29 Extraterrestrial arid surface processes

The development of arid zone geomorphology has progressed over the past century from ground-level observations of a limited number of locations through the field traverse mapping of sizeable arid areas to a broad-scale overview of global desert regions assisted by aerial photography and satellite imagery. Extraterrestrial investigations of planetary surfaces and landforms have proceeded in exactly the opposite manner. Telescopic observations first provided global overviews of distant planets. The early spacecraft missions of the 1960s relayed hemispheric and regional views, while later missions returned more detailed images comparable to satellite images of the Earth. To date, detailed views seen from the surfaces of other terrestrial planets are restricted to two locations on Mars and four landing sites on Venus. As a result, the approach and scope of our inquiry into arid processes are necessarily different when studying extraterrestrial surfaces. In this review, we attempt to build a picture of the arid surface of Mars starting with a discussion of possible microscale and mesoscale processes and later examining the creation of individual arid landforms and terrains. We conclude with an overview of recent discoveries regarding the surface of Venus and attempt to forecast the course of future planetary missions of interest to arid zone scientists. Spacecraft exploration of Mars began with three Mariner flyby missions during the 1960s. The initial impression was of a surface much like that of the Moon. This judgement changed following the Mariner 9 mission in 1971-72, which relayed more detailed images showing a surface with many landforms common to Earth. Beginning in the summer of 1976, views from the surface were transmitted by two landing craft. On 20 July 1976, Viking 1 landed on the vast volcanic plain of Chryse Planitia (22.5°N, 47.8°W). Several weeks later, Viking 2 touched down upon Utopia Planitia (48.0°N, 225.6°W), another northern hemisphere plain. During the time of Viking lander data collection, two Viking orbiters imaged the surface from an altitude of about 1500 km. Though Mars is a small planet, its landsurface area is nearly equal to that of the Earth. Another close similarity is daylength. Because the Martian day is slightly longer than Earth's, events at the Viking landing sites were measured in Sols, or Martian solar days, following each landing. Mars also has distinct seasons, but due to the eccentricity of its orbit, the seasons are of unequal length. The