The thumbprint terrain: What will Mars Observer tell us?

Some of the more puzzling features seen on Mars are those known as curvilinear features or 'thumb print' terrain, types of patterned ground found in the northern plains. The thumbprint terrain, named for its resemblance to the lines of a human thumbprint is found on what appears to be level, relatively low-lying ground near the crustal dichotomy boundary. It is often found near the termini of large channels. There are three types of thumbprint terrain in the classification of Rossbacher and Judson. The first consists of ridges and depressions about a kilometer wide, separated by a few kilometers, and with an apparent relief of 40-100 m. The second consists of steep-sided, flat-floored depressions about a kilometer wide, separated by a few kilometers, and with an apparent relief of 10-290 m. The third consists only of albedo markings, with a typical scale of about a kilometer. Many models were proposed to explain the origin of this terrain. It was suggested that it was caused by lava flows, removal of debris mantles, glaciers, or karst. However, the most popular models at present involve the action of subsurface ice to form such thermokarst features as striped ground, solifluction lobes, and/or linear, ice-cored ridges. There are several instruments on the Mars Observer spacecraft that will be able to provide us with information useful in distinguishing between these models.