Looking for life in salty Martian streams
Microbes in Chile’s Atacama Desert might hold clues about life on Mars. Viviane Richter reports.
In 1895 Percival Lowell pointed his telescope at Mars and saw canals. Ever since, Earthlings have debated whether the Red Planet has running water. With great fanfare, this September NASA concluded it does. And where there’s water, there may be life. But what form might it take and where should we would look? Astrobiologists are using clues from Earth’s cold deserts to find out.
The smoking gun for water on Mars was dark streaks, hundreds of metres long, that decorated the walls of canyons and crater walls, growing and shrinking with the seasons. Researchers first noticed these streaks in 2010, in images taken by the Mars Reconnaissance Orbiter. But what they were composed of? To find out Matt Chojnacki, a planetary geologist from the University of Arizona, and colleagues, had to analyse data from another type of onboard camera – a spectrometer that detects chemical signatures in the rocks.
Their stunning conclusion was published in Nature Geoscience in September. The streaks were a brine, flowing downhill in the warmer months. They were composed of hydrated salts, a mix of chlorates and perchlorates. The spectrometer detected the salts in dark streaks in walls of the Hale, Palikir and Horowitz craters, and a large canyon called Coprates Chasma, but did not detect them in the surrounding terrain.
While scientists didn’t doubt that water flowed on Mars in the distant past, finding fresh flows is startling. The planet has lost most of its atmosphere – it’s a mere 1% of that on Earth. That means any water reaching the surface should rapidly evaporate. But because it’s mixed with salt, it evaporates more slowly. In addition, the salt lowers the water’s freezing point – the briny streaks are found in a region that reaches minus 23°C even in the warmer months.
If liquid water still exists, might life have defied the odds too? Four billion years ago Mars, like Earth, had lakes, oceans, a warmer atmosphere, and a protective magnetic field. All have since been lost. But if life took hold in these hospitable conditions, it didn’t just disappear; it would have likely retreated underground, says Malcolm Walter, an astrobiologist at the University of New South Wales. “It’s impossible to sterilise a planet.”
For Kathleen Campbell, an astrobiologist at the University of Auckland, hunting for life on Mars is like looking for a needle in a haystack. But as far as haystacks go, these streaks may be the best bet. “Now, you go to the haystack and look for the needle.”
Walter agrees the briny flows might be sweeping deeply nestled microbes up to the planet’s surface.
“Mars and Earth were very similar more than three billion years ago.
If life is here, why not on Mars?"
But could life exist in such cold, dry, salty conditions? Yes. In 2012 researchers from Spain’s Centre of Astrobiology discovered a “microbial oasis” two metres beneath the surface of Chile’s cold dry Atacama desert. These microbes – several species of bacteria and archaea – can cling to life thanks to salty crystals in the sand. The crystals suck moisture out of the air percolating through the sand. This condenses on their surface as a thin liquid film that is enough to sustain the microbes. If that strategy works for Atacama microbes, perhaps it could work for Martians.
The possibility that Earthly and Martian microbes might not be dissimilar, raises a problem. If we find something, how do we know it’s not a contaminant from Earth? A spacecraft’s surfaces have so far proved impossible to sterilise. Campbell thinks rovers are not the way we’ll find life anyway – rovers are just too slow and inefficient to collect and process all the samples we’ll need to find one containing traces of life, and so we’ll have to send humans. “It could be Matt Damon, it could be anyone. You can leave Matt there, but send back the rock samples to Earth – please!” she says.
Meanwhile, there’s still a lot left to discover about Mars’ briny flows. One theory is the streaks are subterranean water seeping up to the surface. But another possibility – and one that Chojnacki is now investigating – is that the dried up salts along the steep Martian slopes absorb water from the thin atmosphere to form the salty streams – just the way it happens in the Atacama desert.
The team don’t yet know the full extent of the briny flows. The orbiter only observes the region at 4 pm local solar time – by which time the damp streaks have been warmed by the Sun all day and much of the water would have evaporated, Chojnacki says. “There’s probably a lot more there than what we’ve been able to document.”
The next Mars orbiter mission, scheduled to launch in 2022, should provide more information about water on the Red Planet. But in 2020, NASA will launch its next Mars rover. Could it be sent to examine these briny patches to look for life? It’s one of the potential landing sites being discussed, says Campbell – though the steep slopes would be very challenging terrain for the rover to try to traverse, reducing the likelihood NASA will send the rover here.
Walter, for one, is betting we will find life on the Red Planet, sooner or later. “Mars and Earth were very similar more than three billion years ago; if life is here, why not on Mars?”