of South America and the Scotia Plate in the southern Atlantic Ocean. The Australian Plate actually fused with Indian Plate between 50 and 55 million years ago. The fastest-moving plates are the oceanic plates, with the Cocos Plate advancing at a rate of 75 mm/yr^[53] and the Pacific Plate moving 52–69 mm/yr. At the other extreme, the slowest-moving plate is the Eurasian Plate, progressing at a typical rate of about 21 mm/yr.^[54]

Surface

Main articles: Landform and Extreme points of Earth

The Earth’s terrain varies greatly from place to place. About 70.8%^[55] of the surface is covered by water, with much of the continental shelf below sea level. The submerged surface has mountainous features, including a globe-spanning mid-ocean ridge system, as well as undersea volcanoes,^[37] oceanic trenches, submarine canyons, oceanic plateaus and abyssal plains. The remaining 29.2% not covered by water consists of mountains, deserts, plains, plateaus, and other geomorphologies.

The planetary surface undergoes reshaping over geological time periods due to the effects of tectonics and erosion. The surface features built up or deformed through plate tectonics are subject to steady weathering from precipitation, thermal cycles, and chemical effects. Glaciation, coastal erosion, the build-up of coral reefs, and large meteorite impacts^[56] also act to reshape the landscape.

Present day Earth altimetry and bathymetry. Data from the National Geophysical Data Center‘s TerrainBase Digital Terrain Model.

As the tectonic plates migrate across the planet, the ocean floor is subducted under the leading edges. At the same time, upwellings of mantle material create a divergent boundary along mid-ocean ridges. The combination of these processes continually recycles the oceanic crustal material. Most of the ocean floor is less than 100 million years in age. The oldest oceanic crust is located in the Western Pacific, and has an estimated age of about 200 million years. By comparison, the oldest fossils found on land have an age of about 3 billion years.^[57][58]

The continental crust consists of lower density material such as the igneous rocks granite and andesite. Less common is basalt, a denser volcanic rock that is the primary constituent of the ocean floors.^[59] Sedimentary rock is formed from the accumulation of sediment that becomes compacted together. Nearly 75% of the continental surfaces are covered by sedimentary rocks, although they form only about 5% of the crust.^[60] The third form of rock material found on Earth is metamorphic rock, which is created from the transformation of pre-existing rock types through high pressures, high temperatures, or both. The most abundant silicate minerals on the Earth’s surface include quartz, the feldspars, amphibole, mica, pyroxene and olivine.^[61] Common carbonate minerals include calcite (found in limestone), aragonite and dolomite.^[62]

The pedosphere is the outermost layer of the Earth that is composed of soil and subject to soil formation processes. It exists at the interface of the lithosphere, atmosphere, hydrosphere and biosphere. Currently the total arable land is 13.31% of the land surface, with only 4.71% supporting permanent crops.^[5]Close to 40% of the Earth’s land surface is presently used for cropland and pasture, or an estimated 1.3×10⁷ km² of cropland and 3.4×10⁷ km² of pastureland.^[63]

The elevation of the land surface of the Earth varies from the low point of −418 m at the Dead Sea, to a 2005-estimated maximum altitude of 8,848 m at the top of Mount Everest. The mean height of land above sea level is 840 m.^[64]

Hydrosphere

Main article: Hydrosphere

Elevation histogram of the surface of the Earth—approximately 71% of the Earth’s surface is covered with water.

The abundance of water on Earth’s surface is a unique feature that distinguishes the “Blue Planet” from others in the solar system. The Earth’s hydrosphere consists chiefly of the oceans, but technically includes all water surfaces in the world, including inland seas, lakes, rivers, and underground waters down to a depth of 2,000 m. The deepest underwater location is Challenger Deep of the Mariana Trench in the Pacific Ocean with a depth of −10,911.4 m.^[65][66] The average depth of the oceans is 3,800 m, more than four times the average height of the continents.^[64]

The mass of the oceans is approximately 1.35×10¹⁸ metric tons, or about 1/4400 of the total mass of the Earth, and occupies a volume of 1.386×10⁹ km³. If all of the land on Earth were spread evenly, water would rise to an altitude of more than 2.7 km.^[67] About 97.5% of the water is saline, while the remaining 2.5% is fresh water. The majority of the fresh water, about 68.7%, is currently in the form of ice.^[68]

About 3.5% of the total mass of the oceans consists of salt. Most of this salt was released from volcanic activity or extracted from cool, igneous rocks.^[69] The oceans are also a reservoir of dissolved atmospheric gases, which are essential for the survival of many aquatic life forms.^[70] Sea water has an important influence on the world’s climate, with the oceans acting as a large heat reservoir.^[71] Shifts in the oceanic temperature distribution can cause significant weather shifts, such as the El Niño-Southern Oscillation.^[72]

Atmosphere

Main article: Earth’s atmosphere

The atmospheric pressure on the surface of the Earth averages 101.325 kPa, with a scale height of about 8.5 km.^[8] It is 78% nitrogen and 21% oxygen, with trace amounts of water vapor, carbon dioxide and other gaseous molecules. The height of the troposphere varies with latitude, ranging between 8 km at the poles to 17 km at the equator, with some variation due to weather and seasonal factors.^[73]

Earth’s biosphere has significantly altered its atmosphere. Oxygenic photosynthesis evolved 2.7 billion years ago, forming the primarily nitrogen-oxygen atmosphere that exists today. This change enabled the proliferation of aerobic organisms as well as the formation of the ozone layer which, together with Earth’s magnetic field, blocks ultraviolet solar radiation, permitting life on land. Other atmospheric functions important to life on Earth’s include transporting water vapor, providing useful gases, causing small meteors to burn up before they strike the surface, and moderating temperature.^[74] This last phenomenon is known as the greenhouse effect: trace molecules within the atmosphere serve to capture thermal energy emitted from the ground, thereby raising the average temperature. Carbon dioxide, water vapor, methane and ozone are the primary greenhouse gases in the Earth’s atmosphere. Without this heat-retention effect, the average surface temperature would be −18 °C and life would likely not exist.^[55]

Weather and climate

Main articles: Weather and Climate

The Earth’s atmosphere has no definite boundary, slowly becoming thinner and fading into outer space. Three-quarters of the atmosphere’s mass is contained within the first 11 km of the planet’s surface. This lowest layer is called the troposphere. Energy from the Sun heats this layer, and the surface below, causing expansion of the air. This lower density air then rises, and is replaced by cooler, higher density air. The result is atmospheric circulation that drives the weather and climate through redistribution of heat energy.^[75]

The primary atmospheric circulation bands consist of the trade winds in the equatorial region below 30° latitude and the westerlies in the mid-latitudes between 30° and 60°.^[76] Ocean currents are also important factors in determining climate, particularly the thermohaline circulation that distributes heat energy from the equatorial oceans to the polar regions.^[77]

Source regions of global air masses.

Water vapor generated through surface evaporation is transported by circulatory patterns in the atmosphere. When atmospheric conditions permit an uplift of warm, humid air, this water condenses and settles to the surface as precipitation.^[75] Most of the water is then transported back to lower elevations by river systems, usually returning to the oceans or being deposited into lakes. This water cycle is a vital mechanism for supporting life on land, and is a primary factor in the erosion of surface features over geological periods. Precipitation patterns vary widely, ranging from several meters of water per year to less than a millimeter. Atmospheric circulation, topological features and temperature differences determine the average precipitation that falls in each region.^[78]

The Earth can be sub-divided into specific latitudinal belts of approximately homogeneous climate. Ranging from the equator to the polar regions, these are the tropical (or equatorial), subtropical, temperate and polar climates.^[79] Climate can also be classified based on the temperature and precipitation, with the climate regions characterized by fairly uniform air masses. The commonly-used Köppen climate classification system (as modified by Wladimir Köppen‘s student Rudolph Geiger) has five broad groups (humid tropics, arid, humid middle latitudes, continental and cold polar), which are further divided into more specific subtypes.^[76]

Upper atmosphere

This view from orbit shows the full Moon partially obscured by the Earth’s atmosphere. NASA image.

