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Planetary science, also known as planetology and closely related to planetary astronomy, is the science of planets, or planetary systems, and the solar system. Incorporating an interdisciplinary approach, planetary science draws from diverse sciences and may be considered a part of the Earth sciences, or more logically, as its parent field. Research tends to be done by a combination of astronomy, space exploration (particularly robotic spacecraft missions), and comparative, experimental and meteorite work based on Earth. There is also an important theoretical component and considerable use of computer simulation. Astrogeology is a major component of planetary sciences.

Photograph of Apollo 15 orbital unit about the rilles in the vicinity of the crater Aristarchos. The arrangement of the two valleys is very similar, although one third in size, to Great Hungarian Plane rivers Danube and Tisza.

Planetology is an interdisciplinary science growing out from astronomy and earth science. Its development was determined by the increasing importance of robotics and measuring technology. In general, planetary science studies the planets, their moons, all the bodies and radiations of the Solar System, the various force fields and interactions between the several components of the Solar system.

Its relation to earth sciences

The earth science has a new discipline: geonomy, strongly related to planetary science. Geonómia is a comprehensive earth science about earth science disciplines, extending a synthesis between them. Geonomy integrates the knowledge collected from the Earth. However, the sequence of collecting data from Earth and from the planets was different. Earth sciences began their studies in the vicinity of men, and it later grew up gradually to embrace the planet Earth.

Planetary science began in astronomy from studies of the light point planets and later it has got better and better resolution about the atmospheric or surface details. One exception was the Moon, which always exhibited details on its surface, because its small distance orbiting. The Moon has been known from the whole picture, from the size of a spherical body. The gradually better and better resolution resulted in more and more detailed geological knowledge about our natural satellite. In this scientific process astronomical telescope, (later radio telescope) and finally space probe robots played important role.

Planetary science involved all disciplines sometimes directed only terrestrial studies of mineralogy, petrology, geocehemistry. So we can speak today about cosmochemistry, cosmopetrography, or cosmo-geochemistry, too. Meteoritics studies the rocky and mineral materials of the Solar System. (There are important journals about it: The Geochimica et Cosmochimica Acta, and the Meteoritics and Planetary Science.)

The most important regular annual conference of this discipline is the Lunar and Planetary Science Conference (LPSC), organized by the Lunar and Planetary Institute in Houston, at NASA Lyndon B. Johnson Space Center (JSC). Since 1970 this (2008) year will see the 39. LPSC.

Investigations of the surface of the Moon, Mars and Venus

The most well known research topics of planetary science are the planetary bodies in the nearest vicinity of the Earth: the Moon, and the two neighbour planets: Venus and Mars. Among them the Moon was the first, where those methods were used earlier developed on the Earth. Two important disciplines are in surface studies: geomorphology and stratigraphy.

Geomorphology

Geomorphology studies the features on the planetary surface and reconstructs their formational processes. It contains studies on: - features originating from the outer space effects, like impacts (multi-ringed basins, craters) - features originating from inner processes like volcanism and tectonism (lavaflows, fissures, rilles) - erosional objects produced by the continuous meteorite bombardment

On the Moon impact structures can be found in the wide size range from the basins with 1000 km diameter till the micrometer sized craterlets on the mineral grains. Volcanism produced extended lava flows, with wrinkle ridges, lava channels, exhibiting the morphological evidences of their formational processes. Erosion on the Moon produced the thin regolithic dust cover on the surface.

The objects of our geomorphological studies can be used to decipher the history of the surface. They can be mapped according their settling sequence from top to bottom, as determined first on terrestrial strata by Nicolas Steno. On the basis of this sequence stratigraphical mapping prepared the Apollo astronauts in their lunar mission works.

Stratigraphy

The stratigraphy studies and arranges the strata according to their settling sequence and summarizes them in stratigraphical maps. In order to identify strata and determine their sequence geology developed the geological (stratigraphical) axioms. They were applied to the Moon. The overlapping sequences were identified first on images and photometric, telescopic measurements, later remote sensing technologies were developed (Lunar Orbiter). The final product of this work was a Lunar stratigraphic column, showing the sequence of the main strata (and events, producing them), and the stratigraphical map of the Moon.

On the top of the lunar stratigraphical sequence rayed impact craters can be found. Such youngest craters belong to the Copernican unit. Below it can be found craters without the ray system, but with rather well developed impact crater morphology. This is the Eratosthenian unit. The two younger stratigraphical units can be found in crater sized spots on the Moon. Below them two extending strat can be found: mare units (earlier defined as Procellarian unit) and the Imbrium basin related ejecta and tectonic units (Imbrian units). Another impact basin raletad unit is the Nectarian unit, defined around the Nectarian Basin. At the bottom of the lunar stratigraphical sequence the pre-Nectarian unit of old crater plains can be found. The surface of Mercury is similar in many aspects to the Lunar one. The stratigraphy of Mercury is very similar to the lunar case, too.

