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.

Mercury’s origin and evolution:- Likely evidence from surface composition

David A Rothery1, J Carpenter2, G Fraser2 & the MIXS team 1Dept of Earth Sciences, Open University, UK

2Space Research Centre, University of Leicester, UK

Possible

•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’

•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’

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 QuestionsTo be addressed by other experiments too (MERTIS, SYMBIO-SYS, MGNS, SERENA ….)

Composition working group science questions

Mercury in context

How was Mercury formed?The role of Giant Impacts (planetary embryo collisions)

Deliberately no scale bar – this probably happened many times

Simulations from Horner et al. (2006)

The final embryo-embryo collision?

Most of Mercury’s crust?Lava flows?

Unlikely on Mercury

crust

mantle

Origin:•Closest terrestrial planet to the Sun•Anomalously high uncompressed density, implies largecore, 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

We need to understand what we are looking at, before we canuse its composition to interpret Mercury’s origin and evolution

PhotogeologySIMBIO-SYS

MineralogyMERTISSIMBIO-SYS

How can we recognise (for example) lava?

We need to understand what we are looking at, before we canuse 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 <7% in lavas refractory-volatile mixture model for Mercury, 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:

Element (wt%) Chondrites Lunar anorthosite Aluminous Apollo 12 basalt

O 37 46 42

Si 18 20.7 21.8

Ti 0.064 0.04 2.0

Al 1.0 18.6 6.6

Fe 25 0.52 14

Mn 0.23 0 0.2

Mg 15 0.48 4.0

Ca 1.2 13.4 8.4

Na 0.62 0.59 0.5

K 0.088 0.0 0.1

P 0.11 0 0.1

S 2.1 0 0

Cr 0.36 0 0.3

Ni 1.5 - 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

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 thesecondary questions before we can answer the primaryquestions

Composition working group science questions

But there are many issues to resolve or understandbefore 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 abundancesand ratios in GIS* format. Will evolve during the mission:MIXS team and/or ESA data distribution? Virtual Organisation? Common GIS formatting of all spatiallyResolved data sets: an issue for ESA/JAXA.*GIS = Geographic Information System

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