Could clays have been a safe place for prebiotic compounds before life on Earth?

07/10/2020

Carbonaceous chondrites, such as Murchison CM2, formed early in the solar system’s history. As such, these extraterrestrial materials record the processes that have operated since their formation. Carbonaceous chondrite parent bodies represent the non-differentiated aggregates of ice, dust and chondrules that were present in the protosolar nebular. Radioactive elements, such as 26Al and/or 60Fe, likely supplied the heat necessary to melt the accreted ice, which in turn reacted with the accreted minerals (mainly olivine and pyroxene and some metals and sulphides) to form secondary minerals, including phyllosilicates, sulphates and carbonates.

Aqueous alteration also effected organic matter through alteration and/or synthesis processes. Exactly what processes effected the organic matter within carbonaceous chondrites (CM, CI and CR) is still debated. Nevertheless, fluid circulation should have impacted the distribution of free organic matter (FOM) found within carbonaceous chondrites. Previous studies have shown associations between phyllosilicate phases and organic matter and employed aqueous alteration as a process that can move and concentrate organic matter onto these phases. However, no studies have demonstrated the effects of aqueous alteration on a specific set of organic molecules in relation to phyllosilicates.


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FOM distribution during aqueous alteration. FOM and water ice are accreted by the parent body. The ice melts and HMW FOM (red) adsorbs onto phases close to the site of accretion, but LMW and IMW FOM (light orange) are transported further into the matrix, generating a geochromatographic effect (red to light orange). With the generation of phyllosilicates, adsorption of the remaining organic matter (LMW and IMW), as well as any previously adsorbed, occurs on these new charged mineral surfaces.

Homologues are organic compounds that differ by their hydrocarbon chain length, but possess the same functionalities. In the Murchison meteorite alkyimidazoles appear to be fractionated in terms of their low molecular weight (LMW) and high molecular weight (HMW) homologues. The LMW homologues have a wider distribution and are associated with phyllosilicates, whereas HMW homologues are limited in their spatial distribution and do not show as stronger correlation with phyllosilicates. Such an observation indicates that LMW alkylimidazoles were preferentially mobilised by aqueous rich fluids over their HMW counterparts, presumably due to differences in solubility. The aqueous fluids then preceded to alter silicate minerals to phyllosilicates, which later adsorped the LMW homologues in solution. The adsorption of LMW organic matter onto phyllosilicates may protect these compounds from further alteration by aqueous fluids. Therefore, the adsorption and concentration of organic matter on phyllosilicates may allow the availability of biogenically important molecules on the Earth’s surface during the period when life originated.


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