The Deuteronilus level (“contact 2”) is an inner contact almost entirely within the northern plains thought to represent a smaller and younger ocean stand (Parker et al., 1989, 1993). The Arabia level (“contact 1”) roughly follows the dichotomy transition region and represents a hypothesized larger and older ocean stand. ( 1989, 1993) were described roughly following the topographic dichotomy along the northern plains boundary and interpreted as corresponding to different hypothesized ocean highstands. Two distinct contacts identified by Parker et al.

The question of whether or not such an ocean (or oceans) existed has profound implications for the past climate of Mars (e.g., Banfield et al., 2015 Jakosky & Phillips, 2001 Lammer et al., 2018 Ramirez & Craddock, 2018), the possibility of the planet harboring ancient life (e.g., Beaty et al., 2005), and the evolution and role of liquid water in its surficial geology (e.g., Carr & Head, 2010, 2015). These observations, coupled with hand-mapped contacts along the edges of the northern plains on comparatively low-resolution images, led to the interpretation of these contacts as paleoshorelines (Parker et al., 1989, 1993). Support for vanished oceans historically stems from the recognition that large outflow channels and valley networks empty into the northern plains, suggesting these lowlands served as a basin where water pooled (Baker et al., 1991 Jöns, 1985 Lucchitta et al., 1986). The extent and duration of large open reservoirs of liquid water on the surface of Mars are some of the most intriguing questions about the planet's past. Application of our methods to similar sites could shed further light on the ocean hypothesis. Our findings do not mean that oceans never existed on Mars, only that this specific site does not support the ocean hypothesis. Instead, features at this site are better explained as exposed and eroded rock layers that mimic shoreline shapes in low-resolution images. 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. 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. 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.

However, new high-resolution data allow for better and more complete analyses. Features have previously been traced out in relatively low-resolution images and interpreted as shorelines of episodic 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. There is an ongoing controversy about whether Mars had ancient oceans. 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. These features are more consistent with material boundaries for lithological units exposed along a degraded crater wall. Lineaments appear blocky in nature at small scales, and they bifurcate, truncate, and merge along track. 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. Our results show these curvilinear features are inconsistent with a wave-generated shoreline interpretation. (2001, ) 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, ). In particular, we apply the validated topographic expression analysis of Hare et al. We present a toolkit for quantitatively identifying paleoshorelines using topographic, morphological, and spectroscopic evaluations. Primary support for ancient Martian oceans has relied on qualitative interpretations of hypothesized shorelines on relatively low-resolution images and data.

Face of mars from different angle