The HiRISE camera just keeps snapping cool things, and in the latest issue of Geology, Milazzo et al. have spotted something particularly cool in this image of the rim of this crater. It seem this particular impact punched right through a large basalt lava flow, and tilted the exposed edges skywards, allowing us to see: columnar basalts!
Below I’ve sketched in some interpretation of the features in these images, which show multiple layers of columnar basalt separated by massive (featureless) layers. In at least one place (in the middle figure), you can see columns which are much more irregularly spaced and oriented.
These are all features you can see within large basalt flows on Earth: take, for example, the Isle of Staffa, off the west coast of Scotland, which formed as part of the same volcanic episode responsible for the Giant’s Causeway. The cliffs of this island are basically a section through a single basaltic lava flow, with lovely regular columns at the base and more massive basalt, with irregular fanning columns seen in places, at the top.
The columns in a columnar basalt are basically joints; cracks that formed during cooling and shrinkage of the lava. Their regular spacing is due to the cooling rate being fairly constant throughout the entire flow. The more irregular, radiating columns found close to the top of a flow are thought to record interaction with water percolating through cracks in the surface and causing localised cooling. By analogy, Milazzo et al. argue that the flows they see on Mars were also water-cooled. Of course, the age of these lavas is presently unknown, so I’m not sure how signficant this is in terms of charting the history of water on the the Martian surface; but it shows that looking at volcanic features, as well as sedimentary and mineralogical ones, might be useful in working out that story.
M.P. Milazzo, L.P. Keszthelyi, W.L. Jaeger, M. Rosiek, S. Mattson, C. Verba, R.A. Beyer, P.E. Geissler, A.S. McEwen (2009). Discovery of columnar jointing on Mars Geology, 37 (2), 171-174 DOI: 10.1130/G25187A.1