Posted on by Evan Gough

A Rover Could Weave its Way Between Patches of Sunlight on the Lunar South Pole

In any plan to establish a presence on the Moon, the South Pole is key. There, in the deep permanent shadows of the region’s craters, are voluminous quantities of water ice. And water ice means water, oxygen, and even rocket fuel.

But the region is shrouded in shadows.

Science has advanced considerably in the decades since the Apollo era, the last time astronauts walked on the Moon. Soon NASA’s Artemis mission will bring another generation of explorers to the lunar surface. But while the Apollo astronauts faced many unknowns on their missions, including the fear that their landers could sink into the dust, we now have a much clearer understanding of the lunar environment.

This photograph of the Apollo 11 Lunar Module Eagle shows the three sensor probes on three of the four landing pods. They told the crew when the Eagle was safely on the Moon and when the descent engine could be shut down. They were 67.2-inch (1.71 m) long because nobody was certain if the Eagle would sink into a thick layer of fine Lunar dust. (There were originally four probes, but they tended to bend, so the one next to the ladder was removed because of the risk of puncturing a spacesuit.) Image Credit: NASA.
This photograph of the Apollo 11 Lunar Module Eagle shows the three sensor probes on three of the four landing pods. They told the crew when the Eagle was safely on the Moon and when the descent engine could be shut down. They were 67.2-inch (1.71 m) long because nobody was certain if the Eagle would sink into a thick layer of fine Lunar dust. (There were originally four probes, but they tended to bend, so the one next to the ladder was removed because of the risk of puncturing a spacesuit.) Image Credit: NASA.

We’ve mapped the surface of the Moon in detail, and we know how much water ice is there and where it’s located. A 2018 study based on lunar orbiter data showed that the Moon holds vast quantities of water in its shadowy polar craters. When that study was published, NASA Administrator Jim Bridenstine said, “We know that there’s <sic> hundreds of billions of tons of water ice on the surface of the moon.”

There are billions of tons of water ice at the lunar poles, with the bulk of it at the south pole. Image Credit: NASA/ Shuai Li.
There are hundreds of billions of tons of water ice at the lunar poles, with the bulk of it at the south pole. Image Credit: NASA/ Shuai Li.

We also know where minerals are located and that some regions on the Moon are 10 times richer in titanium than Earth rocks. We know there’s abundant iron and rich sources of aluminum, silicon, and magnesium. We’ve also mapped the lunar KREEP terrain that holds rare-Earth elements that are important in electronics manufacturing.

This image of the Moon shows the near side (left) and the far side (right.) It shows thorium concentrations which indicate where the KREEP terrain is. Image Credit: By NASA – http://solarsystem.nasa.gov/multimedia/display.cfm?Category=Planets&IM_ID=13643, Public Domain, https://commons.wikimedia.org/w/index.php?curid=32868958

We have topographic maps that allow us to plot pathways that rovers can follow. We know where safe landing spots are and where the most intriguing locations for exploration are. The sunlit areas of the Moon are open for exploration by solar-powered rovers, though they’d have to contend with two-week periods of darkness.

But the poles are a different matter. Unlike Earth, with its axial tilt of 23.4 degrees and its concomitant seasons, the Moon is seasonless. With an axial tilt of only about 1.5 degrees, the Moon basically experiences one continuous season. Unlike Earth’s polar regions, the Moon’s polar regions have no well-lit winter to aid exploration by a solar-powered rover. But though the variations at the equator are minor, they’re more pronounced at the poles.

Because of the Sun’s angle at the south pole, the lower elevation in craters is almost continuously dark, while high elevation regions are almost continuously lit. The problem is the high-elevation areas are not smooth terrain easily navigated by a rover. But a new study based on detailed lunar maps shows how a clever string of routes and traverses could open up the lunar south pole to exploration by rover.

