The streaks that turned back into sand
NASA announced briny water seeping down warm slopes. Two years later the same streaks were shown to stop exactly where dry sand stops, and the claim collapsed.
On 28 September 2015 NASA announced liquid water flowing on Mars today. Lujendra Ojha, a graduate student at Georgia Tech, had pointed Mars Reconnaissance Orbiter's CRISM spectrometer at four sites where dark streaks appear on steep slopes in the warm season; at their longest, they showed hydrated salts — magnesium perchlorate, magnesium chlorate and sodium perchlorate. Perchlorates pull water vapour from the air and hold it as brine far below freezing.
The streaks, recurring slope lineae, were identified in 2011 by Alfred McEwen's HiRISE team: a few metres wide, lengthening downhill over weeks, fading as the ground cools, returning next year in the same places. RSL sites became candidate "special regions" under planetary protection rules, closed to insufficiently sterilised spacecraft.
The spectral detections were marginal: a CRISM pixel is about 18 metres across, wider than the features it was resolving, so the signal came from mixed ground; other groups read the absorption features as processing artefacts. In November 2017 Colin Dundas's group published stereo topography for 151 lineae at ten sites. Almost every one stopped on a slope of 27 degrees or steeper, the angle at which dry sand stops moving; water does not stop at the angle of repose, it runs onto flatter ground.
RSL are now read as largely or entirely dust and sand avalanches, perhaps started by small amounts of water or by gas from the soil. The restrictions stayed in place.
In July 2018 Roberto Orosei's team reported a bright radar reflection beneath the south polar ice at Ultimi Scopuli, about 1.5 kilometres down and 20 kilometres across, read as a subglacial lake. By 2022 other groups had shown that frozen smectite clay, or interference between thin ice and dust layers, reproduces the reflection with no liquid present. That argument is open.
Ancient water is not in question; the rock record is unambiguous. Across most of the planet surface pressure sits at or below water's triple point, so liquid is hard to keep there.