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.

Where are Mars’ Hypothesized Ocean Shorelines? Large Lateral and Topographic Offsets Between Different Versions of Paleoshoreline Maps

“…The wide topographic and spatial range of the Arabia Level, even when considering the range of different mappings, does not strongly support an ocean hypothesis and may simply be the result of the degradation of the highlands or exposure of different lithological units along the topographic dichotomy (Sholes et al, 2019;Tanaka, 1997).…”

“…(2010, their Figure 9.3) review). The Arabia Level is difficult to map because the level exhibits a range of geomorphic expressions along track and is often discontinuous (Parker et al., 2010; Sholes et al., 2019). For mapping, we use the level description provided in Parker et al.…”

“…To test whether published maps of the Arabia Level capture the defining characteristics of the putative paleoshoreline, we remap one of its type localities where features are coherent and observable (located in Deuteronilus Mensae as presented in Parker et al (1989, their Figure 4b) and again in the Parker et al (2010, their Figure 9.3) review). The Arabia Level is difficult to map because the level exhibits a range of geomorphic expressions along track and is often discontinuous (Parker et al, 2010;Sholes et al, 2019). For mapping, we use the level description provided in Parker et al (2010) adapted from Parker et al (1989, their Figure 4b): A sharp albedo contact between the dark-toned northern plains material and the light-toned upper highlands material.…”

“…Past oceans would imply many constraints on the past climate, habitability, and hydrological evolution of the planet. Putative paleoshorelines have been described as “the most compelling evidence that Mars once had oceans” (Zuber, 2018), but two major problems confront their interpretation: 1) detailed localized geomorphological studies of the putative shorelines consistently find little to no evidence of coastal landforms (e.g., Ghatan & Zimbelman, 2006; Malin & Edgett, 1999; Sholes, 2019; Sholes et al., 2019) contrary to limited studies of south Isidis (Erkeling et al., 2014, 2012) and broader regional analyses (e.g., Clifford & Parker, 2001; Parker et al., 2010, 1993, 1989), and 2) the mapped features vary by multiple kilometers in elevation across the planet in contrast to an expected equipotential surface (Carr & Head, 2003) (Figure 2). Here, we set aside the validity of these features as paleoshorelines and, rather, address the mapped locations of the features and how that affects their topographic expression and, by extension, their interpretation.…”

Circumpolar ocean stability on Mars 3 Gy ago

“…One also has to consider the arguments that confront the stable ocean scenario. First, the interpretation and location of the paleo shoreline itself are matters of controversy ( 68 , 69 ). Some studies claim that the putative shorelines are mutually inconsistent ( 70 ) but it mainly concerns contact 1 (Arabia potential shoreline), not contact 2 (younger Deuteronilus shoreline) that we use in our study.…”