The study is “Sunlit Pathways between South Pole Sites of Interest for Lunar Exploration.” The paper will be published in the journal Acta Astronautica. The authors are from NASA’s Goddard Space Flight Centre, the NASA Johnson Space Centre, NASA Headquarters, and Jacobs Technology in Houston, Texas. The lead author is Erwan Mazarico from Goddard.

The Moon's axial tilt is only about 1.5 degrees, so it doesn't have seasons. The polar regions see very little sunlight in their deep craters. Image Credit: NASA/Richard Pappa, Geoff Rose, Dave Paddock, and Roger Lepsch.
The Moon’s axial tilt is only about 1.5 degrees, so it doesn’t have seasons. The polar regions see very little sunlight in their deep craters. Image Credit: NASA/Richard Pappa, Geoff Rose, Dave Paddock,
and Roger Lepsch.

NASA’s upcoming Artemis Moon missions are designed not only to explore the Moon but to understand how it can be used as a base for further exploration of the Solar System. Launching everything needed for Solar System exploration from Earth is cumbersome and expensive. If we can use the resources on the Moon, then we can more effectively explore the rest of the Solar System.

Another Artemis goal is to use the Moon as a proving ground to develop technologies that will further NASA’s exploration goals. The development of expertise goes hand in hand with that, as does making cutting-edge discoveries along the way.

A critical piece in this endeavour is water. From the Moon’s water ice, we get oxygen for breathing, hydrogen for fuel, and of course, water itself. Since the bulk of the Moon’s water ice is at the south pole, it’s critical that NASA opens up the region to exploration. But the deep shadows that are responsible for all that coveted water are also the obstacles to its exploitation.

Almost everything about Artemis requires mobility. Acquiring samples, curating them, deploying instruments, and all other activities require mobile rovers. Mars rovers like Curiosity and Perseverance don’t need solar power. They have MMRTGs— Multi-Mission Radioisotope Thermoelectric Generators. But MMRTGs are expensive and cost over $100 million to build. Their fuel—Plutonium 238—is also complicated and expensive to produce. And it’s difficult to stockpile because it decays.

There’s ample solar energy available on the Moon because it’s so close to the Sun. Scientists have identified elevated locations at the south pole that are free from shadows and could provide energy. Their favourable positioning in regard to solar power makes them strategic positions for exploration. The question is, how accessible are they? “However, it has not been previously established whether travel between these sites is possible, given the complexity of the lighting conditions and dynamic shadows cast by distant topography sweeping through the area,” the authors of the new paper write.

This image from the Lunar and Planetary Institute shows the permanently shadowed regions in craters on the Moon's south pole. Image Credit: Stopar J. and Meyer H. (2019) Topography and Permanently Shaded Regions (PSRs) of the Moon’s South Pole (80°S to Pole), Lunar and Planetary Institute Regional Planetary Image Facility, LPI Contribution 2170
This image from the Lunar and Planetary Institute shows the permanently shadowed regions in craters on the Moon’s south pole. Image Credit: Stopar J. and Meyer H. (2019) Topography and Permanently Shaded Regions (PSRs) of the Moon’s South Pole (80°S to Pole), Lunar and Planetary Institute Regional Planetary Image Facility, LPI Contribution 2170

That’s where this new paper comes in. This study points out how finely-planned routes through the lunar south pole could eke out enough sunlight to traverse the region. “The goal of our study is to demonstrate, with minimal assumptions about the capabilities of the rover, that trips exist between selected distant well-lit sites,” they explain.

The researchers identified feasible “trips” a rover could follow that minimize the time spent in shadow, cut off from solar power. They point out that more challenging trips might be possible depending on rover design. For example, higher speeds or the capability to survive darkness or even drive in the dark might expand these trips.

The team based their trips on data from the Lunar Orbiter Laser Altimeter (LOLA) instrument onboard the
Lunar Reconnaissance Orbiter (LRO) spacecraft. They used software to find the routes, and the software “… computes the least-cost path from a source to a destination, considering the route length, the terrain slope, and the terrain average solar illumination,” they explain.

