On lunar asymmetries 1. Tilted convection and crustal asymmetry

Abstract

[1] It is proposed that lunar crustal asymmetries are the result of convective processes acting early in the Moon's history, during the magma-ocean phase and after synchronous rotation was established. Buoyant anorthositic crystals were transported to the farside by a large-scale circulation, called tilted convection, known to occur spontaneously in chaotic convective systems. The circulation was oriented toward the farside of the Moon by a small temperature contrast produced by radiative thermal shielding due to the proximity of Earth. Crustal thickening near the equator resulted from the modification of tilted convection by the Coriolis force. The anorthosite that was transported to the farside and equator may be a major cause of the observed crustal asymmetries and of the associated offset of the center of figure from the center of mass and bimodal hyposgram. The selective transport may have resulted in a nearside excess of incompatible elements, leading to the formation of the Procellarium KREEP terrane.

Anorthosite formation by plagioclase flotation in ferrobasalt and implications for the lunar crust

“…The variable amount of mafic minerals in Sept Iles anorthosites is explained by a non-uniform distribution of the interstitial liquid (10-50%), due to convection within the anorthosite mush (Tait et al, 1984;Morse, 1986). A similar model may be proposed for the lunar crust where anorthosite is formed by flotation atop a convecting magma ocean (Herbert et al, 1977;Morse, 1982Morse, , 1987Solomatov, 2000;Loper and Werner, 2002). This hypothesis of crystallization of the mafic minerals from the interstitial liquid is reasonable in light of the trapped liquid-content determined for lunar soils of the Apollo 16 landing site (30%; Jolliff and Haskin, 1995).…”

A new model of lunar crust: asymmetry in crustal composition and evolution

“…Binder, 1978Binder, , 1982Ryder, 2002;Garrick-Bethell et al, 2006). In the latter case, some mechanism to accumulate the early-crystallizing component on the farside needs to be considered, since a simple convection seems unlikely to contribute a large-scale lateral migration across the hemispheres (Loper and Werner, 2002). Fig.…”

On lunar asymmetries 2. Origin and distribution of mare basalts and mascons

“…The Moon has a number of interesting asymmetries of uncertain origin. In a companion paper [ Loper and Werner , 2002] we describe a novel mechanism, called tilted convection, possibly responsible for the creation of the observed thicker crust on the farside and the geochemical signature of the nearside crust [ Jolliff et al , 2000; Wieczorek and Phillips , 2000]. In this paper we focus attention on the unusual mass concentrations (mascons) in mare basins associated with impact craters on the nearside [ Muller and Sjogren , 1986; Arkani‐Hamed , 1998], addressing two related questions: why are mare basalts confined to the nearside, and how were they emplaced at the surface?…”