In October 2009, the upper stage of NASA’s Lunar Crater Observation and Sensing Satellite (LCROSS) mission plummeted into the lunar south pole, spreading a cloud of debris across the surface. After the dust settled, all trained eyes turned to the Moon and found exactly what they hoped for: water ice and other volatiles.
Since then, NASA has confirmed plans to send rovers to this polar region and take a closer look to gain insight into the evolution of the solar system and potential for future crewed missions.
Among the shaky politics surrounding NASA missions, a recent paper published in the Journal of Geophysical Research: Planets provided some much needed solid ground. The study, conducted by an international team of planetary scientists, used images from the Lunar Reconnaissance Orbiter Camera (LROC) to conclude that the proposed landing sites are compact enough to support rover wheels with a radius of at least 30 centimeters.
David Kring, principal scientist at the Universities Space Research Association’s Lunar and Planetary Institute in Houston and head researcher of the study, said, “The study’s most important result is that we can explore the regions and measure, in situ, the abundance of ices.”
Permanently Shadowed Regions
The landing sites under scrutiny here are some of the most elusive craters on the Moon’s southern pole, known as permanently shadowed regions (PSRs), where the Sun hasn’t shone for billions of years owing to the Moon’s tiny axial tilt.
By studying the tracks and dimensions of boulders that had tumbled into these PSRs, the researchers calculated physical properties of the lunar surface and also suggested the presence of water ice within the cracks of the topsoil, or regolith, on the Moon.
Tumbling into Darkness
Without direct in situ access to the Moon, lunar scientists have had to come up with remote methods to understand its history, mainly by focusing on its most ubiquitous features: craters and boulders. Boulders, which can be seen everywhere on the Moon, range anywhere from 1 to 31 meters in diameter. They have been used to date craters, record moonquakes, and, in this study’s case, infer the physical properties of the regolith on the lunar surface.
“What I like about our method is that it was used pre-Apollo,” said Hannah Sargeant, a Ph.D. researcher at The Open University in Milton Keynes in the United Kingdom and lead author of the study. “It was really cool to revive this method.”
The tracks that boulders make as they traverse the lunar surface reveal much more than their path. Characteristics like track depth, width, and slope can be mathematically combined to calculate the “bearing capacity” of the regolith, a physical property that describes how much boulders (and rover wheels) sink into the lunar surface. Using the highest-resolution images available from the LROC data, Sargeant and her colleagues did exactly this for the tracks of 13 boulders that had tumbled into PSRs.
By comparing the track properties inside and outside the PSRs, the researchers determined how traversable the regolith within the dark craters is compared to the normal, sunlit regions of the Moon. And what they found was very promising: The bearing capacity of the regolith within PSRs is roughly the same as the highland and mare surfaces that the Apollo missions successfully touched down on decades ago.
Sources of Friction
As promising as these results may be, they come after many years of less promising research. Lab simulations, data from the LCROSS orbiter, and thermal measurements had all concluded that the regolith at these polar regions was highly porous and likely too loose for rovers to safely traverse. The consensus had been that PSRs did not experience enough variations in sunlight to cycle around and thus compact the regolith for a rover to safely traverse them.
This new study suggests that water ice may fill in the cracks of the regolith and make it sturdier, thus reconciling the high-porosity measurements with the newer bearing capacity values.
However, as all planetary scientists would say, we need to collect samples in situ to really know. As Philip Metzger, a planetary scientist at the Florida Space Institute in Orlando not associated with the study, puts it, “We need to get back to the Moon and its polar regions to get ground truth on the soil properties.”
Although the researchers couldn’t study the pitch-dark PSRs at the extreme north and south polar regions that NASA and other space agencies have their eyes on, they could study similar regions at low latitudes between 70° and 75° with a tiny bit of reflected sunlight to work with. These regions aren’t expected to have as much water ice as the southernmost PSRs, but their surficial properties are expected to be similar.
After many years of doubt, researchers at NASA’s Johnson Space Center in Houston were “infused” with hope for future missions after hearing the results of this new study, according to Sargeant.
NASA’s first mission to the PSRs, Volatiles Investigating Polar Exploration Rover (VIPER), is currently under construction at Johnson Space Center and slated to launch in 2022.
—Christian Fogerty (@ChristianFoger1), Science Writer