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| Titel |
Mercury's thermo-chemical evolution from numerical models constrained by Messenger observations |
| VerfasserIn |
N. Tosi, D. Breuer, A. C. Plesa, F. Wagner, M. Laneuville |
| Konferenz |
EGU General Assembly 2012
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| Medientyp |
Artikel
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| Sprache |
Englisch
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 14 (2012) |
| Datensatznummer |
250065427
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| Zusammenfassung |
| The Messenger spacecraft, in orbit around Mercury for almost one year, has been delivering a
great deal of new information that is changing dramatically our understanding of the solar
system’s innermost planet. Tracking data of the Radio Science experiment yielded improved
estimates of the first coefficients of the gravity field that permit to determine the normalized
polar moment of inertia of the planet (C-MR2) and the ratio of the moment of inertia of the
mantle to that of the whole planet (Cm-C). These two parameters provide a strong constraint
on the internal mass distribution and, in particular, on the core mass fraction. With
C-MR2 = 0.353 and Cm-C = 0.452 [1], interior structure models predict a core radius as
large as 2000 km [2], leaving room for a silicate mantle shell with a thickness of only ~ 400
km, a value significantly smaller than that of 600 km usually assumed in parametrized [3] as
well as in numerical models of Mercury’s mantle dynamics and evolution [4]. Furthermore,
the Gamma-Ray Spectrometer measured the surface abundance of radioactive elements,
revealing, besides uranium and thorium, the presence of potassium. The latter, being
moderately volatile, rules out traditional formation scenarios from highly refractory
materials, favoring instead a composition not much dissimilar from a chondritic
model.
Considering a 400 km thick mantle, we carry out a large series of 2D and 3D numerical
simulations of the thermo-chemical evolution of Mercury’s mantle. We model in a
self-consistent way the formation of crust through partial melting using Lagrangian tracers to
account for the partitioning of radioactive heat sources between mantle and crust and
variations of thermal conductivity. Assuming the relative surface abundance of
radiogenic elements observed by Messenger to be representative of the bulk mantle
composition, we attempt at constraining the degree to which uranium, thorium and
potassium are concentrated in the silicate mantle through a broad exploration of the
parameter space. We analyze how different rheologies, buoyancy variations associated
with mantle depletion and the absence or presence of a primordial crust influence
the thermal history of Mercury, the duration of convection and the formation of
partial melting with its associated crustal production. Additionally, we calculate
the global radial contraction of the planet resulting from secular cooling, mantle
differentiation and inner core growth, and compare it with the traditional estimate
of 1-2 km which was recently confirmed by the analysis of Messenger’s images
[5].
[1] Smith D.E. et al., 2011. Mercury’s Gravity Field After The First Months
Of MESSENGER’S Orbital Phase. AGU Fall Meeting, San Francisco, Abstract
P43E-02.
[2] Riner M.A. et al., 2008. Internal structure of Mercury: Implications of a molten core. J.
Geophys. Res., 113, E08013, doi:10.1029/2007JE00299.
[3] Hauck S.A. et al., 2004. Internal and tectonic evolution of Mercury. Earth Planet. Sci.
Lett., 222, 713–728.
[4] Redmond H. and King S.D., 2004. Does mantle convection currently exist on Mercury?
Phys. Earth Planet. Int., 164, 221–231.
[5] Watters et al., 2009. The tectonics of Mercury: The view after MESSENGER’s first flyby.
Earth Planet. Sci. Lett., 285, 283-296. |
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