A Roadmap for Planetary Caves Science and Exploration

To the Editor— 2021 is the International Year of Caves and Karst (IYCK). What better way to celebrate the majesty of the subterranean realm than to cast our gaze heavenward to the vast scientific and exploration potential embodied in planetary substrata. While researchers have pondered the possibility of otherworldly caves for more than half a century, we are in the incipient phase of examining the caves of alien worlds. Our knowledge of planetary caves varies tremendously across the Solar System. Earth represents the most advanced level of exploration, but many unanswered questions remain concerning terrestrial caves, and how these features might serve as planetary analogs. Off terra firma, identification of planetary caves is most advanced for the Moon and Mars1, as hundreds of candidate cave entrances have been documented and several mission concepts have been proposed. Beyond the Moon and Mars, identifying potential cave entrances becomes increasingly more difficult due to fewer spacecraft images and lower spatial resolution – although potential subsurface access points (SAP) and cave-forming processes have been identified. To date, we have identified 2,418 SAPs across the solar system (Fig. 1). In honor of the IYCK, we propose a planetary cave exploration roadmap composed of three phases: (1) identification, (2) characterization, and (3) exploration. 1. Identification: Terrestrial caves can be identified using a combined thermal, visible, and LiDAR approach. This should be further developed and expanded upon to detect caves on other planetary bodies. Identifying cave-bearing landscapes, followed by systematically searching for caves via standard remote context imaging has been successfully used2. However, to specifically identify and examine possible cave entrances, skylights, and collapse pits, high spatial resolution multispectral imaging is needed. Once candidates have been identified, radar, electrical resistivity imaging3, and gravimetric4 approaches may be used to estimate cave extent and volume. As these techniques have successfully employed on Earth and the Moon, this approach could be used to examine cave candidates on other planetary bodies. 2. Characterization: Prior to formal exploration at depth, a candidate cave entrance must be evaluated and determined to be a high priority science target. Mars Curiosity and Perseverance rovers, the Mars Ingenuity Helicopter, Titan’s Dragonfly (planned for 2034), and other proposed missions (e.g., Moon Diver) could be used to evaluate and/or confirm scientifically interesting potential SAPs. Surface missions should be used for entrance characterization given favorable surrounding terrain and could potentially map internal cave structure if equipped with ground-penetrating sensors. These technologies will be required to delineate the cave structure for future mission design and to reduce mission risk. 3. Exploration: While there are no currently planned missions to examine or enter a planetary cave, the investment in long lead robotic technological development will be required to ultimately explore a planetary substratum. Various mission concepts have been proposed that include limbed robots5, flying robot swarms6, tethered rovers7, microbot swarms8, and deployable stationary payloads9. Each platform type has unique capabilities and limitations, and selection will depend on the cave and scientific objectives. Regardless of the robotic platform, additional engineering challenges are outstanding – including communications, autonomous navigation, and power. Once inside a cave, direct line-of-sight communication is no longer possible. Communication will require extra steps such as: relaying between nodes, communication bundled within the tether, transmitting low frequency radio waves through the overburden, or returning to the cave entrance10. Limited communications will require autonomous navigation and decision making to detect and avoid hazards and potentially identify and select sampling locations (for life detection and habitability assessments). Devoid of natural lighting, active sensors will also be required. Ideally, navigational instruments would perform the dual role of both navigation and science. Power will be needed for all aspects discussed above and will determine mission duration. Innovation will be required to address these challenges. Planetary caves research and exploration has the potential to exponentially expand over the next decade. On Earth, analog studies and technological research and development will be crucial. For most of the Solar System, orbiting spacecraft capable of identifying caves and cave bearing landscapes and resolving potential cave entrances is lacking. The advent of aerial drones in planetary surface exploration is a potential game-changer as these systems could be used for both detection and entrance characterization. For rovers, the development of flight-qualified instruments that can resolve (and characterize) cave entrances and internal structure using remote sensing is needed. For the Moon and Mars, entering a cave within the next decade is an achievable goal, but will require significant investment in robotic development. To achieve the flexibility required to conduct a successful mission, the platforms discussed should be developed to a ‘flight qualified’ level. By applying this roadmap and advancing these key technologies, we will be able to investigate the planetary substrata – one of the most promising habitats to search for evidence of life.