Figure 1. (top) Geological map of Iceland with black stars marking approximate cave locations. Map projection is ISN93. Courtesy of Anett Blischke, Icelandic GeoSurvey. (bottom) Lava fields visited and examples of cave entrances.
The Icelandic caves play host to a variety of microbial communities, as seen by microbial mats scattered across the cave ceilings, walls, and floors in all colors of the rainbow. Although the lava tubes themselves originate from relatively young flows (Figure 1), they also show some secondary mineral alteration. What is microbial mat and what is secondary mineral can often be difficult to distinguish, as their textures and colors can be interchangeable to the naked eye, and is compounded by the knowledge that they may be intricately linked.
Blue-colored mats/mineral precipitates observed in Icelandic lava tubes were often found on surfaces of flowing water or along cracks in cave walls or ceilings, where water seeps in from the ground above (Figure 2). Some blue features had a viscous texture (Figure 2b), while others were brittle mineral crusts attached to the lava rock beneath (Figure 2a). The blue features found in caves B, C, and H (Figure 4) are the subject of this paper, as well as a detailed account of their acquisition in an effort to inform the search for biosignatures in future missions to lava caves on Mars.