Mars

 Mars has a more varied history than most other
bodies. With twice the diameter of Earth's Moon, and half that
of Earth, it is not surprising that it shares geological characteristics
of both. First mapped in detail in the 1970's by Mariner 9 and
Vikings 1 and 2, Mars exhibits a variety of landscapes, geologic
materials, and geologic structures that reveal a long history
of activity. Most significant is the global division into two
geographic terrain's: the old, cratered, southern highlands that
stand 1 to 5 km above the average elevation, and the younger,
sedimentary and volcanic, northern lowlands, 0 to 3 km below the
average.

  The ancient cratered highlands formed in the
first of three major historical periods (before 3.5 b.y. ago).
Many stream valleys are seen that date from this period, indicating
that a warmer, thicker atmosphere probably covered Mars at that
time, or at least that the crustal rocks were warmer. Volcanoes
also erupted sporadically in the highlands. Rock units formed
in this early period show the scars of being battered by the still-numerous
impacts that followed the formation of the Solar System. The history
of the low northern plains is not preserved from this period,
perhaps, as some suggest, because they were carved out by an enormous
impact.

  The next major period in Mars' history was
dominated by the effects of internal heat loss. This led to extensive
volcanism and tectonism at the surface, and also to catastrophic
erosional events-the sudden releases of huge stores of millions
of cubic kilometers of ground water, causing ice and entrained
rock debris to scour enormous channels on their way to the low
northern plains basin. At the headwaters of these channels are
chaotic, jumbled blocks positioned in steep-walled canyons, where
the ground collapsed as the water was released, taking much of
the surrounding rock with it. Lesser releases of water occurred
on flanks of volcanoes. There, geothermal heating seems to have
driven ground water to the surface, where it easily cut through
the soft volcanic ash deposits. Still farther upstream the Solar
System's largest canyon, Valles Marineris (or "Mariner Valley"-named
for the spacecraft that discovered it), formed during this period,
probably by rifting of the crust and subsequent sideward erosion
of the canyon's walls. Elsewhere, lava flows continued to pour
onto the surface, covering over one-third of the planet. The Tharsis
rise, for example, a broad 3- to 10-km-high volcanic plateau straddling
the boundary between the cratered highland and low plains, formed
during this period.

  Geologic activity declined during the last
period; large volcanoes, some of the Solar System's largest, formed
at this time. With the small planet having lost much of its internal
heat by now, the crust thickened and shifted less, so that outpourings
of lava's continued to reach the surface via only a few access
conduits to the surface. Massive landslides also continued to
occur in areas of steep terrain. Today, Mars' atmosphere-reduced
to only 1/100 the density of Earth's-has lost its ability to cause
significant erosion, although solar heating still imparts enough
energy to seasonally move dust and small amounts of atmospheric
water around. These latter effects nonetheless continue to vary
in intensity due to long-term climate fluctuations driven by Mars'
erratic orbital behavior.

  New knowledge about Mars' surface composition,
coupled with advances in chemical analysis, has led to the recent
recognition that some meteorites found on Earth are very likely
rocks from Mars-knocked free by impact events.