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Cloudy skies may have allowed water to flow on Mars long ago

Though the Red Planet is a frigid desert today, around four billion years ago it hosted relatively long-lived lakes and river systems, as observations by NASA's Curiosity rover and other Mars robots have shown.

Mars' atmosphere was much thicker back then, which helped to keep liquid surface water from boiling away. However, it remains unclear how that water could have been liquid in the first place; the sun was about 30% dimmer during that ancient epoch than it is today, so the Martian surface should seemingly have been a permanent winter wonderland.

A new study may help to solve this puzzle. A thin layer of icy clouds high in the Martian atmosphere could have provided a greenhouse effect, warming the planet for long stretches, researchers found.

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