Geology of Mercury - AbsoluteAstronomy.com

The surface of Mercury

Mercury (planet)

Mercury is the innermost and smallest planet in the Solar System, orbiting the Sun once every 87.969 Earth days. The orbit of Mercury has the highest eccentricity of all the Solar System planets, and it has the smallest axial tilt. It completes three rotations about its axis for every two orbits...

is dominated by impact craters, and lava plains similar in some respects to the lunar maria. Other notable features include scarps and mineral deposits (possibly ice) inside craters at the poles. Currently, the surface is presumed to be geologically inactive. At present only about 55% of the surface has been mapped in sufficient detail to say much about its geology (by the Mariner 10

Mariner 10

Mariner 10 was an American robotic space probe launched by NASA on November 3, 1973, to fly by the planets Mercury and Venus. It was launched approximately two years after Mariner 9 and was the last spacecraft in the Mariner program...

spacecraft in 1974-5 and the MESSENGER

MESSENGER

The MErcury Surface, Space ENvironment, GEochemistry and Ranging space probe is a robotic NASA spacecraft in orbit around the planet Mercury. The spacecraft was launched aboard a Delta II rocket in August 2004 to study the chemical composition, geology, and magnetic field of Mercury...

spacecraft in 2008). Mercury's interior contains a very large metal core that accounts for about 42% of its volume. Part of this core may still be liquid as evidenced by a weak but global magnetosphere

Magnetosphere

A magnetosphere is formed when a stream of charged particles, such as the solar wind, interacts with and is deflected by the intrinsic magnetic field of a planet or similar body. Earth is surrounded by a magnetosphere, as are the other planets with intrinsic magnetic fields: Mercury, Jupiter,...

Difficulties in exploration

Of all the terrestrial planets in the Solar System

Solar System

The Solar System consists of the Sun and the astronomical objects gravitationally bound in orbit around it, all of which formed from the collapse of a giant molecular cloud approximately 4.6 billion years ago. The vast majority of the system's mass is in the Sun...

, the geology of Mercury is the least understood. This stems largely from Mercury

Mercury (planet)

's proximity to the Sun

Sun

The Sun is the star at the center of the Solar System. It is almost perfectly spherical and consists of hot plasma interwoven with magnetic fields...

which makes reaching it with spacecraft technically challenging and Earth-based observations difficult.

Virtually all that is known about Mercury's geology is based on the data from the three Mariner 10

Mariner 10

flybys in 1974 and 1975.

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

Spacecraft

A spacecraft or spaceship is a craft or machine designed for spaceflight. Spacecraft are used for a variety of purposes, including communications, earth observation, meteorology, navigation, planetary exploration and transportation of humans and cargo....

launched from Earth must travel 91 million kilometers into the Sun’s gravitational potential well

Potential well

A potential well is the region surrounding a local minimum of potential energy. Energy captured in a potential well is unable to convert to another type of energy because it is captured in the local minimum of a potential well...

. Starting from the Earth’s orbital speed

Orbital speed

The orbital speed of a body, generally a planet, a natural satellite, an artificial satellite, or a multiple star, is the speed at which it orbits around the barycenter of a system, usually around a more massive body...

of 30 km/s, the change in velocity

Velocity

In physics, velocity is speed in a given direction. Speed describes only how fast an object is moving, whereas velocity gives both the speed and direction of the object's motion. To have a constant velocity, an object must have a constant speed and motion in a constant direction. Constant ...

(delta-v

Delta-v

In astrodynamics a Δv or delta-v is a scalar which takes units of speed. It is a measure of the amount of "effort" that is needed to change from one trajectory to another by making an orbital maneuver....

) the spacecraft must make to enter into a Hohmann transfer orbit

Hohmann transfer orbit

In orbital mechanics, the Hohmann transfer orbit is an elliptical orbit used to transfer between two circular orbits, typically both in the same plane....

that passes near Mercury is large compared to other planetary missions. The potential energy

Potential energy

In physics, potential energy is the energy stored in a body or in a system due to its position in a force field or due to its configuration. The SI unit of measure for energy and work is the Joule...

liberated by moving down the Sun’s potential well becomes kinetic energy

Kinetic energy

The kinetic energy of an object is the energy which it possesses due to its motion.It is defined as the work needed to accelerate a body of a given mass from rest to its stated velocity. Having gained this energy during its acceleration, the body maintains this kinetic energy unless its speed changes...

