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

2. Missions: Mariner 10 flybys 1973-4 MESSENGER flybys 2008-9, orbit 2011-12 BepiColombo orbit 2020-2021 KEY FACT: despite very large, presumably iron-rich, core, Mercury’s surface has <3 wt% FeO

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

7. Mixed crust types (if you don’t recognise the distinction!) oxideHighlands Average basalt Average nearside Average globe Bulk Moon (mantle) SiO 2 454144 47 TiO 2 0.567.82.71.80.3 Al 2 O 3 24.68.419.721.86 FeO6.620.710.99.013 MgO6.811. 78.37.629 CaO15.89.513.914.74.5 Na 2 O0.450.290.400.420.09 K2OK2O0.030.050.0360.0330.01 Mn (ppm)570211010308301200 Cr (ppm)8004140180013704200 Ni (pm)100338089400 Primary crust (70% of nearside) Secondary crust (30% of nearside) Lunar crust examples

8. 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

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

11. Raditladi

15. Caloris basin

17. 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

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

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

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