Petrological constraints on the density of the Martian crust

Abstract

New insights into the chemistry of the Martian crust have been made available since the derivation of crustal thickness maps from Mars Global Surveyor gravity and topography data that used a conservative range of density values (2700-3100 kg/m 3 ). A new range of crustal density values is calculated from the major element chemistry of Martian meteorites (3100-3700 kg/m 3 ), igneous rocks at Gusev crater (3100-3600 kg/m 3 ) and from the surface concentration of Fe, Al, Ca, Si, and K measured by the Gamma-Ray Spectrometer on board Mars Odyssey (3250-3450 kg/m 3 ). In addition, the density of mineral assemblages resulting from low-pressure crystallization of primary melts of the primitive mantle are estimated for plausible conditions of partial melting corresponding to the Noachian to Amazonian periods (3100-3300 kg/m 3 ). Despite the differences between these approaches, the results are all consistent with an average density above 3100 kg/m 3 for those materials that are close to the surface. The density may be compatible with the measured mass of Mars and the moment of inertia factor, but only if the average crustal thickness is thicker than previously thought (approaching 100 km). A thicker crust implies that crustal delamination and recycling could be possible and may even control its thickness, globally or locally. Alternatively, and considering that geoid-to-topography ratios argue against such a thick crust for the highlands, our results suggest the existence of a buried felsic or anorthositic component in the southern hemisphere of Mars.

Mars: a small terrestrial planet

“…The martian basalts are relatively enriched in iron, in comparison with terrestrial basalts, which are direct consequence of the low Mg# of the mantle and smaller core. The grain (pore-free) density of these basalts is about 3100-3300 kg/m 3 (Baratoux et al 2014). An average crustal thickness of ∼50 km was derived from Mars Orbiter Laser Altimeter data assuming a range of crustal density of 2700-3100 kg/m 3 and considering that the young basaltic shergottites were not representative of the igneous crust (Wieczorek and Zuber 2004).…”

“…However, the density of martian basalts, irrespective of their age, would be comparable to that of the basaltic shergottites (Baratoux et al 2014). This leaves us with two possible alternatives: a thick (>100 km) and non-porous basaltic crust, or a thin (∼50 km) stratified crust with a non-basaltic and/or porous component (Nimmo and Tanaka 2005;Baratoux et al 2014). The present-day average crustal thickness is poorly constrained by numerical simulations, with values ranging from several tens of kilometers to more than 100 km, depending on hypotheses made on the heat budget, water content, and mantle rheology (Hauck and Phillips 2002;Breuer and Spohn 2006;Fraeman and Korenaga 2010;Morschhauser et al 2011).…”

“…21, Sohl and Spohn 1997;Zuber 2001;Wieczorek and Zuber 2004;Sohl et al 2005;Khan and Connolly 2008;Rivoldini et al 2011;Baratoux et al 2014). Models of crustal thickness, assuming a constant crust-mantle density contrast, produce a crustal volume corresponding to 4.4% of the planet (average thickness of ∼50 km) and a north-south dichotomy in the crustal structure.…”

“…An average crustal thickness of ∼50 km was derived from Mars Orbiter Laser Altimeter data assuming a range of crustal density of 2700-3100 kg/m 3 and considering that the young basaltic shergottites were not representative of the igneous crust (Wieczorek and Zuber 2004). However, the density of martian basalts, irrespective of their age, would be comparable to that of the basaltic shergottites (Baratoux et al 2014). This leaves us with two possible alternatives: a thick (>100 km) and non-porous basaltic crust, or a thin (∼50 km) stratified crust with a non-basaltic and/or porous component (Nimmo and Tanaka 2005;Baratoux et al 2014).…”

A steamy proposal for Martian clays

“…The simulations also show that the dearth of clays in the northern Martian hemisphere can be explained by the disruption of the primordial layer by the Borealis impact -the collision of a single, large body with Mars that is thought to have occurred in this region. In the southern hemisphere, the buried clay layer might correspond to a lowdensity crustal layer that has been identified by studies of the gravity and topography of Mars 9 . Models of magma-ocean evolution on Earth have sometimes included crustal hydration 10 , but, unlike for Mars, there is no geological record for Earth that goes back more than 3.8 billion years.…”