Quantitative High-Resolution Reexamination of a Hypothesized Ocean Shoreline in Cydonia Mensae on Mars

Abstract

Primary support for ancient Martian oceans has relied on qualitative interpretations of hypothesized shorelines on relatively low-resolution images and data. We present a toolkit for quantitatively identifying paleoshorelines using topographic, morphological, and spectroscopic evaluations. In particular, we apply the validated topographic expression analysis of Hare et al. (2001, https://doi.org/10.1029/2001JB000344) for the first time beyond Earth, focusing on a test case of putative shoreline features along the Arabia level in northeast Cydonia Mensae, as first described by Clifford and Parker (2001, https://doi.org/10.1006/icar.2001.6671). Our results show these curvilinear features are inconsistent with a wave-generated shoreline interpretation. The topographic expression analysis identifies a few potential shoreline terraces along the historically proposed contacts, but these tilt in different directions, do not follow an equipotential surface (even accounting for regional tilting), and are not laterally continuous. Lineaments appear blocky in nature at small scales, and they bifurcate, truncate, and merge along track. These features are more consistent with material boundaries for lithological units exposed along a degraded crater wall. These results do not preclude the existence of an ancient hemispheric ocean, but the geophysical and high-resolution topographic data at this site do not support such an interpretation.