Past lunar missions have detected evidence of large ice deposits in permanently shadowed regions near the Moon’s south pole. Such ice could provide astronauts with drinking water, oxygen, and rocket propellants, reducing the cost of lunar operations.
But new research has found that astronauts might not have to dig very deep or journey especially close to the Moon’s poles to find water ice. A recent study published in Communications Earth and Environment says the critical resource for future lunar explorers might lurk tantalizingly close to the surface on pole-facing slopes at lower latitudes. The Sun shines at a low angle on such regions, which may allow ice to accumulate just centimeters below the surface, where it would be insulated by lunar regolith.
The Moon’s low axial tilt means that craters and low-lying areas near the south pole never see direct sunlight. This lack of sunlight would allow even surface ice deposits to remain frozen for a long time—perhaps billions of years. Because of the likely presence of ice, both NASA and China’s space agency have announced plans to land astronauts near the south pole and eventually establish permanent outposts there.
Locations farther from the poles “can also become potential locations for future human habitats, with better illumination and smoother topography than the poles. These regions pose less technical challenges for landing and operations.”
“Our study reveals that the poles are not the only options for future exploration,” said K. Durga Prasad, lead author of the report and a planetary scientist at the Physical Research Laboratory in Ahmedabad, India. Locations farther from the poles “can also become potential locations for future human habitats, with better illumination and smoother topography than the poles. These regions pose less technical challenges for landing and operations.”
“This result is very much consistent with both theoretical modeling studies and observations made by Lunar Reconnaissance Orbiter,” said Timothy McClanahan, an emeritus planetary scientist at NASA’s Goddard Space Flight Center who was not involved with the study.
First Measurements Since Apollo
The new lunar temperature data come from Chandra’s Surface Thermophysical Experiment (ChaSTE), an instrument aboard the Vikram lander, which itself was part of India’s Chandrayaan-3 mission. Vikram touched down on 23 August 2023 at 69° south latitude, the most southerly landing site at that time. (Two subsequent landers, both built by the American company Intuitive Machines, landed farther south, but both tipped over on landing and were unable to achieve all of their science goals.)
ChaSTE collected data continuously from 24 August to 2 September, shortly before the Sun set on the solar-powered lander (a lunar day lasts about 29.5 Earth days). The probe penetrated 10 centimeters into the regolith, with temperature sensors spaced at 1-centimeter intervals. The instrument also heated the regolith to measure its thermal conductivity.
ChaSTE provided the first direct subsurface lunar temperature measurements since the Apollo 15 and 17 missions of the early 1970s. The Apollo heat probes drilled deeper than ChaSTE did but provided fewer measurements of the top 10 centimeters. The Apollo sites also were close to the equator, where temperatures are likely to remain too warm for water ice even well below the surface, Prasad said.
Vikram landed on the rim of a shallow crater in a Sun-facing area with a 6° slope. ChaSTE and other instruments aboard the lander recorded a peak daytime surface temperature of 355 K (81.85°C). That was higher than expected on the basis of both models and observations by Diviner, an infrared instrument aboard the Lunar Reconnaissance Orbiter that has compiled temperature maps of much of the lunar surface. (Temperatures at the probe’s maximum depth ranged from 55 K to 85 K colder than surface temperatures, depending on the time of day.)
However, the temperature on a flat area just 1 meter from the ChaSTE site peaked at only 332 K (58.85°C), suggesting that a location’s slope could play a significant role in its subsurface temperatures.
The findings “validated the idea that topographic variation, even toward meter scales, has an important impact on locations where we might expect water ice to occur,” McClanahan said.
Taking the Right Angle
Modeling showed that at high latitudes, poleward-angled slopes of 14° or greater could remain cold enough to preserve ice at depths of just a few centimeters. The Sun would hit such tilted regions at a low angle, minimizing heating, and the fine-grained top layer of the regolith would be an efficient thermal blanket, effectively insulating the shallow subsurface.
“Depending on the slope, you can have a lot of temperature variation even in craters as small as a meter. One side might be quite warm, but…you could have conditions that are suitable for water ice on the poleward-facing slope.”
“Depending on the slope, you can have a lot of temperature variation even in craters as small as a meter,” McClanahan said. “One side might be quite warm, but given the low thermal conductivity of the regolith, you could have conditions that are suitable for water ice on the poleward-facing slope.” The slope angle suitable for hosting ice increases as you move farther from the pole, he added.
The Vikram team is continuing to analyze the ChaSTE observations to learn more about the thermal characteristics of the landing site and of high lunar latitudes in general, Prasad said. In addition, because temperatures are important for any lunar lander, “future missions will definitely carry similar instruments that will also help substantiate our results,” he said.
—Damond Benningfield, Science Writer
