And guess how they know? Opal! (Here’s another article, and here is a link to a PDF of the original article in Geology.) Score for mineralogy!

This interests me at the moment for a number of reasons, besides the fact that finding out anything new about Mars is just cool. First reason: I’m taking a remote sensing class right now, and we’re getting ready to do projects that involve tasks like identifying minerals by their spectral properties – just what the Compact Reconnaissance Imaging Spectrometer (CRISM) did to find the opal.

Second reason: I’m sure that for some people, the word “opal” conjures up images of shimmering, multihued gems strewn about the landscape. Unfortunately, what CRISM recorded on Mars won’t be anywhere near as pretty. In fact, it will probably look something like this:

While this photo (taken just outside the caldera of Kilauea Volcano, Hawaii – and yes, I’ve shown it before) is a pretty good approximation of what the Martian surface looks like anyway, the important thing to note is the ground. Don Swanson is walking on the surface of an eruption deposit that’s made up of ash, lithic fragments, and ballistic blocks. He should be sinking into something like that, which is usually pretty unconsolidated, but he’s not – because the ground he’s walking on is all crusted over. And that crust is made of…wait for it…opal! The explanation (which I gave here, back in March) is that acid rain forms from volcanic gases, dissolves SiO2 in the environment, and redeposits it again, which cements the ash and lithics into the hardpan surface you see here.

The articles discuss the possibility that low-temperature acidic water may have been at least partially responsible for forming the opal deposits – precisely what happens in Hawaii, although the source of the acid is probably not just from volcanoes. There are areas where there doesn’t appear to have been any acid involved, however, and the jury’s still out on what processes were operating to deposit opal there.