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

Volcanism on Mercury asa tool for determining itsevolution, internal composition and origin

Missions: Mariner 10 flybys 1973-4MESSENGER flybys 2008-9, orbit 2011-12BepiColombo orbit 2020-2021

KEY FACT: despite very large,presumably iron-rich, core, Mercury’s surface has <3 wt% FeO

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

Mixed crust types(if you don’t recognise the distinction!)

oxide HighlandsAverage basalt

Average nearside

Average globe

Bulk Moon(mantle)

SiO2 45 41 44 44 47TiO2 0.56 7.8 2.7 1.8 0.3Al2O3 24.6 8.4 19.7 21.8 6

FeO 6.6 20.7 10.9 9.0 13

MgO 6.8 11. 7 8.3 7.6 29

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

Mn (ppm) 570 2110 1030 830 1200

Cr (ppm) 800 4140 1800 1370 4200

Ni (pm) 100 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

‘Average crust’:• reflects arbitrary proportions of exposedareas of primary and secondary crust• conflates two entirely different processesof crust-formation

Spatial resolution 10s to 100s kmdepending on abundance and solar state

BepiColombo: Mercury Imaging X-ray Spectrometer MIXS

<100

km

>

Raditladi

< 100 km >

Calorisbasin

< 100 km >

Matisse

pi

pi

Matisse ejectaoverlies intercrater plains

EmbaysMatisseejecta

Primarycrust exposedin walls?

< 200 km >

pi lava

Matisse-fill lava

ps lava

Primary crust

Matisse ejectapost-Matisse ejecta

Look for primary crust here

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