This is exciting for several reasons: first, these are very tiny craters – only a few meters (yards) across. This means they’re not excavating very deep into the crust of Mars. So the ice has to be really shallow – less than a few feet below the surface! Secondly, the location of these craters is surprising – they’re between 40-55 degrees north latitude. This is far from the polar regions, where we’d expect to find ice (for example, where the Phoenix mission landed at 68 degrees north, ice was found by digging down into the dirt).
The third exciting aspect of this ice is its purity. We’d expect this ice to be mixed in with dirt and dust and rock. Instead, we found that it’s 99% pure ice! (Only 1% is dirt mixed in.) This can be measured because we watched the ice disappear over time. By taking repeated images of the same spot, HiRISE got a time sequence as the ice slowly faded. It faded so slowly that it has to be almost all ice – a dirtier mixture would have faded much faster as it sublimated (went directly from a solid to a gas) in Mars’s extremely dry atmosphere.
Speaking of dry atmospheres, this also has interesting implications about the history of the Martian climate – there had to have been more water vapor in the atmosphere in the recent past than we previously thought. We still have lots of questions about how this ice formed, how much of it there is, and how many more of these craters we’ll find. Luckily, we’ve got a long mission ahead of us to explore these issues!
This discovery is also a great example of how the instruments on MRO work together. CTX initially detected these new craters as “dark spots,” and HiRISE followed up to confirm that they’re really impact craters. Some of those HiRISE images revealed some very bright white material, and then CRISM confirmed that material really is water ice. The instruments worked together to accomplish the best combined science. Go team! ☺
Here are some more detailed stories, images, and multi-media:
• Really nice movie with Shane Byrne talking about the discovery and excellent animations showing the locations of the craters and the time-evolution of the ice disappearing: NASA multimedia – then go to “Video Gallery” on the right, and click on “Mars – Exposed”.