; requiring another large delta-v 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

Aerobraking

Aerobraking is a spaceflight maneuver that reduces the high point of an elliptical orbit by flying the vehicle through the atmosphere at the low point of the orbit . The resulting drag slows the spacecraft...

is ruled out because the planet has very little atmosphere. A direct trip to Mercury actually requires more rocket fuel than that required to escape

Escape velocity

In physics, escape velocity is the speed at which the kinetic energy plus the gravitational potential energy of an object is zero gravitational potential energy is negative since gravity is an attractive force and the potential is defined to be zero at infinity...

the solar system completely. As a result, only two space probes, Mariner 10

Mariner 10

and MESSENGER

MESSENGER

, which are both NASA

NASA

The National Aeronautics and Space Administration is the agency of the United States government that is responsible for the nation's civilian space program and for aeronautics and aerospace research...

, have flown-by the planet so far.

Furthermore, the space environment near Mercury is demanding, posing the double dangers to spacecraft of intense solar radiation and high temperatures.

Historically, a second obstacle has been that Mercury's period of rotation

Rotation period

The rotation period of an astronomical object is the time it takes to complete one revolution around its axis of rotation relative to the background stars...

is a slow 58 Earth

Earth

Earth is the third planet from the Sun, and the densest and fifth-largest of the eight planets in the Solar System. It is also the largest of the Solar System's four terrestrial planets...

days, so that spacecraft flybys are restricted to viewing only a single illuminated hemisphere. In fact, unfortunately, even though Mariner 10

Mariner 10

space probe flew past Mercury three times during 1974 and 1975, it observed the same area during each pass. This was because Mariner 10's orbital period was almost exactly 3 sidereal Mercury days, and the same face of the planet was lit at each of the close approaches. As a result, less than 45% of the planet’s surface was mapped.

Earth-based observations are made difficult by Mercury's constant proximity to the Sun. This has several consequences:

Whenever the sky is dark enough for viewing through telescopes, Mercury is always already near the horizon, where viewing conditions are poor anyway due to atmospheric factors.
The Hubble Space Telescope
Hubble Space Telescope
The Hubble Space Telescope is a space telescope that was carried into orbit by a Space Shuttle in 1990 and remains in operation. A 2.4 meter aperture telescope in low Earth orbit, Hubble's four main instruments observe in the near ultraviolet, visible, and near infrared...

and other space observatories are usually prevented from pointing close to the Sun for safety reasons (Erroneously pointing such sensitive instruments at the Sun is likely to cause permanent damage).

It is hoped that NASA's MESSENGER

MESSENGER

probe, launched in August 2004, will greatly contribute to our understanding when it enters orbit around Mercury in March 2011.

Mercury's geological history

Like the Earth, Moon and Mars

Mars

Mars is the fourth planet from the Sun in the Solar System. The planet is named after the Roman god of war, Mars. It is often described as the "Red Planet", as the iron oxide prevalent on its surface gives it a reddish appearance...

, Mercury's geologic history is divided up into eras. From oldest to youngest, these are: the pre-Tolstojan, Tolstojan, Calorian, Mansurian, and Kuiperian. These ages are based on relative dating

Relative dating

Relative dating is the science determining the relative order of past events, without necessarily determining their absolute age.In geology rock or superficial deposits, fossils and lithologies can be used to correlate one stratigraphic column with another...

only.

After the formation of Mercury along with the rest of the solar system

Solar System

4.6 billion years ago, heavy bombardment by asteroids and comets ensued. The last intense bombardment phase, the Late Heavy Bombardment

Late Heavy Bombardment

The Late Heavy Bombardment is a period of time approximately 4.1 to 3.8 billion years ago during which a large number of impact craters are believed to have formed on the Moon, and by inference on Earth, Mercury, Venus, and Mars as well...

came to an end about 3.8 billion years ago. Some regions or massif

Massif

In geology, a massif is a section of a planet's crust that is demarcated by faults or flexures. In the movement of the crust, a massif tends to retain its internal structure while being displaced as a whole...

s, a prominent one being the one that formed the Caloris Basin

Caloris Basin

The Caloris Basin, also called Caloris Planitia, is a large impact crater on Mercury about in diameter, one of the largest impact basins in the solar system. Caloris is Latin for heat and the basin is so-named because the Sun is almost directly overhead every second time Mercury passes perihelion...

