Looking for a convenient place to set up a research station on the moon? Look no further than the inner reaches of the moon pits and caves. Although the lack of air will be a problem, new research shows that these underground sanctuaries have constant temperatures that hover around 17 Celsius, or 63 Fahrenheit, even though the surface of the Moon heats up to about 127 C (260 F) during the day and cools to minus 173 C (minus 280 F) at night.
The lunar craters, or lava tubes, were discovered in 2009 by the Lunar Reconnaissance Obiter and the Japanese Kaguya spacecraft. These are deep holes on the moon that can open into huge underground tunnels. They could possibly serve as safe protection from cosmic rays, solar radiation and micrometeorites for future human lunar explorers. But now we know they could provide thermally stable sites for lunar exploration.
These long, winding lava tubes are like the structures we have on Earth. They are created when the top of a flow of molten rock solidifies and the lava inside drains away, leaving a hollow tube of rock. Years before their existence was confirmed, scientists thought there were hints that the moon had lava tubes based on observations of long, winding depressions carved into the lunar surface by the lava flow, called meander shafts.
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Thurston Lava Tube on the Big Island of Hawaii. Courtesy: P. Mouginis-Mark, LPI
About 200 lunar craters have been discovered so far, and at least 16 of them are likely collapsed lava tubes with the potential for “habitable” space, said Tyler Horvath, a doctoral student in planetary sciences at UCLA who led the new study. Two of the most prominent pits have visible overhangs that clearly lead to some sort of cave or void, and there is strong evidence that the overhang of another may also lead to a large cave.
Horvath processed images from the Diviner Lunar Radiometer Experiment — a thermal camera and one of LRO’s six instruments — to find out if the temperature in the pits differs from that on the surface. Diviner was designed to measure temperatures on the moon’s surface, and Horvath’s team had to focus on extremely small areas to get their data.
They focused on a pit found in the Sea of Tranquility (Mare Tranquillitatis). This image below was taken when the Sun was almost directly overhead, illuminating the area. By comparing this image with previous images that have different lighting, scientists can estimate the depth of the pit. They think it is over 100 meters.
This is a spectacular high-sun view of the pit crater Mare Tranquillitatis, revealing an overhang and a deep, dark pit. This image from LRO’s narrow-angle camera is 400 meters (1,312 ft) wide, north is up. Credits: NASA/Goddard/Arizona State University
The researchers used computer modeling to analyze the thermal properties of the rock and lunar dust and to plot the pit’s temperatures over time. Their research, recently published in the journal Geophysical Research Letters, revealed that temperatures in the permanently shaded parts of the pit fluctuate only slightly throughout the lunar day, remaining around 17 C (63 F). If a cave extends from the bottom of the pit, as the images taken by the Lunar Reconnaissance Orbiter camera suggest, it would also have this relatively comfortable temperature. Researchers believe the overhang is responsible for the constant temperature, limiting how hot things get during the day and preventing heat from radiating at night.
However, if this particular pit is to be used as a habitat or research station, there will likely be a heat problem right inside the pit. The sun-baked portion of the pit floor, unprotected by an overhang, reaches daytime temperatures near 150 C (300 F), which is even hotter than the surface of the Moon.
“Because the Tranquillitatis pit is closest to the lunar equator, the illuminated floor at noon is probably the hottest spot on the entire moon,” Horvath said.
Two views of another moon pit in Mare Ingenii. Credit: NASA/GSFC/Arizona State University.
Since a day on the Moon lasts nearly 15 Earth days, the lunar surface is constantly bombarded by sunlight and is often hot enough to boil water. Conversely, equally long lunar nights (also 15 Earth days) reach incredibly low temperatures. Any habitat or base would mean inventing heating and cooling equipment that could operate in those conditions, as well as ways to produce enough energy to power it without interruption. This could prove an insurmountable barrier to lunar exploration or habitation.
However, researchers say building bases in the shadowy parts of these pits allows scientists to focus on other challenges, such as growing food, providing oxygen for astronauts, gathering resources for experiments and expanding the base.
“Humans evolved living in caves, and we may return to caves when we live on the moon,” said UCLA planetary sciences professor David Page, who led the Diviner lunar radiometer experiment and participated in the study.
Additional information: Press releases from UCLA and NASA and the team’s research paper
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