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.
The ice is blue in the pictures. Against ground the colour of rust, in the enhanced colour that HiRISE returns, eight steep scarps in the Martian mid-latitudes show a pale blue-white face where erosion has cut the mantling deposit open and left its interior standing in cross-section. Colin Dundas of the USGS Astrogeology Science Center and eleven colleagues published them in Science on 12 January 2018. The ice runs more than 100 metres deep. Its top sits 1 to 2 metres below the surface.
Latitude is the whole point. Odyssey's neutron counts had found hydrogen poleward of about 60 degrees in 2002, and Phoenix scraped down to ice at 68.2 north in 2008 — both of them in the arctic, 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 pitched as steeply as 55 degrees. CRISM, the Mars Reconnaissance Orbiter's spectrometer, found water-ice absorptions in the exposed walls, so this was not an inference from hydrogen. It was ice, seen edge-on.
Radar had already said something was down there. In November 2008 John Holt's team published SHARAD soundings of the lobate aprons ringing massifs in eastern Hellas: not rock piles but debris-covered glaciers, one of them up to half a mile thick, the largest reservoir of water ice on Mars outside the caps. The same instrument found northern equivalents in Deuteronilus Mensae. What radar could not tell you was how far down you would have to dig or how much grit was mixed in. The scarps answered both by falling apart.
They are still falling apart. The exposed ice is sublimating, which is why the faces stay clean and why boulders keep dropping out of them. And 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. Nobody has dated a single band. An ice core lying open on a cliff face is worth nothing until something goes and reads it.
The practical line was blunter. Astronauts, the team said, could get their water with "a bucket and a shovel." That is the piece missing under every in-situ resource plan — the 26-month window, the propellant you have to manufacture after you arrive, MOXIE's oxygen. A 2021 follow-up by the same group found such exposures are widespread rather than eight lucky accidents. Where to actually land is unsettled: NASA's 2019 shallow-ice map leaned north toward Arcadia Planitia, partly because lower ground gives a descending spacecraft more atmosphere to brake in.