David Rothery, Dept of Earth & Environmental Sciences The Open University With thanks to the ESA Mercury Surface & Composition Working Group Volcanism on Mercury as a tool for determining its evolution, internal composition and origin

Missions: Mariner 10 flybys MESSENGER flybys , orbit BepiColombo orbit KEY FACT: despite very large, presumably iron-rich, core, Mercury’s surface has <3 wt% FeO

Primary & Secondary crust defined by Taylor, S. R. (1982, 1989)

Mixed crust types (if you don’t recognise the distinction!) oxideHighlands Average basalt Average nearside Average globe Bulk Moon (mantle) SiO TiO Al 2 O FeO MgO CaO Na 2 O K2OK2O Mn (ppm) Cr (ppm) Ni (pm) Primary crust (70% of nearside) Secondary crust (30% of nearside) Lunar crust examples

We cannot back-track from crust composition to mantle composition unless we: recognise how the crust formed measure and model primary crust and secondary crust separately ‘Average crust’: reflects arbitrary proportions of exposed areas of primary and secondary crust conflates two entirely different processes of crust-formation

Spatial resolution 10s to 100s km depending on abundance and solar state BepiColombo: Mercury Imaging X-ray Spectrometer MIXS

Raditladi

Caloris basin

Scale of variation within secondary crust (Caloris, MESSENGER) Older Younger Dark halo & walls (exposed/excavated primary crust?) Fresh ejecta Deposit on floor of Sander crater

Matisse pi Matisse ejecta overlies intercrater plains Embays Matisse ejecta Primary crust exposed in walls?

pi lava Matisse-fill lava ps lava Primary crust Matisse ejecta post-Matisse ejecta Look for primary crust here ?

Secondary crust Primary crust? (probably not in this example) Mariner-10 PIA02443