HOW MODERN HOT SPRING DEPOSITS ON EARTH INFORM THE SEARCH FOR PAST LIFE ON MARS

In a 1996 Ciba Foundation Symposium publication, Jack Farmer presented a set of observations that showed evidence for candidate ancient hydrothermal systems on Mars [1]. Such settings are ideal places to look for evidence of microbial life given their high habitability and preservation potential, as documented in a companion paper Jack co-authored from the same symposium [2]. Though predicted at the time, definitive evidence of surficial hydrothermal deposits had not yet been identified on Mars. Then in 2007, the Spirit rover in the Columbia Hills of Gusev crater serendipitously stumbled upon compelling evidence for this type of deposit. As a member of the science team, Jack emphasized the significance of the opaline silica that had been discovered there, noting its association with hot spring deposits on Earth. But the full significance was not recognized until he saw opaline silica digitate structures in the outflow channels of active hot springs at El Tatio, Chile, features that are remarkably similar to those observed by Spirit. In the terrestrial case, the digitate forms arise through mediation by microbial communities that inhabit the outflow channels. This discovery led to a provocative paper 20 years after the Ciba symposium, which suggested that the digitate structures discovered in the Martian silica deposits are potential biosignatures [3]. From a paleontological perspective, any evidence for Martian hot springs deposits represents a compelling astrobiological target [4]. And though no currently available datasets provide definitive evidence of past life on Mars, perhaps future observations will, answering a driving question at the center of Jack Farmer’s long career.

[1] Farmer, J.D. (1996), Hydrothermal Processes on Mars: An Assessment of Present Evidence, in Evolution of Hydrothermal Ecosystems on Earth (and Mars?), Wiley & Sons. [2] Cady, S. L. & J.D. Farmer (1996), Fossilization processes in siliceous thermal springs: Trends in preservation along thermal gradients, ibid. [3] Ruff, S. W. & J. D. Farmer (2016), Silica deposits on Mars with features resembling hot spring biosignatures at El Tatio in Chile, Nature Communications, 7, 13554. [4] Cady, S. L. et al. (2018), Siliceous Hot Spring Deposits: Why They Remain Key Astrobiological Targets, in From Habitability to Life on Mars, Elsevier.