Crust-atmosphere coupling and carbon sequestration on palaeo-Mars
By Science Hooker, Mar 24 2016 01:29PM
The modern surface of Mars displays evidence for past liquid water flows, with mounds and polygons in the Chryse-Acidalia region possibly indicating large bodies of ancient standing liquid (Oehler and Allen, 2012). For liquid water to be stable at the planet’s surface, temperatures of >273.2K and a saturation water vapour pressure of >6.1 mbar are required (Martínez and Renno, 2013). To achieve such conditions, atmospheric pressures >1 bar CO2 have been hypothesised during the late Noachian/early Hesperian period (i.e. ~1.4-3.0 Ga e.g. Fernández-Remolar et al. 2011). Mars' currently thin (6 mbar) atmosphere poses the question of the fate of the hypothesised multi-bar CO2 atmosphere. Estimates for ~270 mbar lost to space (Niles and Michalski, 2011), with ~5 mbar at the poles (Phillips et al. 2011), leaves a minimum 750 mbar unaccounted for.
The nakhlite martian meteorites display clear evidence of low water to rock (W/R) ratio isochemical silicate mineral carbonation (Tomkinson et al. 2013). Such carbonation processes can also be observed in basic to ultra-basic terrestrial rock exposures, such as the Samail peridotite (Oman; Kelemen et al. 2011) and Del Puerto ophiolites (Californian; Blank et al. 2009). The present research project aims to compare martian meteorite petrology and geochemistry with basic to ultra-basic carbonated terrestrial ophiolite analogues, conduct carbonation experiments on terrestrial Mars analogue samples and construct a crust-atmosphere coupling model. By analysing carbon sequestration processes at differing scales and environments, carbonation rates and total silicate carbonation volumes under palaeo-martian atmospheric and mineralogical conditions can be estimated.
Hydration and carbonation of silicate rocks is an important negative feedback process in the terrestrial carbon cycle. Significant atmospheric CO2 removal via silicate weathering partly balances the volcanic CO2 output. Peridotite contains >40% olivine, which can hydrate to form quartz, magnesite and serpentine via reactions 1 to 3 of table 1. These reactions may be followed by carbon sequestration via reactions 4 to 7 of table 1.
Basic to ultra-basic rocks also contain abundant pyroxenes which can hydrate and carbonate via reaction 8 of table 1, usually in two discreet steps through reactions 9 and 10 of table 1. These reactions form highly alkaline travertine springs (pH>11), which have been observed in terrestrial ophiolites worldwide.
Carbonation is exothermic, with the total fully carbonated solid products possessing 44% greater mass than the reactants (Kelemen et al. 2011). This causes cracking (Kelemen and Hirth, 2012), exposing fresh reactant surfaces, although this can be offset by expansion causing reduced porosity (Alt and Teagle, 1999). The raised temperatures increase reaction rates, and a positive feedback mechanism of sustained carbonation can develop.
The crust of Mars is composed of similarly basic to ultra-basic minerals, mostly basalt on the surface (Taylor et al. 2010). By investigating carbonated terrestrial analogues a deeper understanding of Martian crust-atmosphere dynamics can be achieved, possibly resolving currently unexplained martian anomalies, as well as accounting for the loss of an early >1bar CO2 atmosphere. For example, the recent crater wall slumps observed by the Mars Global Surveyor could potentially be explained as alkaline spring extrusions from subsurface carbonation processes.
Objectives of the present research
1.) Conduct detailed petrographic observational analysis of terrestrial carbonation from collected samples of the Oman and Californian ophiolites, comparing the results with data available for Martian meteorites.
2.) Conduct chambered carbonation experiments on terrestrial ophiolite samples using a variety of mineralogical compositions under incrementally increased CO2 pressures, repeating with increasing ratios of CO2/SO2 mixtures.
3.) The results of objectives 1 and 2 will be synthesised to create a quantitative Martian CO2 model with variable parameters of atmospheric compositions, pressures and crustal compositions.
4.) Further mineralogical evidence to validate the model will be sought using the Curiosity Rover's Mars Science Laboratory, and collaborating with partners, new tools (rock polishing and deep drilling capabilities) will be designed, built and tested for planned use on future rovers.
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