David Rothery, Dept of Earth & Environmental Sciences With thanks to the ESA Mercury Surface & Composition Working Group Spatial heterogeneity in Mercury’s surface

Simulations from Horner et al. (2006) The final embryo-embryo collision?

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

Mixed crust types (if you don’t recognise the distinction!) oxideHighlands Low-Ti basalt High-Ti basalt Average basalt Average nearside Average globe Bulk Moon 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 Mantle composition is essential for understanding Mercury’s origin.

BepiColombo MIXS elements

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

Scale of variation within secondary crust (Caloris, MESSENGER) Older Younger Dark halo Fresh ejecta Deposit on floor of Sander crater