Some of the moon’s craters may contain “double-shadowed” regions that are so dark they would be among the coldest places in the solar system.
The small tilt of the moon – just 1.5 degrees – as it orbits with Earth around the sun means that it has hundreds of craters where direct sunlight never reaches. We know that inside these craters, located near the moon’s poles, temperatures can drop below -170°C, making them prime locations for water ice to collect and optimum locations for future human missions, as future astronauts could use the ice as a source of water for their missions.
Even though the insides of these craters don’t receive direct sunlight, they can be heated by sunlight reflecting off their rims, which can melt some of their more exotic ices, such as carbon dioxide ice.
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Now, and at the University of Arizona in Tucson think they have found even darker craters that are shielded from this reflected sunlight. These double-shadowed regions would be rare, a fraction of a per cent of the total area of craters that don’t receive direct sunlight, with temperatures dropping to -250°C.
“Their main source of light is starlight,” says O’Brien, who presented the work at the Lunar and Planetary Science Conference in Texas on 7 March. “They could be the coldest places in the solar system.”
Evidence for these frigid craters comes from NASA’s Lunar Reconnaissance Orbiter, which uses a laser to study the moon’s surface. Billions of pulses have been fired at the moon for more than a decade, says O’Brien, allowing detailed measurements of the lunar surface to be made. The team used this data to search for and examine these double-shadowed craters.
“They’ve been predicted, but we’re the first to actually look for them on the moon,” he says.
For a double-shadowed crater to exist it must be sufficiently deep and set at an angle that wouldn’t allow sunlight to be reflected in. In total, the researchers found hundreds of suitable craters that could host double-shadowed regions, ranging in size from 100 to 600 metres across, but the team says there could be many more smaller ones, with diameters of just tens of centimetres.
at the University of Colorado Boulder, who wasn’t involved in the research, says these colder regions could help us work out where water ice or other exotic ices on the moon and other bodies came from.
Temperatures of -170°C “are great to preserve water ice, but too warm for other ices like carbon dioxide, or organic species that might be a fingerprint of a comet impact”, she says. Such impacts could have been a source of Earth’s water.
“It has massive implications for the amount of water that Earth got from non-Earth sources,” says Landis.
An upcoming NASA lunar rover may be able to drive into some of these regions. Called VIPER, it is planned to arrive at the moon’s south pole in November 2023 and will drive into three regions that never receive direct sunlight for up to 10 hours at a time. It will use a drill and headlights to look for ice, and it may also discover some of these double-shadowed craters.
“One of our objectives is to locate and observe multi-shadowed craters,” says Anthony Colaprete at NASA’s Ames Research Center, the VIPER mission’s lead scientist. “It’s going to be pretty awesome.”
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