On the origin of the Vastitas Borealis Formation in Chryse and Acidalia Planitiae, Mars

The geologic history of the northern plains of Mars has been extensively debated. Specifically, the Vastitas Borealis Formation (VBF) represents an enigmatic surface unit that exhibits flow morphologies at its boundary, extensive surface fracturing, tens of thousands of small mounds, and unique crater morphologies. Here we test the hypothesis that the VBF in the region of Chryse and Acidalia Planitiae, Mars, originated through the compaction and later expulsion of fluid-laden sediments sourced from the shallow subsurface. We find that the morphological, thermophysical, and spectral properties of the VBF marginal unit, in addition to the recent identification of fine-grained sedimentary layers in the shallow subsurface, are all consistent with such a formation mechanism. Estimates of volume loss based on "collar-like" morphologies present on high-standing buttes suggest that a minimum fluid volume of ∼13,500 km³ was expelled from the subsurface, making the VBF a significant ancient hydrologic reservoir on the Martian surface. This formation mechanism lends additional insight into the sedimentary and aqueous history of the northern plains of Mars and unites morphologic, spectral, thermophysical, and structural observations under one consistent formation hypothesis.