Moon Diver: A Discovery Mission Concept for Understanding the History of Secondary Crusts through the Exploration of a Lunar Mare Pit

When the Apollo astronauts collected samples from Tranquility Base in 1969, they provided an unprecedented window into the processes that shaped the Moon. Ever since, the Moon has served as a “keystone” for understanding planetary geological processes throughout the Solar System. Like all samples that have been returned from the Moon, the Apollo 11 samples were collected from the lunar regolith, a layer of jumbled and pulverized rocks that blankets and obscures the Moon's bedrock geology. When geologists reconstruct the history of the Moon, these samples are like scattered puzzle pieces, each representing important information, but removed from the context of their formation and isolated from the bigger picture of how the Moon's crust was formed. The goal of Moon Diver is to return to Mare Tranquillitatis, taking advantage of the discovery of a natural pit cave entrance exposing a deep cross-section through both the lunar regolith as well as tens of meters of bedrock lava layers. Collecting information on the chemistry, mineralogy, and morphology of these intact bedrock layers would allow us to investigate where rocky crusts come from, how they are emplaced, and the process by which they are transformed into the regolith layer that we see from space. In doing so, the mission would combine the deep knowledge gained by Apollo with the unprecedented in situ access to secondary crust granted by the lunar mare pit to understand these fundamental processes on the Moon, and to use this knowledge as a keystone for understanding the same processes across the Solar System. The success of the Moon Diver concept hinges on accessing the subsurface. The existence of the mare pit provides a cross-section through the lunar maria. Access to the record exposed in the wall of this pit is provided by two critical space technologies: pinpoint landing (allowing the delivery of the payload close to the pit) and extreme terrain mobility (allowing the delivery of capable instruments to the cliff wall). Pinpoint landing is a closed-loop guidance and navigation capability that repeatedly matches visual features from a downward-facing camera to a priori acquired terrain maps. This body-relative navigation is then used with closed-loop control to guide the spacecraft toward its landing target, yielding a tight landing ellipse. Once on the surface, an extreme-terrain robotic explorer, called Axel, would egress from the lander and traverse tens of meters to the pit. The lander provides mechanical support, power and communication to the rover through its umbilical tether. Anchored to the lander, the two-wheeled, tail-dragger rover would pay out its tether as it traverses toward the pit. With the aid of its 300-meter tether, the rover can traverse the steep slopes of the pit funnel and rappel its vertical walls. The rover carries a surface preparation tool together with a suite of three instrument types: (a) a trio of high-resolution cameras (Mars 2020's EECAMs) for acquiring context images of the near and far walls with the near-wall pair in a stereoscopic configuration, (b) an alpha-particle-X-ray Spectrometer (MSL's APXS) for elemental composition, and (c) a multi-spectral microscopic imager (MMI) that uses controlled lighting for minerology. The surface-preparation tool removes dust and patina that may be present on the rock wall by grinding a small area. The surface-preparation tool, the MMI and the APXS would be deployed from their instrument bays in one of the wheel wells. The rover would independently point each of its instruments at the same target of interest on the wall with millimeter-level repeatability. Confidence in the technologies of pinpoint landing and extreme-terrain access is based on helicopter testing of terrain-relative navigation and field testing of extreme terrain mobility respectively. The latter was tested using Axel rover prototypes with integrated science instruments at multiple terrestrial analog sites including a basaltic pit in Arizona. Landing shortly after sunrise, the surface mission timeline is just shy of a lunar day (14 Earth days). Upon landing, the rover would egress from the lander, traverse toward the pit, descend along the pit funnel and rappel down its wall. Throughout its traverse, the rover would acquire multiple measurements of both regolith and mare layers. After descending to the bottom of the layers, the rover will reach a significant overhang. This void space may open into a large cave or lava tube, which could someday provide a protected location for a lunar base. For these reasons, lunar pits provide an exciting new target for lunar exploration.