The Significance of Sub-Milankovitch Signals in the Martian Northern Polar Layered Deposits

Abstract:

Our understanding of the paleoclimate of Mars and the dominant forcing functions which drive large-scale surface and atmospheric changes remains incomplete. The earliest observations of the layering in the polar caps suggested that it was due to climate forcing. While previous studies have searched for an interpretable climate signal in the PLD, definitive evidence of correlation of the stratigraphic record to the recent orbital history of Mars remains elusive. We conduct a new investigation of potential paleoclimate signals within the Polar Layered Deposits (PLD) on the northern polar ice cap of Mars. We present a study of 12 sites at the northern polar cap ranging from 82 to 88 deg latitude and outline new and updated techniques used to reconstruct a stratigraphic column from HiRISE stereo imagery, including geometric corrections for non-horizontal stratigraphy. Using time-series analysis, we find a significant signal representative of excess power at approximately the 1-meter scale, consistent with the occurrence of fine-bedding noted by previous authors. Under most assumed deposition rates for the NPLD, we note that this quasi-periodic signal must occur at centennial to millennial timescales, which would imply that the layers can not result from orbital forcing of solar insolation. These "sub-Milankovitch" layers must arise from a yet-unknown Martian climate signal intermediate between orbital and annual periods. While the observational record of Mars is not long enough to provide a definitive explanation, we speculate on potential causal mechanisms within the Martian climate system.