, were filled by magma eruptions from within the planet. These created smooth intercrater plains similar to the maria

Lunar mare

The lunar maria are large, dark, basaltic plains on Earth's Moon, formed by ancient volcanic eruptions. They were dubbed maria, Latin for "seas", by early astronomers who mistook them for actual seas. They are less reflective than the "highlands" as a result of their iron-rich compositions, and...

found on the Moon

Moon

The Moon is Earth's only known natural satellite,There are a number of near-Earth asteroids including 3753 Cruithne that are co-orbital with Earth: their orbits bring them close to Earth for periods of time but then alter in the long term . These are quasi-satellites and not true moons. For more...

.
Later, as the planet cooled and contracted, its surface began to crack and form ridges; these surface cracks and ridges can be seen on top of other features, such as the craters and smoother plains – a clear indication that they are more recent.
Mercury's period of vulcanism

Vulcanism

Vulcanism may refer to* Volcanism or volcanic activity.* Plutonism, a scientific theory of the Earth....

ended when the planet's mantle had contracted enough to prevent further lava

Lava

Lava refers both to molten rock expelled by a volcano during an eruption and the resulting rock after solidification and cooling. This molten rock is formed in the interior of some planets, including Earth, and some of their satellites. When first erupted from a volcanic vent, lava is a liquid at...

from breaking through to the surface. This probably occurred at some point during its first 700 or 800 million years of history.

Since then, the main surface processes have been intermittent impacts.

Surface features

Mercury’s surface is overall similar in appearance to that of the Moon, with extensive mare

Lunar mare

-like plains and heavily cratered terrains similar to the Lunar highlands

Geology of the Moon

The geology of the Moon is quite different from that of the Earth...

Impact basins and craters

Craters on Mercury range in diameter from small bowl-shaped craters 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 — the extent of their ejecta blankets is much smaller, which is a consequence of the 2.5 times stronger surface gravity on Mercury.

The largest known crater is the enormous Caloris Basin, with a diameter of 1550 km, A basin of comparable size, tentatively named Skinakas Basin

Skinakas Basin

The Skinakas Basin is the informal name given to a structure on Mercury that appeared to be an extremely large impact basin. The traditional name for this region of Mercury is Solitudo Aphrodites...

had been postulated from low resolution Earth-based observations of the non-Mariner-imaged hemisphere, but has not been observed in MESSENGER imagery of the corresponding terrain. The impact which created the Caloris Basin was so powerful that its effects are seen on a global scale. It caused lava

Lava

eruptions and left a concentric ring over 2 km tall surrounding the impact crater

Impact crater

In the broadest sense, the term impact crater can be applied to any depression, natural or manmade, resulting from the high velocity impact of a projectile with a larger body...

. At the antipode

Antipode

Antipode, Antipodes, or Antipodeans may refer to:* Antipodal point, the diametrically opposite point on a sphere* Antipodes Water Company, a premium bottled water brand...

of the Caloris Basin lies a large region of unusual, hilly and furrowed terrain, sometimes called “Weird Terrain”. The favoured hypothesis for the origin of this geomorphologic unit is that shock waves generated during the impact traveled around the planet, and when they converged at the basin’s antipode (180 degrees away) the high stresses were capable of fracturing the surface. A much less favoured idea was that this terrain formed as a result of the convergence of ejecta at this basin’s antipode. Furthermore, the formation of the Caloris Basin appears to have produced a shallow depression concentric around the basin, which was later filled by the smooth plains (see below).

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.

As on the Moon

Moon

, fresh craters on Mercury show prominent bright Ray system

Ray system

A ray system comprises radial streaks of fine ejecta thrown out during the formation of an impact crater, looking a bit like many thin spokes coming from the hub of a wheel. The rays can extend for lengths up to several times the diameter of their originating crater, and are often accompanied by...

s. These are made by ejected debris, which while they remain relatively fresh tend to be brighter because of a lesser amount of space weathering

Space weathering

Space weathering is a blanket term used for a number of processes that act on any body exposed to the harsh space environment. Airless bodies incur many weathering processes:* collisions of galactic cosmic rays and solar cosmic rays,* irradiation, implantation, and sputtering from solar wind...

than the surrounding older terrain.

