Summary of Chapter

Surfaces are investigated through mapping that establishes the topography. Mapping can be performed using images, altimetry, and synthetic aperture radar. For bodies that are rotationally flattened, the zero of altitude can be defined by a surface of hydrostatic equilibrium, such as mean sea level on Earth. The gravitational potential is used on some bodies. For irregular bodies, a reference ellipsoid is used.

The composition and other characteristics of a surface are investigated by photometry and reflectance spectrometry, through the use of radar, by X-ray and gamma ray fluorescence spectrometry, and by neutron spectrometry Direct analysis of surface samples has only been

Differentiation, melting, Subduction

Differentiation, melting, Subduction

Figure 6.17 A summary of the processes described in Section 6.2.
Table 6.2 Dominant surface processes today in planets and large satellites

Planetary body

Dominant surface processes today

Mercury

Impact cratering; dry gradation"

Venus

V + Mb ; tectonics; gradation; metamorphic rock formation?

Earth

Plate tectonics; other tectonics; V + M; gradation; S + Mc

Mars

Gradation; impact cratering; tectonics?

Pluto

Impact cratering; dry gradation; cryo-V+M?

Moon

Impact cratering; dry gradation

Io

V+M; gradation; impact cratering

Europa

Cryo-V+M; cryotectonics; impact cratering; gradation

Ganymede

Impact cratering; dry gradation

Callisto

Impact cratering; dry gradation

Titan

Cryo-V+M; cryotectonics; gradation; cryo-S+M?

a Dry gradation is gradation that does not involve fluids (liquids or gases). b Volcanic and magmatic processes, including the formation of igneous rocks. c S denotes sedimentary rock formation, and M denotes metamorphic rock formation.

achieved for the Earth, the Moon, Venus, and Mars. Meteorites and micrometeorites provide samples of asteroids and comets, and also of the Moon and Mars.

There are several different types of process that create and modify the surface of a planetary body:

• melting, fractional crystallisation, and partial melting;

• impact cratering - in which bodies from interplanetary space impact the surface;

• (cryo)magmatism and (cryo)volcanism - all processes by which gases, liquids, or solids are expelled from the interior;

• tectonic processes - all processes that cause relative motion or distortion of the lithosphere;

• gradation - all processes by which material is eroded from a surface, and transported and deposited elsewhere, sometimes with chemical modification, notably the production of clays;

• the formation of sedimentary and metamorphic rocks.

Impact craters can be used to place surfaces on a given body in a sequence of ages: the greater the crater density, the older the surface, i.e. the further into the past it was last resurfaced by lava or by melting. Care has to be taken to allow for gradational effects, and to exclude volcanic craters and secondary impact craters. Absolute ages of surfaces with widely different crater densities have been obtained for the Moon. In comparing one body with another we have to allow for any differences in the bombardment history of the two bodies, their surface properties, and in the rates of gradation.

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