See also: Outer space

Above the troposphere, the atmosphere is usually divided into the stratosphere, mesosphere, and thermosphere.^[74] Each of these layers has a different lapse rate, defining the rate of change in temperature with height. Beyond these, the exosphere thins out into the magnetosphere (where the Earth’s magnetic fields interact with the solar wind).^[80] An important part of the atmosphere for life on Earth is the ozone layer, a component of the stratosphere that partially shields the surface from ultraviolet light. The Kármán line, defined as 100 km above the Earth’s surface, is a working definition for the boundary between atmosphere and space.^[81]

Due to thermal energy, some of the molecules at the outer edge of the Earth’s atmosphere have their velocity increased to the point where they can escape from the planet’s gravity. This results in a slow but steady leakage of the atmosphere into space. Because unfixed hydrogen has a low molecular weight, it can achieve escape velocity more readily and it leaks into outer space at a greater rate.^[82] For this reason, the Earth’s current environment is oxidizing, rather than reducing, with consequences for the chemical nature of life which developed on the planet. The oxygen-rich atmosphere also preserves much of the surviving hydrogen by locking it up in water molecules.^[83]

Magnetic field

The Earth’s magnetic field, which approximates a dipole.

Main article: Earth’s magnetic field

The Earth’s magnetic field is shaped roughly as a magnetic dipole, with the poles currently located proximate to the planet’s geographic poles. According to dynamo theory, the field is generated within the molten outer core region where heat creates convection motions of conducting materials, generating electric currents. These in turn produce the Earth’s magnetic field. The convection movements in the core are chaotic in nature, and periodically change alignment. This results in field reversals at irregular intervals averaging a few times every million years. The most recent reversal occurred approximately 700,000 years ago.^[84][85]

The field forms the magnetosphere, which deflects particles in the solar wind. The sunward edge of the bow shock is located at about 13 times the radius of the Earth. The collision between the magnetic field and the solar wind forms the Van Allen radiation belts, a pair of concentric, torus-shaped regions of energetic charged particles. When the plasma enters the Earth’s atmosphere at the magnetic poles, it forms the aurora.^[86]

Orbit and rotation

Main articles: Earth’s Orbit and Earth’s rotation

An animation showing the rotation of the Earth as seen from the northern hemisphere of the solar system.

Relative to the background stars, it takes the Earth, on average, 23 hours, 56 minutes and 4.091 seconds (one sidereal day) to rotate around the axis that connects the north and the south poles from west to east.^[87] From Earth, the main apparent motion of celestial bodies in the sky (except that of meteors within the atmosphere and low-orbiting satellites) is to the west at a rate of 15°/h = 15′/min. This is equivalent to an apparent diameter of the Sun or Moon every two minutes. (The apparent sizes of the Sun and the Moon are approximately the same.)

Earth orbits the Sun at an average distance of about 150 million kilometers every 365.2564 mean solar days (1 sidereal year). From Earth, this gives an apparent movement of the Sun with respect to the stars at a rate of about 1°/day (or a Sun or Moon diameter every 12 hours) eastward. Because of this motion, on average it takes 24 hours—a solar day—for Earth to complete a full rotation about its axis so that the Sun returns to the meridian. The orbital speed of the Earth averages about 30 km/s (108,000 km/h), which is fast enough to cover the planet’s diameter (about 12,600 km) in seven minutes, and the distance to the Moon (384,000 km) in four hours.^[8]

The Moon revolves with the Earth around a common barycenter every 27.32 days relative to the background stars. When combined with the Earth–Moon system’s common revolution around the Sun, the period of the synodic month, from new moon to new moon, is 29.53 days. Viewed from the celestial north pole, the motion of Earth, the Moon and their axial rotations are all counter-clockwise. The orbital and axial planes are not precisely aligned: Earth’s axis is tilted some 23.5 degrees from the perpendicular to the Earth–Sun plane (which causes the seasons); and the Earth–Moon plane is tilted about 5 degrees against the Earth-Sun plane (without this tilt, there would be an eclipse every two weeks, alternating between lunar eclipses and solar eclipses).^[88][8]

Because of the axial tilt of the Earth, the position of the Sun in the sky (as seen by an observer on the surface) varies over the course of the year. For an observer at a northern latitude, when the northern pole is tilted toward the Sun the day lasts longer and the Sun climbs higher in the sky. This results in warmer average temperatures from the increase in solar radiation reaching the surface. When the northern pole is tilted away from the Sun, the reverse is true and the climate is generally cooler. Above the arctic circle, an extreme case is reached where there is no daylight at all for part of the year. (This is called a polar night.)

Earth seen as a tiny dot by the Voyager 1 spacecraft, more than 6 billion kilometers from Earth.

This variation in the climate (because of the direction of the Earth’s axial tilt) results in the seasons. By astronomical convention, the four seasons are determined by the solstices—the point in the orbit of maximum axial tilt toward or away from the Sun—and the equinoxes, when the direction of the tilt and the direction to the Sun are perpendicular. Winter solstice occurs on about December 21, summer solstice is near June 21, spring equinox is around March 20 and autumnal equinox is about September 23. The axial tilt in the southern hemisphere is exactly the opposite of the direction in the northern hemisphere. Thus the seasonal effects in the south are reversed.

The angle of the Earth’s tilt is relatively stable over long periods of time. However, the tilt does undergo a slight, irregular motion (known as nutation) with a main period of 18.6 years. The orientation (rather than the angle) of the Earth’s axis also changes over time, precessing around in a complete circle over each 25,800 year cycle; this precession is the reason for the difference between a sidereal year and a tropical year. Both of these motions are caused by the varying attraction of the Sun and Moon on the Earth’s equatorial bulge. From the perspective of the Earth, the poles also migrate a few meters across the surface. This polar motion has multiple, cyclical components, which collectively are termed quasiperiodic motion. In addition to an annual component to this motion, there is a 14-month cycle called the Chandler wobble. The rotational velocity of the Earth also varies in a phenomenon known as length of day variation.^[89]

In modern times, Earth’s perihelion occurs around January 3, and the aphelion around July 4 (for other eras, see precession and Milankovitch cycles). The changing Earth-Sun distance results in an increase of about 6.9%^[90] in solar energy reaching the Earth at perihelion relative to aphelion. Since the southern hemisphere is tilted toward the Sun at about the same time that the Earth reaches the closest approach to the Sun, the southern hemisphere receives slightly more energy from the Sun than does the northern over the course of a year. However, this effect is much less significant than the total energy change due to the axial tilt, and most of the excess energy is absorbed by the higher proportion of water in the southern hemisphere.^[91]

The Hill sphere (gravitational sphere of influence) of the Earth is about 1.5 Gm (or 1,500,000 kilometers) in radius.^[92][93] This is maximum distance at which the Earth’s gravitational influence is stronger than the more distant Sun and planets. Objects must orbit the Earth within this radius, or they can become unbound by the gravitational perturbation of the Sun.

Earth, along with the Solar System, is situated in the Milky Way galaxy, orbiting about 28,000 light years from the center of the galaxy, and about 20 light years above the galaxy’s equatorial plane in the Orion spiral arm.^[94]

Moon

Main article: Moon

The Moon is a relatively large, terrestrial, planet-like satellite, with a diameter about one-quarter of the Earth’s. It is the largest moon in the solar system relative to the size of its planet. (Charon is larger relative to the dwarf planet Pluto.) The natural satellites orbiting other planets are called “moons” after Earth’s Moon.

The gravitational attraction between the Earth and Moon causes tides on Earth. The same effect on the Moon has led to its tidal locking: its rotation period is the same as the time it takes to orbit the Earth. As a result, it always presents the same face to the planet. As the Moon orbits Earth, different parts of its face are illuminated by the Sun, leading to the lunar phases: The dark part of the face is separated from the light part by the solar terminator.

