Charity M. Phillips-Lander

Nearly half a century ago, two papers postulated the likelihood of lunar lava tube caves using mathematical models. Today, armed with an array of orbiting and fly‐by satellites and survey instrumentation, we have now acquired cave data across our solar system—including the identification of potential cave entrances on the Moon, Mars, and at least nine other planetary bodies. These discoveries gave rise to the study of planetary caves. To help advance this field, we leveraged the expertise of an interdisciplinary group to identify a strategy to explore caves beyond Earth. Focusing primarily on astrobiology, the cave environment, geology, robotics, instrumentation, and human exploration, our goal was to produce a framework to guide this subdiscipline through at least the next decade. To do this, we first assembled a list of 198 science and engineering questions. Then, through a series of social surveys, 114 scientists and engineers winnowed down the list to the top 53 highest priority questions. This exercise resulted in identifying emerging and crucial research areas that require robust development to ultimately support a robotic mission to a planetary cave—principally the Moon and/or Mars. With the necessary financial investment and institutional support, the research and technological development required to achieve these necessary advancements over the next decade are attainable. Subsequently, we will be positioned to robotically examine lunar caves and search for evidence of life within Martian caves; in turn, this will set the stage for human exploration and potential habitation of both the lunar and Martian subsurface.

Enceladus’s long-lived plume of ice grains and water vapor makes accessing oceanic material readily achievable from orbit (around Saturn or Enceladus) and from the moon’s surface. In preparation for the National Academies of Sciences, Engineering and Medicine 2023–2032 Planetary Science and Astrobiology Decadal Survey, we investigated four architectures capable of collecting and analyzing plume material from orbit and/or on the surface to address the most pressing questions at Enceladus: Is the subsurface ocean inhabited? Why, or why not? Trades specific to these four architectures were studied to allow an evaluation of the science return with respect to investment. The team found that Orbilander, a mission concept that would first orbit and then land on Enceladus, represented the best balance. Orbilander was thus studied at a higher fidelity, including a more detailed science operations plan during both orbital and landed phases, landing site characterization and selection analyses, and landing procedures. The Orbilander mission concept demonstrates that scientifically compelling but resource-conscious Flagship-class missions can be executed in the next decade to search for life at Enceladus.

Snow algae growth and mineral dust deposition are both known to strongly decrease snow albedo. Previous work has shown snow algae growth is enhanced in the presence of trace nutrient-containing minerals. Increased dust deposition with aridification due to climate change may therefore additionally enhance snow algae growth. Here we performed batch growth experiments with a snow alga (Chloromonas brevispina) and bacteria co-culture and the minerals andradite (Ca_3.0(Fe_0.6,Al_0.43)₂(SiO₄)₃), olivine (Mg_1.8Fe_0.2SiO₄), and quartz (SiO₂), to observe differences in snow algae attachment to mineral surfaces with differing iron (Fe) contents. In the presence of olivine, which has a low Fe content (∼9%), the number of snow algae on the surface of the olivine decreased with decreasing concentrations of snow algae in the medium. In contrast, in the presence of andradite, which has a higher Fe content (∼16%), the number of snow algae on the surface of andradite remained high even when concentrations of snow algae in the medium were greatly decreased. Additionally, µX-ray Fluorescence (µXRF) measurements indicate greater formation of Fe-containing precipitates on the mineral surfaces in the presence of the snow algae cells than in abiotic controls. These results show that the snow alga both preferentially grows on Fe-rich surfaces and forms Fe-containing precipitates, indicating the presence of a potential positive feedback system that could amplify the effect of dust deposition on snow albedo and therefore climate change.