Planetary Geology pdf - NASA

Unit
Three
Earth is not unique in possessing an atmosphere.
Venus, Mars, Pluto, and two of the
satellites of the outer planetsÑTitan (a moon
of Saturn) and Triton (a moon of Neptune) have
atmospheres that envelop their surfaces. In addition,
the giant planets of the outer solar systemÑ
Jupiter, Saturn, Uranus, and NeptuneÑare composed
predominantly of gases. Other bodies in the
solar system possess extremely thin atmospheres.
Such bodies are the Moon (sodium gas), Mercury
(sodium gas), Europa (oxygen) and Io (sulfur).
The compositions of planetary atmospheres are
different, for a variety of reasons. First, surface gravity,
the force that holds down an atmosphere, differs
significantly among the planets. For example, the
large gravitational force of the giant planet Jupiter is
able to retain light gases such as hydrogen and helium
that escape from lower gravity objects. Second,
the distance from the sun determines the energy
available to heat atmospheric gas to a planetÕs
escape velocity, the speed at which gas molecules
overcome a planetÕs gravitational grasp. Thus, the
Introduction to
Planetary Atmospheres
EG-1998-03-109-HQ Activities in Planetary Geology for the Physical and Earth Sciences
85
distant and cold Titan, Triton, and Pluto are able to
retain their atmospheres despite relatively low gravities.
Finally, we know that the chemistry and geologic
history are different for each planet. Because
atmospheric makeup is generally related to chemistry
and temperature during planetary formation
and the subsequent escape of interior gases, the constituents
and total pressures of planetary atmospheres
are likely to be different. Moreover, on Earth,
atmospheric composition is largely governed by the
by-products of the very life that it sustains.
From the perspective of the planetary geologist,
atmospheres are important in the ways they shape
planetary surfaces. Wind can transport particles,
both eroding the surface and leaving deposits. Frost
and precipitation can leave direct and indirect
marks on a planetary surface. Climate changes can
influence a planetÕs geological history. Conversely,
studying surface geology leads to an understanding
of the atmosphere and climate of a
planetÑboth its present state and its past.