Atmospheric Thickness on Ancient Mars: Constraints from Snc Meteorites

Introduction: Mars atmospheric pCO2 levels between 4.5-3.8 Ga have commonly been estimated at 13 bar [1,2] based on the existence of fluvial features or sedimentary rocks on the surface of Mars. Most of the original atmospheric CO2 (90-95%) has been lost through impact erosion, sputtering processes associated with the solar wind and sequestration within the crust [3]. This has been correlated with isotopic fractionations identified within SNC meteorites [4]. Here we consider the amount of CO2 that could be sequestered within the martian crust – using carbonate abundances in some SNC meteorites as a guide – in order to provide constraints on past atmospheric conditions and total CO2 inventory. Carbonate in SNC meteorites: Of the >32 currently known martian meteorites the 7 nakhlites and ALH 84001 contain evaporite mineral assemblages [5,6]. ALH 84001 contains ~1 vol% carbonate grains 100 μm diameter which are zoned from Ca-rich cores to Mgand Fe-rich rims. They have a complex history, having undergone post-formational shock alteration but originally crystallised (3.9 Ga) rapidly (ie in days or weeks) from 25-150C brines [6,7]. The equivalent phases in the nakhlites (mainly Fe-rich smectite clays, Fe-carbonate, sulphates, halite) have been modelled as products of progressive brine evaporation through the nakhlite parent rocks. Such thermodynamically-based modelling can provide information about the composition of martian fluids and the pressure (pCO2) of the atmosphere at the time of the fluid's activity. For instance, modelling brines associated with the nakhlites studied so far suggests that the atmospheric pressure of CO2 at the time of the evaporite minerals' crystallisation was 50-100 mbar. Radiometric dating work [8] suggests that these evaporite assemblages are approximately 670 Ma. We use this modelling as a way of providing an estimate of pCO2 in the Mars atmosphere 670 Ma. Calculation of past pCO2: We can calculate the past atmospheric pCO2 on >3.8 Ga Mars (Table 1) by using the following observations. 1. The average abundance of martian carbonate in SNC meteorites varies up to 1 vol.% in ALH 84001 [6,9]. 2. ALH 84001 is Noachian in age (4.5 Ga rock crystallisation with carbonates 3.9 Ga, see [7]). This provides an estimate for average crustal abundances of carbonate (which we take here as MgCO3, consistent with the composition of carbonate within ALH 84001). For instance, within the uppermost 1 km of the martian crust, for 1 vol. % MgCO3, this is equivalent to 5.2 x 10 mol or 2.3 x 10 kg CO2. Over the planet’s surface this quantity on Mars is equivalent to an atmospheric pressure pCO2 of 580 mbar. Some CO2 may also be absorbed directly within the uppermost 1 km of Mars. Lesser amounts are present within the polar ice caps and the remnant caps are dominated by H2O. Carbonate is assumed to be the largest crustal reservoir of carbon e.g. the regolith probably contains much less than 280 mbar of CO2 in the form of ice [10]. A potential alternative to meteorites for estimating average carbonate abundances within the martian crust is to use thermal emission data from Mars Global Surveyor and Mars Express. The former has reported ~2 wt% MgCO3 within martian dust [11], which if it is representative of the uppermost 1 km of crust is equivalent to 1200 mbar pCO2. However, the OMEGA experiment onboard Mars Express, has not confirmed this during its current mapping phase [12]. The salt deposits of the Eagle Crater sedimentary layers contain sulphate but not carbonate. Similarly, the Spirit Lander has detected only possible traces of carbonate [13]. However, within an acid-sulphate fluid system, carbonate will be replaced by sulphate minerals and this is a likely explanation for the lack of any more than uncertain traces of carbonate being found by the MER rovers. Carbonate associated with the evaporative brines is expected to be present at deeper levels i.e. tens of meters to km depth in the crust. About 56% of the martian surface (the ancient highlands in the southern hemisphere) consists of terrains of ancient, Noachian age. By using the concentration of carbonate within ALH 84001 and the proportion of Mars over which Noachian terrains are exposed in the ancient highlands, the equivalent of 2300 mbar pCO2 is trapped within the 7 km depth layered units of the Noachian terrains. The 7 km figure is taken from the thickness of layered units exposed on the sides of Valles Marineris canyons. The photographic evidence of Mars [14] suggests that a large proportion of the ancient highlands may be underlain by layered rocks, some of which, but not all, will be sedimentary in origin deposited from flowing water and likely to contain carbonate. However, the SNCs show that igneous rocks on Mars also contain carbonate. Such figures of trapped CO2 are small in comparison with the amount of CO2 trapped within the Earth’s crust as carbonate, or present within the current VenuLunar and Planetary Science XXXVII (2006) 1990.pdf