River water spills down snow-capped mountains into evaporating oceans and then pours back onto mountaintops as rain or snow. Thus, the water cycle on Earth turns. A new model proposes that a similar sequence may have occurred 3.5 billion years ago on Mars, even though the temperature was a frosty -25°. Many scientists had thought that ancient Mars needed to be warm in order to be wet. But a paper published in this month's Icarus suggests that heavy doses of salts from the mineral-rich martian soil and an atmosphere 10,000 times thicker than present-day allowed flowing rivers and open oceans despite subfreezing temperatures. What did a frozen water cycle look like? Streams gushed from the melting underside of highland glaciers into basins, according to the author, creating ice-packed oceans that sublimated and then snowed back onto the highlands. Over the eons, Mars grew too cold for continuous flows; it's current average temperature is -55°C. But modern satellites have spotted trickles at edges of martian craters (left panel) that look similar to icy brine flows in the Canadian Arctic (right panel), suggesting that small amounts of open water may still exist today and potentially provide a home for extraterrestrial life.
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