Because of their tidal interaction, the Moon recedes from Earth at the rate of approximately 38 mm a year. Over millions of years, these tiny modifications—and the lengthening of Earth’s day by about 23 µs a year—add up to significant changes.^[95] During the Devonian period, for example, (approximately 410 million years ago) there were 400 days in a year, with each day lasting 21.8 hours.^[96]

The Moon may have dramatically affected the development of life by moderating the planet’s climate. Paleontological evidence and computer simulations show that Earth’s axial tilt is stabilized by tidal interactions with the Moon.^[97] Some theorists believe that without this stabilization against the torques applied by the Sun and planets to the Earth’s equatorial bulge, the rotational axis might be chaotically unstable, exhibiting chaotic changes over millions of years, as appears to be the case for Mars.^[98] If Earth’s axis of rotation were to approach the plane of the ecliptic, extremely severe weather could result from the resulting extreme seasonal differences. One pole would be pointed directly toward the Sun during summer and directly away during winter. Planetary scientists who have studied the effect claim that this might kill all large animal and higher plant life.^[99] However, this is a controversial subject, and further studies of Mars—which has a similar rotation period and axial tilt as Earth, but not its large Moon or liquid core—may settle the matter.

Viewed from Earth, the Moon is just far enough away to have very nearly the same apparent-sized disk as the Sun. The angular size (or solid angle) of these two bodies match because, although the Sun’s diameter is about 400 times as large as the Moon’s, it is also 400 times more distant. This allows total and annular eclipses to occur on Earth.

A scale representation of the relative sizes of, and distance between, Earth and Moon.

The most widely accepted theory of the Moon’s origin, the giant impact theory, states that it formed from the collision of a Mars-size protoplanet called Theia with the early Earth. This hypothesis explains (among other things) the Moon’s relative lack of iron and volatile elements, and the fact that its composition is nearly identical to that of the Earth’s crust.^[100]

Earth has at least two co-orbital asteroids, 3753 Cruithne and 2002 AA₂₉.^[101]

Habitability

See also: Planetary habitability

A planet that can sustain life is termed habitable, even if life did not originate there. The Earth provides the (currently understood) requisite conditions of liquid water, an environment where complex organic molecules can assemble, and sufficient energy to sustain metabolism.^[102] The distance of the Earth from the Sun, as well as its orbital eccentricity, rate of rotation, axial tilt, geological history, sustaining atmosphere and protective magnetic field all contribute to the conditions necessary to originate and sustain life on this planet.^[103]

Biosphere

Main article: Biosphere

The planet’s life forms are sometimes said to form a “biosphere“. This biosphere is generally believed to have begun evolving about 3.5 billion years ago. Earth is the only place in the universe where life is known to exist. Some scientists believe that Earth-like biospheres might be rare.^[104]

The biosphere is divided into a number of biomes, inhabited by broadly similar plants and animals. On land primarily latitude and height above the sea level separates biomes. Terrestrial biomes lying within the Arctic, Antarctic Circle or in high altitudes are relatively barren of plant and animal life, while the greatest latitudinal diversity of species is found at the Equator.^[105]

Natural resources and land use

Main article: Natural resource

The Earth provides resources that are exploitable by humans for useful purposes. Some of these are non-renewable resources, such as mineral fuels, that are difficult to replenish on a short time scale.

Large deposits of fossil fuels are obtained from the Earth’s crust, consisting of coal, petroleum, natural gas and methane clathrate. These deposits are used by humans both for energy production and as feedstock for chemical production. Mineral ore bodies have also been formed in Earth’s crust through a process of Ore genesis, resulting from actions of erosion and plate tectonics.^[106] These bodies form concentrated sources for many metals and other useful elements.

The Earth’s biosphere produces many useful biological products for humans, including (but far from limited to) food, wood, pharmaceuticals, oxygen, and the recycling of many organic wastes. The land-based ecosystem depends upon topsoil and fresh water, and the oceanic ecosystem depends upon dissolved nutrients washed down from the land.^[107] Humans also live on the land by using building materials to construct shelters. In 1993, human use of land is approximately:

The estimated amount of irrigated land in 1993 was 2,481,250 km².^[5]

Natural and environmental hazards

Large areas are subject to extreme weather such as tropical cyclones, hurricanes, or typhoons that dominate life in those areas. Many places are subject to earthquakes, landslides, tsunamis, volcanic eruptions, tornadoes, sinkholes, blizzards, floods, droughts, and other calamities and disasters.

Many localized areas are subject to human-made pollution of the air and water, acid rain and toxic substances, loss of vegetation (overgrazing, deforestation, desertification), loss of wildlife, species extinction, soil degradation, soil depletion, erosion, and introduction of invasive species.

A scientific consensus exists linking human activities to global warming due to industrial carbon dioxide emissions. This is predicted to produce changes such as the melting of glaciers and ice sheets, more extreme temperature ranges, significant changes in weather conditions and a global rise in average sea levels.^[108]

Human geography

The Earth at night, a composite of DMSP/OLS ground illumination data on a simulated night-time image of the world. This image is not photographic and many features are brighter than they would appear to a direct observer.

Main article: Human geography

See also: World

Earth has approximately 6,707,000,000 human inhabitants as of July 2008.^[109] Projections indicate that the world’s human population will reach seven billion in 2013 and 9.2 billion^[110] in 2050. Most of the growth is expected to take place in developing nations. Human population density varies widely around the world, but a majority live in Asia. By 2020, 60% of the world’s population is expected to be living in urban, rather than rural, areas.^[111]

It is estimated that only one eighth of the surface of the Earth is suitable for humans to live on—three-quarters is covered by oceans, and half of the land area is either desert (14%),^[112] high mountains (27%),^[113] or other less suitable terrain. The northernmost permanent settlement in the world is Alert, on Ellesmere Island in Nunavut, Canada.^[114] (82°28′N) The southernmost is the Amundsen-Scott South Pole Station, in Antarctica, almost exactly at the South Pole. (90°S)

Independent sovereign nations claim all of the planet’s land surface, with the exception of some parts of Antarctica. As of 2007 there are 201 sovereign states, including the 192 United Nations member states. In addition, there are 59 dependent territories, and a number of autonomous areas, territories under dispute and other entities. Historically, Earth has never had a sovereign government with authority over the entire globe, although a number of nation-states have striven for world domination and failed.

The United Nations is a worldwide intergovernmental organization that was created with the goal of intervening in the disputes between nations, thereby avoiding armed conflict. It is not, however, a world government. While the U.N. provides a mechanism for international law and, when the consensus of the membership permits, armed intervention,^[115] it serves primarily as a forum for international diplomacy.

In total, about 400 people have been outside the Earth’s atmosphere as of 2004, and, of these, twelve have walked on the Moon. Normally the only humans in space are those on the International Space Station. The station’s crew of three people is usually replaced every six months.

Cultural viewpoint

The first photograph ever taken of an “Earthrise,” on Apollo 8.

Etymology

The name Earth originates from the 8th century Anglo-Saxon word erda, which means ground or soil. In Old English the word became eorthe, then erthe in Middle English.^[116] Earth was first used as the name of the sphere of the Earth around 1400.^[117] It is the only planet whose name in English is not derived from Greco-Roman mythology.

The standard astronomical symbol of the Earth consists of a cross circumscribed by a circle. This symbol is known as the wheel cross, sun cross, Odin’s cross or Woden’s cross. Although it has been used in various cultures for different purposes, it came to represent the compass points, earth and the land. Another version of the symbol is a cross on top of a circle; a stylized globus cruciger that was also used as an early astronomical symbol for the planet Earth.^[118]

Religious beliefs

Earth has often been personified as a deity, in particular a goddess. In many cultures the mother goddess, also called the Mother Earth, is also portrayed as a fertility deity. See also Graha. To the Aztec, Earth was called Tonantzin—”our mother”; to the Incas, Earth was called Pachamama—”mother earth”. The Chinese Earth goddess Hou-T’u^[119] is similar to Gaia, the Greek goddess personifying the Earth. To Hindus it is called Bhuma Devi, the Goddess of Earth. In Norse mythology, the Earth goddess Jord was the mother of Thor and the daughter of Annar. Ancient Egyptian mythology is different from that of other cultures because Earth is male, Geb, and sky is female, Nut.

