An overview of geological results from Mariner 9


  • Harold Masursky


Mariner 9 acquired pictures of all of Mars at a resolution of 1–3 km; 1–2% of the planet is covered by pictures of 100- to 300-meter resolution. From these data, preliminary 1:5,000,000 scale photomosaics have been made of the entire planet, and a 1:25,000,000 scale shaded relief map published. Geologic maps of the planet have also been made at a variety of scales. The more than 7300 pictures acquired indicate that Mars is more varied and dynamic than previously inferred. About one half of the surface consists of ancient cratered terrain; the largest circular feature, Hellas, is almost twice the size of the largest basin on the moon, Imbrium. The remainder of the surface is covered either by younger volcanic rocks and constructs that stand as much as 17km above the mean level or by extensive tracts of plains deposits, some of which are sedimentary in origin. The volcanic piles with summit calderas have fresh flank flows and appear to be geologically young. The great equatorial chasm or canyon system, comparable in size to the East African rift valley system, terminates in a complexly faulted plateau to the west and in large patches of chaotic terrain on the east. Large fluvial channels originate in this chaotic terrain possibly by melting of permafrost and appear to flow northward into the Chryse region. Other large sinuous channels with many tributaries have no such obvious source areas and many small dendritic channel networks abound in the equatorial regions and imply possible collection of rainfall. In addition, many small lava channels with distinctive characteristics are present like those on the moon and earth. Many of the basin floors are underlain by lava flows inferred to be basaltic from the form of the flows, ridges, and domes that characterize their surface. The polar regions are covered by glacio-eolian layered sediments that appear to be still forming under the present climatic regime. Older, layered, somewhat different deposits are being eroded into large pits and troughs around the margins of the poles. A mantle of eolian debris presumably derived from these eroded circumpolar zones thins equatorward. Both eolian erosional features such as yardangs and depositional features such as dunes have been identified. Eolian erosion and deposition processes are currently active, as is seen by numerous changes in the albedo patterns that were monitored after the clearing of the planetwide dust storm. The largest planetary scale differences in crustal style are between the southern highlands, presumably underlain by less dense rocks, and the northern lowlands or ‘oceanic’ basins, underlain by more dense rocks. The greatest difference along the equator is between the high ‘continental’ block of the Tharsis ridge with its volcanoes aligned along its margin and the oceanic floor of the Amazonis basin in which the Nix Olympica volcanic pile lies.