Science1 publisher2 min readPublished
Ray tracing finds week-long microbe refuges in the shadows at Artemis IV's candidate landing sites
Researchers at NASA Goddard and the University of Maryland mapped polar sunlight down to five metres per pixel and found small shadows where five skin-borne bacteria and fungi could sit dormant for days.
The Scientist · Science desk
What happened
- Prabal Saxena of NASA Goddard and Stefano Bertone of the University of Maryland modelled how five bacteria and fungi found on human skin would fare at three candidate Artemis IV landing sites.
- The team screened for spots where a microorganism could last at least 24 hours, and the simulations returned niches where the organisms could survive up to or potentially beyond seven days.
- Aspergillus, the most UV-resistant of the group, could survive across 15 to 30 per cent of the areas assessed, including some that receive sunlight during the lunar winter.
- Bacillus, Deinococcus, Staphylococcus, Aspergillus and Fusarium could all survive in parts of the permanently shaded De Gerlache Rim, even with scattered UV light added to the model.
- Saxena stresses that the survival modelled is dormancy, a cryptobiotic state, and not growth, which would require active metabolism and possible reproduction.
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Why it matters
- constraint Sterilising hardware before launch does not address the main source, because the crew carry millions of microbes on their skin and more inside, so any control has to be exercised in how the surface mission is planned.
- decision The cold, shadowed ground that can hold water ice is the same ground this model flags for microbial persistence, so ice access and contamination control now compete over the same terrain in site selection.
- exposure The south pole is also under consideration as a site for a human outpost. An outpost would keep a source of Earth organisms on that ground continuously instead of for one visit.
Geometry is what separates the poles from the ground crews have already walked. The Moon's tilt to the Sun is 1.5 degrees, so at high latitude the Sun stays low and the shadows stay where they are [4]. "Since the Moon has a very small axial tilt, the Sun appears to hover just above the horizon here, so even small hills and rocks can prevent light from reaching lower-lying ground, creating small, shadowed areas protected from UV rays," Saxena said [5]. Every previous crewed landing went to equatorial terrain, where high ultraviolet flux and temperature make the surface hard on microorganisms [21].
Which stressor to model finely followed from that. "Since the temperatures at the lunar poles are rarely high enough to kill bacteria and low temperatures can preserve microbes on Earth, UV radiation is generally likely to be more dangerous in these regions," Saxena said [6].
"We therefore, analysed UV fluxes at finer spatial scales using a technique called ray tracing, and for some regions, included optical effects such as reflection and refraction as sunlight hits the Moon's surface, leveraging updated LOLA-based topography maps on a scale of 5 m/pixel," he said [7]. The regional pass had used averaged illumination maps at 60 m per pixel, covering UV at 320 nm and shorter [9]. Going from 60 m to 5 m is twelve times finer on a side, and it resolves a shaded patch 144 times smaller in area [19].
The thermal side is coarser. Seasonal temperatures came from the Lunar Reconnaissance Orbiter's Diviner instrument on 240 m length scales [8], so a single temperature cell spans roughly 2,300 of the smallest UV cells [20]. The refuges the ray tracing picks out are smaller than the grid on which their temperature is specified [22].
The fungal percentage is measured against the terrain the team assessed inside the candidate landing sites, not against the polar region [3]. And the whole result is a simulation assembled from orbital topography, illumination and thermal data; the Physics World account of the work does not describe any test of the predictions on the Moon itself [23].
The researchers say crewed missions to the lunar south pole will have to be carefully thought out to limit the unintended transfer of Earth life forms [16].
What to watch
- Whether a peer-reviewed version publishes site-by-site survivability maps and the UV dose thresholds assumed for each organism.
- Whether NASA turns the modelled refuges into site-selection or traverse guidance for Artemis IV's landing choice.
- Whether anyone flies an exposure experiment to check the seven-day figure against real polar shadow.