GEOLOGIC MAPPING RESOLVES THE ENIGMA OF MOTTLED TERRAIN IN CHRYSE AND ACIDALIA PLANITIAE

The history of infilling in the Martian northern lowlands remains poorly understood despite decades of focused geologic mapping and topical science investigation. In particular, the units in Chryse and Acidalia Plantiae (CAP) remain enigmatic due partly to the presence of “mottled terrain” – a suite of landforms and units identified in Mariner and Viking images by sharp albedo changes. The mottled terrain has been interpreted as remnant highland strata eroded into knobs, intermixed aeolian and volcanic units, and exhumed bright mantles. Recently acquired high resolution images indicate the “bright” component of mottled terrain is a meters-thick, fine-grained mantle that has undergone periglacial and aeolian modification. However, the dark component has never been satisfactorily addressed insofar as geologic process and evolution. As part of recent global geologic mapping efforts and regional thermophysical assessments, we observe that the bulk of the mottled terrain in CAP is comprised of bright mantles overlying dark, lobe-forming units whose shape implies emplacement through flow to both the north and south. We mapped >700 individual dark lobes using a controlled THEMIS daytime IR mosaic. Lobes are defined by dark outer margins, intermediate to bright interiors, and digitate margins that are discontinuous, self-overlapping, and often levee bound. Spot measurements of the MOLA DEM indicate levees can be up to 10 meters high. The mapped lobes range from 6 to >100 km in length. Polygonal fractures and pitted cones – common lowland landforms – generally do not coincide with dark lobes. Rather, lobes tend to bury polygons while pitted cones tend to occur in bright, superposing mantle-like units. Though dark lobes dominantly face northwest (41°), a small but significant set of lobes face southwest (228°), suggesting emplacement via flow in both directions. Bimodally oriented lobes not only occur in close proximity to one another but also are centered along SE-NW trending ridge- and trough-like lineaments. Therein, dark lobes form elongate edifices a few tens of meters high. We previously interpreted the dark lobes in CAP as digitate terminations of debris flows sourced from the outflow channels. However, recent work suggests some dark lobes were emplaced via fissure-fed eruption of either magma or fluidized sediment.