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| Titel |
Mercury's thermo-chemical evolution constrained by MESSENGER observations |
| VerfasserIn |
Nicola Tosi, Matthias Grott, Doris Breuer, Ana-Catalina Plesa |
| Konferenz |
EGU General Assembly 2013
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| Medientyp |
Artikel
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| Sprache |
Englisch
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 15 (2013) |
| Datensatznummer |
250076812
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| Zusammenfassung |
| Low-degree coefficients of Mercury’s gravity field as obtained from the MESSENGER’s
Radio Science experiment combined with estimates of Mercury’s spin state permit to
compute the normalized polar moment of inertia of the planet (C-MR2) as well as the ratio
of the moment of inertia of the mantle to that of the planet (Cm-C). These two parameters
provide a strong constraint on the internal mass distribution. With C-MR2 = 0.346 and
Cm-C = 0.431 [1], interior structure models predict a large core radius but also a large
mantle density. The latter requirement can be met with a relatively standard composition of
the silicate mantle for which a core radius of ~ 2000 km is expected [2]. Alternatively, the
large density of the silicate shell has been interpreted as a consequence of the presence of a
solid FeS layer that could form atop the liquid core under suitable temperature conditions
[3]. According to this hypothesis, the thickness of the mantle would be reduced to
~ 300 km only. Additionally, the Gamma-Ray Spectrometer measured a surface
abundance of U, Th and K, which hints at a bulk mantle composition comparable to
other terrestrial planets [4]. Geological evidence also suggests that volcanism was a
globally extensive process even after the late heavy bombardment (LHB) and that
northern plains were likely emplaced in a flood lava mode by high-temperature,
low-viscosity lava. Finally, the analysis of previously unrecognized compressional tectonic
features as revealed by recent MESSENGER images yielded revised estimates
of the global planetary contraction, which is calculated to be as high as 4–5 km
[5].
We employed the above pieces of information to constrain the thermal and magmatic
history of Mercury with numerical simulations. Using 1D-parameterized thermo-chemical
evolution models, we ran a large set of Monte-Carlo simulations (~ 10000) in which we
varied systematically the thickness of the silicate shell, intial mantle and CMB temperatures,
mantle rheology, thermal conductivity of the crust, volume change upon differentiation
associated with depletion and crustal enrichment factor of radiogenic elements. We
considered as successful models yielding less than 5 km of global contraction after
the LHB and a phase of magmatic activity extending beyond the end of the LHB
along with the production of at least a 5 km thick crust. We found a small subset of
admissible models (~ 1%) characterized by a dry olivine rheology, enrichment
factors between 2.5 and 3.5 and a production of up to 35 km of secondary crust
extending up to 3.5 Ga. In a few models convection persists until present day. Models
with a 300 km thick silicate shell are generally incompatible with the contraction
constraints.
From the set of successful models, we selected few representative ones that we
investigated in detail with numerical simulations in 2D cylindrical and 3D spherical
geometry, which confirmed the validity of the 1D approach. In addition, we found that the
thin mantle produces a convection planform whose spectrum is dominated by short
wavelengths. These persist throughout the planet’s evolution and their gravity signature
would be difficult to reveal with the present resolution of gravity data delivered by
MESSENGER. Furthermore, long-wavelength convective features that have been
proposed as a plausible source of compressional tectonic landforms [6] are not
confirmed.
References
[1] Margot et al., 2012. J. Geophys. Res., 117, E00L09.
[2] Rivoldini et al., 2011. Icarus, 201, 12-30.
[3] Peplowski et al., 2012. J. Geophys. Res., 117, E00L04.
[4] Smith et al., 2012. Science, 336, 214-217.
[5] di Achille et al., 2012. Icarus, 221, 456-460.
[6] King, 2008. Nature Geosci., 1, 229-232. |
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