THE EFFECT OF SOLUTION CHEMISTRY AND FREEZING TEMPERATURE ON THE MORPHOLOGY OF CRYOGENIC OPAL-A (COA): IMPLICATIONS FOR MICROBE PRESERVATION AND PAST CLIMATES ON MARS

Cryogenic opal-A (COA) is a type of amorphous opal that forms when silica-rich geothermal fluid erupts and is exposed to sub-zero temperatures. As the fluid reaches its freezing point, silica precipitates in the brine veins between water ice crystals. COA is found embedded within frozen fluids near hydrothermal sources such as geysers and hot springs at Yellowstone and in Iceland, and has been shown to preserve signs of past biological activity in natural and lab synthesized samples. COA is plausible on Mars because of the existence of hydrothermal systems and sub-zero temperatures in the present and the past. It is currently unknown how freezing temperature and solution chemistry affect COA morphology and its ability to preserve microbes.

The objective of this study was to determine the effect of initial solution chemistry and freezing temperature on the morphology of COA without microbes. Two solution chemistries and three freezing temperatures were used. Synthetic hydrothermal solutions were made by dissolving sodium metasilicate in 18.2 MΩ water and adding either HCl or sulfuric acid until the pH was 7.5. The synthetic hydrothermal solutions were heated to 50ºC and then placed in a -20ºC as well as -80ºC freezer overnight, and also flash frozen in liquid nitrogen. Frozen solutions were thawed and the COA was pipetted onto SEM stubs, which were then air-dried. The COA SEM stubs were washed with milli-Q water to remove salts and then dried for SEM analysis.

SEM analyses showed that COA frozen at -20ºC in HCl solution produced smooth silica branches similar to what was observed in previous studies. The COA’s made with lower freezing temperatures produced progressively smaller particle sizes and unique morphologies such as halite dendrites within the silica, dendritic silica, and porous silica. COA made in a sulfuric acid solution was more angular and fragmented than COA made in HCl solutions. Particle size also decreased with decreasing freezing temperature. COA morphology has implications for future return samples from Mars because it can provide information about past climate and fluid chemistry.

Future work will include testing the ability of COA made with these solution chemistries and freezing temperatures to preserve microbes, as well as testing the ability of the different COA’s to shield microbes from UV degradation.