Geology of Mercury
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Of all the terrestrial planets in the Solar System, the geology of Mercury is the least understood. Reasons for this include Mercury's proximity to the Sun and the resultant dangers to spacecraft of intense solar radiation and high surface temperatures. Also, Mercury's period of rotation is a slow 58 Earth days, so that when NASA's Mariner 10 space probe flew past Mercury three times during 1974 and 1975, it was only able to observe the side facing the Sun during each pass. It is hoped that NASA's MESSENGER probe, launched in August 2004, will greatly contribute to our understanding when it enters orbit around Mercury in March 2011.
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[edit] Mercury's geological history
Like the Moon and Mars, 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 only.[1][2]
After the formation of Mercury over four billion years ago, it received heavy bombardment of comets and asteroids that came to an end 3.8 billion years ago. During this period of intense crater formation, the surface received many impacts. Other massifs, such as the one that formed the Caloris Basin, were filled by magma from within the planet, which produced smooth intercrater plains similar to the maria found on the Moon. As the planet cooled and contracted, its surface began to crack; these surface cracks 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 ended when the planet's mantle had contracted enough to prevent further lava 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 only impacts have been the result of the planet being struck by stray asteroids and comets.
[edit] General characteristics of the surface
With a surface density of 5.44 g/cm3, the surface of Mercury is highly cratered, and those craters are uniformly distributed over the surface. The present surface exhibits such an abundant quantity of craters because Mercury's atmosphere is so thin that meteorites can easily reach the surface without disintegrating. The surface of Mercury has accumulated meteoric impacts since its creation over 4 billion years ago, and for this reason its surface, like those of the Moon and Mars registers the importance such impacts have in determining the duration of this period of craterization, which was very intense for about 3 billion years.
Apart from craters of diameters in the range of hundreds of meters to hundreds of kilometers, there are others of gigantic proportions such as Caloris, the largest structure on the surface of Mercury with a diameter of 1,300 km. The impact was so powerful that it caused lava eruptions from the crust of the planet and left a concentric ring surrounding the impact crater over 2 km tall. The consequences of Caloris are also impressive: it is widely accepted as the cause for the fractures and leaks on the opposite side of the planet.
These kinds of craters which have been filled with lava are known as seas in lunar geology.
As on the Moon, craters in Mercury show the typical characteristics of an impact: the ejected debris forms banks around the crater shaped as linear extensions known as radii (or rays), the brightness of which is stronger because the terrain is relatively younger than the surrounding surface.
Other escarpments have also been seen crossing the planet's surface, in both the heavily cratered and flat areas. They are attributed to the cooling that Mercury has undergone since its formation, shrinking and causing the crust to realign itself.
The planet's high density (5.44 g/cm3) indicates that it has a core of 65% iron, accounting for somewhere about 75% of its diameter. This core is then surrounded by a 600 km thick mantle. As the cooling of the planet caused the core and mantle to shrink after its initial formation, Mercury' lost an estimated 2 to 4 km of its radius, which created the network of fissures visible on its surface.
[edit] Origin of the planet's high luminosity and the presence of ice
The first radar observations of Mercury were carried out by the radiotelescopes at Arecibo (Puerto Rico) and Goldstone (California, United States), with assistance from the U.S. National Radio Astronomy Observatory Very Large Array (VLA) facility in New Mexico. The transmissions sent from the NASA Deep Space Network 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 (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 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: craters 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, and other small craters containing reflective areas have also been identified.
At the North Pole, the situation is more complicated; no one can correlate the radar images with the data from Mariner 10, due to minor differences in the images. It should also be emphasized that there are areas of high reflectivity that do not correspond to any known craters.
The radar reflection of ice on Mercury is minor 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.
[edit] Origin of ice
Mercury is not unique in having craters that stand in permanent shadow; at the south pole of Earth's Moon there is a large crater (Aitken) where ice is possibly thought to exist. Ice on both Mercury and the Moon must have originated from external sources: comets in the case of the Moon, or meteorites in the case of Mercury. The existence of ice on certain meteorites has been proven; it is therefore conceivable for meteorite impacts to have deposited water in the permanent-shadow craters, where it would have remained for millions or billions of years.
Another hypothesis, which has not been confirmed, is that Mercury has an important flow of water from its interior. It has also not been proved that any mechanism, such as photodissociation, erosion due to solar wind, or small meteorite impact, causes the loss of ice in on the surface.
The behavior of ice on other celestial bodies has its peculiarities. The high temperatures on the surface of Mercury, near 420°C, the emptiness of space (the atmosphere of Mercury is almost imperceptible), and solar rays contribute to the sublimation of ice into vapor and its escape into space.
Nevertheless, it is not thought that such behavior occurs with ice on Mercury because the location of ice at high latitudes makes it so that the temperature is very low. 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.
The evidence for ice on Mercury has not been irrefutably corroborated, but is simply scientific speculation provoked by images of areas of high reflectivity and the coincidence of the existence of large craters in polar zones. It must be made clear, however, that this anomalous reflection could also be due to the existence of metallic sulfates or other materials with the same capacity for reflection.
[edit] Mercury's atmosphere
The existence of an atmosphere on a planet is of great geological relevance, since the erosion caused by wind, changes in temperature levels, moisture levels, etc contribute to the modification of the landscape and the deterioration of materials.
Mercury's atmosphere dissipated shortly after the planet's formation over four billion years ago because of the low level of gravity on the planet and, mainly, the effects of the solar wind. However, there are still traces of a very thin atmosphere with a pressure level of 10-15 bar (which can be considered negligible). The existence of an atmosphere would also keep temperatures more or less stable despite the variations in sunlight levels between night and day; consequently, temperature variations in bodies without atmospheres (or with extremely weak atmospheres) are more pronounced. For example, during the day Mercury's surface reaches a temperature of 420°C, while at night it dips to –180°C.
Due to the abrupt changes in the temperature, the type of interaction over the surface is related to the thermal agitation produced on the materials.
[edit] See also
[edit] References
- ^ Map of Mercury (PDF, large image; bilingual)
- ^ Paul Spudis, "The Geological History of Mercury" (PDF)
- Stardate, Guide to the Solar System. Publicación de la University of Texas at Austin McDonald Observatory
- Our Solar System, A Geologic Snapshot. NASA (NP-157). May 1992.
- Fotografía: Mercury. NASA (LG-1997-12478-HQ)
- This article draws heavily on the corresponding article in the Spanish-language Wikipedia, which was accessed in the version of 26 June 2005. It was translated by the Spanish Translation of the Week collaboration.
[edit] Original references for Spanish article
- Ciencias de la Tierra. Una Introducción a la Geología Física (Earth Sciences, an Introduction to Physical Geology), by Edward J. Tarbuck y Frederick K. Lutgens. Prentice Hall (1999).
- "Hielo en Mercurio" ("Ice on Mercury"). El Universo, Enciclopedia de la Astronomía y el Espacio ("The Universe, Encyclopedia of Astronomy and the Space"), Editorial Planeta-De Agostini, p. 141-145. Volume 5. (1997)
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