The pits and caves they might lead to would make safer, more thermally stable base camps for lunar exploration and long-term habitation than the rest of the moon’s surface, which heats up to 260 degrees during the day and drops to 280 degrees below zero at night. Pits were first discovered on the moon in 2009, and since then, scientists have wondered if they led to caves that could be explored or used as shelters. About 16 of the more than 200 craters are likely collapsed lava tubes, said Tyler Horvath, a UCLA doctoral student in planetary science who led the new research. Two of the more prominent pits have visible ledges that clearly lead to some sort of cave or void, and there is strong evidence that another ledge may also lead to a large cavern. Lava tubes, also found on Earth, form when molten lava flows under a cold lava field or crust forms over a lava river, leaving a long, hollow tunnel. If the roof of a solidified lava tube collapses, it opens a pit that can lead to the rest of the cave-like tube. Horvath processed images from the Diviner Lunar Radiometer Experiment — a thermal camera and one of six instruments on NASA’s robotic Lunar Reconnaissance Orbiter — to find out if the temperature inside the craters deviates from that on the surface. Focusing on a cylindrical cavity roughly 100 meters deep the length and width of a football field in a region of the moon known as Mare Tranquillitatis, Horvath and his colleagues used computer modeling to analyze the thermal properties of lunar rock and dust. and record pit temperatures over a period of time. The results, recently published in the journal Geophysical Research Letters, revealed that temperatures inside the permanently shadowed areas of the pit fluctuate only slightly during the lunar day, staying at about 63 degrees. If a cave extends from the bottom of the pit, as images taken by the Lunar Reconnaissance Orbiter’s camera show, it would also have this relatively comfortable temperature. The research team, which also included UCLA planetary science professor David Paige and Paul Hayne of the University of Colorado Boulder, believes the shadow overhang is responsible for the constant temperature, limiting how hot things get during the day. and preventing heat from radiating away. Night. Meanwhile, the sunlit part of the pit’s floor hits daytime temperatures near 300 degrees, about 40 degrees higher than the moon’s surface. “Because the Tranquillitatis crater is the closest to the lunar equator, the illuminated floor at noon is probably the hottest place on the entire moon,” Horvath said. A day on the moon lasts nearly 15 Earth days, during which the surface is constantly bombarded by sunlight and is often hot enough to boil water. The unimaginably cold nights also last about 15 Earth days. Inventing heating and cooling equipment that can operate under these conditions and generate enough energy to power it non-stop could prove an insurmountable obstacle to lunar exploration or habitation. Solar power — NASA’s most common form of power generation — doesn’t work at night, after all. (NASA currently has no plans to build an exploration base camp or habitats on the moon.) Building bases in the shadowed parts of these pits allows scientists to focus on other challenges, such as growing food, providing oxygen for the astronauts, gathering resources for experiments, and expanding the base. Pits or caves will also provide some protection from cosmic rays, solar radiation, and micrometeorites. “Humans evolved by living in caves, and we may go back to caves when we live on the moon,” said Paige, who leads the Diviner Lunar Radiometer Experiment. Diviner has been mapping the moon continuously since 2009, producing NASA’s second-largest planetary data set and providing the most detailed and comprehensive thermal measurements of any object in our solar system, including Earth. The team’s current work on lunar craters has improved the data from the Diviner experiment. “Because no one else had looked at such small things with the Diviner, we found that it had a bit of double vision, making all our maps a bit blurry,” Horvath said. The team worked to align the many images taken by the instrument until they could achieve an accurate thermal reading down to the level of a single pixel. This process yielded much higher resolution maps of the moon’s surface. Data from the early stages of this lunar crater thermal modeling project were used to help develop the rover’s thermal management system for NASA’s proposed Moon Diver mission. Horvath and Hayne were part of the science team for this mission, which aims to rappel the rover into Tranquillitatis crater to investigate the layers of lava flows seen on its walls and to explore any existing caverns. Horvath and Paige are part of the science team for a new lunar docking thermal camera led by Paul Hayne called L-CIRIS, which will head to the lunar south pole in late 2023 to take the first thermal images ground.
