Lunar Pits Stay Warmer than Surface During Night, Finds New Study

Lunar pits, and caves to which they may lead, would make thermally stable sites — around a comfortable 17 degrees Celsius (63 degrees Fahrenheit) — for lunar exploration compared to areas at the Moon’s surface, which heat up to 127 degrees Celsius (260 degrees Fahrenheit) during the day and cool to minus 173 degrees Celsius (minus 260 degrees Fahrenheit) at night.

Temperature measurements for the Mare Tranquillitatis pit. Image credit: Horvath et al., doi: 10.1029/2022GL099710.

Pit craters are formed from the collapsed ceilings of subsurface void spaces, such as natural caves or lava tubes.

Collapse pits have been found on every major rocky body of the inner Solar System. However, distinct overhangs have only been observed on Earth and the Moon.

They are typically the result of a structural instability triggered by seismic activity, tectonism, and/or impacts.

Since the discovery of lunar pits by JAXA’s SELENE spacecraft in 2009, there has been interest in whether they provide access to caves that could be explored by rovers and astronauts.

“About 16 of the more than 200 pits are probably collapsed lava tubes,” said lead author Tyler Horvath, a doctoral student at the University of California, Los Angeles.

Two of the most prominent lunar pits have visible overhangs that clearly lead to caves or voids, and there is strong evidence that another’s overhang may also lead to a large cave.

“Humans evolved living in caves, and to caves we might return when we live on the Moon,” said co-author Dr. David Paige, a researcher in the Department of Earth, Planetary, and Space Sciences at the University of California, Los Angeles.

To find out if the temperature within the lunar pits diverged from those on the surface, the study authors analyzed data from the Diviner Lunar Radiometer Experiment aboard NASA’s Lunar Reconnaissance Orbiter (LRO).

Focusing on a roughly cylindrical 100-m- (328- foot) deep depression in the Mare Tranquillitatis, they used computer modeling to analyze the thermal properties of the rock and lunar dust and to chart the pit’s temperatures over time.

Their results revealed that temperatures within the permanently shadowed reaches of the pit fluctuate only slightly throughout the lunar day, remaining at around 17 degrees Celsius (63 degrees Fahrenheit).

If a cave extends from the bottom of the pit, as images taken by LRO’s Lunar Reconnaissance Orbiter Camera suggest, it too would have this relatively comfortable temperature.

“The shadowing overhang is responsible for the steady temperature, limiting how hot things gets during the day and preventing heat from radiating away at night,” the researchers said.

“A day on the Moon lasts about 15 Earth days, during which the surface is constantly bombarded by sunlight and is frequently hot enough to boil water. Brutally cold nights also last about 15 Earth days.”

The team’s paper was published in the journal Geophysical Research Letters.

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Tyler Horvath et al. Thermal and Illumination Environments of Lunar Pits and Caves: Models and Observations From the Diviner Lunar Radiometer Experiment. Geophysical Research Letters, published online July 8, 2022; doi: 10.1029/2022GL099710

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