Space and planetary geology

Space and planetary geology comprises that branch of the discipline of geology that applies basic scientific principles to the study of the origin, development, and characteristics of solar system objects such as planets, satellites, asteroids, comets , meteorites, and interplanetary dust particles. On Earth, space and planetary geology investigations are generally limited to impact craters and impact effects upon Earth, and to the study of Earth as an analogue for other planets and their processes. As technology progresses in the future, the field of space and planetary geology will likely expand to include extra-solar system objects as well. Space and planetary geology is also called astrogeology.

Space and planetary geology has it origins in telescopic observations of planets, satellites, and comets and in the study of meteorites. Telescopic maps of the Moon date from early work c. 1612 by Galileo Galilei (1564–1642). The first true lunar geologic map was that of Michael van Langren (1598–1675), which he completed in 1645. After this, many other such telescopic maps were made of the Moon over the next three centuries. During the period 1880 to 1925, several telescopic maps of topographic and geological features of Mercury and Mars were produced. The most famous of these were maps by Percival Lowell (1855–1916), which showed his interpreted "canals" on Mars. Meteoritics, which is the study of meteorites and their origins, traces its origin as a science back to the German physicist Ernst Florens Chladni (1756–1827). Chladni proposed convincing (but highly debated) arguments (c. 1794) that stones and masses of iron that were seen falling from the sky were in fact objects from space that produced fireballs as the fell through Earth's atmosphere. That small objects (asteroids) were orbiting the Sun was confirmed shortly thereafter (1801–1807) by a group of Italian astronomers dubbed the "celestial police," who discovered the first four known asteroids, Ceres, Pallas, Juno, and Vesta.

Space and planetary geology received an essential boost with the advent of rocketry and space flight. Beginning with missions to the Moon in the late 1950s and early 1960s (i.e., Luna 2 and 3 in 1959 and Ranger 7 in 1964), detailed orbital photographs of the near side and far side of the Moon were obtained. From these photographs, the first detailed geological maps of the Moon were made, thus establishing a new area of planetary photo-geologic mapping. During the 1960s, spacecraft made other missions to Mars and Venus. Mariner 4 (1964) and Mariner 6 (1969) took the first detailed photographs of Mars, and Mariner 2 (1964) landed on Venus and recorded surficial conditions. In the late 1960s and early 1970s, spacecraft returned samples from the Moon (Apollo 11, 12, 14–17 and Luna 16, 17, 20, 21), thus ushering in a new era of geological sample studies of the Moon. Study of these samples allowed radiometric dating and careful chemical and physical analysis that led to the first comprehensive description of the geological history of the Moon. In the 1970s and 1980s, spacecraft made radar maps of Venus (Venera 15 and 16), imaged the outer planets and some of their satellites (Voyager 1 and 2), imaged and landed upon Mars (Viking 1 and 2), and imaged part of Mercury (Mariner 10). These data further expanded planetary geological mapping. In the 1980s and 1990s, spacecraft photographed Halley's comet (Vega 1 and 2, Giotto ), made detailed radar maps of Venus (Magellan ), imaged asteroids Gaspara and Ida (Galileo, NEAR), imaged and landed on Mars (Mars Pathfinder ), and imaged Jupiter and its satellites (Galileo ). With each new photographic set, geological mapping of planet and satellite surfaces was expanded. More spacecraft observations are planned or are underway for Mercury, Mars, Saturn, the Moon, and various asteroids and comets in the near future.

Imagery from the various rocky planets and satellites has led to detailed topographic and geologic mapping of all the imaged bodies, and such maps, published mainly by the U.S. Geological Survey, are available to the public. Relative age relationships among geologic units on the planetary surfaces, deduced from photographic imagery, has led to development of preliminary geological time scales for Mercury, Venus, the Moon, and Mars. Similar studies are underway for the large Jovian satellites (Callisto, Ganymede, and Europa), the Saturnian satellites (Mimas, Enceladus, Tethys, Dione, Rhea, and Iapetus), the Uranian satellites (Ariel, Umbriel, Titania, Miranda, and Oberon), and the Neptunian satellite, Triton.

The International Astronomical Union (IAU) is in charge of standardized nomenclature for planetary surface features and geological units. There are approximately forty IAU-approved, generic feature terms in use in planetary nomenclature. For example, a ridge on a planetary surface is a dorsum (plural = dorsa) and a chain of craters is a catena. A distinctive area of broken terrain is a chaos. The IAU has approved certain themes for assigning names to generic features on planets, satellites, and asteroids. For example, all craters on Venus shall be named for famous women and all dorsa for sky goddesses. Also, for example, on Mars all large craters are named for deceased scientists who have contributed to the study of Mars, all small craters are named for villages of the earth, all large valleys are named for Mars in various languages, and all small valleys are named for Earth's classical or modern rivers . On Jupiter's satellite, Europa, all craters are named for Celtic gods and heroes. There is a comprehensive, IAU-approved list of such themes and all new suggested names must be approved for use on maps by an IAU Task Group specific to the planetary body at issue.

See also Astronomy; History of manned space exploration; Meteoroids and meteorites; Space probe