The global average crustal composition of Mercury – a

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The global average crustal composition of Mercury – a Powered By Docstoc
					                           The global average
                           crustal composition of
                           Mercury –
                           a useful measurement
                           or not?

David Rothery, Dept of Earth & Environmental Sciences
D.A.Rothery@open.ac.uk
With thanks to the ESA Mercury Surface & Composition Working Group
         The final embryo-embryo collision?




Simulations from Horner et al. (2006)
                    Lunar crust examples
             Primary crust               Secondary crust
                                                                     Mixed crust types
           (70% of nearside)            (30% of nearside)

                               Low-Ti       High-Ti    Average      Average    Average   Bulk Moon
oxide         Highlands        basalt       basalt      basalt      nearside    globe
SiO2              45            44             38            41       44          44        47
TiO2             0.56           2.6            13           7.8       2.7        1.8        0.3
Al2O3            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       4.5
Na2O             0.45           0.23          0.36          0.29     0.40        0.42      0.09
K2O              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
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 for
understanding Mercury’s origin.
    Primary crust?




      Secondary crust

<         350 km                     >
                   Mariner-10 PIA02443
                           Variation within
                          secondary crust


Fresh ejecta
                              Older
                              Younger
Crater-floor      Dark halo
deposit



      < 40 km >