Microbes break survival records in conditions of extreme aridity
Astrobiologists have made a crucial discovery that could reshape our approach to finding life on Mars. An experiment using a Martian regolith simulant showed that terrestrial microorganisms can replicate at catastrophically low humidity levels previously considered uninhabitable.
Testing in a Mars Simulant
Researchers used Mojave Mars Simulant 2 (MMS-2), a fine-grade basaltic soil modified with sulfates and oxides to closely mimic the composition of Martian regolith. Because standard optical methods (like colony counting) don't work well in opaque dirt, scientists monitored microbiome growth by quantifying extracted DNA mass over a 60-day period.
Pushing Biological Boundaries
For years, the absolute lower limit of "water activity" (aw) required for cell replication on Earth was thought to be 0.585. However, during the experiment, the regolith microbiome demonstrated DNA accumulation (indicative of replication) at a water activity level of just aw = 0.34. Growth only ceased entirely at the extreme value of aw = 0.12.
Key Findings of the Study
- aw = 1.00: Exponential growth with DNA peaking on day 15.
- aw = 0.34: DNA accumulation peaking on day 30 (a new survival record).
- aw = 0.12: Zero detectable growth throughout the entire 60-day observation.
Planetary Protection Implications: Liquid water is unstable on Mars, but atmospheric vapor actively interacts with salts in the regolith, increasing local humidity at night. If Earth bacteria can survive at aw = 0.34, the risk of biological contamination of the Red Planet by our rovers is much higher than previously assumed.
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