Morphologic Observations of Chasma Boreale, Mars Using Moc, Themis, and Mola: Origins Revisted

Introduction: Chasma Boreale, a large reentrant in the martian North Pole, is distinct from other polar troughs by its large size and counterclockwise orientation [5]. The ~ 1300 m deep chasma extends for ~ 500 km from its proposed origin at 85° N, 2o E and is on average ~ 60 km wide. The head of the feature is marked by two major steep, arcuate scarps. Extending from the mouth of Chasma Boreale is a large lobate feature [11]. This Amazonian age deposit is ~ 250 m thick and appears to overlay polygonal terrain of the Vastitas Borealis Formation (Hvg) [10]. There have been several proposed formation mechanisms for Chasma Boreale, including katabatic wind erosion [8,9], accumulation/ablation [6], and catastrophic outflooding [1,3,5]. Here we examine the latest image data from the Mars Orbiting Camera (MOC) and Thermal Emssion Imaging System (THEMIS) to describe the overall morphology of the Chasma Boreale system and to attmept to reconcile its mechanism of formation. THEMIS VIS images provide complete coverage for the chasma at an ideal 18 – 40 m/pixel resolution. Mars Orbiting Laser Altimeter (MOLA) topographic profiles were also constructed across observed features. General Morphology: The floor of the chasma has an overall negative slope of ~0.1°, measured from the center of the source arcuate scarp (85° N, 2° E) to the furthest margin of the lobate deposit (79° N, 53° W). The lowest elevations are found within the depressions at the base of the arcuate scarps [5]. The steep-walled, arcuate scarps appear as single continuous arcs in lower resolution Viking Orbiter images. These arcs cut through the polar layered material (PLD) and perennial cap. However, higher resolution THEMIS images reveal that each major scarp actually consists of a series of smaller, adjacent scarps. Depressions at the base of each scarp expose an older unit with a cratered, low albedo surface, and wrinkle ridge-like features that suggest similarities to ridged lava plains [10]. Extending from the mouth of Chasma Boreale is a distinctive lobate scarp. This scarp is approximately ~250 m in height and has a slope of ~ 5° [5]. The lobate material exhibits meter scale layering at MOC and THEMIS resolutions. Although it may have a different origin, this layering is similar in scale to the layering of the PLD displayed in the chasma walls. The lobate material overlies polygonal terrain of the Upper Hesperian Vastitas Borealis Formation (grooved member, Hvg) [10]. Based on crater counts, the lobate material itself is likely Amazonian in age. THEMIS images reveal, however, that the polygonal pattern, although subdued, is also present on the younger lobate material (Figure 1). Several troughs, forming orthogonal intersection patterns, are visible in the western exposures of this deposit. These troughs are not as obvious along the southern and eastern margins, where they appear to be mantled by a surface deposit. In many cases, troughs extend outward from the base of the lobate deposit at small reentrants. These troughs join with other troughs to form the pologynol terrain of Hvg. Aeolian Features: Aeolian features, including sand dunes and seasonal frost streaks, dominate the floor of Chasma Boreale. For this research, aoelian dunes were mapped, described, and orientations determined. Three dune forms are abundant within the chasma. These include transeverse, barchan, and linear dunes. Slip face orientations of the transverse and