Rocky materials from the planetary bodies with rigid surface

A branch of planetary science is the materials science studying rocks and minerals from the Solar System. There are three main source types of these materials: meteorites, lunar samples, and Martian samples.

Meteorites

First the meteorites were the known extraterrestrial materials. Since 200 years they are continuously collected and studied collecting data about their parent bodies. Meteorites mostly originated from smaller asteroidal bodies of the solar system. Therefore they are beneficial to study the evolution of these asteroidal bodies. Chondrites in particular (containing chondtuels, small "grains"- Greek word) are very important to see primordial materials from the early solar system age.

Lunar rocks

During the Apollo era, in the Apollo program, lunar samples were collected and transported to the Earth (384 kilogramm) and 3 Luna-robots also delivered regolith samples from the Moon. Finally lunar meteorites were also found among the Antarctic meteorites. Today about 100 paired lunar meteorites are known (in 2008).

Martian meteorites

A third group of planetary materials are the Martian meteorites. Today about 50 paired Martian meteorites are known (in 2008).

Studies of the force fields of the planets

Space probes made it possible, that data should be collected not only the visible light region, but in other parts of the electromagnetic spectrum. It became possible to study the outer empty and rarely populated material space around planets too.

The planets are characterized by their force fields, too: by the gravity and by the magnetic field. The magnetic field and the interaction with the solar wind forms the magnetosphere around a planet with strong magnetic field. Some examples will be shown.

Magnetic force field

Early space probes discovered the main aspects of the terrestrial magnetic field. It extends about 10 terrestrial radius distance towards the Sun. The corpuscular radiations of the Sun streams around the terrestrial magnetic field forming a magnetic chamber (magnetosphere), and close again after forming the magnetic tail, hundreds of terrestrial radius distance.

Inside the magnetosphere radiation belts are forming where the density of the charged particles is high (Van Allen radiation belt).

Gravity fields

Spatial acceleration of the space probes was used to measure the fine details of the gravity field of the planets. First the gravity field disturbances above lunar maria were measured and the mascons were discovered. Lunar Orbiters found 5 lunar mascons which are at Imbrium, Serenitatis, Crisium, Nectaris and Humorum basins. They are known since the 1970s.

Effects of the rotational force field on the atmospheres

The atmosphere is an important transitional material zone between the solid planetary surface and the higher rare ionizing and radiation belts. Not all of the planets have atmospheres, because its existence depends on the mass of the planet. Besides the four giant planets, Earth, Venus, and Mars have atmospheres, and Saturn's moon Titan also has one.

The rotation of the planet affects its body shape, too. More expressed effects of rotation can be seen on the atmospheric streams. The cloudy system in the atmosphere exhibits well these affects by its striped features. Even amateur telescopes show the stripe system of Jupiter and Saturn. Such belts has the name in the terrestrial circulation: Hadley-cells.

Comparative planetary science

The bodies of the Solar System gradually formed and reached their recent state we observe today. These bodies started in their evolution in different initial conditions, considering their composition and mass, solar distance and other parameters. Therefore it is important to follow and describe the evolutionary path of these individual objects and the comparison of them. The comparative planetology is a discipline of various celestial planetary "laboratories", the planets and other bodies themselves. Many decades of future space probe investigations are necessary to make detailed evolutionary history of planetary bodies and comparative work may help to interpolate some of the hiatus in the known evolutionary histories. For example lunar history has helped us to see the first 3 aeons of the Solar System events which were mostly destroyed on Earth.

Planetary science studies objects ranging in size from micrometeoroids to gas giants, their composition, dynamics and history.

Terminology

When the discipline concerns itself with a celestial body in particular, a specialized term is used, as shown in the table below (only heliology, geology, selenology, and areology are currently in common use):

! Body ||  Planetary science  || Source of root |- | Sun || heliology || Greek Helios |- | Mercury || hermeology || Greek Hermes |- | Venus || cytherology || Greek Aphrodite |- ||||| |- | Earth || geology || Greek Gaia |- |  ( Moon || selenology || Greek Selene ) |- ||||| |- | Mars || areology || Greek Ares |- | Ceres || demeterology || Greek Demeter |- | Jupiter || zenology || Greek Zeus |- | Saturn || kronology || Greek Cronus |- | Uranus || uranology || Greek/Latin Uranus |- | Neptune || poseidology || Greek Poseidon |- | Pluto || hadeology || Greek Hades |- | Eris || eridology || Greek Eris |}

Basic Concepts

• Asteroid
• Brown dwarfs
• Celestial mechanics
• Comets
• Earthquake
• Equatorial bulge
• Extrasolar planets
• Gas giant
• Geophysics
• Icy moons
• Kuiper belt
• Magnetosphere
• Planet
• Planetary differentiation
• Planetary system
• The Pluto debate
• Precession
• Space weather
• Space weathering
• Star system
• Sun
• Synchronous rotation
• Terrestrial planets

