Icy Twin Craters on Mars

I've mentioned a few times here that I write a biweekly column for Sen.com, a site that has lots of news and blogs dealing with space exploration (Sen stands for "Space Exploration Network"). The news articles are free, but the blogs are subscription only. My latest article there is one I've been meaning to write for some time: What phenomenon sculpted this weird pair of craters, each with a central pit in it? It turns out the answer is ... layered. Sen.com is worth the subscription price. There...