Widespread evidence points to the existence of an ancient Martian ocean1,2,3,4,5,6,7,8. Most compelling are the putative ancient shorelines in the northern plains2,7. However, these shorelines fail to follow an equipotential surface, and this has been used to challenge the notion that they formed via an early ocean9 and hence to question the existence of such an ocean. The shorelines’ deviation from a constant elevation can be explained by true polar wander occurring after the formation of Tharsis10, a volcanic province that dominates the gravity and topography of Mars. However, surface loading from the oceans can drive polar wander only if Tharsis formed far from the equator10, and most evidence indicates that Tharsis formed near the equator11,12,13,14,15, meaning that there is no current explanation for the shorelines’ deviation from an equipotential that is consistent with our geophysical understanding of Mars. Here we show that variations in shoreline topography can be explained by deformation caused by the emplacement of Tharsis. We find that the shorelines must have formed before and during the emplacement of Tharsis, instead of afterwards, as previously assumed. Our results imply that oceans on Mars formed early, concurrent with the valley networks15, and point to a close relationship between the evolution of oceans on Mars and the initiation and decline of Tharsis volcanism, with broad implications for the geology, hydrological cycle and climate of early Mars.
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We thank J. T. Perron for providing the data for the Arabia shoreline (originally from ref. 7), and M. A. Ivanov for providing the data for the Deuteronilus and Isidis shorelines. We thank I. Matsuyama for discussions regarding this research. R.I.C. and M.M. are supported by NSF EAR-1135382. D.J.H. is supported by the Miller Institute for Basic Research in Science.
The authors declare no competing financial interests.
Reviewer Information Nature thanks S. Bouley and M. Zuber for their contribution to the peer review of this work.
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Extended data figures and tables
Extended Data Figure 1 Illustration of the feasibility of post-Tharsis TPW depending on the location of Tharsis’ formation.
a–c, Tharsis (orange zone) forms far from the palaeo-equator (a), causing large-scale TPW as the planet reorients so Tharsis is at the equator (b). After the reorientation, the fossil bulge is far from the current equatorial bulge, making the rotation pole sufficiently unstable to allow for ocean loading (blue zone) to cause subsequent TPW along an arc 90° from Tharsis (c, blue dashed line)10. d–f, Alternatively, Tharsis forms near the palaeo-equator (d), causing limited (approximately 20°) TPW (e). The position of the fossil bulge near the equator stabilizes the planet against subsequent TPW caused by oceans and other surface loads (f, see supplementary figure 1 of ref. 10). Formation of Tharsis near the equator is supported by refs 14 and 15.
Arabia (magenta) shoreline data are from ref. 10 (data originally from ref. 7). Deuteronilus (white) and Isidis (cyan) shoreline data and regional names are from ref. 18. The contribution of Tharsis to Mars’ topography up to degree-5 (equation (2)) is displayed as 1-km dark grey contours (dashed contours are negative).
a, Current Arabia shoreline topography compared to displacement due to TPW and Tharsis deformation (equation (3)). Tharsis gravity and shape coefficients are computed separately for Te = 26 km, 58 km and 92 km (see Methods), which each yield a corresponding best-fit offset Z and error σrms. Dashed lines show the best fit when the percentage of Tharsis topography added after shoreline formation was allowed to vary by a factor C. Solid lines assume 100% percent of Tharsis topography was emplaced after shoreline formation (C = 1). b, Deuteronilus shoreline topography compared to the best-fit displacement due to Tharsis loading (equation (4)) for Te = 26 km, 58 km and 92 km.
Current Isidis shoreline topography (elevation data from ref. 18) compared to the Perron et al.10 model for Te = 200 km and our model of deformation due to partial Tharsis emplacement (0.17∆TTharsis − 3.95 km). The topography of the Isidis shoreline can be explained by subsequent loading of the Utopia basin (see Methods). The starting point for the shoreline is (82.32° E, 7.36° N), near the southwest (SW) rim, with shoreline data proceeding clockwise through the northeast (NE) rim.
Current shoreline elevations are plotted against displaced elevations for the Arabia shoreline (a), the Deuteronilus shoreline (b) and the Isidis shoreline (c).
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Citron, R., Manga, M. & Hemingway, D. Timing of oceans on Mars from shoreline deformation. Nature 555, 643–646 (2018). https://doi.org/10.1038/nature26144
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