Bad news, Martian-seekers. What some planetary scientists thought were trickling salty streams on the Red Planet, potentially harbouring microbial life, could be nothing but avalanches of dry sand.
Researchers in the US and UK analysed images of so-called “recurring slope lineae” — dark streaks on slopes that appear and fade with seasons — and found they don’t fit models that involve liquid water.
The findings come just over a year after Mars’ gullies — another potential sign of liquid water — were found to be dry too.
With the latest findings on recurring slope lineae, the odds that life could be found on Mars today aren’t looking great, but not impossible, said US Geological Survey astrogeologist Colin Dundas.
“There’s plenty of evidence for past habitable environments, such as rover observations of rock units deposited by water, and the deeper crust could also be more hospitable.”
But, he adds, whether those habitable conditions ever led to life is unknown.
Water, water everywhere, but not a drop to drip
Mars looks like a desert, but it’s far from water-free. There are massive ice deposits at the poles, water molecules locked up in hydrated salts in the soil and traces of water vapour in the atmosphere.
But while research suggests the Red Planet once boasted oceans, lakes and streams, planetary scientists haven’t yet found definitive evidence of recent liquid water, an important ingredient to life as we know it, according to astrogeologist and president of Mars Society Australia Jonathan Clarke.
So when recurring slope lineae were reported in 2015, the finding was very exciting.
At first blush, it looked as though liquid water was involved — perhaps melting ice below the surface, or salts in the soil absorbing water and dribbling downhill.
“On a humid day, your table salt gets sticky because it’s absorbed moisture from the atmosphere,” Dr Clarke said.
“Some salts will actually suck enough moisture out of the air that they liquefy.”
But in the years since the discovery, ‘dry hypotheses’, which don’t need a role for liquid water, have also been put forward.
New analysis of Mars images, published in Nature Geoscience today, by Dr Dundas and colleagues, supports the dry side.
If recurring slope lineae were formed by liquid water, there’s no reason they shouldn’t be triggered on low, gentler slopes. But you only see them start on very steep slopes — more in line with sandy avalanches.
The reason they appear dark is due to coarse sand grains brought to the surface by the landslide contrasting with the paler, smoother dust either side.
Still, Dr Dundas said, there may still be a role for small amounts of liquid water in triggering the avalanches.
Evaporating water or dehydrating salts could make grains of sand less sticky and destabilise the slope.
Subsurface frost, evaporating in the thin Martian atmosphere, may also dislodge grains and kick off an avalanche.
It’s not all bad, though. If recurring slope lineae are dry, they’re open for exploration.
A few years ago, space agencies around the world agreed to steer clear of patches on Mars which may contain liquid water, in case their rovers inadvertently contaminated them with Earth microbes, said Monash University planetary geologist Alistair Tait.
Unanswered questions remain
There are still a few mysteries, the researchers concede.
Why do recurring slope lineae appear every year? With each avalanche, there should be fewer sand-sized particles at the top of the slope to create the streaks, so the effect should fade over time. Yet it doesn’t appear to.
Strangely, the colour of the streaks also seems to match the colour of the dust next to it.
The work isn’t the final nail in the coffin for the damp streaks theory, Dr Clarke said.
It’s quite hard to prove if recurring slope lineae are damp or dry without going to Mars and seeing it happen.
“Hopefully during the discussion that emerges we find the better description — or they’re both wrong.