An RAS Daytime Public Lecture: The planet Mercury, newly revealed Professor David RotheryGeological Society Lecture Theatre Many questions remain for ESA's BepiColombo to answer ten years from now, but thanks to NASA's MESSENGER (orbiting Mercury March 2011-March 2015) we now know far more about the closest planet to the Sun than was possible from ground-based astronomy and the Mariner-10 flybys in the 1970s. Mercury is a rocky planet with a disproportionately large iron core. The outer core is molten, and dynamo processes there generate a magnetic field (unique among the terrestrial planets apart from Earth). It has a rich and dynamic exosphere. The surface is perplexingly rich in volatile elements such as S, K, Na and Cl, and there is widespread evidence of explosive volcanic eruptions (mostly more than 3 billion years ago, but extending into the past billion years) that must be driven by expanding volatiles. Such volatile abundance is hard to reconcile with models for Mercury's origin that call for much of its primordial rocky fraction to have been stripped away, possibly in a giant impact. Thermal contraction of the planet has led to widespread development of 'lobate scarps' at the surface that have taken up at least 7 km of radial contraction. The most recent macroscopic process to sculpt the surface (other than ongoing impact cratering) is the formation of 'hollows' – occurring as fields of steep-sided, flat-bottomed depressions tens of metres deep where a surface layer has been removed, seemingly by some sort of sublimation process. The volatile phase involved has not been identified. David Rothery is Professor of Planetary Geosciences at the Open University, and currently 'Lead Co-Investigator' for MIXS (Mercury Imaging X-ray Spectrometer), the only UK-led instrument on ESA's BepiColombo Mercury orbiter (to be launched 2016). He is also co-leader of ESA's Mercury Surface & Composition Working Group.His research interests centre on the study of volcanic activity by means of remote sensing, and volcanology and geoscience in general on other planets. Copies of his book, Planets: A Very Short Introduction will be available at the lecture.