Creation myths in many religions recall a story involving the creation of the Earth by a supernatural deity or deities. A variety of religious groups, often associated with fundamentalist branches of Protestantism^[120] or Islam,^[121] assert that their interpretations of the accounts of creation in sacred texts are literal truth and should be considered alongside or replace conventional scientific accounts of the formation of the Earth and the origin and development of life.^[122] Such assertions are opposed by the scientific community^[123][124] and other religious groups.^{[125][126][127]} A prominent example is the creation-evolution controversy.

Exploration and mapping

In the ancient past there were varying levels of belief in a flat Earth, with the Mesopotamian culture portraying the world as a flat disk afloat in an ocean. The spherical form of the Earth was suggested by early Greek philosophers; a belief espoused by Pythagoras. By the Middle Ages—as evidenced by thinkers such as Thomas Aquinas—European belief in a spherical Earth was widespread.^[128] Prior to circumnavigation of the planet and the introduction of space flight, belief in a spherical Earth was based on observations of the secondary effects of the Earth’s shape and parallels drawn with the shape of other planets.^[129]

Cartography, the study and practice of map making, and vicariously geography, have historically been the disciplines devoted to depicting the Earth. Surveying, the determination of locations and distances, to a lesser extent navigation, the determination of position and direction, have developed alongside cartography and geography, providing and suitably quantifying the requisite information.

Modern perspective

The technological developments of the latter half of the 20th century are widely considered to have altered the public’s perception of the Earth. Before space flight, the popular image of Earth was of a green world. Science fiction artist Frank R. Paul provided perhaps the first image of a cloudless blue planet (with sharply defined land masses) on the back cover of the July 1940 issue of Amazing Stories, a common depiction for several decades thereafter.^[130]

Earth and Moon from Mars, imaged by Mars Global Surveyor. From space, the Earth can be seen to go through phases similar to the phases of the Moon.

Earth was first photographed from space by Explorer 6 in 1959.^[131] Yuri Gagarin became the first human to view Earth from space in 1961. The crew of the Apollo 8 was the first to view an Earth-rise from lunar orbit in 1968. In 1972 the crew of the Apollo 17 produced the famous “Blue Marble” photograph of the planet Earth from cislunar space (see top of page). This became an iconic image of the planet as a marble of cloud-swirled blue ocean broken by green-brown continents. NASA archivist Mike Gentry has speculated that “The Blue Marble” is the most widely distributed image in human history. A photo taken of a distant Earth by Voyager 1 in 1990 inspired Carl Sagan to describe the planet as a “Pale Blue Dot.”^[132]

Since the 1960s, Earth has also been described as a massive “Spaceship Earth,” with a life support system that requires maintenance,^[133] or, in the Gaia hypothesis, as having a biosphere that forms one large organism.^[134]

Over the past two centuries a growing environmental movement has emerged that is concerned about humankind’s effects on the Earth. The key issues of this socio-political movement are the conservation of natural resources, elimination of pollution, and the usage of land. Environmentalists advocate sustainable management of resources and stewardship of the environment through changes in public policy and individual behavior. Of particular concern is the large-scale exploitation of non-renewable resources. Changes sought by the environmental movements are sometimes in conflict with commercial interests due to the additional costs associated with managing the environmental impact of those interests.^{[original research?][135]}

Future

See also: Risks to civilization, humans and planet Earth

The future of the planet is closely tied to that of the Sun. As a result of the steady accumulation of helium ash at the Sun’s core, the star’s total luminosity will slowly increase. The luminosity of the Sun will increase by 10 percent over the next 1.1 Gyr (1.1 billion years), and by 40% over the next 3.5 Gyr.^[136] Climate models indicate that the rise in radiation reaching the Earth is likely to have dire consequences, including the possible loss of the planet’s oceans.^[137]

The Earth’s increasing surface temperature will accelerate the inorganic CO₂ cycle, reducing its concentration to the lethal levels for plants (10 ppm for C4 photosynthesis) in 900 million years. The lack of vegetation will result in the loss of oxygen in the atmosphere, so animal life will become extinct within several million more years.^[27] But even if the Sun were eternal and stable, the continued internal cooling of the Earth would have resulted in a loss of much of its atmosphere and oceans (due to lower volcanism).^[138] After another billion years the surface water will have completely disappeared^[139] and the mean global temperature will reach 70°C.^[27] The Earth is expected to be effectively habitable for another 500 million years or so.^[140]

The Sun, as part of its evolution, will expand to a red giant in about 5 Gyr. Models predict that the Sun will expand out to about 250 times its present size, roughly 1 AU (150,000,000 km).^[136][141] Earth’s fate is less clear. As a red giant, the Sun will lose roughly 30% of its mass, so, without tidal effects, the Earth will be in an orbit 1.7 AU (250,000,000 km) from the Sun when the star reaches it maximum radius. Therefore, the planet is expected to escape envelopment by the expanded Sun’s sparse outer atmosphere, though most, if not all, existing life will be destroyed because of the Sun’s increased luminosity.^[136] However, a more recent simulation indicates that Earth’s orbit will decay due to tidal effects and drag, causing it to enter the red giant Sun’s atmosphere and be destroyed.^[141]

From Wikipedia, the free encyclopedia

Mercury

MESSENGER false color image of Mercury
Designations
Adjective	Mercurian, Mercurial[1]
Orbital characteristics[2]
Epoch J2000
Aphelion	69,816,900 km 0.466697 AU
Perihelion	46,001,200 km 0.307499 AU
Semi-major axis	57,909,100 km 0.387098 AU
Eccentricity	0.205630[3]
Orbital period	87.9691 d (0.240846 a)
Synodic period	115.88 d[3]
Average orbital speed	47.87 km/s[3]
Mean anomaly	174.796°
Inclination	7.005° 3.38° to Sun’s equator
Longitude of ascending node	48.331°
Argument of perihelion	29.124°
Satellites	None
Physical characteristics
Mean radius	2,439.7 ± 1.0 km[4][5] 0.3829 Earths
Flattening	< 0.0006[5]
Surface area	7.48×107 km² 0.108 Earths[4]
Volume	6.083×1010 km³ 0.054 Earths[4]
Mass	3.3022×1023 kg 0.055 Earths[4]
Mean density	5.427 g/cm³[4]
Equatorial surface gravity	3.7 m/s² 0.38 g[4]
Escape velocity	4.25 km/s[4]
Sidereal rotation period	58.646 day 1407.5 h[4]
Equatorial rotation velocity	10.892 km/h
Axial tilt	2.11′ ± 0.1′[6]
North pole right ascension	18 h 44 min 2 s 281.01°[3]
North pole declination	61.45°[3]
Albedo	0.119 (bond) 0.106 (geom.)[3]
Surface temp. 0°N, 0°W 85°N, 0°W	min mean max 100 K 340 K 700 K 80 K 200 K 380 K
Apparent magnitude	up to −1.9[3]
Angular diameter	4.5″ – 13″[3]
Atmosphere
Surface pressure	trace
Composition	42% Molecular oxygen 29.0% sodium 22.0% hydrogen 6.0% helium 0.5% potassium Trace amounts of argon, nitrogen, carbon dioxide, water vapor, xenon, krypton, & neon[3]

Mercury (pronounced [ˈmɝkjʊəri] (help·info)) is the innermost and smallest planet in the solar system (since Pluto was re-labelled as a dwarf planet), orbiting the Sun once every 88 days. Mercury is bright when viewed from Earth, ranging from −2.0 to 5.5 in apparent magnitude, but is not easily seen as its greatest angular separation from the Sun (greatest elongation) is only 28.3°: It can only be seen in morning and evening twilight. Comparatively little is known about it; the first of two spacecraft to approach Mercury was Mariner 10 from 1974 to 1975, which mapped only about 45% of the planet’s surface.^[7] The second was the MESSENGER spacecraft, which mapped another 30% of the planet during its flyby of January 14, 2008.^[7] MESSENGER will make two more passes by Mercury, followed by orbital insertion in 2011, and will survey and map the entire planet.

