Mercury’s origin and evolution:Likely evidence from surface composition
Possible
David A Rothery1, J Carpenter2, G Fraser2 & the MIXS team
1Dept
of Earth Sciences, Open University, UK
2Space Research Centre, University of Leicester, UK
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’
•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)
Composition
groupQuestions
science questions
MIXS working
Science
To be addressed by other experiments too (MERTIS, SYMBIO-SYS, MGNS, SERENA ….)
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?
Mercury in context
How was Mercury formed?
The role of Giant Impacts (planetary embryo collisions)
Deliberately no scale bar – this probably happened many times
The final embryo-embryo collision?
Simulations from Horner et al. (2006)
Unlikely on Mercury
Most of
Lava
Mercury’s
flows? crust?
mantle
crust
Mercury salient facts
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)
We need to understand what we are looking at, before we can
How
can
we
recognise
(for
example)
lava?
use its composition to interpret Mercury’s origin and evolution
Photogeology
SIMBIO-SYS
Mineralogy
MERTIS
SIMBIO-SYS
Wewhen
need to
understand
what wethat,
are looking
at,elements
before we can
But,
we
have achieved
the key
are:
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 <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]
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
Composition
group
science questions
MIXS working
Science
Questions
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?
We have to ‘see through’ the evidence bearing on the
secondary questions before we can answer the primary
questions
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
Collaboration with SERENA, MERMAG and others?
•Viewing geometry and physical state of the surface
Jyri Näränen’s experiments, and others
•Spatial resolution and noise levels varying with solar state
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
Tomorrow composition splinter group (working
group) meeting
- includes surface geology & geophysics ?