When Mariner 9 became the first craft to orbit another planet in 1971, the photographs it returned of dry river beds and canyons seemed to indicate that water had once existed on the Martian surface. Scientists hypothesise that with Mars having less gravity and a thinner atmosphere, liquid water would have evaporated into space over millions of years, allowing less to fall back to the surface of the planet.

Evidence for liquid water on Mars first came to light in 2000, with the appearance of gullies that suggested a liquid origin. Their formation has been hotly debated over the ensuing years. The exploration for liquid water on the Red Planet took more than 15 years to turn up what appeared to be definitive signs that liquid flows on the surface today.



In 2015, Scientists at NASA advanced a theory that features on Mars known as recurring slope lineae (RSL) were signs of salty water running on the surface of the planet today. They noted that the liquid water was briny, being bonded to a mix of compounds known as “chlorates” and “percholorates”.

In August 2016, the space agency reported that new analysis of data gathered by Mars Odyssey's Thermal Emission Imaging System (THEMIS) suggests that whatever makes RSLs is not water.

The key THEMIS finding is that the parts of Mars where RSLs form are at the same temperature as parts of Mars where there are no RSLs. This puts the liquid water theory into doubt since it would be expected that warmer zones to be more conducive to liquids forming and flowing.

THEMIS analysis is now thought to indicate “an upper limit on how much water is present at these darkened streaks: about as much as in the driest desert sands on Earth.” That's three grams of water per kilogram of soil.

RSLs may still be related to water

Despite the latest conclusions, scientists remain confident in their theory that RSLs may have something to do with water. Christopher Edwards of Northern Arizona University, who with colleagues analysed the THEMIS data has stated the following:

“Our findings are consistent with the presence of hydrated salts, because you can have hydrated salt without having enough for the water to start filling pore spaces between particles.”

“Salts can become hydrated by pulling water vapor from the atmosphere, with no need for an underground source of the water.”

"Some type of water-related activity at the uphill end still might be a factor in triggering RSL, but the darkness of the ground is not associated with large amounts of water, either liquid or frozen,"

"Totally dry mechanisms for explaining RSL should not be ruled out."

Next steps

The question now is, what makes RSLs? A new theory suggests they are made from “cascades of dry material with different thermal properties than the pre-existing slope material, such as would be the case with annual avalanching of powdery dust that accumulates from dusty air.” This Martian hypothesis will need a lot more exploration before it can be confirmed with certainty.

Whilst NASA has announced they have not found liquid water on Mars, the new observations do not contradict Martian wet patch theories. The next NASA rover launch is due in 2020 and will look for further evidence.

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