Organic Adsorption onto Iron Hydroxide and Sulfide Minerals: Implications for Ceres Sample Return Analysis

Characterizing the past and present habitability potential of Ceres is an important goal of a future NASA mission to the dwarf planet. Signs of organic molecules (potentially amino acids, carboxylic acids, etc.) and their potential interactions with surrounding minerals may be present on the surface of Ceres. In addition, iron minerals are postulated to be present based on Dawn's gamma ray and neutron detector measurements, and iron minerals are known to be able to adsorb organic molecules. To understand if these iron minerals could adsorb prebiotically relevant organics on the surface of Ceres, we conducted an experimental study to test the adsorption of a set of simple organics (alanine, glycine, acetate, malic acid, and aspartic acid) on two classes of iron minerals found in carbonaceous chondrites (Fe-sulfides and Fe-hydroxides, synthesized via coprecipitation). Our experiments revealed little to no adsorption of amino acids onto each of the Fe-minerals. Our results also revealed that the redox state of the Fe-hydroxide minerals remained stable throughout the experiment; however, Fe-sulfide precipitates exhibited reduction of Fe(III) to Fe(II). Additionally, there was negligible adsorption of glycine onto mixed-valent iron mineral systems in a synthetic brine solution. This work shows that organic acids remain stable in the presence of Fe-bearing minerals and are mobile when dried and rehydrated by brines, meaning that these organics may be stable on Ceres' surface but could be further mobilized during cryovolcanic events, the products of which may be used for further reactivity toward other complex molecules with astrobiological relevance.