Effects of Organics on Chemical Garden Structures Relevant to Planetary Environments.

Chemical gardens are inorganic structures that spontaneously form via the addition of metal salts to a solution of a precipitating ion.^1,2 These structures, especially those using FeCl₂, are sometimes similar to those found in geological settings such as deep sea hydrothermal vents and mineral deposits in Mars analog environments. As an important aspect of planetary exploration is the search for organics and life; understanding how biologically relevant organic compounds, such as amino acids, interact with chemical gardens is important for understanding and interpreting mission-based data as well as eliminating "false positives" in samples looking for life detection.³ In order to better understand the organic/inorganic interactions, we explored the effect of five different amino acids on the growth and structure of iron-silicate chemical gardens via scanning electron microscopy (SEM), nuclear magnetic resonance (NMR), and energy dispersive X-ray analysis (EDX).⁴ Our results show that the amino acids appeared as a smooth layer on the otherwise rough chemical garden surface (Figure 1). The SEM images indicated that concentration of the amino acids had more of an impact on the structure in comparison to the composition (side chain) of the amino acids - the thickness of the smooth layer correlated with the concentration of amino acid added. The EDX spectra of chemical gardens grown with 20 mM cysteine indicate that the smooth outer layer contains sulfur but the internal rough layers were free of amino acid. Therefore, the amino acids are likely concentrating on the outside layer of the iron-silicate membrane, which has implications for organic/inorganic interaction as well as the reactivity of these surfaces.

[1] Barge, L.M., Cardoso, S.S., Cartwright, J.H., Cooper, G.T., Cronin, L., De Wit, A., Doloboff, I.J., Escribano, B., Goldstein, R.E., Haudin, F., Jones, D.E., Mackay, A.L., Maselko, J., Pagano, J.J., Pantaleone, J., Russell, M.J., Sainz-Diaz, C.I., Steinbock, O., Stone, D.A., Tanimoto, Y., and Thomas, N.L. (2015) Chem. Rev. 115, 8652-8703.

[2] Barge, L.M., Doloboff, I.J., White, L.M., Stucky, G.D., Russell, M.J., and Kanik, I. (2012) Langmuir 28, 3714-3721.

[3] McMahon, S. (2019) Proc. R. Soc. B. 286, 20192410.