5 Conclusion
The Planetary Analogues and Exobiology Lava Tube Expedition (PELE) was
formed to investigate biosignatures in terrestrial lava tubes and inform
the search for extant and extinct life in Martian lava tubes. OurStudy in Blue , a case study of blue, copper-rich speleothems
collected in Icelandic lava tubes, gave insights into how we might
approach sampling these analogue environments, and what we might expect
to find. A portable XRF gave us a good indication of elemental
composition inside the caves and helped us distinguish regions of
interest, i.e., secondary minerals with obvious variations in
composition to that of the background lava rock. 16S rRNA sequencing of
microbial mats revealed Proteobacteria as the dominant bacterial phylum,
several carotenoid-containing genera, chemolithoautotrophic genera, and
genera known for their high metal resistance, all characteristics which
make them good candidates for surviving on Mars, but also potential
contaminants in Mars missions. Raman analysis produced a prominent
carotenoid signal, which is a promising biosignature feasibly detectable
for robotic lava tube explorers carrying Raman instruments. The
morphology and elemental composition of the blue secondary mineral
precipitates were elucidated by SEM/EDS, identifying chrysocolla crusts,
and a complex, layered precipitate consisting of chrysocolla,
pseudomalachite, manganese (hydr)oxide, and a carbonate-bearing species.
It appears that copper may be leached from hyaloclastite and ash
deposits, enriching the rainwater and groundwater that filters into the
caves. Copper-rich speleothems are then precipitated out and serve as
biotopes for metal-resistant organisms. As chrysocolla and other
oxidized copper mineral phases have been detected on the surface of
Mars, they may also be present in Martian lava tubes, and if so, would
be worthy targets for astrobiological investigations by future missions.