Identifying the topographic signature of early Martian oceans | Nature

Download PDF

Subjects

- Geomorphology

Abstract

Planet-wide interpretations of shorelines suggest that Mars once hosted an early ocean covering one-third of its surface1,2,3,4,5,6,7,8,9. However, the elevations of these shorelines deviate from an equipotential surface by several kilometres, challenging that interpretation3,7,10,11,12. Here we investigate whether a planet that once hosted an ocean should be expected to leave discernible shorelines. We show that on Earth, the most prominent topographic signature of a global ocean is not a shoreline. Rather, it is a band of low slope and curvature values that comprises coastal plains and the continental shelf, with an elevation range of −410 m to −15 m. When applying a similar analysis to the Martian surface, we observe a comparably flat zone between approximately –1,800 m and –3,800 m elevation, potentially marking a partially preserved Martian coastal shelf. Although other processes, such as lava flows13, might explain flat regions locally, a coastal shelf best explains the circumglobal band of flat topography, in addition to river delta deposits4,14,15,16,17, coastal deposits18, thick sequences of layered rock19,20 and aqueously altered minerals20,21, all observed within the putative coastal shelf zone. Our results support the presence of an ancient ocean on Mars and indicate that topographic shelves rather than shorelines may be better indicators of long-lived oceans.

Main

The northern plains of Mars feature distinct geological boundaries between the southern highlands and northern lowlands extending for thousands of kilometres, which have been used to infer ancient ocean shorelines1,2,3,4,5,6,7,8,9. These are known as the Arabia (contact 1) and Deuteronilus (contact 2) shorelines3,7. However, several kilometres of deviation in the elevation of these proposed shorelines from equipotential surfaces called into question the interpretation of these features as shorelines and, consequently, the presence of a vast ocean covering one-third of the Martian surface3,7,10. Two explanations have been proposed to explain this long-wavelength deviation in putative shoreline elevation. The first is true polar wander, which formed a new equatorial bulge and redistributed mass from the palaeoequatorial bulge to the new equator, thereby deforming the surface topography and palaeoshorelines following Tharsis volcanic growth11. The second explanation is the Tharsis-induced deformation model, which proposes that the uplift from the formation of Tharsis altered the shape of Mars and geoid, leading to the deformation of the shorelines12. Moreover, misidentification of shoreline features or resurfacing due to tsunamis or lava flows has been raised as possibilities10,13,22,23. However, none of these explanations fully accounts for the global elevation deviations.

Previous work reconstructed Martian oceans by mapping topographic breaks2,3,7,22, assuming they represent shorelines, in line with palaeolake reconstructions on Earth. However, reconstructing even geologically recent palaeolake levels based on putative shorelines is challenging because of erosion and deformation24,25,26,27,28. Moreover, it is unclear if a shoreline is the topographic fingerprint of a long-lived ocean. Rather than a distinct shoreline imprint, the main topographic feature of the modern ocean is the band of low-gradient terrain that bounds most continents at an elevation between tens and a few hundreds of metres below the sea level, and it comprises coastal plains and the continental shelf29,30.

The origin of continental shelves is debated, and multiple erosion and deposition processes probably contribute to their formation. The processes include (1) fluvial deposition to build low-gradient deltaic and coastal plains, and the extension and contraction of these plains during sea-level fluctuations31,32,33,34; (2) wave bevelling to create a low-gradient platform and efficient offshore transport of sediment at depths above wave base through wave-supported suspensions33,35; and (3) the formation of buoyant freshwater river plumes over saline ocean water that move sediment long distances along the coast because of geostrophic steering36. On Earth, plate tectonics is responsible for differentiating the continental and oceanic lithosphere that has created continental margin relief37,38 and also for developing the faulted basement platform during rift extension on which the continental shelf on passive margins is built37,39. Active tectonics can also influence shelf morphology through faulting and folding37. Despite the lack of plate tectonics on Mars, the erosion and deposition processes that are the primary controls on shelf morphology on Earth may also have been active on early Mars. The question arises as to whether Mars has a coastal shelf.

To address this question, we analysed the global topography of Earth to identify the characteristic signature of the modern ocean (see flowchart in Supplementary F