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Ice cliffs expose clean water in the mid-latitudes

Eight eroding scarps at 55 to 58 degrees latitude cut open water ice more than 100 metres thick, its top just 1 to 2 metres down — and visibly banded.

An orbital view of a steep north-facing cliff inside a crater, pale material exposed in cross-section along its face.
Not one of the eight scarps in the 2018 survey, but the same thing elsewhere: icy material exposed in cross-section on a mid-latitude crater wall. · Image: NASA/JPL-Caltech/University of Arizona

Colin Dundas of the USGS Astrogeology Science Center and eleven colleagues reported eight eroding scarps in Science on 12 January 2018. Erosion has cut the mantling deposit open, leaving its interior in cross-section, pale blue-white against rust-coloured ground in enhanced colour HiRISE. The ice runs more than 100 metres deep, its top 1 to 2 metres below the surface.

Odyssey's neutron counts found hydrogen poleward of about 60 degrees north and south in 2002, and Phoenix scraped down to ice at 68.2 north in 2008, where a solar-powered machine freezes to death in its first winter. These scarps are at 55 to 58 degrees in both hemispheres, on faces as steep as 55 degrees. CRISM, the Mars Reconnaissance Orbiter's spectrometer, found water-ice absorptions in the walls, so this was not an inference from hydrogen.

Radar had already found buried ice: in November 2008 John Holt's team read SHARAD soundings of the lobate aprons ringing massifs in eastern Hellas as debris-covered glaciers, one up to half a mile thick, and collectively the largest water-ice reservoir on Mars outside the caps; the same instrument found northern equivalents in Deuteronilus Mensae. Radar could not say how far down to dig; the scarps answer that.

The sublimating ice keeps the faces clean and sheds boulders. The walls are banded; Dundas's team read the layering as snow, laid down when Mars' axial tilt swung far wider than Earth's ever does and drove polar ice toward the middle latitudes, then compacted into massive, fractured ice. No band has been dated.

Astronauts, the team said, could get their water with "a bucket and a shovel" — which bears on every in-situ resource plan: the 26-month window, propellant made after arrival, MOXIE's oxygen. A 2021 follow-up by the same group found such exposures widespread, not eight isolated cases. Where to land is unsettled: NASA's 2019 shallow-ice map leaned north toward Arcadia Planitia, partly because lower ground gives more atmosphere to brake in.