Geology of Mercury
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The surface of Mercury 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 spacecraft in 1974-5 and the MESSENGER spacecraft in 2008). Mercury's interior contains a very large metal core that accounts for abut 42% of its volume. Part of this core may still be liquid as evidenced by a weak but global magnetosphere.
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[edit] Difficulties in exploration
Of all the terrestrial planets in the Solar System, the geology of Mercury is the least understood. This stems largely from Mercury's proximity to the Sun 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 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 launched from Earth must travel 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. The potential energy liberated by moving down the Sun’s potential well becomes kinetic energy; 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 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 the solar system completely. As a result, only two space probes, Mariner 10 and MESSENGER, which are both NASA, 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 is a slow 58 Earth days, so that spacecraft flybys are restricted to viewing only a single illuminated hemisphere. In fact, unfortunately, even though 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 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 probe, launched in August 2004, will greatly contribute to our understanding when it enters orbit around Mercury in March 2011.
[edit] Mercury's geological history
2. Mantle - 600km thick
3. Nucleus - 1,800km radius
Like the Earth, 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 along with the rest of the solar system 4.6 billion years ago, heavy bombardment by asteroids and comets ensued. The last intense bombardment phase, the Late Heavy Bombardment came to an end about 3.8 billion years ago. Some regions or massifs, a prominent one being the one that formed the Caloris Basin, were filled by magma eruptions from within the planet. These created smooth intercrater plains similar to the maria found on the Moon. 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 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 main surface processes have been intermittent impacts.
[edit] Timeline
[edit] Surface features
Mercury’s surface is overall similar in appearance to that of the Moon, with extensive mare-like plains and heavily cratered terrains similar to the Lunar highlands.
[edit] 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. [3]
The largest known craters are the enormous Caloris Basin, with a diameter of 1550 km,[4] and the Skinakas Basin with a diameter of 1600 km, but known only from low resolution Earth-based observations of the non-Mariner-imaged hemisphere. The impact which created the Caloris Basin was so powerful that its effects are seen on a global scale. It caused lava eruptions and left a concentric ring over 2 km tall surrounding the impact crater. At the antipode 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.[5] 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 [3].
As on the Moon, fresh craters on Mercury show prominent bright Ray systems. These are made by ejected debris, which while they remain relatively fresh tend to be brighter because of a lesser amount of space weathering than the surrounding older terrain.
[edit] Plains
There are two geologically distinct plains units on Mercury. [6] [3]:
- Inter-crater plains are the oldest visible surface [3], predating the heavily cratered terrain. They are gently rolling or hilly and occurs in the regions between larger craters. The inter-crater plains appears to have obliterated many earlier craters, and show a general paucity of smaller craters below about 30 km in diameter[6]. It is not clear whether they are volcanic or debris of impact origin[6]. 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.[3]
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 [3].
[edit] 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.[7] Mercury’s surface is also flexed by significant tidal bulges raised by the Sun—the Sun’s tides on Mercury are about 17% stronger than the Moon’s on Earth.[8]
[edit] Terminology
Non-crater surface features are given the following names:
- Albedo features — areas of markedly different reflectivity
- Dorsa — ridges (see List of ridges on Mercury)
- Montes — mountains (see List of mountains on Mercury)
- Planitiae — plains (see List of plains on Mercury)
- Rupes — scarps (see List of scarps on Mercury)
- Valles — valleys (see List of valleys on Mercury)
[edit] High luminosity polar patches and possible 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, 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 yet definitive. The anomalous reflective properties could also be due to the existence of deposits of metallic sulfates or other materials with high reflectance.
[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 some possible signs of the presence of ice 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 comets. 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.
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.
[edit] See also
[edit] References
- ^ Map of Mercury (PDF, large image; bilingual)
- ^ Paul Spudis, "The Geological History of Mercury" (PDF)
- ^ a b c d e f P. D. Spudis (2001). "The Geological History of Mercury". Workshop on Mercury: Space Environment, Surface, and Interior, Chicago: 100.
- ^ Shiga, David. "Bizarre spider scar found on Mercury's surface", NewScientist.com news service, 30 January 2008.
- ^ Schultz P.H., Gault D.E. (1975), Seismic effects from major basin formations on the moon and Mercury, The Moon, vol. 12, Feb. 1975, p. 159-177
- ^ a b c R.J. Wagner et al. (2001). "Application of an Updated Impact Cratering Chronology Model to Mercury's Time-Stratigraphic System". Workshop on Mercury: Space Environment, Surface, and Interior, Chicago: 106.
- ^ Dzurisin D. (1978), The tectonic and volcanic history of Mercury as inferred from studies of scarps, ridges, troughs, and other lineaments, Journal of Geophysical Research, v. 83, p. 4883-4906
- ^ Van Hoolst, T., Jacobs, C. (2003), Mercury’s tides and interior structure, Journal of Geophysical Research, v. 108, p. 7.
- 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)
[edit] External links
- Mariner 10
- MESSENGER probe
- Mercury on Nineplanets.org
- USGS Geology of Mercury Retrieved 5 August 2007
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