Water could have flowed on the surface of Mars despite its below-freezing temperatures with the help of salts in the Martian soil that can melt ice, a new study has found.
In chambers that mimic Mars' conditions, University of Michigan researchers have shown how small amounts of liquid water could form on the planet.
Liquid water is an essential ingredient for life as we know it. Mars is one of the very few places in the solar system where scientists have seen promising signs of it - in gullies down crater rims, in instrument readings, and in Phoenix spacecraft self portraits that appeared to show wet beads on the lander's leg several years ago.
No one has directly detected liquid water beyond Earth, though. The experiments are among the first to test theories about how it could exist in a climate as cold as Mars' climate.
The researchers found that a type of salt present in Martian soil can readily melt ice it touches - just like salts do on Earth's slippery winter walkways and roads.
But this Martian salt cannot, as some scientists suggested, form liquid water by sucking vapour out of the air through a process called deliquescence.
"For me, the most exciting thing is that I can now understand how the droplets formed on the Phoenix leg," said Nilton Renno, who led the research.
In 2008, Renno was the first to notice strange globules in photos Phoenix sent back. Over several weeks, the globules seemed to grow and coalesce.
While Renno deemed them water and suggested that salts on the planet's surface might make it so, many of his colleagues disagreed.
Among the salts that Phoenix detected is calcium perchlorate, a mixture of calcium, chlorine and oxygen that's found in arid places like the Atacama Desert in Chile.
Years later, the Curiosity rover found it elsewhere on Mars. Now scientists believe it and other salts are sprinkled across the planet's surface.
Researchers used a technique called Raman scattering spectroscopy that involves shining lasers onto the surface and examining the reflected light.
Different substances and states of matter have different reflective signatures.
The findings show how small amounts of liquid water could exist across a large swath of Mars' surface and shallow subsurface, from its polar regions to its mid-latitudes, for several hours a day during the spring and early summer.
Such a cycle could form gullies, Renno said, flowing, freezing, thawing and flowing again. Water could also form just beneath the surface.
Renno said the water wouldn't necessarily need to stay liquid indefinitely for it to support microbial life now or have supported it in the past.
The research was published in the journal Geophysical Research Letters.