Pit-floor craters

Some impact craters on Mercury have non-circular, irregularly shaped depressions or pits on their floors. Such craters have been termed pit-floor craters, and MESSENGER

MESSENGER

team members have suggested that such pits formed by the collapse of subsurface magma chamber

Magma chamber

A magma chamber is a large underground pool of molten rock found beneath the surface of the Earth. The molten rock in such a chamber is under great pressure, and given enough time, that pressure can gradually fracture the rock around it creating outlets for the magma...

s. If this suggestion is correct, the pits are evidence of volcanic processes at work on Mercury. Pit craters are rimless, often irregularly shaped, and steep-sided, and they display no associated ejecta

Ejecta

Ejecta can mean:*In volcanology, particles that came out of a volcanic vent, traveled through the air or under water, and fell back on the ground surface or on the ocean floor...

or lava flows but are typically distinctive in color. For example, the pits of Praxiteles

Praxiteles (crater)

Praxiteles is a crater on Mercury.MESSENGER's high-resolution images obtained during the mission's second Mercury flyby have revealed a number of irregularly shaped depressions on the floor of Praxiteles crater, making it a pit-floor crater...

have an orange hue. Thought to be evidence of shallow magmatic activity, pit craters may have formed when subsurface magma drained elsewhere and left a roof area unsupported, leading to collapse and the formation of the pit. Major craters exhibiting these features include Beckett

Beckett (crater)

Beckett is a pit-floored crater on Mercury, which was discovered in January 2008 during the first flyby of the planet by MESSENGER spacecraft. Its floor is not smooth and displays a telephone or arc-shaped collapse feature, which is also called a central pit. The size of the pit is...

, Gibran

Gibran (crater)

Gibran is a crater on Mercury, which was discovered in January 2008 during the first flyby of the planet by MESSENGER spacecraft. It contains a large , nearly circular pit crater. Multiple examples of pit craters have been observed on Mercury on the floors of impact craters, leading to the name...

and Lermontov

Lermontov (crater)

Lermontov is an impact crater on the planet Mercury, 152 kilometers in diameter. It is located at 15.2°N, 48.1°W, southwest of the crater Proust and northeast of the crater Giotto. It has a circular rim and a flat crater floor. Lermontov is likely a mature crater, but it remains a bright feature...

, among others.

Plains

There are two geologically distinct plains units on Mercury:

Inter-crater plains are the oldest visible surface, predating the heavily cratered terrain. They are gently rolling or hilly and occur in the regions between larger craters. 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. It is not clear whether they are of volcanic or impact origin. The inter-crater plains are distributed roughly uniformly over the entire surface of the planet.
Smooth plains are widespread flat areas resembling the lunar maria, which fill depressions of various sizes. Notably, they fill a wide ring surrounding the Caloris Basin. An appreciable difference to the lunar maria is that the smooth plains of Mercury have the same albedo as the older intercrater plains. Despite a lack of unequivocally volcanic features, their localisation and lobate-shaped colour units strongly support a volcanic origin. 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.

The floor of the Caloris Basin

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.

Tectonic features

One unusual feature of the planet’s surface is the numerous compression folds 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. Mercury’s surface is also flexed by significant tidal bulges raised by the Sun

Sun

The Sun is the star at the center of the Solar System. It is almost perfectly spherical and consists of hot plasma interwoven with magnetic fields...

—the Sun’s tides on Mercury are about 17% stronger than the Moon’s on Earth.

