David Rothery, Dept of Earth & Environmental SciencesD.A.Rothery@open.ac.ukWith thanks to the ESA Mercury Surface & Composition Working Group

Spatial heterogeneityin 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!)

oxide HighlandsLow-Ti basalt

High-Ti basalt

Average basalt

Average nearside

Average globe

Bulk Moon

SiO2 45 44 38 41 44 44 47TiO2 0.56 2.6 13 7.8 2.7 1.8 0.3Al2O3 24.6 7.87 8.8 8.4 19.7 21.8 6

FeO 6.6 21.7 19.7 20.7 10.9 9.0 13

MgO 6.8 14.9 8.4 11. 7 8.3 7.6 29

CaO 15.8 8.3 10.7 9.5 13.9 14.7 4.5Na2O 0.45 0.23 0.36 0.29 0.40 0.42 0.09K2O 0.03 0.05 0.05 0.05 0.036 0.033 0.01

Mn (ppm) 570 2150 2080 2110 1030 830 1200

Cr (ppm) 800 5260 3030 4140 1800 1370 4200

Ni (pm) 100 64 2 33 80 89 400

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 forunderstanding Mercury’s origin.

BepiColombo MIXS elements

Secondary crust

Primary crust?(probably not in this example)

Mariner-10 PIA02443< 350 km >

< 40 km >

Scale of variation within secondary crust(Caloris, MESSENGER)

OlderYounger

Dark halo

Fresh ejecta

Deposit onfloor of Sandercrater