THE GEOMICROBIAL LIFE OF LAVATUBES: BIOSIGNATURES ON EARTH AND BEYOND

Much of the terrain of the Moon and Mars is recognizably and ruggedly volcanic in nature. Lunar lava rilles and possible lavatubes were clearly visible even in early Apollo-era lunar images. Subsequent work has extended our appreciation of lavatubes and other salient volcanic features on the moon. The case for lavatubes on Mars has been greatly strengthened recently, and the presence of these structures on Venus, Io, and possibly even Mercury has been suggested. Naturally, the value of Earth's many lavatubes as models for planetary structures has become a focus of attention. The volcanic caves of Earth are much less well understood than many other cave types, notably limestone caves. However, recent attention is beginning to rectify this situation including study of the microbial inhabitants of Earth tubes. Such microbial communities can range from visually striking to invisible but comprise a vast menu of differing physical appearances, mineral-precipitating behaviors, and fundamental biological properties. Mapping biosignatures (i.e. the traces of microbial presence) can enable us to infer the presence of organisms even long after they have departed the scene. Unique mineral precipitates and textures associated primarily or solely with microbial activity are major targets of astrobiological interest in volcanic caves. We are comparing the microbial biosignatures produced in lavatube caves with each other across a wide geographic range and with similar microbial/mineral biosignatures produced in other cave environments. For example, there are many parallels like Opal-A speleothems seen in lavatubes, granite, and quartzite caves, to Fe and Mn precipitating organisms seen in lavatubes, limestone, and gypsum caves. The geochemistry and lithology may be dominated by sulfates and sulfides, iron and manganese, carbonates, copper minerals, or silica, but although the details of the chemistry and microbial diversity may differ, a suite of similar ecological themes with specific speleological and geomicrobial consequences can be seen at work in each. These unifying themes may be useful in assessing the potential biological interactions to be expected in new types of cave environments both on Earth and other Solar System bodies.