Terminology

Non-crater surface features are given the following names:

Albedo
Albedo
Albedo , or reflection coefficient, is the diffuse reflectivity or reflecting power of a surface. It is defined as the ratio of reflected radiation from the surface to incident radiation upon it...

features — areas of markedly different reflectivity
Dorsa — ridge
Ridge
A ridge is a geological feature consisting of a chain of mountains or hills that form a continuous elevated crest for some distance. Ridges are usually termed hills or mountains as well, depending on size. There are several main types of ridges:...

s (see List of ridges on Mercury)
Montes — mountain
Mountain
Image:Himalaya_annotated.jpg|thumb|right|The Himalayan mountain range with Mount Everestrect 58 14 160 49 Chomo Lonzorect 200 28 335 52 Makalurect 378 24 566 45 Mount Everestrect 188 581 920 656 Tibetan Plateaurect 250 406 340 427 Rong River...

s (see List of mountains on Mercury)
Planitiae — plain
Plain
In geography, a plain is land with relatively low relief, that is flat or gently rolling. Prairies and steppes are types of plains, and the archetype for a plain is often thought of as a grassland, but plains in their natural state may also be covered in shrublands, woodland and forest, or...

s (see List of plains on Mercury)
Rupes
Rupes
Rupes is the Latin word for 'cliff'. It is used in planetary geology to refer to escarpments on other planets, such as Mercury, and moons, such as Luna, Earth's natural satellite.How rupes are formed is, as of 2008, a matter of speculation...

— scarps
Escarpment
An escarpment is a steep slope or long cliff that occurs from erosion or faulting and separates two relatively level areas of differing elevations.-Description and variants:...

(see List of scarps on Mercury)
Valles
Vallis
Vallis is the Latin word for valley. It is used in planetary geology for the naming of landform features on other planets....

— valley
Valley
In geology, a valley or dale is a depression with predominant extent in one direction. A very deep river valley may be called a canyon or gorge.The terms U-shaped and V-shaped are descriptive terms of geography to characterize the form of valleys...

s (see List of valleys on Mercury)

High luminosity polar patches and possible presence of ice

The first radar observations of Mercury were carried out by the radiotelescopes at Arecibo

Arecibo Observatory

The Arecibo Observatory is a radio telescope near the city of Arecibo in Puerto Rico. It is operated by SRI International under cooperative agreement with the National Science Foundation...

(Puerto Rico

Puerto Rico

Puerto Rico , officially the Commonwealth of Puerto Rico , is an unincorporated territory of the United States, located in the northeastern Caribbean, east of the Dominican Republic and west of both the United States Virgin Islands and the British Virgin Islands.Puerto Rico comprises an...

) and Goldstone

Goldstone Deep Space Communications Complex

The Goldstone Deep Space Communications Complex — commonly called the Goldstone Observatory — is located in California's Mojave Desert. Operated by ITT Corporation for the Jet Propulsion Laboratory, its main purpose is to track and communicate with space missions. It includes the Pioneer...

(California

California

California is a state located on the West Coast of the United States. It is by far the most populous U.S. state, and the third-largest by land area...

, United States

United States

The United States of America is a federal constitutional republic comprising fifty states and a federal district...

), with assistance from the U.S.

United States

The United States of America is a federal constitutional republic comprising fifty states and a federal district...

National Radio Astronomy Observatory

The National Radio Astronomy Observatory is a Federally Funded Research and Development Center of the United States National Science Foundation operated under cooperative agreement by Associated Universities, Inc for the purpose of radio astronomy...

Very Large Array

The Very Large Array is a radio astronomy observatory located on the Plains of San Agustin, between the towns of Magdalena and Datil, some fifty miles west of Socorro, New Mexico, USA...

(VLA) facility in New Mexico

New Mexico

New Mexico is a state located in the southwest and western regions of the United States. New Mexico is also usually considered one of the Mountain States. With a population density of 16 per square mile, New Mexico is the sixth-most sparsely inhabited U.S...

. The transmissions sent from the NASA Deep Space Network

Deep Space Network

The Deep Space Network, or DSN, is a world-wide network of large antennas and communication facilities that supports interplanetary spacecraft missions. It also performs radio and radar astronomy observations for the exploration of the solar system and the universe, and supports selected...

site at Goldstone were at a power level of 460 kW at 8.51 GHz; the signals received by the VLA multi-dish array detected points of radar reflectivity (radar luminosity) with depolarized waves from Mercury's north pole.

Radar maps of the surface of the planet were made using the Arecibo radiotelescope. The survey was conducted with 420 kW UHF band

Ultra high frequency

Ultra-High Frequency designates the ITU Radio frequency range of electromagnetic waves between 300 MHz and 3 GHz , also known as the decimetre band or decimetre wave as the wavelengths range from one to ten decimetres...

