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For a long time, the climate of ancient Mars remained a mystery: the beds of dried-up rivers spoke of water, but models couldn't explain how it got there — after all, 4 billion years ago the Sun shone 30% dimmer. New research shows that the planet swayed, as if on a swing, between long glaciations and short wet epochs.

The culprit behind these swings was carbon dioxide. During cold periods it got bound up in rocks, and Mars froze. Then volcanoes spewed the gas back out, causing abrupt warming. Streams of water flowed across the surface, carving the valleys we see today. Each thaw lasted less than the entire existence of the human species, just a few million years, but that was enough to leave a noticeable mark.

Scientists accounted for the fact that the Sun brightened over time, and that Mars' axis wobbled greatly. Calculations showed that the climate swings occurred several times, explaining the different ages of the valleys. However, with each cycle, some of the atmosphere escaped into space, and eventually the planet froze for good. Today Mars is an icy desert, but remnants of ancient ice may still lurk beneath the surface.