References

• Basilevsky, A. T.,& J. W. Head (1995): Regional and global stratigraphy of Venus: a preliminary assessment and implications for the geological history of Venus Planetary and Space Science 43/12, pp. 1523-1553
• Basilevsky, A. T.,& J. W. Head (1998): The geologic history of Venus: A stratigraphic view JGR-Planets Vol. 103 , No. E4 , p. 8531
• Basilevsky, A. T.,& J. W. Head (2002): Venus: Timing and rates of geologic activity Geology; November 2002; v. 30; no. 11; p. 1015?1018;
• Frey, H. V., E. L. Frey, W. K. Hartmann & K. L. T. Tanaka (2003): Evidence for buried "Pre-Noachian" crust pre-dating the oldest observed surface units on Mars Lunar and Planetary Science XXXIV 1848
• Gradstein, F. M., James G. Ogg, Alan G. Smith, Wouter Bleeker & Lucas J. Lourens (2004): A new Geologic Time Scale, with special reference to Precambrian and Neogene Episodes, Vol. 27, no. 2.
• Hansen V. L. & Young D. A. (2007): Venus's evolution: A synthesis. Special Paper 419: Convergent Margin Terranes and Associated Regions: A Tribute to W.G. Ernst: Vol. 419, No. 0 pp. 255?273.
• Hartmann, W. K. & Neukum, G. (2001): Cratering Chronology and the Evolution of Mars. Space Science Reviews, 96, 165?194.
• Hartman, W. K. (2005): Moons and Planets. 5th Edition. Thomson Brooks/Cole.
• Head J. W. & Basilevsky, A. T (1999): A model for the geological history of Venus from stratigraphic relationship: comparison geophysical mechanisms LPSC XXX #1390
• Mutch T.A., Arvidson R., Head J., Jones K.,& Saunders S. (1977): The Geology of Mars Princeton University Press
• Offield, T. W. & Pohn, H. A. (1970): Lunar crater morphology and relative-age determiantion of lunar geologic units U.S. Geol. Survey Prof. Paper No. 700-C. pp. C153-C169. Washington;
• Phillips, R. J., R. F. Raubertas, R. E. Arvidson, I. C. Sarkar, R. R. Herrick, N. Izenberg, and R. E. Grimm (1992): Impact craters and Venus resurfacing history, J. Geophys. Res., 97, 15,923-15,948
• Scott, D. H. & Carr, M. H. (1977): The New Geologic Map of Mars (1:25 Million Scale). Technical report.
• Scott, D. H. & Tanaka, K. L. (1986): Geological Map of the Western Equatorial Region of Mars (1:15,000,000), USGS.
• Shoemaker, E.M., & Hackman, R.J., (1962):, Stratigraphic basis for a lunar time scale, in *Kopal, Zdenek, and Mikhailov, Z.K., eds., (1960): The Moon - Intern. Astronom. Union Symposium 14, Leningrad, 1960, Proc.: New York, Academic Press, p. 289- 300.
• Spudis, P.D. & J.E. Guest, (1988):. Stratigraphy and geologic history of Mercury, in Mercury, F. Vilas, C.R. Chapman, and M.S. Matthews, eds., Univ. of Arizona Press, Tucson, pp. 118-164.
• Spudis, P. D.& Strobell, M. E. (1984): New Identification of Ancient Multi-Ring Basins on Mercury and Implications for Geologic Evolution. LPSC XV, P. 814-815
• Spudis, P. (2001): The geological history of mercury. Mercury: Space Environment, Surface, and Interior, LPJ Conference, #8029.
• Tanaka K. L. (ed.) (1994): The Venus Geologic Mappers? Handbook. Second Edition. Open?File Report 94-438 NASA. Tanaka K. L. 2001: The Stratigraphy of Mars LPSC 22, #1695
• Tanaka K. L. & J. A. Skinner (2003): Mars: Updating geologic mapping approaches and the formal stratigraphic scheme. Sixth International Conference on Mars #3129
• Wagner R. J., U. Wolf, & G. Neukum (2002): Time-stratigraphy and impact cratering chronology of Mercury. Lunar and Planetary Science XXXIII 1575
• Wilhelms D. E. (1970): Summary of Lunar Stratigraphy - Telescopic Observations. U.S. Geol. Survey Prof. Papers No. 599-F., Washington;
• Wilhelms D. (1987): Geologic History of the Moon, US Geological Survey Professional Paper 1348, http://ser.sese.asu.edu/GHM/
• Wilhelms D. E.& McCauley J. F. (1971): Geologic Map of the Near Side of the Moon. USGS Maps No. I-703, Washington;

See also

• Astrogeology
• Planetary habitability
• Europlanet

External links

• NASA Mars Exploration Home
• NASA Cassini Mission to Saturn
• NASA DAWN Mission to the Asteroid Belt
• NASA MESSENGER Mission to Mercury
• European Space Agency
• Lunar and Planetary Lab at the University of Arizona
• Planetary Sciences at UCLA
• Planetary Science Research Discoveries

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