THE MARTIAN SURFACE:
like the moon and Mercu ry, Mars has a different topographic pattern in each hemisphere, but it is more diverse and complex than either the moon or Mercury. Mars's northern hemisphere is generally lower than the mean radius of the planet by a couple of kilometers, possesses few craters, and has been altered by intense volcanic activity and subsequent lava flooding. The extensive lava flooding occurred at various times after the cessation of the heavy bombardment period in the early solar system. On the other hand the southern hemisphere has a densely cratered surface, averaging a couple of kilometers greater than the mean radius. Its crust has not changed appreciably throughout the planet's life.
There are about 16 smooth circular basins, or maria, on Mars. One that has long been observed from the earth is Hellas, an almost craterless basin about 1600 kilometers wide, or about one and a half times the size of the largest lunar sea, Mare Imbrium. The small number of impact craters on the maria suggests that they formed after the heavy bombardment period ceased in the inner solar system some 4 billion years ago. Estimates of age for them are about 3 billion years or less.
The abundance of craters in some of the regions of the southern hemisphere is comparable to that in the bright highlands of the moon. The similarities between the crate red Martian southern hemisphere and the lunar highlands has prompted the speculation that the two are of about the same age. Thus almost half the surface of Mars is ancient terrain, with many of its landforms having remained essentially unchanged over the last 4 billion years.
From spacecraft photographs, one sees that Mars has a wide variety of channels in the regions of oldest terrain. They appear to have formed between 3 and 4 billion years ago, shortly after the end of the heavy bombardment period. is an oblique-angle photograph of a channel that has all the appearance (left center) of some drainage systems on earth. Thus it is speculated that water flowed on the Martian surface and is now held as subsurface ice. Since both polar caps contain water ice, it is not unreasonable to hypothesize that a time existed in the past when a thicker and warmer atmosphere permitted liquid water on the surface.
An important departure from the character of the major portion of the Martian terrain is the Tharsis ridge, shown in Figure 8.8. This area is different because of three large volcanoes, running diagonally along the crest of the ridge, and a spectacular isolated volcanic structure, Olympus Mons (Figure 8.9), which is similar to, but much larger than, Mauna Loa and Mauna Kea in Hawaii as seen from the bottom of the Pacific Ocean. In addition to these large shield volcanoes rising some 20 kilometers above the surrounding plains, there are flattish saucer-shaped volcanoes, some which lack impact craters on their slopes suggesting that they are relatively young. Tharsis ridge is 10 kilometers or so above the average Martian radius; the volcanoes extend above it.
In the equatorial region lies the spectacular canyon Valles Marineris, cutting across the middle of a plateau. It is nearly 4000 kilometers long, up to 100 kilometers wide in some places, and at least 4 kilometers deep (Figure 8.8). At its western end lies a complex pattern of intersecting fault valleys. Valles Marineris runs radially away from Tharsis ridge and probably results from the faulting that accompanied the evolution of Tharsis since its formation. However, thermaltectonic activity on Mars is much weaker than that on earth.
From orbit the dominant features of the region around the Viking 1 lander are craters. From the ground there are only a few obvious craters to be seen in the immediate vicinity of the lander. If Mars were like the moon, then there should be visible several small craters, tens of meters in diameter. Their absence indicates that the Martian atmosphere is dense enough to burn up small meteoroids or material ejected from large impacts before they reach the surface; and that surface erosion is vigorous enough to obliterate small craters. Thus there is not the profusion of small craters seen on the moon.
The area photographed by the Viking 1 lander in the Chryse region is a gently rolling landscape, yellowish brown in color, strewn with rocks and dotted with drifts of fine-grained material. Within 30 meters or so of the lander lie several outcrops of bedrock, which in many ways resemble the semidesert regions of the American Southwest but without vegetation. Chemical analysis by the lander found the elements common in crustal rocks on the earth, but organic analysis failed to detect any organic molecules (those containing carbon, one of the essential ingredients for life). On earth, the soil of even the most dry, sterile-looking valleys contains many organic compounds.
The multilayered polar regions are still another type of Martian topography. The layered deposits probably hold appreciable quantities of frozen water mixed with dust beneath a carbon dioxide coating. Periodic changes in Martian climate may be responsible for the deposition of the successive layers of material.
