Given the year we've just had, you may not think there's too much to smile about right now (we beg to differ) but that hasn't stopped Mars's "Happy Face Crater" from widening its Cheshire Cat grin. 

The cheerful-looking crater is helping scientists track climate trends on the Red Planet, and for once a melting frost is not a sign of bad news. The crater is located in Mars's south pole region, which is frosty, but changes due to thermal erosion. A decade of documenting this feature means we now have a good side-by-side comparison of how much thermal erosion has occurred over the last decade.

NASA's Mars Reconosence Orbiter has been studying Mars since its arrival in 2006. Its HiRISE (High-Resolution Imaging Science Experiment) instrument is the most powerful camera ever sent to another planet, regularly returning incredibly detailed images of Mars's features, from ancient rivers to avalanches as they happen. 

It first snapped the crater in 2011. Because humans love seeing faces and recognizable objects in the unlikeliest of places, the smiley feature was dubbed the Happy Face Crater. Mars is a veritable treasure trove of unexpected items "spotted" on another world, ranging from gorillas and Muppets to Batman's signal and (OK, this one is pretty good) a dragon. 

Another photo of the crater taken on December 13, 2020, during roughly the same season as the first, has shown how much frost has been lost to thermal erosion in that time as more of the red ground is revealed.

According to HiRISE team member Ross Beyer, "The 'blobby' features in the polar cap are due to the Sun sublimating away the carbon dioxide into these round patterns." Sublimation is when a solid bypasses the liquid phase and turns into a gas. Mars is a very cold planet due to its thin atmosphere and lack of oceans to moderate temperatures. During a Martian winter, carbon dioxide freezes, building up a thin layer of dry ice on the ground. When the Sun shines on it, the ice melts, skipping the liquid phase and turning directly into a vapor, causing erosion on the surface.

As you can see in the image above, the "nose" in 2011 consisted of two circular depressions. By 2020, these dips had grown larger and merged into one. Measuring these changes helps scientists understand the annual cycle of deposits and removal of the polar frost. Longer observations give an insight into long-term climate trends on Mars. 

Understanding these climate trends is important in our quest for human exploration and the possibility of populating another planet by determining if Mars has ever or could ever have the right conditions to support life.