Physically, Mercury is similar in appearance to the Moon. It is heavily cratered, has no natural satellites and no substantial atmosphere. It has a large iron core, which generates a magnetic field about 1% as strong as that of the Earth.^[8] It is an exceptionally dense planet due to the large size of its core. The surface temperatures on Mercury range from about 90 to 700 K (-183 ºC to 427 ºC),^[9] with the subsolar point being the hottest and the bottoms of craters near the poles being the coldest.

Recorded observations of Mercury date back to at least the first millennium BC. Before the 4th century BC, Greek astronomers believed the planet to be two separate objects: one visible only at sunrise, which they called Apollo; the other visible only at sunset, which they called Hermes.^[10] The English name for the planet comes from the Romans, who named it after the Roman god Mercury, which they equated with the Greek Hermes. The astronomical symbol for Mercury is a stylized version of Hermes’ caduceus.^[11]

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Internal structure

Mercury is one of four terrestrial planets, and is a rocky body like the Earth. It is the smallest planet in the solar system, with an equatorial radius of 2439.7 km.^[3] Mercury is even smaller—albeit more massive—than the largest natural satellites in the solar system, Ganymede and Titan. Mercury consists of approximately 70% metallic and 30% silicate material.^[12] Mercury’s density is the second highest in the Solar System at 5.427 g/cm³, only slightly less than Earth’s density of 5.515 g/cm³.^[3] If the effect of gravitational compression were to be factored out, the materials of which Mercury is made would be denser, with an uncompressed density of 5.3 g/cm³ versus Earth’s 4.4 g/cm³.^[13]

1. Crust—100–300 km thick
2. Mantle—600 km thick
3. Core—1,800 km radius

Mercury’s density can be used to infer details of its inner structure. While the Earth’s high density results appreciably from gravitational compression, particularly at the core, Mercury is much smaller and its inner regions are not nearly as strongly compressed. Therefore, for it to have such a high density, its core must be large and rich in iron.^[14] Geologists estimate that Mercury’s core occupies about 42% of its volume; for Earth this proportion is 17%. Recent research strongly suggests Mercury has a molten core.^[15][16]

Surrounding the core is a 600 km mantle. It is generally thought that early in Mercury’s history, a giant impact with a body several hundred kilometers across stripped the planet of much of its original mantle material, resulting in the relatively thin mantle compared to the sizable core.^[17]

Based on data from the Mariner 10 mission and Earth-based observation, Mercury’s crust is believed to be 100–300 km thick.^[18] One distinctive feature of Mercury’s surface is the presence of numerous narrow ridges, some extending over several hundred kilometers. It is believed that these were formed as Mercury’s core and mantle cooled and contracted at a time when the crust had already solidified.^[19]

Mercury’s core has a higher iron content than that of any other major planet in the Solar System, and several theories have been proposed to explain this. The most widely accepted theory is that Mercury originally had a metal-silicate ratio similar to common chondrite meteors, thought to be typical of the Solar System’s rocky matter, and a mass approximately 2.25 times its current mass.^[17] However, early in the solar system’s history, Mercury may have been struck by a planetesimal of approximately 1/6 that mass.^[17] The impact would have stripped away much of the original crust and mantle, leaving the core behind as a relatively major component.^[17] A similar process has been proposed to explain the formation of Earth’s Moon (see giant impact theory).^[17]

Alternatively, Mercury may have formed from the solar nebula before the Sun’s energy output had stabilized. The planet would initially have had twice its present mass, but as the protosun contracted, temperatures near Mercury could have been between 2,500 and 3,500 K, (2,227 ºC to 3,227 ºC) and possibly even as high as 10,000 K (9,727 ºC).^[20] Much of Mercury’s surface rock could have been vaporized at such temperatures, forming an atmosphere of “rock vapor” which could have been carried away by the solar wind.^[20]

A third hypothesis proposes that the solar nebula caused drag on the particles from which Mercury was accreting, which meant that lighter particles were lost from the accreting material.^[21] Each of these hypotheses predicts a different surface composition, and two upcoming space missions, MESSENGER and BepiColombo, both aim to make observations to test them.^[22][23]

Surface geology

First high-resolution image of Mercury transmitted by MESSENGER (false color)

Main article: Geology of Mercury

Mercury’s surface is overall very similar in appearance to that of the Moon, showing extensive mare-like plains and heavy cratering, indicating that it has been geologically inactive for billions of years. Since our knowledge of Mercury’s geology has been based on the 1975 Mariner flyby and terrestrial observations, it is the least understood of the terrestrial planets.^[16] As data from the recent MESSENGER flyby is processed this knowledge will increase. For example, an unusual crater with radiating troughs has been discovered which scientists are calling “the spider.”^[24]

Albedo features refer to areas of markedly different reflectivity, as seen by telescopic observation. Mercury also possesses Dorsa (also called “wrinkle-ridges“), Moon-like highlands, Montes (mountains), Planitiae, or plains, Rupes (escarpments), and Valles (valleys).^[25][26]

Mercury was heavily bombarded by comets and asteroids during and shortly following its formation 4.6 billion years ago, as well as during a possibly separate subsequent episode called the late heavy bombardment that came to an end 3.8 billion years ago.^[27] During this period of intense crater formation, the planet received impacts over its entire surface,^[26] facilitated by the lack of any atmosphere to slow impactors down.^[28] During this time the planet was volcanically active; basins such as the Caloris Basin were filled by magma from within the planet, which produced smooth plains similar to the maria found on the Moon.^[29][30]

Impact basins and craters

Mercury’s Caloris Basin is one of the largest impact features in the Solar System.

Craters on Mercury range in diameter from small bowl-shaped cavities to multi-ringed impact basins hundreds of kilometers across. They appear in all states of degradation, from relatively fresh rayed craters to highly degraded crater remnants. Mercurian craters differ subtly from lunar craters in that the area blanketed by their ejecta is much smaller, a consequence of Mercury’s stronger surface gravity.^[31]

The largest known craters are Caloris Basin, with a diameter of 1550 km,^[32] and the Skinakas Basin with an outer-ring diameter of 2,300 km.^[33] The impact that created the Caloris Basin was so powerful that it caused lava eruptions and left a concentric ring over 2 km tall surrounding the impact crater. At the antipode of the Caloris Basin is a large region of unusual, hilly terrain known as the “Weird Terrain”. One hypothesis for its origin is that shock waves generated during the Caloris impact traveled around the planet, converging at the basin’s antipode (180 degrees away). The resulting high stresses fractured the surface.^[34] Alternatively, it has been suggested that this terrain formed as a result of the convergence of ejecta at this basin’s antipode.^[35]

Overall, about 15 impact basins have been identified on the imaged part of Mercury. Other notable basins include the 400 km wide, multi-ring, Tolstoj Basin which has an ejecta blanket extending up to 500 km from its rim, and its floor has been filled by smooth plains materials. Beethoven Basin also has a similar-sized ejecta blanket and a 625 km diameter rim.^[31] Like the Moon, the surface of Mercury has likely incurred the effects of space weathering processes, including Solar wind and micrometeorite impacts.^[36]

Plains

There are two geologically distinct plains regions on Mercury.^[37][31] Gently rolling, hilly plains in the regions between craters are Mercury’s oldest visible surfaces,^[31] predating the heavily cratered terrain. The inter-crater plains appear to have obliterated many earlier craters, and show a general paucity of smaller craters below about 30 km in diameter.^[37] It is not clear whether they are of volcanic or impact origin.^[37] The inter-crater plains are distributed roughly uniformly over the entire surface of the planet.

