1. Mercury’s origin and evolution:- Likely evidence from surface composition David A Rothery 1, J Carpenter 2, G Fraser 2 & the MIXS team 1 Dept of Earth Sciences, Open University, UK 2 Space Research Centre, University of Leicester, UK Possible

2. Origin and evolution of a planet close to its parent star Mercury’s figure, interior structure, and composition Interior dynamics and origin of its magnetic field Exogenic and endogenic surface modifications, cratering, tectonics, and volcanism Composition, origin and dynamics of Mercury’s exosphere and polar deposits Structure and dynamics of Mercury’s magnetosphere Test of Einstein’s theory of general relativity (BepiColombo Science Requirements Document v2.2) BepiColombo ‘main issues to be addressed’

3. Origin and evolution of a planet close to its parent star Mercury’s figure, interior structure, and composition Interior dynamics and origin of its magnetic field Exogenic and endogenic surface modifications, cratering, tectonics, and volcanism Composition, origin and dynamics of Mercury’s exosphere and polar deposits Structure and dynamics of Mercury’s magnetosphere Test of Einstein’s theory of general relativity (BepiColombo Science Requirements Document v2.2) BepiColombo ‘main issues to be addressed’

4. Primary Questions: From what material did Mercury form, and how? How and when did it become internally differentiated? Is there both primary and secondary crust on Mercury? Secondary Questions: What is the history of crust formation? How does crustal composition vary (i) across the surface (ii) with depth? How are the surface and the exosphere related? How do the surface and magnetosphere interact? MIXS Science Questions To be addressed by other experiments too (MERTIS, SYMBIO-SYS, MGNS, SERENA ….) Composition working group science questions

5. Mercury in context

6. How was Mercury formed? The role of Giant Impacts (planetary embryo collisions) Deliberately no scale bar – this probably happened many times

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

9. Most of Mercury’s crust?Lava flows? Unlikely on Mercury crust mantle

10. Origin: Closest terrestrial planet to the Sun Anomalously high uncompressed density, implies large core, 42% of its volume (Earth’s core is 16% volume) Despite giant core, crust appears very poor in FeO (1-3 wt%) Formation models: thermal/oxidation gradient in solar nebula? (metal enrichment, or vaporization of silicates?) giant impact stripping away most of original mantle? mantle = enstatite chondrite? Evolution: Heavily cratered. No signs of recent activity. Lacks obvious dark lava terrain like the lunar maria. lava not dark because no Fe-O? lava not present? Spacecraft: Mariner-10 (flybys 1974-5), Messenger (flybys 2008-9, orbit 2011-12) BepiColombo (orbit 2019-2020) Mercury salient facts

11. We need to understand what we are looking at, before we can use its composition to interpret Mercury’s origin and evolution Photogeology SIMBIO-SYS Mineralogy MERTIS SIMBIO-SYS How can we recognise (for example) lava?

12. We need to understand what we are looking at, before we can use its composition to interpret Mercury’s origin and evolution Ti: if <0.1% in lavas  enstatite chondrite model for Mercury Fe: expect 2% = primary crust, >7% = secondary crust, but if <0.3% in lavas  enstatite chondrite model for Mercury Mg more abundant in secondary crust than in primary crust, if 10% in lavas  other models Ca expect 18-20% in primary crust, 8-14% in secondary crust, if <9% in lavas  enstatite chondrite model for Mercury Al expect 18% in primary crust, 4-10% in secondary crust P partitions as Ti during partial melting, but is siderophile during differentiation. Ti/P ~1 in chondrites. Ti/P if ~10 in volcanic units  early core formation Cr if ~1% in lavas  refractory-volatile mixture model, if ~0.1% in lavas  other models [Taylor, G. J. and Scott, E. R. D., Mercury, p. 477-486 in Treatise in Geochemistry, Vol. 1. Meteorites, Comets, and Planets, Davis, A. M. (ed), Elsevier, 2004] But, when we have achieved that, the key elements are:

13. Element (wt%)Chondrites Lunar anorthositeAluminous Apollo 12 basalt O374642 Si1820.721.8 Ti0.0640.042.0 Al1.018.66.6 Fe250.5214 Mn0.2300.2 Mg150.484.0 Ca1.213.48.4 Na0.620.590.5 K0.0880.00.1 P0.1100.1 S2.100 Cr0.3600.3 Ni1.5-0 All these elements are potentially detectable by MIXS (may need solar flares for some) There is no element >0.1 wt % in chondritic meteorites or lunar crust missed by MIXS By assuming occurrence as ‘oxides’ we could map absolute abundances on the surface, provided we can eliminate, or take account of, roughness and phase angle effects

14. Primary Questions: From what material did Mercury form, and how? How and when did it become internally differentiated? Is there both primary and secondary crust on Mercury? Secondary Questions: What is the history of crust formation? How does crustal composition vary (i) across the surface (ii) with depth? How are the surface and the exosphere related? How do the surface and magnetosphere interact? MIXS Science Questions We have to ‘see through’ the evidence bearing on the secondary questions before we can answer the primary questions Composition working group science questions

15. But there are many issues to resolve or understand before MIXS can even do that. Solar incident X-ray flux – that’s why SIXS is vital Particle-induced X-ray emission (PIXE) from the surface Viewing geometry and physical state of the surface Spatial resolution and noise levels varying with solar state Collaboration with SERENA, MERMAG and others? Jyri Näränen’s experiments, and others Need to be able to provide element abundances and ratios in GIS* format. Will evolve during the mission: MIXS team and/or ESA data distribution? Virtual Organisation? Common GIS formatting of all spatially Resolved data sets: an issue for ESA/JAXA. *GIS = Geographic Information System

16. Tomorrow - composition splinter group (working group) meeting - includes surface geology & geophysics ?