1. Quantitative High‐Resolution Reexamination of a Hypothesized Ocean Shoreline in Cydonia Mensae on Mars.
- Author
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Sholes, Steven F., Montgomery, David R., and Catling, David C.
- Subjects
COASTS ,WATER on Mars ,OCEAN ,CYDONIA (Mars) ,MARTIAN craters ,TOPOGRAPHY ,SPECTRUM analysis - 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. Plain Language Summary: There is an ongoing controversy about whether Mars had ancient oceans. Abundant evidence for past surface liquid water exists, but it is debated how much and for how long it persisted on Mars' surface. Features have previously been traced out in relatively low‐resolution images and interpreted as shorelines of episodic ancient oceans. However, new high‐resolution data allow for better and more complete analyses. Here we present methods to determine whether any proposed ancient shoreline feature meets the requirements to be classified as a shoreline using these improved data. Shorelines should form flat "step‐like" landforms at a near‐constant elevation, so one critical analysis looks at the shape and elevation of proposed features. We examine a site that has been promoted in the literature as an exemplar of ancient shorelines and find that this site lacks a diagnostic shoreline shape. Instead, features at this site are better explained as exposed and eroded rock layers that mimic shoreline shapes in low‐resolution images. Our findings do not mean that oceans never existed on Mars, only that this specific site does not support the ocean hypothesis. Application of our methods to similar sites could shed further light on the ocean hypothesis. Key Points: We apply a proposed paleoshoreline‐identification toolkit to newer high‐resolution data of an exemplar site for paleoshorelines on MarsAny wave‐generated paleoshorelines should exhibit expressions identifiable in the residual topography from an idealized slopeOur analysis of these curvilinear features does not support a paleoshoreline interpretation and is more consistent with eroded lithologies [ABSTRACT FROM AUTHOR]
- Published
- 2019
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