The so-called “Weird Terrain” was formed by the Caloris Basin impact at its antipodal point.

Smooth plains are widespread flat areas which fill depressions of various sizes and bear a strong resemblance to the lunar maria. Notably, they fill a wide ring surrounding the Caloris Basin. An appreciable difference between these plains and lunar maria is that the smooth plains of Mercury have the same albedo as the older inter-crater plains. Despite a lack of unequivocally volcanic characteristics, the localisation and rounded, lobate shape of these plains strongly support volcanic origins.^[31] All the Mercurian smooth plains formed significantly later than the Caloris basin, as evidenced by appreciably smaller crater densities than on the Caloris ejecta blanket.^[31] The floor of the Caloris Basin is also filled by a geologically distinct flat plain, broken up by ridges and fractures in a roughly polygonal pattern. It is not clear whether they are volcanic lavas induced by the impact, or a large sheet of impact melt.^[31]

One unusual feature of the planet’s surface is the numerous compression folds, or rupes, which crisscross the plains. It is thought that as the planet’s interior cooled, it contracted and its surface began to deform. The folds can be seen on top of other features, such as craters and smoother plains, indicating that they are more recent.^[38] Mercury’s surface is also flexed by significant tidal bulges raised by the Sun—the Sun’s tides on Mercury are about 17 times stronger than the Moon’s on Earth.^[39]

Surface conditions and “atmosphere” (exosphere)

The mean surface temperature of Mercury is 442.5 K,^[3] but it ranges from 100 K to 700 K,^[40] due to the absence of an atmosphere. On the dark side of the planet, temperatures average 110 K.^[41] The intensity of sunlight on Mercury’s surface ranges between 4.59 and 10.61 times the solar constant (1370Wm⁻²).^[42]

Radar image of Mercury’s north pole

Despite the generally extremely high temperature of its surface, observations strongly suggest that ice exists on Mercury. The floors of some deep craters near the poles are never exposed to direct sunlight, and temperatures there remain far lower than the global average. Water ice strongly reflects radar, and observations by the 70m Goldstone telescope and the VLA in the early 1990s revealed that there are patches of very high radar reflection near the poles.^[43] While ice is not the only possible cause of these reflective regions, astronomers believe it is the most likely.^[44]

The icy regions are believed to be covered to a depth of only a few meters, and contain about 10¹⁴–10¹⁵ kg of ice.^[45] By comparison, the Antarctic ice sheet on Earth has a mass of about 4×10¹⁸ kg, and Mars’ south polar cap contains about 10¹⁶ kg of water.^[45] The origin of the ice on Mercury is not yet known, but the two most likely sources are from outgassing of water from the planet’s interior or deposition by impacts of comets.^[45]

Size comparison of terrestrial planets (left to right): Mercury, Venus, Earth, and Mars

Mercury is too small for its gravity to retain any significant atmosphere over long periods of time; however, it does have a “tenuous surface-bounded exosphere“^[46] containing hydrogen, helium, oxygen, sodium, calcium and potassium. This exosphere is not stable—atoms are continuously lost and replenished from a variety of sources. Hydrogen and helium atoms probably come from the solar wind, diffusing into Mercury’s magnetosphere before later escaping back into space. Radioactive decay of elements within Mercury’s crust is another source of helium, as well as sodium and potassium. Water vapor is present, being brought to Mercury by some combination of processes such as: comets striking its surface, sputtering creating water “where none existed before from the ingredients of solar wind and Mercury rock” (both contain hydrogen and oxygen), and “reservoirs of water ice in small areas of Mercury’s poles where local topography creates permanently shadowed spots in crater walls that might trap water over the age of the solar system”. MESSENGER found high proportions of calcium, helium, hydroxide, magnesium, oxygen, potassium, silicon, sodium, and water. The detection of high amounts of water-related ions like O+, OH-, and H2O+ was a surprise.^[47][48] Because of the quantities of these ions that were detected in Mercury’s space environment, scientists surmise that these molecules were blasted from the surface or exosphere by the solar wind.^[49]

Sodium and potassium were discovered in the atmosphere during the 1980s, and are believed to result primarily from the vaporization of surface rock struck by micrometeorite impacts. Due to the ability of these materials to diffuse sunlight, Earth-based observers can readily detect their composition in the atmosphere. Studies indicate that, at times, Sodium emissions are localized at points that correspond to the planet’s magnetic dipoles. This would indicate some interaction between the magnetosphere and the planet’s surface.^[50]

Magnetic field and magnetosphere

Graph showing relative strength of Mercury’s magnetic field

Despite its small size and slow 59-day-long rotation, Mercury has a significant, and apparently global, magnetic field. According to measurements taken by Mariner 10, it is about 1.1% as strong as the Earth’s. The magnetic field strength at the Mercurian equator is about 300 nT.^[51][52] Like that of Earth, Mercury’s magnetic field is dipolar in nature.^[50] Unlike Earth, however, Mercury’s poles are nearly aligned with the planet’s spin axis.^[53] Measurements from both the Mariner 10 and MESSENGER space probes have indicated that the strength and shape of the magnetic field are stable.^[53]

It is likely that this magnetic field is generated by way of a Dynamo effect, in a manner similar to the magnetic field of Earth.^[54][55] This dynamo effect would result from the circulation of the planet’s iron-rich liquid core. Particularly strong tidal effects caused by the planet’s high orbital eccentricity would serve to keep the core in the liquid state necessary for this dynamo effect.^[56]

Mercury’s magnetic field is strong enough to deflect the solar wind around the planet, creating a magnetosphere. The planet’s magnetosphere, though small enough to fit within the Earth,^[50] is strong enough to trap solar wind plasma. This contributes to the space weathering of the planet’s surface.^[53] Observations taken by the Mariner 10 spacecraft detected this low energy plasma in the magnetosphere of the planet’s nightside. Bursts of energetic particles were detected in the planet’s magnetotail, which indicates a dynamic quality to the planet’s magnetosphere.^[50]

Orbit and rotation

Orbit of Mercury (yellow)

Mercury has the most eccentric orbit of all the planets; its eccentricity is 0.21 with its distance from the Sun ranging from 46,000,000 to 70,000,000 kilometers. It takes 88 days to complete an orbit. The diagram on the left illustrates the effects of the eccentricity, showing Mercury’s orbit overlaid with a circular orbit having the same semi-major axis. The higher velocity of the planet when it is near perihelion is clear from the greater distance it covers in each 5-day interval. The size of the spheres, inversely proportional to their distance from the Sun, is used to illustrate the varying heliocentric distance. This varying distance to the Sun, combined with a 3:2 spin-orbit resonance of the planet’s rotation around its axis, result in complex variations of the surface temperature.^[12]

Mercury’s orbit is inclined by 7° to the plane of Earth’s orbit (the ecliptic), as shown in the diagram on the right. As a result, transits of Mercury across the face of the Sun can only occur when the planet is crossing the plane of the ecliptic at the time it lies between the Earth and the Sun. This occurs about every seven years on average.^[57]

Orbit of Mercury as seen from the ascending node (bottom) and from 10° above (top)

Functionally, Mercury’s axial tilt is nonexistent,^[58][59] with measurements as low as 0.027°.^[6] This is significantly smaller than that of Jupiter, which boasts the second smallest axial tilt of all planets at 3.1 degrees. This means an observer at Mercury’s equator during local noon would never see the Sun more than approximately 1/30^[a] of one degree north or south of the zenith. Conversely, at the poles the Sun never rises more than 2.1′ above the horizon.^[6]

At certain points on Mercury’s surface, an observer would be able to see the Sun rise about halfway, then reverse and set before rising again, all within the same Mercurian day. This is because approximately four days prior to perihelion, Mercury’s angular orbital velocity exactly equals its angular rotational velocity so that the Sun’s apparent motion ceases; at perihelion, Mercury’s angular orbital velocity then exceeds the angular rotational velocity. Thus, the Sun appears to move in a retrograde direction. Four days after perihelion, the Sun’s normal apparent motion resumes at these points.^[12]

