Planetary geology is a branch of geology devoted to the study of structure, composition, processes, and origin of major and minor planetary bodies traditionally inside and now, in the twenty-first century, outside the solar system, and to the effects of interaction between planetary bodies within the solar system. Planetary geology interfaces with many other fields including mathematics, astronomy, biology, chemistry, and physics. Planetary geologists work in the field, laboratory, and—indirectly—in outer space through imagery and other data returned by spacecraft. One planetary geologist has visited another planetary body (Dr. Harrison Schmitt worked on the moon during the Apollo 17 mission in 1974) and perhaps someday in the near future other scientific explorers will visit the moon, Mars, and other planetary bodies.
The goal of planetary geology studies of other planetary bodies is to understand the origin of the features seen (and samples returned, if such are available). Planetary geology studies usually relate to the quest for an understanding of the geological history of the body from its formation during accretion from the early solar nebula to its present condition. Most planetary bodies in the solar system of any significant size have passed through stages of accretion, internal heating and differentiation, and surficial and tectonic evolution. However, some objects have not, for example, small asteroids, comets, and smaller solar system debris such as meteorites. These objects can give important clues to the nature of the origin of the solar system, as they contain unaltered primordial material from the original nebular dust cloud. An important sub-discipline within planetary geology is meteoritics, the study of meteorites.
Most initial planetary geology studies help establish a temporal (time) framework for major events in the history of a planetary body. This framework, called a geologic time scale, is used to help fit events into a sequential order. On Earth, this kind of framework was established over a period of about 200 years using fossils, age relations among rocks, and absolute (radiometric) age dating. For planetary bodies like the moon and Mars, for which scientists have some surface samples (note—there are a few Martian and lunar meteorites plus the samples returned by lunar landing missions), some radiometric age dating is possible. Mainly, planetary age relations are established by examining photographic evidence such as impact-crater densities, cross-cutting relationships, and superposition relationships (layers lying on top of one another). This is worked out by careful geologic mapping using photographic mosaics of planetary surfaces.
Planetary geologists are also interested in studying interactions among planetary bodies. When one planetary body impacts another, great energy can be released. Large body impacts represent the most energetic geologic process known. Very large body impacts can release the energy equivalent to thousands or even millions of megatons of TNT (megaton = 1 million tons) in a few seconds. Impact events produce impact craters, whose size and shape are controlled by factors like planetary gravity, strength of crustal materials, and density of planetary atmosphere. The impact crater is the most common feature on rocky and icy planetary surfaces in the solar system, except where resurfacing processes (like volcanism, erosion and sedimentation, and crustal melting) remove impact craters faster than they can form.
Studies of large impact craters on Earth have lead geologists to new understandings of how large body impacts, and the ecological disasters that they can cause, have affected evolution of life on Earth. Some large body impact events have been strongly connected to mass extinctions of life on Earth, a driving force in evolutionary change. The best modern example of this is the great Cretaceous-Tertiary mass extinction event (65 million years ago), which has been dated as the same age as two large impact craters on the Earth (Chicxulub crater in Mexico, nearly 112 mi [180 km] in diameter, and Boltysh crater in the Ukraine, about 15 mi [24 km] in diameter). Planetary geologists are currently working on theories about possible comet swarms due to perturbations in the Oort cloud (the region about halfway between the sun and the helio-pause) or other disruptive mechanisms which may have made time intervals in the past more dangerous for life on Earth due to cosmic impacts.
Studies of impact craters hold considerable academic interest for investigators, but on the practical side, considerable natural resources have been discovered within impact craters. For this reason, their study has been supported by petroleum, mining, and ground-water concerns. The intensive fracturing caused by impact creates avenues for fluid movement within the affected crustal area, causing entrapment of hydrocarbons and emplacement of some types of fluid-borne mineral resources. Groundwater accessibility and quality is either enhanced or degraded in impact areas, depending upon geological conditions.
Studies of impacts of large comets may have other implications for solar system evolution, as recent theories suggest such impacts may have delivered significant amounts of water and primitive organic material to Earth and Mars during a very early phase of their evolution. Recent investigations of the origin of Earth’s moon suggest that a major impact event that penetrated Earth’s crust ejected material into orbit that eventually became the moon. This theory helps explain some of the unusual chemical characteristics of lunar rocks and their similarity to Earth’s upper mantle.
As of October 2006, National Aeronautics and Space Administration’s (NASA’s) Mars Reconnaissance Orbiter (MRO), which was launched on August 12, 2005, is expected to begin its primary mission in November 2006. It will conduct exploration and reconnaissance of Mars’ landforms, minerals, water/ice, stratigraphy (rocks) while in orbit about the planet. Its explorations will help to pave the way for future manned and unmanned missions to the planet.
In 2007, the Kepler spacecraft, a space telescope especially designed to scan large areas of the sky for transits by planets as small as Earth, will be launched by the U.S. National Aeronautics and Space Administration (NASA). By 2011, Kepler should have gathered enough data to pinpoint hundreds of extrasolar planets and to determine how typical the own solar system is in the universe. This is of interest to scientists because estimates of the probability that life exists elsewhere in the universe depend strongly on the existence of planets, and their physical geology, not too different from Earth. Intelligent life is unlikely to evolve on large gas giants or on bodies of any type that orbit very near to their stars or follow highly eccentric orbits. If solar systems like Earth’s are rare in the universe, then life (intelligent or otherwise) may be correspondingly rare. Theoretical models of the formation of solar systems have been in a state of rapid change under the pressure of the rush of extra-solar planet discoveries, and revised models indicate that solar systems like Earth’s may be abundant. However, these models supply only educated guesses, and must be checked against observation.
Consequently, in the 2000s, planetary geology is rapidly expanding into the study of extra-solar system bodies. This future area for planetary geology investigation will help shed more light on the origin of the own solar system and on solar system formation in this part of the Milky Way galaxy.
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David T. King, Jr.