Hollows on the floor of the crater Sander

Volatiles at Mercury – LPSC 2013

Prokofiev crater - there's ice down there in the dark (Source: NASA/JHUAPL/Carnegie)

The evidence keeps on mounting that Mercury, once thought dry and dead, actually has a load of volatile substances like water and sulphides within it and on its surface.  It was big news a few months ago that several lines of evidence show there’s water ice in permanently-shadowed craters at its poles, and I, amongst others, have been looking at the fascinating hollows which seem to be forming even now at the surface as some volatile layer is stripped away.

The Volatiles at Mercury session kicked off with some awesome pictures from Nancy Chabot: she and her team have managed to enhance images of craters near the poles so we can actually see those ice deposits.  It’s a work in progress, since it’s very hard to get a look into something that’s getting no direct illumination from the sun, but what they do have is very convincing. Cool stuff.

A stunning view of hollows on the central peak of the crater Eminescu - the picture's about 50km across (Source: NASA/JHUAPL/Carnegie)

The session also had three talks on hollows, which, as well as being very intriguing scientifically, are about the most photogenic features on Mercury. These shallow depressions are incredibly fresh-looking and are haloed by bright, bluish material which we’re still trying to identify. Jorn Helbert didn’t solve the problem, but he did show us just how difficult it is: he’s show experimentally that the high temperatures at Mercury make sulphides even harder to recognise from orbit than they usually would be. After that, Jeff Kargel pointed out similar landforms caused by solar heating on the Earth and Mars, supporting the theory that it’s the sun which leads to hollow formation in some way.

That fits in well with my own work, which I presented next: where hollows form on a particular slope, it’s almost always on one that faces the sun.  I also showed the distribution of hollowing I’ve seen in surveys of the planet’s surface – they form mostly in the dark material we call Low Reflectance Material (LRM), which may be an older layer in the crust which is overlain with younger volcanics in the areas where you don’t see hollowing. And how does that stuff get to the surface? People have been saying it’s just brought up when the surface gets impacted and a deep crater is formed, but I’m seeing evidence that it can migrate or even erupt up through the faults that underlie craters. That makes Mercury a dynamic place indeed.

Volcanic pits and their reddish pyroclastic deposits cohabit with bright bluish hollows in the crater Praxiteles (Source: NASA/JHUAPL/Carnegie)

Christian Klimczak came next and he agrees with me that those crater-related faults are an ideal conduit for volatiles from the subsurface. In his case, he’s been looking at gas-laden volcanic eruptions which form in craters that are crossed by tectonic thrusts. You often see a pit right in the centre of these craters, and those pits are surrounded by pyroclastic deposits – fine material which forms when magma explodes up out of the pits and is redeposited as a rain of fragments.  It makes perfect sense really: if you have a shrinking planet, stresses on its surface are going to be too high for volcanic materials to escape in most places, but somewhere which is pre-fractured like an impact crater is a perfect release valve.  And are the hollow-forming volatiles coming along for the ride too?  Watch this space…