A breakthrough study reveals pits and caves on the Moon to call 'home'

To find out if the temperature within the pits diverged from those on the surface, Horvath processed images from the Diviner Lunar Radiometer Experiment - a thermal camera and one of six instruments on NASA’s robotic Lunar Reconnaissance Orbiter.

The results that were recently published in the journal Geophysical Research Letters, revealed that temperatures within the permanently shadowed reaches of the pit fluctuate only slightly throughout the lunar day, remaining at around 63 degrees. And if a cave extends from the bottom of the put, it too would have a comfortable temperature.

Hottest days and coldest nights

Horvath and his colleagues, UCLA professor of planetary science David Paige and Paul Hayne of the University of Colorado Boulder, focused on a roughly cylindrical 100-meter–deep depression about the length and width of a football field in an area of the moon known as 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 a period of time.

The research team believes the shadowing overhang in a cave is responsible for the stable temperature, which prevents heat from radiating away at night. Contrastingly, the pit floor hits daytime temperatures close to 300 degrees, some 40 degrees hotter than the moon’s surface.

"Because the Tranquillitatis pit is the closest to the lunar equator, the illuminated floor at noon is probably the hottest place on the entire moon," said Horvath.

Why is this discovery important?

A day on the Moon lasts almost 15 Earth days, during which the temperature can be hot enough to boil water. Unimaginably cold nights also last about 15 Earth days. Conceptualizing and inventing heating and cooling equipment that can operate under such circumstances and producing enough energy to sustain it would be an enormous barrier to lunar exploration and habitation. Meanwhile, NASA's most common form of power generation, which is solar power, doesn't work at night.

And to caves we return

Building 'homes' in these pits will allow scientists to focus on other challenges like growing food, providing oxygen for astronauts, gathering resources for experiments and expanding the base. The pits or caves would also offer some protection from cosmic rays, solar radiation and micrometeorites.

"Humans evolved living in caves, and to caves we might return 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 dataset. The team’s current work on lunar pits has improved data from the Diviner experiment.

"Because nobody else had looked at things this small with Diviner, we found that it had a bit of double vision, causing all of our maps to a be a bit blurry," said Horvath. The team had 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 eventually yielded much higher resolution maps of the moon’s surface.

Data from the early stages of this lunar pit thermal modeling project helped develop the thermal management system of the rover for NASA’s proposed Moon Diver mission. Horvath and Hayne were part of the science team for this mission, which aims to have the rover rappel into the Tranquillitatis pit to research the layers of lava flows seen in its walls and to explore any existing cave.

Abstract:

Lunar collapse pits may provide access to subsurface lava tubes of unknown extent. We present Diviner Lunar Radiometer measurements showing that the Mare Tranquillitatis and Mare Ingenii pits exhibit elevated thermal emission during the night, ∼100 K warmer than the surrounding surface. Using these data, along with computational thermophysical models, we characterize the thermal environment inside pits and potential caves. Near the equator, peak day-time temperatures on regolith-covered pit floors can potentially reach >420 K, whereas temperatures beyond the opening in permanent shadow would maintain a nearly constant temperature of ∼290 K, similar to that of a blackbody cavity in radiative equilibrium. Thermal IR measurements such as those of Diviner can readily detect pit thermal signatures but would be insensitive to the existence of caves they may host, as the latter would only induce a 0.1 K increase to night-time temperatures of the overlying surface.