The relation of this crater (nicknamed the Spider) to the radial troughs is still sparking debate.
"Although they may look very similar, we shouldn't carry assumptions of the Moon to Mercury," says Dave Rothery of the Open University, who kicked off the Mercury session with a review of MESSENGER findings from the first two flybys of 2008. Rothery says some of the key geological results are that the innermost planet has a large core and magnetic field and is dominated by volcanic activity.
Rothery commented that the suspiciously convenient location of a crater in the centre of the giant Caloris basin, and which infamously became known as the Spider Crater because of the fractures and troughs, or 'legs', radiating from the crater 'body', was still being debated in the planetary community. Did the impact cause the radial troughs to form or where they there already? Rothery comments that there are multiple generations of troughs so they formed at different times, and therefore suspects that they are related to volcanic activity, where a dome of magma bulged up beneath the surface and subsequently collapsed, causing the surface to fracture.
Rothery also highlighted a curious 80 kilometre wide crater containing white deposits. "I haven't a clue what that is," he jokes, but suggests impact melt or some form of volatile deposit as a possible cause.
As more and more MESSENGER data is analysed what is certain is that processes on Mercury are very different to those on the Moon. "Therefore the more we learn about Mercury the more we learn about planets in general."
This was a theme continued by John Bridges from Leicester University who touched on the idea that Mercury could represent a planetary embryo, since it shares remarkable compositional similarities with primitive meteorites that condensed from the primordial solar nebula. What became clear in the discussion session that followed this talk is that the study of exoplanets, specifically learning where planets form in relation to their host planets, will shed enormous light on models for rocky planet formation and evolution.
Future Mercury mission Bepi-Colombo will build upon many of the foundations that MESSENGER has set, and will look in detail at the planet's compositional diversity, as well as the possibility of water ice being present in polar craters. Bepi-Colombo will launch by the end of the next decade.