Advance of perihelion

Main articles: Tests of general relativity#Perihelion_precession_of_Mercury and Perihelion precession of Mercury

During the 19th century, French mathematician Le Verrier noticed that the slow precession of Mercury’s orbit around the Sun could not be completely explained by Newtonian mechanics and perturbations by the known planets. He proposed that another planet might exist in an orbit even closer to the Sun to account for this perturbation. (Other explanations considered included a slight oblateness of the Sun.) The success of the search for Neptune based on its perturbations of the orbit of Uranus led astronomers to place great faith in this explanation, and the hypothetical planet was even named Vulcan. However, no such planet was ever found.^[60]

In the early 20th century, Albert Einstein’s General Theory of Relativity provided the explanation for the observed precession. The effect is very small: the Mercurian relativistic perihelion advance excess is just 42.98 arcseconds per century, therefore it requires a little over twelve million orbits for a full excess turn. Similar, but much smaller effects, operate for other planets, being 8.62 arcseconds per century for Venus, 3.84 for Earth, 1.35 for Mars, and 10.05 for 1566 Icarus.^[61][62]

After one orbit, Mercury has rotated 1.5 times, so after two complete orbits the same hemisphere is again illuminated.

Spin–orbit resonance

For many years it was thought that Mercury was synchronously tidally locked with the Sun, rotating once for each orbit and keeping the same face directed towards the Sun at all times, in the same way that the same side of the Moon always faces the Earth. However, radar observations in 1965 proved that the planet has a 3:2 spin–orbit resonance, rotating three times for every two revolutions around the Sun; the eccentricity of Mercury’s orbit makes this resonance stable—at perihelion, when the solar tide is strongest, the Sun is nearly still in Mercury’s sky.^[63]

The original reason astronomers thought it was synchronously locked was that whenever Mercury was best placed for observation, it was always at the same point in its 3:2 resonance, hence showing the same face. Due to Mercury’s 3:2 spin–orbit resonance, a solar day (the length between two meridian transits of the Sun) lasts about 176 Earth days.^[12] A sidereal day (the period of rotation) lasts about 58.7 Earth days.^[12]

Orbital simulations indicate that the eccentricity of Mercury’s orbit varies chaotically from 0 (circular) to a very high 0.47 over millions of years.^[12] This is thought to explain Mercury’s 3:2 spin-orbit resonance (rather than the more usual 1:1), since this state is more likely to arise during a period of high eccentricity.^[64]

Observation

Mercury’s apparent magnitude varies between about −2.0—brighter than Sirius—and 5.5.^[65] Observation of Mercury is complicated by its proximity to the Sun, as it is lost in the Sun’s glare for much of the time. Mercury can be observed for only a brief period during either morning or evening twilight. The Hubble Space Telescope cannot observe Mercury at all, due to safety procedures which prevent its pointing too close to the Sun.^[66]

Like the Moon, Mercury exhibits phases as seen from Earth, being “new” at inferior conjunction and “full” at superior conjunction. The planet is rendered invisible on both of these occasions by virtue of its rising and setting in concert with the Sun in each case. The first and last quarter phases occur at greatest elongation east and west, respectively, when Mercury’s separation from the Sun ranges anywhere from 17.9° at perihelion to 27.8° at aphelion.^[67][68] At greatest elongation west, Mercury rises earliest before the Sun, and at greatest elongation east, it sets latest after the Sun.^[69]

Mercury attains inferior conjunction every 116 days on average,^[3] but this interval can range from 111 days to 121 days due to the planet’s eccentric orbit. Mercury can come as close as 77.3 million km to the Earth,^[3] but currently it does not come closer than 82 million km from the Earth.^[68] Its period of retrograde motion as seen from Earth can vary from 8 to 15 days on either side of inferior conjunction. This large range also arises from the planet’s high orbital eccentricity.^[12]

Mercury is more often easily visible from Earth’s Southern Hemisphere than from its Northern Hemisphere; this is because its maximum possible elongations west of the Sun always occur when it is early autumn in the Southern Hemisphere, while its maximum possible eastern elongations happen when the Southern Hemisphere is having its late winter season.^[69] In both of these cases, the angle Mercury strikes with the ecliptic is maximized, allowing it to rise several hours before the Sun in the former instance and not set until several hours after sundown in the latter in countries located at southern temperate zone latitudes, such as Argentina and New Zealand.^[69] By contrast, at northern temperate latitudes, Mercury is never above the horizon of a more-or-less fully dark night sky. Mercury can also, like several other planets and the brightest stars, be seen during a total solar eclipse.^[70]

Mercury is brightest as seen from Earth when it is at a gibbous phase, between either quarter phase and full. Although the planet is further away from Earth when it is gibbous than when it is a crescent, the greater illuminated area visible more than compensates for the greater distance.^[65] The opposite is true for Venus, which appears brightest when it is a thin crescent, because it is much closer to Earth than when gibbous.^[71]

Studies of Mercury

Ancient astronomers

The earliest known recorded observations of Mercury are from the MUL.APIN tablets. These observations were most likely made by an Assyrian astronomer around the 14th century BC.^[72] The cuneiform name used to designate Mercury on the MUL.APIN tablets is transcribed as UDU.IDIM.GU₄.UD (“the jumping planet”).^[b][73] Babylonian records of Mercury date back to the 1st millennium BC. The Babylonians called the planet Nabu after the messenger to the Gods in their mythology.^[74]

The ancient Greeks of Hesiod‘s time knew the planet as Στίλβων (Stilbon), meaning “the gleaming”, and Ἑρμάων (Hermaon).^[75] Later Greeks called the planet Apollo when it was visible in the morning sky and Hermes when visible in the evening. Around the 4th century BC, however, Greek astronomers came to understand that the two names referred to the same body. The Romans named the planet after the Roman messenger god, Mercury (Latin Mercurius), which they equated with the Greek Hermes.^[10][76]

In ancient China, Mercury was known as Ch’en-Hsing, the Hour Star. It was associated with the direction north and the phase of water in the Wu Xing.^[77] Hindu mythology used the name Budha for Mercury, and this god was thought to preside over Wednesday.^[78] The god Odin (or Woden) of Germanic paganism was also associated with the planet Mercury and the name Wednesday was derived from Woden’s day.^[79] The Maya may have represented Mercury as an owl (or possibly four owls; two for the morning aspect and two for the evening) that served as a messenger to the underworld.^[80]

Ground-based telescopic research

Transit of Mercury. Mercury is the small dot in the lower center, in front of the sun. The dark area on the left of the solar disk is a sunspot.

