The effect of large impacts on the mantle dynamics and volcanism of Mercury

The flybys of Mercury by MESSENGER, along with those of Mariner 10, have now imaged over 90% of the planet's surface [1]. Images of the Caloris and Rembrandt impact basins, along with several smaller impact structures reveal evidence for interior volcanism subsequent to impact formation [1,2]. Furthermore, crater counts indicate that a smooth plain just beyond the rim of Caloris are at least coeval and possibly younger than volcanic materials within the basin [2,3]. These plains are probably volcanic in origin, and might be associated with the long-term aftermath of Caloris' formation. The broad influence of Caloris on the surface of Mercury indicates that it might also affect heat flow within the mantle and thereby, the core dynamics. Here we investigate possible links between large impacts on Mercury, the volcanism within (and in the case of Caloris, surrounding) the basins, and effects on the planet's dynamo. While the impact cannot have formed the melts directly, the thermal impulse from such a large impact can alter the underlying mantle dynamics, potentially producing subsequent volcanism and altering the heat-flow at the core-mantle boundary. A finite element model of thermochemical convection in a spherical shell [4,5] is used to explore the consequences of the formation of large impacts in the Mercurian mantle. Composition is tracked using a particle tracer method [6]. The impactor size is determined from the observed basin using standard methods of crater scaling [7,8]. The impact is treated as an instantaneous temperature increase that decays away from the impact center [9]. Mantle melting is parameterized as a function of pressure, temperature, and modal cpx based on modeling and experiments of peridotite melting [10]. Melt residue and unmelted mantle are tracked seperately. Melt is removed to the surface; vapor is ignored. We find that even a Caloris-forming impact cannot significantly heat the core, assuming reasonable impact parameters, and should therefore not affect the planet's dynamo. This result differs from investigations on giant impacts on Mars, the largest of which were able to shut down a subcritical dynamo [11]. Despite the relatively thin Mercurian mantle, the expected heating from the Caloris impact cannot penetrate to the core, largely due to the high expected velocity of impactors at Mercury. At 48 km/s, a rocky projectile < 100 km in diameter is sufficient to form the Caloris basin. Since the effective range of heating scales with the impactor size, the heating is limited in extent. Effects of the formation of large impacts on Mercury observed by MESSENGER on the long term evolution of the mantle and volcanism near the impact sites are ongoing. [1] Denevi, B.W. et al. (2009) Science 324, 613-618. [2] Strom, R.G. (2008) et al. Science 321, 79-81. [3] Fasset, C.I. et al. (2009) EPSL 285, 297-308. [4] Zhong, S. et al. (2000) JGR 105, 11,063-11,082. [5] Roberts, J.H. and S. Zhong (2004) JGR 109, E03009. [6] McNamara, A.K. and S. Zhong (2004) JGR 109, B07402. [7] Melosh, H.J. (1989) Impact Cratering, Oxford Univ. Press. [8] Holsapple, K.A. (1993) Ann. Rev. Earth Planet. Sci. 21, 333-373. [9] Watters, W.A. et al. (2009). JGR 114, E02001 [10] Katz, R.F. et al. (2003), G-cubed 4, 1073. [11] Roberts, J.H. et al. (2009) JGR 114, E04009.