Life Detection in the Martian Subsurface: Advancements in Microbial Metabolic Research under Mars Laboratory and Field-Analog Conditions since Viking

Following the Viking experiments in 1976, many of the original inferences of biological metabolism have been replicated via abiotic mechanisms we now know are plausible on Mars' surface. While in many cases subsequent experiments have cast doubt on whether Viking truly detected life, numerous other studies since Viking have greatly expanded our knowledge of life's limits and microbial metabolism. In particular, increased characterization of Earth's subsurface has revealed the astounding complexity and adaptability of life, highlighting chemically based metabolisms as potentially strong targets for future life detection missions. Over the same time frame, we have gained knowledge of putatively more habitable regions in Mars' subsurface, relative to the original Viking lander surface sites, that could host similar organisms. In this review, we discuss the wealth of knowledge concerning the habitability of zones across Mars' surface/subsurface, and we suggest specific microbial metabolisms that should be targeted in future life detection missions based on laboratory and field studies under analogous conditions on Earth and with consideration of recommendations from the larger Astrobiology community. The ability to leverage these advancements in subsurface research toward the incorporation of increased specificity in future life detection efforts is additionally discussed in the context of current Mars subsurface mission progress and planetary protection and defense concerns.