(2.4 GHz) radio waves which allowed for a 15 km resolution. This study not only confirmed the existence of the zones of high reflectivity and depolarization, but also found a number of new areas (bringing the total to 20) and was even able to survey the poles. It has been postulated that surface ice

Ice

Ice is water frozen into the solid state. Usually ice is the phase known as ice Ih, which is the most abundant of the varying solid phases on the Earth's surface. It can appear transparent or opaque bluish-white color, depending on the presence of impurities or air inclusions...

may be responsible for these phenomena.

The belief that Mercury has surface ice may seem absurd at first, given its proximity to the Sun. Regardless, it could very well be ice that is responsible for the high luminosity levels, as the silicate rocks that compose most of the surface of Mercury have exactly the opposite effect on luminosity. The presence of ice may be explained by another discovery of the radar surveys from Earth: crater

Impact crater

In the broadest sense, the term impact crater can be applied to any depression, natural or manmade, resulting from the high velocity impact of a projectile with a larger body...

s at Mercury's higher latitudes may be deep enough to shield the ice from direct sunlight.

At the South Pole, the location of a large zone of high reflectivity coincides with the location of the Chao Meng-Fu crater

Chao Meng-Fu (crater)

Chao Meng-Fu is a 167 kilometer-diameter crater on Mercury named after the Chinese painter and calligrapher Zhao Mengfu . Due to its location near Mercury's south pole and the planet's small axial tilt, an estimated 40% of the crater lies in permanent shadow...

, and other small craters containing reflective areas have also been identified. At the North Pole, a number of craters smaller than Chao-Meng Fu have these reflective properties.

The strength of the radar reflections seen on Mercury are small compared to that which would occur with pure ice. This may be due to powder deposition that does not cover the surface of the crater completely or other causes, e.g. a thin overlying surface layer. However, the evidence for ice on Mercury is not definitive. The anomalous reflective properties could also be due to the existence of deposits of metallic sulfates or other materials with high reflectance.

Possible origin of ice

Mercury is not unique in having craters that stand in permanent shadow; at the south pole of Earth's Moon

Moon

there is a large crater (Aitken

Aitken (crater)

Aitken is a large lunar impact crater that lies on the far side of the Moon, named for Robert Grant Aitken, an American astronomer specializing in binary stellar systems. It is located to the southeast of the crater Heaviside, and north of the unusual formation Van de Graaff. Attached to the...

) where some possible signs of the presence of ice

Lunar ice

Lunar water is water that is present on the Moon. Liquid water cannot persist at the Moon's surface, and water vapour is quickly decomposed by sunlight and lost to outer space...

have been seen (although their interpretation is disputed). It is thought by astronomers that ice on both Mercury and the Moon must have originated from external sources, mostly impacting comet

Comet

A comet is an icy small Solar System body that, when close enough to the Sun, displays a visible coma and sometimes also a tail. These phenomena are both due to the effects of solar radiation and the solar wind upon the nucleus of the comet...

s. These are known to contain large amounts, or a majority, of ice. It is therefore conceivable for meteorite impacts to have deposited water in the permanently-shadow craters, where it would remain unwarmed for possibly billions of years due to the lack of an atmosphere to efficiently conduct heat and stable orientation of Mercury's rotation axis

Axial tilt

In astronomy, axial tilt is the angle between an object's rotational axis, and a line perpendicular to its orbital plane...

.

Despite sublimation into the vacuum of space, the temperature in the permanently shadowed region is so low that this sublimation is slow enough to potentially preserve deposited ice for billions of years. Inside the craters, where there is no solar light, temperatures fall to -171°C; on the polar plains, the temperature does not rise above -106°C.

External links

Mariner 10
MESSENGER probe
Mercury on Nineplanets.org
USGS Geology of Mercury Retrieved 5 August 2007

The source of this article is wikipedia, the free encyclopedia. The text of this article is licensed under the GFDL.

Difficulties in exploration

Mercury's geological history

Surface features

Impact basins and craters

Pit-floor craters

Plains

Tectonic features

Terminology

High luminosity polar patches and possible presence of ice

Possible origin of ice

See also

Original references for Spanish article

External links