Mysterious Moon Flashes Signs of "Last Gasps"?

Astronomer Arlin Crotts is trying to solve a 400-year-old puzzle.

Ever since the invention of the telescope, said the Columbia University astrophysicist, observers around the world have occasionally watched small areas of the moon brighten or "turn fuzzy." Sometimes they even turn reddish.
Because the bright patches are ephemeral, lasting only last a few minutes, these events have come to be known as transient lunar phenomena, or TLPs. "About 1,500 of these have been reported," Crotts said.

Astronomers already know they're not meteorite impacts. Those produce brighter, briefer flashes as the meteorite hits the surface and vaporizes.

But nobody yet knows what TLPs are—or even whether they truly exist. Many astronomers think they're simply optical illusions or figments of observers' tired eyes and overactive imaginations.

Crotts believes the time has come to put the riddle to rest once and for all. Scientists have long believed the moon to be a dead world, but if TLPs are real, then some form of geological activity must still be going on.

In that case, said his colleague Peter Schultz of Brown University, "if we were to go back to the moon, we might be able to go to these places and discover something absolutely new about its ancient history or its deep interior."

Catching a Fleeting Phenomenon in the Act

Funded in part by a grant from the National Geographic Society's Committee for Research and Exploration, Crotts, Schultz, and their team have set up two 10-inch (25-centimeter) robotic telescopes, one in New York City and one at the Cerro Tololo Inter-American Observatory in northern Chile. The instruments are far enough apart that, most nights, at least one will have clear skies.

In a project begun last year and expected to continue at least into 2010, each telescope is photographing the moon every 20 seconds. At each site, the images are fed into computers that can examine them for changes that might signal a TLP.

"We want to start catching these things in the act," Crotts said.

That may sound easy, but it's not. Minor disturbances in Earth's atmosphere can also cause lunar features to brighten, dim, or distort. "The same thing that makes stars twinkle will make every little feature look just a little bit different [each] time," Crotts said.

Still, he believes it's possible for the computers to sort these out, identifying the events most likely to be actual TLPs.
"We've done this sort of stuff for other problems in astrophysics," he said, noting that similar techniques have been used to study variable stars and look at "light echoes" caused by reflected light from supernova explosions.

Are TLPs Real?

Crotts' main goal is to supplement the anecdotal reports dating back hundreds of years with real science. "I'm trying to get an objectively undeniable data set that shows when and where these events take place," he said.

He wouldn't bother if he didn't believe there was already substantial evidence that TLPs are real. A couple of years ago, Crotts did a statistical analysis of all known TLP reports, searching for patterns.

He found that in the most reliable reports, the vast majority appeared to have occurred in a handful of lunar locations. Half, in fact, were in a single crater, Aristarchus, while another 20 percent took place in the crater Plato.

But skeptics like Chuck Wood, a lunar and planetary scientist at Wheeling Jesuit University, said the similarities in location may not mean much.

Aristarchus and Plato, he noted, are among the moon's most dramatic features. "I think one of the reasons [TLP sightings] are concentrated there is [because] those are among the most dramatic craters on the moon that everybody looks at.

"I would be much more impressed with observations of transient phenomena if they occurred at non-spectacular craters" that receive less attention.

Wood also suggests some of these recurring observations may result from the sun being perfectly positioned at sunrise or sunset to slant through a low spot in a crater rim, briefly illuminating areas that would normally be shadowed and making them look as though they are glowing.

As with the Earth, the sun doesn't rise in the same place every lunar "day," but it does eventually return to the same location, producing another opportunity for the same visual flash.

A team in Italy has examined this effect for one of the better-reported TLPs, Wood said, discovering that it repeats on exactly that type of cycle.

Possible Links to "Burps" of Radon Gas

Crotts has carried the research a step further, though. He's compared the areas where TLPs have been most frequently sighted to regions where Apollo space missions observed evidence of a short-lived radioactive gas, radon-222, escaping from the Moon's interior—a clear sign that some type of geological activity was going on.

The Apollo instruments spotted several such outbursts, Crotts said—all in areas where TLPs are frequently reported. "The chances of this happening at random are extremely small," he said.

Crotts and Schultz have developed a theory about how these "burps" of radon might be linked to TLPs. Gases including radon might be migrating up from deep inside the moon, making their way to the surface via fractures in the rock, a team led by Schultz suggested in a 2006 paper in the journal Nature.

Then, a few yards beneath the surface, the gases hit lunar soil, known as regolith. That soil is basically powdered rock, pulverized during eons of bombardment by micro-meteorites.

"It's one of the finest powders you can find," Crotts said. That makes it much less porous than the underlying rock, so it traps the gas beneath the surface. Eventually enough gas collects to burst out in a big puff. Or, more likely, Crotts said, in a ka-boom: "It's a lot like setting off a bomb."

The explosion would blast dust high enough that it would take several minutes to settle, and meanwhile it would shimmer brightly in the sun.


New Lunar Research Opportunities?

Using photos from a 1972 report of findings from Apollo 15, Schultz may even have found the imprint of at least one of these blasts on the lunar surface. It's a depression called the Ina structure, or the D-structure, because it's shaped like a "D" two miles (three kilometers) across.

It's not an impact crater. Those are round. But whatever it is, it's clearly very fresh, Schultz said, which, in lunar terms, means it's possibly less than a million years old.

This indication of relatively recent activity is just one of several features of its type. "I know of four, but there are probably 20 or 30 within [one] area," Schultz said.

Because these features all occur at the intersections of visible fault zones, Schultz thinks they mark places where gases most easily made their way to the surface, producing the type of explosions Crotts is looking for.

And while no TLPs have been observed in this region of the Moon, Schultz doesn't think that means they haven't occurred sometime in the past, when people weren't looking. "We don't need to see it form to know that it happened recently," he said by email. And, he added, "It's clearly a place to watch."

If he catches a TLP in action, Crotts hopes to do more than just photograph it. His computers are fast enough, he believes, to give an alert that would allow moon-orbiting satellites and other spectrometer-equipped telescopes to train their attention on the region in question and measure the wavelengths of light emitted during the episode.

"If we can get a spectrum, that would probably identify what sort of gas is involved," assuming a TLP actually turns out to be a gas burst, Crotts said. "That would be very educational."

Knowing more about the gas would help scientists pinpoint what type of geological processes are producing the eruptions in TLPs and might also reveal interesting information about the processes releasing gas all the way in the interior of the moon.

Wood isn't sure the effort to catch a TLP in action will work. But even this TLP skeptic is rooting for success. "I hope so," he said. "It'll be very exciting if things are discovered."

It might even be useful. Volcanic gases coming out of the moon might include water vapor, sulfur dioxide, and carbon dioxide, all of which would be important resources for future explorers trying to set up lunar bases. "Those would be nice things to find," Crotts said.

But catching a TLP in action is the main goal. "When we look at the moon, we think of [it] as a dead place," Schultz said. "But it may not be dead. It may be having these last gasps." And those last gasps, if confirmed, could breathe new life into lunar research.