Martian ‘salty streams’ are as parched as Earth’s driest deserts

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Dark narrow streaks called ‘recurring slope lineae’ emanate from the walls of Garni Crater on Mars. New observations show they’re not as watery as people may think.
NASA / JPL-Caltech / Univ. of Arizona

There’s liquid water on Mars – at least, that’s what analyses of shifting dark streaks on the red planet suggested last year. But new observations are set to put a bit of a dampener on that conclusion.   

NASA’s Mars Odyssey and its Thermal Emission Imaging System (THEMIS) calculated those streaks contain as much water as the driest places on Earth. 

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The work, by Christopher Edwards from Northern Arizona University and Sylvain Piqueux of NASA’s Jet Propulsion Laboratory in the US, has been accepted for publication in the Geophysical Research Letters.

For the past few years, the biggest clue to the existence of water on Mars was dark streaks – called recurring slope lineae – that adorned edges of canyons and crater walls. They appeared to flow with gravity and grew and shrank with the seasons. 

After investigating further, scientists discovered last year that these mysterious phenomena were made of hydrated salts.

So to calculate exactly how much water the features contained, THEMIS – an infrared camera on board the Odyssey orbiter – examined how much those patches of sandy ground heated and cooled during the Martian day and night.

When water is present in the spaces between the particles of soil or grains of sand, it affects how the temperature swings.

Edwards and Piqueux looked at several years’ worth of THEMIS data for a region within the Valles Marineris canyon system, which has an array of these streaky features sitting close together. 

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Blue dots on this map indicate sites of recurring slope lineae in part of the Valles Marineris canyon network on Mars.
NASA / JPL-Caltech / Univ. of Arizona

They found that the temperatures of patches of ground within streaks and outside them showed no detectable difference, including periods when the streaks were actively growing. 

Even when a maximum margin of error in the THEMIS observations was factored in, the difference in temperature between streaky and non-streaky sites was only one degree Celsius.

This means that if there is an extremely thin layer of water-containing soil in the streaks, they’ll contain an average of just three grams of water in each kilogram of sand – about as much as is found in the Atacama Desert and the Antarctic Dry Valleys, two of the driest places on Earth.

If the depth of water-containing soil is greater, that amount decreases.

Despite this, the researchers write, these results do not contradict the identification of hydrated salts at recurring slope lineae sites, which had previously been regarded as possible markers for the existence of liquid water on Mars. 

This is because hydrated salts are capable of drawing vapour from Mars’ thin atmosphere. Consequently, the salts can still exist without having water in the pore space between particles.

The new results simply put an upper limit on how much water can be found at the red planet’s darkened streaks – and help planetary scientists unravel the mysterious Martian briny streaks.

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