The study is based on prior research that identified four highly-illuminated regions at the lunar south pole. In this paper, the researchers created four distinct paths between the regions.

LOLA topographic maps of the south pole region shown in polar stereographic projection; the South Pole appears at coordinates (0,0). The colour indicates the elevation with respect to the 1737.4-km reference sphere. The paths are shown in black. Image Credit: Mazarico et al. 2023.
LOLA topographic maps of the south pole region shown in polar stereographic projection; the South
Pole appears at coordinates (0,0). The colour indicates the elevation with respect to the 1737.4-km reference sphere. The paths are shown in black. Image Credit: Mazarico et al. 2023.

The paths are very detailed and sometimes require waiting periods in sunlit areas. These periods could be used to recharge batteries or perform local exploration while waiting for the next trip to emerge from the shadows. “It can be advantageous at times to make a ‘stop’ to recharge or to wait out a (current or upcoming) shadow along the path,” the authors explain. “The duration of such segments can be rather long (days), and exploration of the local area may be possible during that time.” Typically, some of the areas around the path are sunlit and open to exploration while the rover waits for the next leg of its journey.

This animation shows Path One, Connecting Ridge to de Gerlache. The horizontal sections in the left panel represent ‘stops’ in the path.

The authors point out that these might not be the most optimal traverses between points at the south pole. Instead, they demonstrate the existence of mostly-sunlit paths. The paths account for “… the extreme time-variable illumination conditions at the lunar south pole.”

They also point out that exact routes will be a combination of optimal sunlight and rover design, with specific scientific points of interest rounding them out. In this work, they avoided or at least minimized travel along shadowed terrain. That meant that some of the trips were of long duration, around 30 days. Depending on the rover and the mission design, there may be more flexibility in the actual paths.

This animation shows an example of a trip from the Connecting Ridge to the Slater rim along the predetermined path 2.

This work doesn’t outline any specific hazards in these trips but acknowledges that hazards could create significant problems and delays. “As with any trip between the few and sparse high-illuminated sites, there would be inherent risks in undertaking these trips, where any delay or anomaly may result in going through many days of darkness, which may not be survivable depending on the rover design,” they write.

Though the Moon’s equatorial regions don’t have much seasonal variability of sunlight, the south pole does. The team explains that these routes are summer routes. “These trips are also clearly limited to the summer seasons, as the paths are generally mostly dark during winter months,” they write in their paper. “The peak summer season could thus be thought of as the migration season from one site to another, where the asset would ‘winter over’ at one of these highly-illuminated sites with minimal darkness durations of a few days.” The rovers would share something in common with Earth’s early people, who migrated with the seasons in order to survive.

This animation shows an example of a trip from the Slater rim to the Connecting Ridge along the predetermined path 3.

Round trips between two sites might not be possible in a single year. Instead, they could be part of a multi-year exploration mission, watched from above by lunar orbiters. Again, rover design would be critical because the ability to withstand long periods of dark while immobile, or even to travel in the dark, would enable shorter trips and make the entire endeavour more flexible.

The way the trips are outlined here, high-speed travel isn’t necessary, but it could also add to mission flexibility. High speed, when combined with shadow survivability or travelling in the dark, could open up the south pole to more ambitious exploration.

This animation shows an example of a trip from the de Gerlache rim to the Malapert massif along the predetermined path 4.

NASA Artemis program is focused on the lunar south pole for a variety of regions. Some of the shadowy regions in deep south pole craters haven’t seen the Sun for billions of years. So there’s more than just water ice there.

“The ability to extract deep core samples and maintain their temperature and vacuum properties all the way back to research facilities on Earth could lead to powerful discoveries—not only about the volatiles but also about the history of our solar system,” said Jake Bleacher, NASA’s Chief Exploration Scientist at NASA Headquarters, in an August 2022 press release.

Hidden in those primordial shadows are clues to the history of the Moon, the Earth, and the Solar System, and important clues to our own place in the Universe. There are exciting discoveries waiting there.

This research helps show us the way.

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