SEARCHING FOR EVIDENCE OF LIFE IN THE DARK: THE ASTROBIOLOGICAL POTENTIAL OF LAVA TUBES (Invited Presentation)

Planetary cave astrobiology sits at the intersection of microbiology, geology, and geochemistry. On Mars, lava tubes have been classified by NASA as special regions because of their astrobiological potential, as they provide protection from surface hazards such as radiation and extreme temperature fluctuation. Martian lava tubes may have served as refugia for life as the post-Noachian (<3.1 Ga) climate became colder and drier, allowing life to persist there long after the surface conditions became inhospitable.

Microorganisms in terrestrial analogs use basaltic glass and crystals as nutrient resources for metabolism (e.g., Popa et al., 2012). To better understand microbial growth and activity in a variety of lava cave conditions, we conducted a year-long microbial colonization study using McKinney Basalt billets, a high phosphate (0.5-0.75 wt. %) Snake River Plain basaltic glass, placed in 4 lava tubes at Craters of the Moon National Monument (Idaho, USA). Caves were selected to offer extremes in internal temperature and humidity conditions: cold wet, cold dry, warm wet, warm dry. Basaltic glass billets were ultraviolet- and 70% ethanol sterilized before incubation in the caves. Billets were retrieved at discrete time intervals (6, 12 months) and preserved in 2.5% glutaraldehyde. Billets were ethanol dehydrated and critically-point dried prior to measuring the amount of accumulated biomass with scanning electron microscopy. Microbial colonization occurred rapidly (i.e., within 6 months) in all cave samples. The highest cell counts correlated with phosphate availability and temperature. Weathering was evident after 12 months, particularly in warmer and wetter caves.

On-going terrestrial lava tube studies provide insight into factors that influence habitability and how to evaluate them. Advances in technology and robotics have made habitability- and astrobiology-focused subsurface exploration mission(s) to lava tubes on Mars feasible within the next decade.