NASA Astrobiology Institute (NAI)

Potentially habitable environments on the Martian surface have been identified by orbital spectroscopy and by landed instruments on the Mars Exploration Rovers (MERs). Identification of evaporite mineral assemblages on Mars provides strong evidence for the widespread role of evaporitic water bodies of water in the past. Evaporite minerals may provide enhanced preservation of biomolecules by sequestration of organic constituents into mineral matrices during crystallization. The utilization of amino acids, the building blocks of proteins, as a distinct biosignature that could be extracted from evaporite phases would provide a strong biosignature for life having existed in the past or persisting to the present on Mars.

Experiments were carried out to mimic the present Martian diurnal cycling of a Mg-Fe-Ca-Na-SO4 brine derived from acidic weathering of olivine-laden basalt. Experimental brines were laced with an enantiomeric excess of amino acids. Testing was done with and without exposure to the photolytic effects of UV radiation and utilizing a diurnal temperature cycle suitable for simulation of evaporation and sublimation processes in a Martian paleolake or permafrost system.

Results indicate that iron containing brines in the presence of UV are prone to increased levels of amino acid degradation due to photo-Fenton oxidation reactions. In the absence of UV, iron-rich brines provide enhanced preservation, with half lives 200-300% longer than systems lacking iron. Racemization half lives are 30 and 50 times greater than corresponding degradation half lives in iron and non-iron samples, respectively. These initial results provide interesting scenarios in the preservation of organic matter on Mars; an iron-rich subsurface groundwater system, such as those attributed to hematite concretion formation, may provide increased organic matter preservation. Additionally, a limiting factor in life detection may not be the detection of an enantiomeric excess of amino acids but detecting a pool of amino acids at all.

In addition to laboratory brines, we are studying a hydrologically closed basin system in southern California as a Mars analog environment. Samples from a long sediment core (30 meters) drilled at Soda Lake in the Carrizo Plain are being analyzed to determine the yield of enantiomeric excess of amino acids in lacustrine sediments dominated by interbedded clay- and sulfate-mineral assemblages. Obtained values will provide rates of amino acid racemization in environments dominated by seasonal to decadal cycles of evaporation and precipitation of sulfate and chloride minerals.