The first telescopic observations of Mercury were made by Galileo in the early 17th century. Although he observed phases when he looked at Venus, his telescope was not powerful enough to see the phases of Mercury. In 1631 Pierre Gassendi made the first observations of the transit of a planet across the Sun when he saw a transit of Mercury predicted by Johannes Kepler. In 1639 Giovanni Zupi used a telescope to discover that the planet had orbital phases similar to Venus and the Moon. The observation demonstrated conclusively that Mercury orbited around the Sun.^[12]

A very rare event in astronomy is the passage of one planet in front of another (occultation), as seen from Earth. Mercury and Venus occult each other every few centuries, and the event of May 28, 1737 is the only one historically observed, having been seen by John Bevis at the Royal Greenwich Observatory.^[81] The next occultation of Mercury by Venus will be on December 3, 2133.^[82]

The difficulties inherent in observing Mercury mean that it has been far less studied than the other planets. In 1800 Johann Schröter made observations of surface features, claiming to observed 20 km high mountains. Friedrich Bessel used Schröter’s drawings to erroneously estimate the rotation period as 24 hours and an axial tilt of 70°.^[83] In the 1880s Giovanni Schiaparelli mapped the planet more accurately, and suggested that Mercury’s rotational period was 88 days, the same as its orbital period due to tidal locking.^[84] This phenomenon is known as synchronous rotation and is also shown by Earth’s Moon. The effort to Map the surface of Mercury was continued by Eugenios Antoniadi, who published a book in 1934 that included both maps and his own observations.^[50] Many of the planet’s surface features, particularly the albedo features, take their names from Antoniadi’s map.^[85]

In June 1962 Soviet scientists at the Institute of Radio-engineering and Electronics of the USSR Academy of Sciences lead by Vladimir Kotelnikov became first to bounce radar signal off Mercury and receive it, starting radar observations of the planet.^[86][87][88] Three years later radar observations by Americans Gordon Pettengill and R. Dyce using 300-meter Arecibo Observatory radio telescope in Puerto Rico showed conclusively that the planet’s rotational period was about 59 days.^[89][90] The theory that Mercury’s rotation was synchronous became widely held, and it was a surprise to astronomers when these radio observations were announced. If Mercury were tidally locked, its dark face would be extremely cold, but measurements of radio emission revealed that it was much hotter than expected. Astronomers were reluctant to drop the synchronous rotation theory and proposed alternative mechanisms such as powerful heat-distributing winds to explain the observations.^[91]

Italian astronomer Giuseppe Colombo noted that the rotation value was about two-thirds of Mercury’s orbital period, and proposed that a different form of tidal locking had occurred in which the planet’s orbital and rotational periods were locked into a 3:2 rather than a 1:1 resonance.^[92] Data from Mariner 10 subsequently confirmed this view.^[93]

Ground-based observations did not shed much further light on the innermost planet, and it was not until space probes visited Mercury that many of its most fundamental properties became known. However, recent technological advances have led to improved ground-based observations. In 2000, high-resolution lucky imaging from the Mount Wilson Observatory 1500 mm telescope provided the first views that resolved some surface features on the parts of Mercury which were not imaged in the Mariner missions.^[94] Later imaging has shown evidence of a huge double-ringed impact basin even larger than the Caloris Basin in the non-Mariner-imaged hemisphere. It has informally been dubbed the Skinakas Basin.^[33] Most of the planet has been mapped by the Arecibo radar telescope, with 5 km resolution, including polar deposits in shadowed craters of what may be water ice.^[95]

The Mariner 10 probe, the first probe to visit the innermost planet

Research with space probes

Main article: Exploration of Mercury

Reaching Mercury from Earth poses significant technical challenges, since the planet orbits so much closer to the Sun than does the Earth. A Mercury-bound spacecraft launched from Earth must travel over 91 million kilometers into the Sun’s gravitational potential well. Starting from the Earth’s orbital speed of 30 km/s, the change in velocity (delta-v) the spacecraft must make to enter into a Hohmann transfer orbit that passes near Mercury is large compared to other planetary missions.^[96]

The potential energy liberated by moving down the Sun’s potential well becomes kinetic energy; requiring another large delta-v change to do anything other than rapidly pass by Mercury. In order to land safely or enter a stable orbit the spacecraft must rely entirely on rocket motors since aerobraking is ruled out because the planet has very little atmosphere. A trip to Mercury actually requires more rocket fuel than that required to escape the solar system completely. As a result, only two space probes have visited the planet so far.^[97] A proposed alternative approach would use a solar sail to attain a Mercury-synchronous orbit around the Sun.^[98]

Mariner 10

Main article: Mariner 10

View of Mercury from Mariner 10

The first spacecraft to visit Mercury was NASA’s Mariner 10 (1974–75).^[10] The spacecraft used the gravity of Venus to adjust its orbital velocity so that it could approach Mercury, making it both the first spacecraft to use this gravitational “slingshot” effect and the first NASA mission to visit multiple planets.^[96] Mariner 10 provided the first close-up images of Mercury’s surface, which immediately showed its heavily cratered nature, and also revealed many other types of geological features, such as the giant scarps which were later ascribed to the effect of the planet shrinking slightly as its iron core cools.^[99] Unfortunately, due to the length of Mariner 10′s orbital period, the same face of the planet was lit at each of Mariner 10’s close approaches. This made observation of both sides of the planet impossible,^[100] and resulted in the mapping of less than 45% of the planet’s surface.^[101]

The spacecraft made three close approaches to Mercury, the closest of which took it to within 327 km of the surface.^[102] At the first close approach, instruments detected a magnetic field, to the great surprise of planetary geologists—Mercury’s rotation was expected to be much too slow to generate a significant dynamo effect. The second close approach was primarily used for imaging, but at the third approach, extensive magnetic data were obtained. The data revealed that the planet’s magnetic field is much like the Earth’s, which deflects the solar wind around the planet. However, the origin of Mercury’s magnetic field is still the subject of several competing theories.^[103]

Just a few days after its final close approach, Mariner 10 ran out of fuel. Since its orbit could no longer be accurately controlled, mission controllers instructed the probe to shut itself down on March 24, 1975.^[104] Mariner 10 is thought to be still orbiting the Sun, passing close to Mercury every few months.^[105]

MESSENGER

Main article: MESSENGER

MESSENGER being prepared for launch

A second NASA mission to Mercury, named MESSENGER (MErcury Surface, Space ENvironment, GEochemistry, and Ranging), was launched on August 3, 2004, from the Cape Canaveral Air Force Station aboard a Boeing Delta 2 rocket. The MESSENGER spacecraft made several close approaches to planets to place it onto the correct trajectory to reach an orbit around Mercury. It made a fly-by of the Earth in August 2005, and of Venus in October 2006 and June 2007.^[106] The first fly-by of Mercury occurred on January 14, 2008.^[107] Two more fly-bys of Mercury are scheduled, in October 2008 and September 2009.^[7] Most of the hemisphere not imaged by Mariner 10 will be mapped during the fly-bys. The probe will then enter an elliptical orbit around the planet in March 2011; the nominal mapping mission is one terrestrial year.^[107]

The mission is designed to shed light on six key issues: Mercury’s high density, its geological history, the nature of its magnetic field, the structure of its core, whether it really has ice at its poles, and where its tenuous atmosphere comes from. To this end, the probe is carrying imaging devices which will gather much higher resolution images of much more of the planet than Mariner 10, assorted spectrometers to determine abundances of elements in the crust, and magnetometers and devices to measure velocities of charged particles. Detailed measurements of tiny changes in the probe’s velocity as it orbits will be used to infer details of the planet’s interior structure.^[22]

BepiColombo

Main article: BepiColombo

The European Space Agency is planning a joint mission with Japan called BepiColombo, which will orbit Mercury with two probes: one to map the planet and the other to study its magnetosphere^[108]. A Russian Soyuz rocket will launch the bus carrying the two probes in 2013 from ESA’s Guiana Space Center to take advantage of its equatorial location.^[108] As with MESSENGER, the BepiColombo bus will make close approaches to other planets en route to Mercury for orbit-changing gravitational assists, passing the Moon and Venus and making several approaches to Mercury before entering orbit.^[108] A combination of chemical and ion engines will be used, the latter thrusting continuously for long intervals.^[109][108] The spacecraft bus will reach Mercury in 2019.^[109] The bus will release the magnetometer probe into an elliptical orbit, then chemical rockets will fire to deposit the mapper probe into a circular orbit. Both probes will operate for a terrestrial year.^[108]

The mapper probe will carry an array of spectrometers similar to those on MESSENGER, and will study the planet at many different wavelengths including infrared, ultraviolet, X-ray and gamma ray. Apart from intensively studying the planet itself, mission planners also hope to use the probe’s proximity to the Sun to test the predictions of General Relativity theory with improved accuracy.^[110]

The mission is named after Giuseppe (Bepi) Colombo, the scientist who first determined the nature of Mercury’s spin-orbit resonance and who was also involved in the planning of Mariner 10’s gravity-assisted trajectory to the planet in 1974.[23