New evidence suggests that Mars' dry climate was once wetter and possibly more Earth-like. The European Space Agency (ESA) has released images of two side-by-side craters offering clues to the dusty, barren Red Planet's past. Here's what you should know about the new theory:

What exactly do the photographs depict?
The high-resolution stereo images showcase the Danielson and Kalocsa craters, which sit side by side in a desert-like region called Arabia Terra. Snapped on June 19, 2011, from aboard the Mars Express spacecraft, the photos highlight new topographical evidence that has led experts to posit that the larger crater was once filled with water. 

What's the evidence?
In the much bigger, 38-mile-across Danielson crater — named after George E. Danielson, who pioneered many of the cameras used to photograph Mars today — steep, wind-carved hills called "yardangs," characterized by distinct layers of sediment, protrude from the crater's bottom. (See a photo below.) ESA scientists believe the sediment that forms these yardangs was deposited there by strong north-northeasterly winds and then hardened (or cemented) by water, "possibly from an ancient deep groundwater reservoir," says Alan Boyle at MSNBC. In contrast, the more elevated, 20-mile-wide Kalosca crater features a smooth bottom with no such structures jutting out, possibly because it was never deep enough to reach the groundwater.

What happened to the water?
The theory is that slight shifts in the planet's axis triggered drastic climate changes over millions of years, leading to intermittent wet and dry periods. Judging from the yardangs' orientation, researchers believe that the same strong north-northeasterly winds that deposited the original sediments battered them with dust and sand during a dry spell after the water disappeared, carving the yardangs into their characteristic shapes. Similar axis changes are thought to have wrought the extreme ice age cycles here on Earth. Take a look at the yardangs in the Danielson crater:

Sources: Astro Bio, Daily Mail, MSNBC, PhysOrg