POST-VIKING ORBITER PERSPECTIVES ON THE GEOLOGY OF THE MARTIAN NORTHERN PLAINS

The diversity of modern Martian data sets affords re-examination of the geologically-enigmatic northern plains. Post-Viking geologic observations reveal greater complexity in the plains deposits than previously appreciated, prompting renewed interest in the refinement of lowland geologic units, stratigraphy, nomenclature, and regional history. We cite five notable refinements in our understanding of northern plains geology. First, the underlying surface of the northern plains is likely to rival the age of the most ancient, exposed cratered highlands (pre-Noachian?). The materials that bury this ancient surface have laterally-contrasting lithology, thickness, and provenance. Second, the hypothesis that most of the plains below ~-3900 meter elevation consist of materials that were emplaced within a temporary ocean during the Hesperian (or earlier) continues to be evaluated. Though Viking data sets alluded to the bulk of northern plains deposits having been emplaced through large-scale, primary (depositional) processes, post-Viking data sets indicate that Amazonian materials at local- to regional-scales formed primarily through secondary (modificational) processes. The primary depositional record may be lost. Third, structurally-discrete expanses of the highland-lowland boundary (HLB) substantially influenced the evolution of the adjacent lowland deposits. The margins of ancient, multi-ring basins (e.g., Utopia basin) facilitated backwasting of the HLB scarp and the development of transitional boundary terrains. Fourth, the Martian mid- to high-latitudes have been the focus of volatile-driven geologic processes throughout the Amazonian, perhaps in a similar fashion to terrestrial glacial and periglacial processes. This is evidenced by spatially-overlapping observations, including elevated surficial hydrogen abundances, widespread smoothing at multiple horizontal wavelengths, gully formation in crater walls, and mantle-like surface deposits. Fifth, and finally, circum-polar outcrops within the northern plains may have formed through and been affected by cyclic deposition and exhumation of weak material. We find evidence that vestiges of these friable sequences may form portions of some of the stratigraphically-lowest polar layered sequences.