Planetary science

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 science encompassing earth science disciplines and extending a synthesis between them. Geonomy integrates the knowledge collected from the Earth. However, the sequence of collecting data from Earth is much different than from other planets. Earth sciences originated studies in the vicinity of human habitation, and it later expanded to embrace the entire Earth.

Planetary science began in astronomy from studies of the unresolved planets and later increased resolution concerning atmospheric and surface details. One exception was the Moon, which always exhibited details on its surface, due to its proximity to the earth. The gradual increase in instrumental resolution resulted in more detailed geological knowledge about our natural satellite. In this scientific process, astronomical telescopes (and later radio telescopes) and finally space probe robots played important roles.

Planetary science involves many disciplines, although many studies such as mineralogy, petrology, and geochemistry mainly concentrate on the earth. Today cosmochemistry, cosmopetrography, and cosmo-geochemistry also are areas of study. Meteoritics studies the rocky and mineral materials of the Solar System. (Journals concerning meteeoritics include: 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). Held since 1970, the 39th LPSC will occur in 2008.

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 to collecte data not only the visible light region, but in other areas of the electromagnetic spectrum.

The planets can be characterized by their force fields: gravity and their magnetic fields. The magnetic field and the interaction with the solar wind forms the magnetosphere around a planet if its magnetic field is sufficiently strong.

Magnetic force field

The solar wind is deflected by the magnetosphere (not to scale)

Main article: Magnetosphere

Early space probes discovered the gross dimensions of the terrestrial magnetic field. It extends about 10 earth radiii towards the Sun. The solar wind, a stream of charged particles, streams around the terrestrial magnetic field forming a magnetic chamber (magnetosphere), and continues behind the magnetic tail, hundreds of earth radii downstream.

Inside the magnetosphere there are relatively dense regions of solar wind particles, these are known as the Van Allen radiation belts.

Gravity fields

Measured changes in acceleration of the space probes was used to map fine details of the gravity fields of the planets. In the 1970s, the gravity field disturbances above lunar maria were measured and concentrations of mass (mascons) were discovered. Lunar Orbiters found 5 lunar mascons which are at Imbrium, Serenitatis, Crisium, Nectaris and Humorum basins.

Effects of the rotational force field on the atmospheres

Cloud bands clearly visible on Jupiter.

The atmosphere is an important transitional zone between the solid planetary surface and the higher rarefied ionizing and radiation belts. Not all planets have atmospheres, its existence depends on the mass of the planet and the planet's mass constituents. Besides the four giant planets, Earth, Venus, and Mars have significant atmospheres. Two moons have significant atmospheres: Saturn's moon Titan and Neptune's moon Triton.

The rotation of a planet about its axis affects its shape, creating a bulge around the equator. The effects of rotation can be seen in atmospheric streams. Seen from space, the cloud system in the atmosphere exhibits these effects as banded features. Even amateur telescopes show the cloud bands of Jupiter and Saturn. On Earth, such belts are also present, although not quite as visible as on the gas giants, and are called Hadley cells.

Comparative planetary science

Main article: Formation and evolution of the Solar System

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. 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

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• Basilevsky, A. T.,& J. W. Head (1998): The geologic history of Venus: A stratigraphic view JGR-Planets Vol. 103 , No. E4 , p. 8531
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• 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.
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See also

• Astrogeology
• Planetary habitability
• Europlanet
• Theoretical planetology

External links

• E. Grayzeck, D. R. Williams (2006-05-11). "Lunar and Planetary Science". NASA. Retrieved 2006-08-21.
• 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