Lunar Dust Toxicity: Regolith Simulant Suitability for Biological Research

The return of mankind to the Moon seems inevitable. Many nations have set their sights on reaching the lunar surface in the near future, and establishing a foothold there will hopefully be a steppingstone to excursions further out into the solar system. While on the Moon, a primary issue we will need to contend with is the ground up regolith covering every last bit of surface. The constant meteoroid bombardment, harsh solar and galactic radiation and vacuum of space has turned the lunar bedrock into a mass of peculiar particles: lunar dust. The frequent impacts have sufficient energy to partially melt the material, causing fragments to be glassbonded together in agglutinates, can send material flying great distances, and continuously churn and grind down material. Particle sizes vary, but many tiny grains, within the respirable (< 2 µm) range exist. At the molecular level, the dust particles exhibit peculiar morphological features, giving them a large surface area and conveying high reactivity. These include sharp edges, vesicles, glass rims, nanophasic iron and dangling bonds. The changing plasma environment on the Moon, varying with the solar wind and passage through the Earth’s magnetotail, as well as the lunar diurnal cycle, charges the surface and regolith, and can make particles levitate electrostatically, up to great heights. Because of its ubiquitous and reactive nature, lunar dust has been found to impact every human activity performed on the moon, and has managed to permeate every spacecraft that has come into contact with it. Furthermore, it has been found to be a substance of grave concern. It can abrade seals, scratch displays, obscure vision, coat heat-transfer surfaces and cause respiratory discomfort. The United States National Aeronautics and Space Administration (NASA) has listed it as a key obstacle to be overcome before the lunar surface can be revisited for extended periods. The regolith can also be a resource, however. It could serve as a structural building material, can be harvested for key elements like O2, metals and water, and could be used to fabricate solar panels or soil to enable life-support systems. It is clear that lunar dust is of key importance to our efforts in returning to moon. Studies of lunar dust, ranging from its potential as in-situ resource utilization (ISRU) resource to its toxic properties are therefore ongoing and increasing in number. Performing these studies directly on lunar material is not possible due to its scarcity. Instead, for decades, we have been making use of lunar dust simulants which mimic properties of actual lunar regolith. Due to the very specific characteristics of lunar dust, it is not feasible to create a cost-effective simulant that can be used for all types of investigations. Instead, the scientific community is searching for a root simulant which could be mass produced at low cost with high fidelity to then be amended with various additives depending on the desired experimental setups. In this thesis we have investigated the potential of two lunar dust simulants, “LHS-1” and “LMS1”, produced by Exolith lab, to be used in biological research. There are many exposure routes through which living organisms might come into contact with lunar dust in a lunar habitat. Of gravest concern are the immediate effects on human astronauts, which could see first and foremost their lungs impacted by breathing in airborne dust. Exposure through eyes, skin, or the gastrointestinal system are also possible, however. In addition, other organisms, such as the biological component parts of closed loop life-support systems could also be negatively affected by contact with the regolith. Lunar dust (simulant) toxicity research has been done for various organisms and tissue types, both in vitro and in vivo, and has so far mostly indicated that the material behaves as terrestrial particulate pollutants. MASTER OF SPACE STUDIES v KU LEUVEN/UGENT In our experiments we have focused on the effects that the Exolith simulants might have on human alveolar A549 cells. We have exposed them to various doses of LHS-1 and LMS-1, for differing durations. Subsequently we have evaluated cytotoxicity, growth, intracellular reactive oxygen species (ROS) and ROS sensitivity and cytokine expression. In addition, we have performed limited SEM investigations of our simulants, to have a look at morphological features. As a benchmark during these activities, we have included the terrestrial particulate toxicant quartz alongside the Exolith simulants. We find that LHS-1 and LMS-1 adequately mimic the particle size and composition of true lunar regolith, but lack certain morphological features that are unique to lunar dust. The simulants do not contain agglutinates or nanophasic iron, for example. Despite these shortcomings, the material still appears to elicit cell responses that are in line with those we would expect for actual lunar dust. A549 cells were found to be less viable, grow more slowly, contain more intracellular ROS and express more pro-inflammatory cytokines compared to controls, after treatment with the simulants. Quartz was found to produce less effects, but this is likely due to the fact that the used quartz sample had a particle size that was far greater than that of our simulants. In order to improve and refine our findings, and characterize the severity and dose-response of the effects, more research could be done. Experimental procedures could be repeated to increase the sample size, and some modifications could be made to further facilitate interpretation. The simulant material could be activated by grinding or milling before the experiments, thus producing dangling bonds that would increase reactivity by mimicking the unsatisfied valences and ions present on lunar dust in the lunar vacuum. In addition, specific other simulants could be included in our experiments, containing morphological features that are lacking in LHS-1 and LMS-1, such as agglutinates or nanophasic iron, in order to improve our understanding of whether these features are truly necessary to facilitate biological research. Lastly, the experimental base could be expanded to other cell types or organisms to be able to make statements about the suitability of Exolith simulants for broader biological research. While doing so, it could be useful to pre-sieve the material to a particle size that is relevant to the tissue being tested in order to obtain more specific results. Overall, we can conclude that the Exolith simulants are relatively cheap and easily obtained, mimic lunar dust to an appreciable degree, and based on our initial results, seem to produce toxic effects in line with what would be expected from true lunar regolith. However, more work should be done to characterize the severity and specificity of these findings