27 May 2008
There has been a lot of excitement about the first glimpses of the martian arctic returned by the Phoenix lander, and particularly lots of “oohs” and “ahhs” over pictures, like the one above of so-called “polygons” on the surface. So, what’s the big deal with these polygons and how do they form?
The short answer is that polygons form when soil with water in it freezes and thaws. The basic idea is that, when water freezes, it expands. When this occurs in rocky soil, it tends to push larger rocks to the surface. More fine-grained soil can hold more water, and therefore can expand more when it freezes. This means that places on the surface with more large rocks expand less than the surroundings, and the large rocks get shoved together. Over many freeze-thaw cycles, as the larger rocks become concentrated, the effect increases, and eventually the result is polygons formed by lines of large rocks that have been forced to the surface and herded into neat rows. Here is an example from Devon Island, Canada:
Why is this a big deal for the Phoenix mission? Well, the primary goal of the mission is to dig up some ice and analyze it, and patterned ground such as the polygons observed at the landing site is a very good indicator that there have been freeze-thaw cycles and that the soil has ice in it. What is weird to me is that the polygons at the Phoenix site don’t seem to have the rocks sorted by size. I see equally large rocks in the middle of the polygons and at the edges. Still, there is no mistaking the patterns at the site. My first, gut instinct explanation of the lack of sorting is that the patterned ground formed a very very long time ago, perhaps billions of years. Since then, there have been impacts nearby (notably, a very large crater just to the east of the landing site) that have scattered rocks of all sizes over the surface. So maybe the rocks just under the surface layer are sorted, but they are covered by ejecta that has not had time to be sorted. That’s just my hypothesis: hopefully we’ll see if I’m right when Phoenix starts digging!