Web Extra:   June 1, 2001

The Moon's last gasps

When amateur astronomers 30 years ago trained their telescopes on the Moon and reported seeing eruptive activity, the general consensus from the scientific community was to lump them into the same group of people who believed the "face on Mars" was a sign of intelligent life. To suggest the Moon was anything other than dead did not fit the evidence of the time

But new evidence garnered from orbiting missions during the 1990s is changing that status. Don't write off the Moon as a cold, dead body just yet. It could still be breathing.

At least four small sections of the crust found in different geologic areas around the Imbrium basin on the near side of the Moon may be vents to the deep interior, connected through deep basin faults. The structures show fine-scale features, details normally lost over time from space weathering, and other evidence of young age. They may even be a place of active degassing from the Moon’s interior.

 Image data from the Apollo missions of the 1970s revealed unusual structures on the Moon’s surface. But it took the spectral images taken during the Clementine mission of 1994 to reinforce their significance. "Clementine gave us color data and additional information that showed the space weathering of the features was very little," says Carlé Pieters of Brown University. Pieters, along with lead author Peter Schultz, also of Brown, and co-author Mathew Staid of the U.S. Geological Survey, presented findings this week during the spring meeting here of the American Geophysical Union (AGU).

Within the basin lies the Ina structure, also called the D-caldera after its shape, which is one structure that is possibly alive and well, Schultz says. In a moonscape where most of the colors astronauts see are different shades of gray, Ina stands out as a "beacon of blue," Schultz says. Through a light filter, this blue jumps out as young, titanium-rich basalt. As volcanic basalts on the Moon age, they change color and become less blue, as seen through a 0.415-micron wavelength filter in the visible light. Or they become redder.

Most of the volcanic eruptions that occurred on the Moon ended 3 billion years ago, with only some areas on the near side erupting up until 1 billion years ago. In the Ina area, freshly exposed titanium-rich basalts are found as blue halos around impact craters. As asteroids and meteoroids strike the lunar surface, they dig out titanium-rich basalt from underneath.

Ina shows the same titanium-rich basalt, but without any signs of a lunar impact event — an indication that Ina's features originated on the Moon. That does not mean they originated from a volcanic eruption. Rather, "it's a reasonable hypothesis that Ina may have formed by the degassing of the interior of the Moon," says Pieters, who also chaired the session at the Boston meeting. Degassing would expose the basalt even without a smashing impact event digging in to uproot it.

Still, there's room for error. Ina is only 2.9 kilometers in diameter and 30 meters deep. "Something that small does not have much area to accumulate craters that can be easily counted," she says. Counting craters is one way to determine how long an area has been exposed to the space environment, but smaller targets are harder to hit — making it difficult to determine their age.

So, for another age test, the geologists turned to a different filter that looks at iron. Weathering on the Moon’s surface changes the optical properties of various iron-bearing minerals such as pyroxene and olivine. The Clementine filter measures the strength of the absorption band in the one-micron wavelength, just beyond the visible in the near infrared. Ina's strong absorption in the ferrous band in pyroxene indicated its rocks have had little exposure to the pelting of micrometeorites from space, which would weaken its absorption capabilities. "The ferrous band of Ina is as strong as that of a fresh crater in basalt," Pieters says.

But, unlike the craters, Ina also shows different morphological features that are sharp structures, devoid of the rounding that occurs over time. "Whatever formed the feature happened relatively recently," Pieters says. Ina looks younger than the 50 million-year-old North Ray crater but has an appearance comparable to the 2 million-year-old South Ray crater, Schultz says.

The idea that Ina-like features on the Moon could be active vents follows a presentation at the Lunar Planetary Science Conference in March, when scientists reported finding radon gas in data still being analyzed from the Lunar Prospector that circled the Moon for a year and a half beginning in 1998, says Bill Feldman of Los Alamos National Laboratory. Radon gas is a product of iridium decay and is visible for only three to four days before it decays into alpha particles. If the data were correct, radon was actively degassing while the Lunar Prospector passed over the Moon. "It would be interesting to look at the Ina feature to see if it's still venting," Feldman says.

"Is the Moon alive?" asks Schultz. "It's not erupting. But what we are seeing are its last gasps. So in that sense: Yes, it is."

Christina Reed

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