Reports of ephemeral flashes of light seen on the Moon, dismissed by some as imaginary, could be due to the explosive discharge of gas beneath its surface. The analysis, by astronomer Arlin Crotts of Columbia University in New York, may breathe new life into investigations of its geological activity and history.

Crotts mapped about 2,000 observations of bright flashes called transient lunar phenomena (TLPs) reported by astronomers during at least the past 350 years. The flashes last too long to be meteorite impacts, and many researchers have dismissed the reports as observational errors.

But when Crotts compared the most commonly reported sites of observation with a map of known gas leaks from the Moon’s surface, he found a strong correlation. “It really boils down to just a small number of sites where [TLPs] are happening consistently,” he says. “That’s almost exactly the same list of sites where people have seen radon [gas].”

“The prevailing paradigm is that there’s very little volatile [gas] activity in the Moon at all,” Crotts says. NASA’s Apollo missions and the more recent Lunar Prospector mission observed wisps of radon and argon gas. Planetary scientists tend to attribute the gas to slow leaks from radioactive decay below the Moon’s surface. But Crotts suggests that the gas may build up in high enough concentrations to burst through the lunar surface, and that these explosions may be the origin of TLPs.

A volatile story

“If you tie all this together in one package, you can convince yourself there’s a story here,” says Paul Spudis of the Lunar and Planetary Institute in Houston, Texas. “At one time the Moon had volatiles; it might still have some remnant of those in the deep interior,” he says. It could be that gravitational forces from Earth and the Sun act to squeeze the gas out through cracks in the Moon’s crust. But nobody knows whether this process might be gradual or sudden. Exactly what makes up the gas — or if it even causes TLPs — is also unknown.

In a paper accepted for publication in the 20 May issue of the Astrophysical Journal, Crotts writes that the debate over whether TLPs are real “unfortunately has taken place in the unrefereed scientific literature”. He compares reports by different observers at different time periods to infer whether TLP reports were imaginary, or if they were copycat reports of a few real sightings.

Crotts rules out such biases, suggesting a natural origin for the flickers. In an accompanying paper published last year, he identified a correlation between TLPs and lunar out-gassing events. Still, Crotts acknowledges in his latest paper that the onus is on “those who would convince us that TLPs are real”.

Enter the robot

Crotts and collaborators are searching for objective evidence of new TLPs using automated telescopes both at the Cerro Tololo Inter-American Observatory in Chile and at Columbia University. The next step will be to compare the images from the telescopes with lunar out-gassing readings from the Japanese lunar mission SELENE (Selenological and Engineering Explorer, or Kaguya), which has an alpha-particle detector for mapping radioactive gases.

In addition to solving the puzzle of TLPs, Crotts would like to analyse the contents of lunar eruptions to find out whether previous studies underestimated the proportion of certain gases. One of Crotts’s collaborators, planetary scientist Peter Schultz of Brown University in Providence, Rhode Island, identified potential evidence for recent out-gassing on the Moon in 2006. “If we were to sample that material, we might find something more than just polonium [a byproduct of radon gas]. We now have much more sophisticated instruments to go after these observations,” Schultz says.

Recorded in 1992 by the Galileo spacecraft enroute to Jupiter, this picture is a mosaic of 15 images combined in an exaggerated color scheme to emphasize composition differences – blue hues reveal titanium rich areas while orange and purple colors show regions relatively poor in titanium and iron. Multicolor images exploring the Moon’s global surface composition were made in 1994 by the Clementine spacecraft. Image credit: NASA

Forming history

“If the volatile content of the Moon is significantly different from what we thought it was … it really relates to the possible origin of the Moon,” says Spudis. The most widely accepted account of the Moon’s formation is that a Mars-sized planet struck Earth billions of years ago, tearing off some of the crust material to give the Moon a distinct geological makeup with its own gas activity. A revision of the Moon’s gas activity could require a fresh look at its history, according to Spudis.

Planetary scientists have yet to embrace Crotts’s idea. The problem is that the vast majority of historical reports of TLPs come from naked-eye observations by astronomers, including amateurs. “You’ve got a bunch of observations on the edge of verifiability,” Spudis says. “Most lunar scientists, including myself, are not against this idea. We’re just sceptical.”

If Crotts’s explanation of TLPs is right, it would answer long-standing questions over their existence and origin, and “at the minimum you’ve got a new area of lunar science,” says Spudis. Ongoing studies of Crott’s proposed gas eruptions might also provoke additional research questions, and could help with the planning of future lunar missions.

Posted by: Soderman/NLSI Staff
Source: Lucas Laursen/Nature News

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