"This is consistent with [liquid] drops freezing and losing water to the atmosphere as it gets colder."
What's more, the images suggest one of the largest globs started to drip down the lander's leg when it got bigger than about 0.4 inch (a centimeter) wide.
"Before it drips it becomes dark, and that's consistent with ice melting," said Renno, referring to the fact that ice is more reflective than liquid water.
It's certainly possible that liquid water could exist at least fleetingly on the Martian surface, said Nicholas Tosca, a geochemist at Harvard University who was not involved in the study.
With perchlorate present, Tosca agrees Mars could support liquid water even down to -94 degrees Fahrenheit (-70 degrees Celsius)—close to the lowest possible temperature around the lander's legs at the time the images were taken.
But daily temperatures fluctuate greatly on Mars, so even very salty water would probably go through cycles of freezing and melting. Liquids therefore wouldn't be present for very long periods of time, he said.
Study leader Renno thinks that the drops seen on Phoenix were liquid during the warmest part of the day but partially froze at night.
Overall, Tosca said, the paper makes a plausible case for liquid water on Mars, "but the nature of the water doesn't bode well for life."
(Read more about Tosca's findings that water on early Mars would have been too toxic for life.)
Even if a layer of liquid does persist deep under Mars's surface, it wouldn't be very hospitable, he said.
"If you make the case that life could have started on Mars and could be hiding out somewhere," Tosca said, "it's not likely to be in this cold, salty water."
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