Thumbprint Terrain and Sinuous Troughs with Medial Ridges in the Northern Lowlands of Mars: Assesment of the Glacial Hypothesis Using New Spacecraft

Introduction: Many workers have described the thumb-print terrain (TT)—terrain consisting of nested curvilinear features visible throughout much of the Northern hemisphere of Mars in Viking data (see Fig. 1)—and the troughs with medial ridges frequently associated with that terrain [1-7, see references within]. Interpretations of TT have included: accumulations of eolian deposits; ice-shoved glaciotectonic features ; chains of cinder cones; moraines; and paleocurrent markers [1-7, see references within]. Kargel et al. [1995] is one of the most comprehensive recent treatments of TT. They attribute the entire system to glacial processes, positing the thumbprint terrain as moraines and the troughs as tunnel channels with medial eskers. This study evaluates the mechanism of origin from [1], testing it against new data available from the Mars Global Surveyor (MGS). Pre-MGS Characterization: Kargel et al. [1], using Vi-king data, characterized the thumbprint terrain (TT) as "whorled ridges, spaced about 2-6 km and forming lobate patters with lobe widths of about 150 km." The authors liken the TT to recessional moraines in central North America and northern Europe, citing similarities of size, spacing, cross-sectional profile, and regional expanse. However, they note that many other landforms share similar characteristics: such as sublacustrine moraines, ice-thrust ridges, and subglacial water-saturated till deformation features. The deep troughs with medial ridges they compare to tunnel channel-esker com-plexes—frequently formed during the collapse of terrestrial glaciers [9, 12]—in scale, sinuosity, and pattern of integration. The authors also commented on the 'knobs' or 'mounds' found interspersed with the troughs in one of their main sites, Arcadia Planitia, interpreting them to be probable kames. All of these features require the presence of glaciers, so the authors concluded that, although the exact formation mechanism could not be ascertained, glacial formation was a very strong probability. Comparing these hypotheses with the regional context of the sites, they found glacial formation hypotheses to be plausible, and even suggested that these features might mark regions where a southerly-advancing ice sheet compressed against the lowlands/highlands border. Kargel et al. [1] point out two major facets of their interpretation that require further research: 1) the absence of drumlin fields, and 2) the unusual convex nature of the TT with respect to the trough complexes. Drumlin fields are expected in localities with extensive glacial movement and erosion [9]. Kargel et al [1] attribute the lack of drumlin fields to resolution and illumination problems in the Viking images. The convexity of the …