PICTURE the scene. Aristarchus crater on the near side of the moon, 20 July 2019. It鈥檚 only a few hours since NASA鈥檚 Altair 2 landing craft touched down and astronauts are walking on the lunar surface for the first time in almost half a century. Suddenly, a large area of the crater floor begins to convulse and a titanic eruption of gas knocks the astronauts off of their feet. They look back to see their landing craft has disappeared in a cloud of dusty debris.
That鈥檚 what fears could happen to the next crew that lands on the moon. Crotts, an astrophysicist and cosmologist at Columbia University in New York, has analysed over 1500 accounts of strange lights on the moon, dubbed transient lunar phenomena. For decades, researchers have argued over whether TLPs are real or merely tricks of the imagination. Now Crotts says he has strong evidence that they are indeed real and caused by giant eruptions of gas. If he is right, the moon is not the stone-cold, dead satellite we thought it was and the eruptions may be due to residual volcanic activity. Not only could that spell danger for future lunar dwellers; we would have to rewrite the moon鈥檚 geological history.
TLPs are one of the moon鈥檚 greatest mysteries. Lasting from a minute to a few hours, they can involve notable brightening, dimming or blurring stretching over a few kilometres of the moon鈥檚 surface. Some observers even report a baffling ruby-red glow.
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Shortly after the start of the to the moon, researchers in the US and UK catalogued the 1500 or so recorded sightings. With the aid of these accounts, Crotts wondered whether it was possible to tease apart any real lunar phenomena from imagined sightings.
One of the first sightings was made in the 10th century by monks at Canterbury cathedral in England, who reported seeing an explosion on the moon. Even the astronauts reported seeing a bright glow from Aristarchus as they passed overhead. The tricky thing is that the majority of sightings are made by amateur astronomers. 鈥淭here is little uniformity in the standard of observation,鈥� says Crotts. 鈥淚t鈥檚 a big hair ball of a data set.鈥�
To untangle the hair ball, Crotts divided up the catalogue according to independent observations made from different locations. Then he looked to see whether there were any common places on the moon identified by multiple observers. He did the same thing with observations made in different centuries and applied a number of other statistical filters. 鈥淭he same six locations on the moon consistently survived my filtering, while other commonly reported sites fell by the wayside,鈥� says Crotts. 鈥淚 was stunned.鈥�
One site accounted for half the sightings: the 45-kilometre-diameter Aristarchus crater. About a quarter were associated with Plato, an impact scar 100 kilometres wide. Most of the rest were linked with the Mare Crisium and craters Grimaldi, Tycho, Copernicus and Kepler.
Was it just a fluke that these sites jumped so dramatically out of the data? 鈥淭he analysis looks correct,鈥� says Chuck Wood of the Planetary Science Unit in Tucson, Arizona. But he warns there might still be a bias towards Aristarchus and the other craters because these are popular spots for amateur astronomers to study.
So Crotts wondered if other events near the lunar surface might correlate with these locations. Apollo 15, Apollo 16 and the probe which orbited the moon in 1998 all carried spectrometers designed to detect alpha particles emitted by the radioactive decay of radon gas on the surface.
Crotts found that episodic emissions of radon were exclusively associated with the six most common sites for TLPs, with Aristarchus accounting for half of all the emission of the gas. 鈥淚t was such a coincidence that the idea hardened in my mind that I was onto something,鈥� he says. 鈥淭he sites of these flashes also seem to be outgassing sources, suggesting one was the cause of the other.鈥�
Crotts suspects that what all these six sites have in common is cracks in the crust. Gas could leak to the surface through these cracks, building up pressure before bursting its plug of lunar soil, or regolith, and exploding into space. 鈥淵ou wouldn鈥檛 want to be standing on top were that to happen,鈥� says Crotts. Apollo 18 was once scheduled for Aristarchus, but it was cancelled along with the Apollo programme.
Crotts and his student Cameron Hummels calculate that a mere half a tonne of gas escaping into the vacuum would puncture the regolith and create a dust-and-gas cloud a few kilometres across that would persist for 5 to 10 minutes 鈥� exactly as often observed. The cloud can change the reflectivity of the surface by either shading features or by spreading the dust grains through the near-vacuum so that they reflect about 20 per cent more light than when they are clumped together on the surface.
So where is the gas coming from? Since radon is a decay product of uranium, the obvious answer would be that it comes from uranium in rocks throughout the moon. 鈥淭he problem is to explain how the products of general radioactivity would become concentrated at so few outlets,鈥� says Crotts.
He favours another explanation, which flies in the face of our accepted view of the moon as a dead satellite whose interior cooled and solidified long ago. 鈥淚 believe we should consider whether the moon still has some residual sites of volcanism in the form of outgassing,鈥� he says. He points to Aristarchus 鈥� sat on a volcanic plateau, it is widely believed to have produced much of the lava of the western maria, about 3 billion years ago. 鈥淚 can鈥檛 think of another place on the moon that screams 鈥榲olcanism鈥� more loudly,鈥� says Crotts.
To test the idea, Crotts calculated the effect of Earth鈥檚 gravity repeatedly stretching and squeezing the moon. 鈥淚t grinds up about 100,000 tonnes of moon rock a year,鈥� he says. 鈥淭hat might release about 100 tonnes of gas.鈥�
That鈥檚 a figure that rings alarm bells. NASA is planning to send humans back to the moon in about 10 years鈥� time. Together, the Altair 2 landing vehicle and its mate, the Orion spacecraft, are expected to eject about 30 tonnes of gas near the moon鈥檚 surface with every landing mission. 鈥淭his level of contamination could wipe out any signal of lunar volcanism,鈥� says Crotts.
It is now a race against time to learn all we can about TLPs and what they might tell us about the moon鈥檚 interior. Crotts and a team of astrophysicists, geologists and spacecraft instrument specialists have secured a robotic telescope with a 10-inch mirror in Cerro Tololo, Chile. They are scanning the moon as often as possible for TLPs and adding similar telescopes around the world as they get funding. Such a network should soon be able to detect a TLP, analyse it and alert other astronomers while it is actually happening.
鈥淲e don鈥檛 understand this aspect of the moon anywhere near as much as we thought and it is trying to tell us something important,鈥� says Crotts. 鈥淲e鈥檝e probably got 10 years to find out what.鈥�
鈥淭he moon is trying to tell us something important. We鈥檝e probably got about 10 years to find out what鈥�
