Physics
Scientific paper
Dec 2010
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2010agufmdi33b..06g&link_type=abstract
American Geophysical Union, Fall Meeting 2010, abstract #DI33B-06
Physics
[5400] Planetary Sciences: Solid Surface Planets, [5430] Planetary Sciences: Solid Surface Planets / Interiors, [5455] Planetary Sciences: Solid Surface Planets / Origin And Evolution, [6235] Planetary Sciences: Solar System Objects / Mercury
Scientific paper
The surface of Mercury exhibits a global system of tectonic landforms called lobate scarps, which bear witness to periods of global planetary contraction. These scarps have been identified on the basis of Mariner 10 imagery [1], and new estimates based on images acquired during the recent MESSENGER flybys indicate a global radius contraction of 1-2 km since the end of the late heavy bombardment (LHB) [2]. The contraction of Mercury has been attributed to a combination of planetary cooling and the growth of an inner core [3, 4], but the very limited amount of contraction documented on the surface poses severe constraints on admissible models. First of all, secular cooling alone causes radial contraction in excess of 2 km if average mantle temperatures drop by more than 50 K, limiting acceptable models to slowly cooling models with a preferably Th rich, refractory composition. Additionally, mantle rheology should be stiff, demanding the concentration of volatiles inside Mercury to be negligible. Furthermore, core freezing contributes a significant amount of contraction which can easily exceed 10 km if the inner core radius exceeds 50% of the total core radius. This limits admissible models to those with high sulphur content >7 wt%, preventing early core freezing and effectively eliminating inner core freeze-out from the radius balance. Therefore, current thermal evolution models require a refractory rich composition, high sulphur and low volatile content to be compatible with the observed low radial contraction [4]. These requirements are at odds with the evidence for pyroclastic eruptions on Mercury documented in recent MESSENGER images, as the magma volatile content required to emplace pyroclasts to the observed distance is estimated to be >0.4 wt% [5]. Here we reinvestigate the coupled thermal and chemical evolution of Mercury taking the presence of a poorly conducting megaregolith layer [6] and a thermally insulating crust into account. Models incorporating a regolith layer cool more slowly than previously considered models [4], thus extending the time span in which global magmatism occurs beyond the end of the LHB. In this way, the mantle phase changes associated with melt extraction can act to compensate some of the contraction caused by secular cooling and inner core formation, allowing for a broader range of models to be compatible with the observed small contraction. This effect has previously been neglected. We find that for refractory models core sulphur contents below 1 wt% and above 6 wt% are compatible with the observations. Low sulphur models freeze most of their cores before the end of the LHB, while high sulphur models only start to form an inner core towards the end of their evolution. Additionally, volatile rich compositions are now also admissible, although they show a phase of global expansion following the LHB. Maximum obtained crustal thicknesses are close to 85 km for the refractory and 90 km for the volatile rich models. [1] Strom et al., JGR, 80, 2478-2507, 1975. [2] Watters et al., EPSL, 285, 283-29, 2009. [3] Schubert et al., Merucry, Univ. Ariz. Press, 1988. [4] Hauck et al., EPSL, 222, 713-728, 2004. [5] Kerber et al., EPSL, 285, 263-27, 2009. [6] Warren and Rassmussen, JGR, 92, 3453-3465, 1987.
Breuer Doris
Grott Matthias
Laneuville M.
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