## 4 Discussion  
In 1969, [PERSON] et al. [56] first suggested that rilles, found in images taken by the Lunar Orbiter V, might be lava caves. A few years later, the existence of such rilles was confirmed in the Marius Hills region on the moon [57], and soon thereafter on Mars [58]. Nearly two decades elapsed before researchers would hypothesize that such lava caves may be potentially habitable places for extant life on Mars [20]. In the search for evidence of extraterrestrial life, Mars remains the strongest candidate because of its appealing evidence of Earth-like conditions, such as the presence of liquid water [59; 60], the existence of numerous potential Martian lava caves [21; 22; 58], and evidence of crustal fluids at the Martian surface [61]. Although Mars is just outside of the traditional circumstellar habitability zone, a set of climate constraints prerequisite to viable life and affected by a planet's proximity to the sun [62], its subsurface is considered potentially habitable by microbial life [63]. Martian lava caves could provide protection from the inhospitable environment at the Martian surface [64] and are likely similar to lava caves found on Earth [20; 56; 58], thus making lava caves on Earth well-suited analogs for those on Mars. Our findings identify critical biosignatures in secondary mineral deposits present in subsurface habitats for programs designing rovers to identify viable sampling targets in the search for life on Mars and elsewhere. There is a large difference in the microbial ecologies between New Mexican and Hawaiian lava caves. Not only do the two locations group separately on NMDS plots (Figure 3), but we also see differences via statistical hypothesis testing applied to the Bray-Curtis dissimilarity measures (see Figures 3 and 4). We detect a difference between Hawai' i and New Mexico, using a PERMANOVA hypothesis test. This is likely due to several different factors. First, the two locations are very different climologically from each other. While both locations experience relatively low precipitation levels, Hawai' i does see almost four times the average rainfall compared to El Malpais in New Mexico. Hawaiian caves are also much warmer, with an average temp of 19 \({}^{\circ}\)C compared to 5 \({}^{\circ}\)C in New Mexico. Furthermore, the lava caves in New Mexico are at a considerably higher altitude--almost five times higher than those found on the island of Hawai' i (Table S1). Despite Hawaii being an active volcanic hot spot, whereas ELMA volcanic activity is related to a deep crustal structure known as the Jemez Linear that extends northeesterly across the state, the two sites have chemically similar tholeiitic lava containing reduced metals [65]. Chemolithotrophic bacteria have been found to utilize reduced metals in the bedrock of caves. Certainly, both systems contain high levels of reduced iron and manganese, and significant levels of copper, all of which may provide energy from oxidation for microbial respiration. Furthermore, Hawaiian caves contain more copper than New Mexican caves, and copper minerals have been observed in previous studies of microbial minerals in Hawaiian lava caves [29]. The cave geometry and total length varied between the two locations, with the Hawaiian tubes all being part of a very large complex of caves while the New Mexican caves were all discrete caves. Together, these factors lead to distinct communities in the locations as shown by the alpha and beta diversities.