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Surface topographic impact of subglacial water beneath the south polar ice cap of Mars


Bright radar reflections observed in the Ultimi Scopuli region of Mars’ south polar layered deposits1,2,3 by the Mars Advanced Radar for Subsurface and Ionosphere Sounding instrument have been interpreted as the signature of areas of subglacial water beneath it. However, other studies put forward alternative explanations, which do not imply the presence of liquid water4,5,6. Here we shed light on the issue by looking at the surface topography of the region. On Earth, reduced or absent basal friction, and consequent ice velocity changes, cause a distinct topographic signature over subglacial lakes7. Using Mars Orbiter Laser Altimeter data8, we identify and characterize an anomaly in the surface topography of the south polar layered deposits overlying the area of the putative lakes, similar to those found above terrestrial subglacial lakes of similar size. Ice flow model results suggest that comparable topographic anomalies form within 0.5–1.5 Myr with locally elevated geothermal heating9 or 2–5 Myr without elevated geothermal heating2. These findings offer independent support for the presence of basal water beneath Ultimi Scopuli and suggest that surface topography could supplement radar returns to help identify other potential subglacial water bodies.

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Fig. 1: Surface topography of Mars’ SPLD, and topographic analysis results.
Fig. 2: Model results for the centre of the region containing the inferred water (red box in Fig. 1b).
Fig. 3: Scatter plots of residuals from the trend surface shown in Fig. 1d against modelled elevation changes within 20 km radius of the centre of the region containing the inferred water.

Data availability

MOLA and MARSIS data are available from the PDS Geosciences node at and at respectively; MARSIS data are also available at the ESA Planetary Science Archive (!Table%20View/MARSIS=instrument). The mean perturbed bed topography16 data used in the area containing the inferred water bodies is available via the University of Cambridge Apollo repository at

Code availability

ISSM is available from NASA/JPL at Code used to calculate slope shading and contributing area is available on reasonable request to the corresponding author.


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The MARSIS instrument and experiment were funded by the Italian Space Agency and NASA. It was developed by the University of Rome, Italy, in partnership with NASA’s Jet Propulsion Laboratory (JPL). The Mars Express and Mars Global Surveyor missions are operated by the space agencies of Europe (European Space Agency), Italy (Agenzia Spaziale Italiana) and the United States (NASA). F.E.G.B. is part of the PALGLAC team of researchers and received funding from the European Research Council (ERC) under the European Union’s Horizon 2020 research and innovation programme (grant agreement 787263). We thank M. Morlighem for help and discussions with ISSM installation and set-up.

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Topographic analysis and modelling was undertaken by N.S.A. F.E.G.B. and S.J.C. assisted with MOLA and MARSIS data download and processing, and with initial discussions on the possibility of detecting surface anomalies on the SPLDs. C.G. and M.R.B. extracted and processed the original MOLA point data from the repository and checked coverage in the study area. The initial draft of the manuscript was written by N.S.A.; all authors contributed to the submitted version, revisions and discussions of the aims and arguments within the paper.

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Correspondence to N. S. Arnold or F. E. G. Butcher.

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Nature Astronomy thanks Ali Bramson and Frank Pattyn for their contribution to the peer review of this work.

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Additional model data and results, Supplementary Figs. 1 and 2 and Tables 1 and 2.

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Arnold, N.S., Butcher, F.E.G., Conway, S.J. et al. Surface topographic impact of subglacial water beneath the south polar ice cap of Mars. Nat Astron 6, 1256–1262 (2022).

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