Physics
Scientific paper
Dec 2011
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2011agufm.p33g..04r&link_type=abstract
American Geophysical Union, Fall Meeting 2011, abstract #P33G-04
Physics
[5420] Planetary Sciences: Solid Surface Planets / Impact Phenomena, Cratering, [5480] Planetary Sciences: Solid Surface Planets / Volcanism, [6235] Planetary Sciences: Solar System Objects / Mercury, [8121] Tectonophysics / Dynamics: Convection Currents, And Mantle Plumes
Scientific paper
MESSENGER has now imaged over 95% of the surface of Mercury, including the entirety of Caloris, the largest impact basin on Mercury [1]. These images reveal evidence for volcanic plains within and exterior to the basin that appear to be younger than the basin rim [2-4], and might be associated with the long-term aftermath of Caloris' formation. The broad influence of Caloris on the surface indicates that it might also affect heating of the deep mantle and therby, the core dynamics. Here we investigate possible links between the Caloris impact on Mercury and volcanism within and surrounding the basin. While the apparent age difference between the rim and plains [3-4] indicates that the plains materials cannot be impact melts, the thermal impulse from such an impact can alter the underlying mantle dynamics, producing volcanism late on. We use standard methods of impact scalings [5-8] and a finite element model of thermochemical convection in a spherical shell [9-11] to explore the melt production, and geodynamic evolution in the Mercurian mantle as a result of the Caloris impact. We find that a single impact may produce multiple episodes of volcanism. The shock heating distribution from larger projectiles penetrates into convective upwellings up to 800 km away; beyond the basin rim. This promotes melt production in these upwellings, beneath a relatively thick stagnant lid. The associated melt is thus produced at depth and may be erupted to the surface much later than the direct impact melts. This is consistent with the relatively young age of the exterior plains [2-4]. We also find that impacts may sample different regions of the mantle. Direct impact melts are primarily in the near surface, while exterior melting is located in deeper mantle plume heads. Volcanic plains may therefore reveal pre-existing compositional stratification of the crust and mantle [2,12]. We further find that an impact capable of forming Caloris cannot significantly heat the core and should not affect the dynamo. This is consistent with observations of a present-day global magnetic field at Mercury [13]. The shock heating decays relatively quickly away from the impact site, due to the high expected impact velocity at Mercury (42.5 km/s [14]), and thus the small size of the projectile. We note, however, that the heating from a slow vertical impact could be mimicked by a fast oblique impact. Furthermore, while the median impact velocity at Mercury is high [14], the distribution is broad and some slower impacts are expected. [1] Murchie, S.L. et al. (2008) Science 321, 73-76. [2] Denevi, B.W. et al. (2009) Science 324, 613-618. [3] Strom, R.G. (2008) et al. Science 321, 79-81. [4] Fassett, C.I. et al. (2009) EPSL 285, 297-308. [5] Melosh, H.J. (1989) Impact Cratering, Oxford Univ. Press. [6] Holsapple, K.A. (1993) AREPS 21, 333-373. [7] Pierazzo, E. et al. (1997) Icarus 127, 408-423. [8] Watters, W. A. et al. (2009) JGR 114, E02001. [9] Roberts, J.H. and S. Zhong (2004) JGR 109, E03009. [10] McNamara, A.K. and S. Zhong (2004) JGR 109, B07402. [11] Roberts, J.H. and O.S. Barnouin (2011) JGR, in revision. [12] Ernst, C.M. et al. (2010) Icarus 209, 210-223. [13] Anderson, B.J. et al. (2008) Science 321, 82-85. [14] Le Feuvre, M. and M. A. Wieczorek. (2008) Icarus 197, 391-206.
Barnouin Olivier S.
Roberts James Hirsch
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