Before looking at some more representative images, we will set up a geologic framework for Mars: The planet, fourth from the Sun, is a little more than half Earth's diameter at 6793 km (4218 miles), about double that of the Moon, and has about 1/10th its mass. Its lower density implies a small metallic core, now rigid as borne out by lack of a distinct magnetic field. It rotates at just over 37 minutes beyond an earth day. Its surface temperatures range from -140 to 20° C. From Earth-based telescopes, Mars was suspected to have oxidized iron-rich materials at its surface, was subject to dust storms, and had ice caps at both poles that alternately expanded and contracted over a martian year (687 earth days). The storms implied at least a thin gaseous envelope; spectroscopic measurements indicated CO₂ and maybe nitrogen. The Mariner/Viking missions greatly modified our concepts of the martian landscapes. Consider the generalized landforms maps of the two martian equatorial hemispheres, drawn on a Lambert equal-area projection from mission results, as shown here:

From T.A. Mutch et al., The Geology of Mars, © 1976. Reproduced by permission of the Princeton University Press, New Jersey.

The lower polar hemisphere is dominated by cratered terrain (cu), considered to be billions of years old (and the likely source of the 3.5 b.y. meteorite containing organics), where the bulk of the larger impact structures survive. Much of the upper hemisphere is dominated by plains units (p), many thought to be volcanic flows (pv) (mainly basalts?) and volcanic constructs (v) (such as shield volcanoes), some of whose ages could be less than one-half billion years; other subdivisions include cratered plains (pc), moderately cratered plains (pm). Unusual or specialized units consist of channel deposits (c), grooved terrain (g), knobby hummocky terrain (hk), fretted hummocky terrain (hf), chaotic hummocky terrain (hc), and mountainous terrain (m). Units confined to the polar regions are: permanent ice (pi), layered deposits (id) (thought to be layers of windblown dust interspersed with ice), and etched plains (ep). As with Venus, structural signs of plate tectonics are absent.

Many features in these categories are also landmarks and physiographic provinces. In the left hemisphere, the three volcanoes in a row (Ascraeus Mons [top], Pavonis Mons [center], and Arsia Mons [bottom]). are in the Tharsis Montes group and the two large ones above and to the left are Olympus Mons (largest volcano in the solar system) and Alba Patera (a caldera-capped volcano). The long, curvilinear feature (c) near the equator in the left hemisphere is the great Valles Marineris, many times longer and much deeper than the Grand Canyon, with associated chasmas or tributary canyons. In the right hemisphere, the large area (p) in the southern half is Hellas Planitia, site of the biggest impact basin on Mars, cut into ancient terrain and backfilled with lava.

The missions confirmed the martian atmosphere to consist of 95.3% CO₂, with the remainder being mostly N₂ (2.7%), argon (1.6%) and minor O₂, CO, , and enough water, if condensed, to fill an average swimming pool. Atmospheric pressure averages 7 millibars or about 0.7% of that of Earth but windstorms within this thin envelope can reach speeds of >200 km/hr (125 mph). Despite the very low density of today's martian atmosphere, its high speed winds can add to erosive sculpturing, and can deposit uplifted dust and sand into vast dune fields. The Viking Orbiter image below contains both transverse (elongate; parallel) and barchan (crescent) dunes laid down in a plains setting.

Both Mariner 10 and the Viking Orbiters gathered many images showing various aspects of atmospheric circulation, such as this view of spiraling clouds above the martian North Polar region; note also the bright patches of ice.

The martian dust and sand is frequently spread out as wind streaks.

The fine particles are often redeposited as dunes. In this scene, both realtime clouds and longitudinal dunes are evident.

The dominant constituent of the polar caps is CO₂ but minor amounts of water may be locked within it and water may also be concentrated subsurface. The next views show the South Polar icecap (top) and North Polar icecap (bottom) (which may contain more water) near their maximum growth stage during a martian winter. This winter recurs about every 685 earthdays at each pole; in about half that time the polar ice at one pole diminishes as summer warming evaporates the frozen gases even as the opposing pole experiences ice condensation and growth:

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