thumb|[[Caravaggio (crater)|Caravaggio is an example of a peak ring impact basin on Mercury.]]

thumb|Several areas on Mercury are extremely dark, such as a small crater within [[Hemingway (crater)|Hemingway crater in the lower right.]]

The geology of Mercury is the scientific study of the surface, crust, and interior of the planet Mercury. It emphasizes the composition, structure, history, and physical processes that shape the planet. It is analogous to the field of terrestrial geology. In planetary science, the term geology is used in its broadest sense to mean the study of the solid parts of planets and moons. The term incorporates aspects of geophysics, geochemistry, mineralogy, geodesy, and cartography.

Historically, Mercury has been the least understood of all the terrestrial planets in the Solar System. This stems largely from its proximity to the Sun which makes reaching it with spacecraft technically challenging and Earth-based observations difficult. For decades, the principal source of geologic information about Mercury came from the 2,700 images taken by the Mariner 10 spacecraft during three flybys of the planet from 1974 to 1975. These images covered about 45% of the planet's surface, but many of them were unsuitable for detailed geologic investigation because of high sun angles which made it hard to determine surface morphology and topography. This dearth of information was greatly alleviated by the Mercury Surface, Space Environment, Geochemistry, and Ranging (MESSENGER) spacecraft which between 2008 and 2015 collected over 291,000 images covering the entire planet, along with a wealth of other scientific data. The European Space Agency's (ESA's) BepiColombo spacecraft, scheduled to go into orbit around Mercury in 2026, is expected to help answer many of the remaining questions about Mercury's geology.

Mercury's surface is dominated by impact craters, basaltic rock and smooth plains, many of them a result of flood volcanism, similar in some respects to the lunar maria, Other notable features include vents which appear to be the source of magma-carved valleys, often-grouped irregular-shaped depressions termed "hollows" that are believed to be the result of collapsed magma chambers, 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 because Mercury has negligible atmosphere. A direct trip to Mercury actually requires more rocket fuel than that required to escape the Solar System completely. As a result, only three space probes, Mariner 10, MESSENGER, and BepiColombo have visited Mercury so far.

alt=|thumb|200x200px|Internal structure of Mercury

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. 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:

  1. 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.
  2. 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).

Geological history

thumb|right|150px|Mercury – Gravity Anomalies – mass concentrations (red) suggest subsurface structure and evolution.

Like the Earth, Moon and Mars, Mercury's geologic history is divided into eras. From oldest to youngest, these are: the pre-Tolstojan, Tolstojan, Calorian, Mansurian, and Kuiperian. Their ages are based on relative dating only.

Boulders that have been found on Mercury as associated with fresh impact craters that are hundreds of meters in diameter or larger.

Impact basins and craters

thumb|right|150px|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 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. 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.

Faculae

Faculae on Mercury are bright areas often surrounding irregular depressions. They are generally interpreted to be pyroclastic in nature. The faculae on Mercury are all named using words in different languages meaning snake.

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)

High-albedo 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.

thumb|Radar image of 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 high luminosity levels, as the silicate rocks that compose most of the surface of Mercury have exactly the opposite effect on luminosity.

In spite of its proximity to the Sun, Mercury may have surface ice, since temperatures near the poles are constantly below freezing point: On the polar plains, the temperature does not rise above −106 °C. Craters at Mercury's higher latitudes (discovered by radar surveys from Earth as well) may be deep enough to shield the ice from direct sunlight. Inside the craters, where there is no solar light, temperatures fall to −171 °C.

See also

References

  • 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)

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