September 28, 2015

Recurring slope lineae

NASA announces spectral evidence for present-day briny water activity on Mars

Nobody photographed flowing water. The decisive observation was an orbital spectrometer's chemical fingerprint of hydrated salts — and two years later the reading shifted.

What was actually announced

On 28 September 2015 NASA announced findings from the Mars Reconnaissance Orbiter that it described as the strongest evidence yet for intermittent liquid-water activity on present-day Mars.

A team led by Lujendra Ojha published CRISM spectrometer observations showing hydrated-salt signatures at four recurring slope lineae locations, taken in seasons when the dark streaks were most extensive.

The signatures were interpreted as magnesium perchlorate, magnesium chlorate and sodium perchlorate — salts that can help water stay liquid in cold Martian conditions.

Streaks that grow and fade

Recurring slope lineae are dark streaks that lengthen down steep slopes during warmer seasons and fade when conditions cool. NASA highlighted typical lengths of about 100 metres, some extending a few hundred, with activity associated with temperatures above roughly −23°C.

Neither the orbiter nor the study directly photographed or sampled flowing liquid. What was measured was hydrated minerals, used to support a hypothesis of contemporary water activity.

Hydrated salts hold water bound inside their crystal structure, and detecting them does not by itself demonstrate macroscopic flowing liquid.

The interpretation did not hold

The listing's phrasing overstates the result. The announcement concerned possible present-day seasonal brine — not a lake, a stream, a usable water supply, or evidence of life.

A 2017 topographic study found that the streaks end at slopes characteristic of dry granular flows and concluded that liquid water probably has a limited role. NASA now presents them as better matched to dry sand or dust movement, while allowing that small amounts of water may help trigger them.

That later work did not erase the salt detection. What it weakened was the claim that the streaks themselves are liquid flows.

Sources

Researched 24 Aug 2026 7 sources not yet audited Date disputed

How this was checked

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Date disputed. Sources disagree about when this happened.

What the sources leave uncertain

The announcement date is certain, but the listed title overstates the result. NASA announced indirect spectral evidence supporting contemporary briny water activity, not direct detection of liquid water.
A 2017 topographic study found that RSL end at slopes characteristic of dry granular flows and concluded that liquid water probably has a limited role. NASA now presents the streaks as better matched to dry sand or dust movement, while allowing that small amounts of water may help trigger them.
Hydrated salts contain water bound within their crystal structure; detecting them does not by itself demonstrate macroscopic flowing liquid.