Over the course of humanity's history as a species, the use of caves, rock shelters, and other natural geological features has played an important role in our survival and cultural development. We suggest that the use of such natural features in future human exploration of Mars and Earth's moon could be a timely and practical solution to a number of potential dilemmas presented by the extreme and challenging nature of the environments on these bodies. Lava tubes, other caves, cavities, and canyon overhangs that are being identified on other planets are sites of intense scientific interest for geology, atmospheric climate records, and potentially biology. They may offer easier subsurface access for direct exploration and drilling, and could provide extractable minerals, gases, and ices. In the past few years, examples of such structures on Mars, the Moon, and potentially other bodies have increasingly come to light. Thus, the real estate is out there waiting for us to modify it for our exploration missions. The present Martian surface environment is extremely cold, dry, chemically active, and high in both ultraviolet and ionizing radiation. Galactic Cosmic Radiation (GCR) and episodic waves of high energy particles from solar proton events (SPE) necessitate the provision of robust radiation protection for habitats, workspaces, vehicles, and personal space suits. The mass penalty of providing this is a major driver in our consideration of the use of natural rock mass for radiation protection for habitats and workspaces, arguably the most massive components of an integrated human exploration equipment suite. Planetary protection considerations emerging from recent studies advocate a localization and zoning of degrees of human impact, much like that being implemented in the Antarctic as Special Regions. Containment of the primary human habitation and work activities within the confines of a subsurface habitat are highly consistent with these new approaches to Planetary Protection forward contamination. To begin to think about caves in the extraterrestrial exploration context, we have developed the notion of a complete, functioning subsurface habitat system. A suite of relatively low technology modifications to caves to improve habitability and safety are suggested. This system can integrate a spectrum of missions from both robotic precursors to human expeditionary missions and ultimately colonization.

This paper presents newly discovered candidate cave entrances into Martian near-surface lava tubes, volcano-tectonic fracture systems, and pit craters and describes their characteristics and exploration possibilities. These candidates are all collapse features that occur either intermittently along laterally continuous trench-like depressions or in the floors of sheer-walled atypical pit craters. As viewed from orbit, locations of most candidates are visibly consistent with known terrestrial features such as tube-fed lava flows, volcano-tectonic fractures, and pit craters, each of which forms by mechanisms that can produce caves. Although we cannot determine subsurface extents of the Martian features discussed here, some may continue unimpeded for many kilometers if terrestrial examples are indeed analogous. The features presented here were identified in images acquired by the Mars Odyssey’s Thermal Emission Imaging System visiblewavelength camera, and by the Mars Reconnaissance Orbiter’s Context Camera. Select candidates have since been targeted by the High-Resolution Imaging Science Experiment. Martian caves are promising potential sites for future human habitation and astrobiology investigations; understanding their characteristics is critical for long-term mission planning and for developing the necessary exploration technologies