We used to think of Mars as a dry, desolate planet devoid of running water. Then, in 2011 astronomers discovered evidence that liquid water might still flow on the Martian surface. This gave scientists hope that life may still exist on the red planet and that humans could more easily set up camp there. However, new evidence suggests past observations might not have been water at all. The dark streaks on Mars could simply be more sand.
Scientists refer to the mysterious dark streaks on Mars as recurring slope lineae, or RSL. They behave like you’d expect water to behave: in the warm season, the dark streaks appear by the thousands on steep slopes. They get longer and darker, until they abruptly vanish as winter takes hold. The leading hypothesis stated that brine (water with high salt content) locked up in the soil would run down the slopes each spring, then recede in the winter.
A new analysis of RSL has been published in Nature Geoscience, conducted by a team from the US Geological Survey, the Planetary Science Institute, the University of Arizona, and the UK’s Durham University. The team used data from the Mars Reconnaissance Orbiter (below), a satellite that has been observing Mars from more than 200 miles above for the last decade. The satellite was used to study 151 RSLs at 10 different sites, and the researchers noticed a strange trend. All the streaks ended at similar points, no matter the length of the slope. If this was seeping water, the streaks should be longer on longer slopes.
The team says RSL compare more accurately to granular flows here on Earth — Martian soil that’s sliding down the slopes. This would involve little or no liquid water in the RSL. The presence of hydrated salts on Mars could play a role, though. These molecules can pull water vapor out of the thin atmosphere, which could cause the darkening and trigger an RSL without “flowing” water.
Whatever the cause, the study notes that we won’t know until an RSL can be investigated first-hand. Earth is the only analog to Mars we have, and there are too many differences to test our current hypotheses here. Even if a future rover determines there’s no flowing water in RSLs, we do know there’s plenty of water on Mars in the form of ice at extreme latitudes. There are also other signs that liquid water could exist in other areas.
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