The researchers--Peter Schultz and Carl Pieters, both geologists at Brown University, and Matthew Staid of the Planetary Science Institute--report three different lines of evidence for their theory in tomorrow's issue of Nature. First, using photographs taken by a panoramic camera on the Apollo 15 mission in 1971, the team noticed that the features inside the crater were very crisp and clear, which indicates that they must be relatively young. "'Gardening' impact bombardment will wear down features on the moon, where they become much more subdued," Schultz explains of impacts that erode the lunar surface. "The structures are unusually well-preserved: steep cliffs only a few meters high surrounding a rough, blocky floor. These features could not stand the test of time."
In support of that finding, the researchers also noticed the general absence of impact craters from asteroids. The team could only make out two impact sites larger than 30 meters on the Ina structure's floor--they say this appearance is roughly equivalent to that of the South Ray Crater, which is located near the landing site of Apollo 16 and is thought to be two million years old. However, Ina holds evidence of other impacts that may have been rounded out over time, so the researchers increased their estimate of the structure's age to no more than 10 million years old. The third piece of evidence came from spectral data gathered by the Clementine satellite in 1994. "You can see that the band depths from minerals that were in reflection; they were deep," Schultz explains. "Typically, mineral band depths are smeared out with time."
Paul Lowman, a geophysicist at NASA's Goddard Space Flight Center, says he is sure that Schultz's team is right in their assessment of the Ina structure's age. In fact, he is in the process of writing up a paper supporting the theory of recent volcanic activity on the moon. "There is very good evidence that the moon is either wholly or partly molten from about 1,000 kilometers down," he explains about his group's work. "When you look at the old data on the moon, in light of what we now know about the moon, it is pretty clear that there is magma generation going on, that there is magma in the moon right now, only 200 to 300 kilometers down, or even shallower."
Schultz says the results show that the moon still has some surprises for its observers, and that even if it turns out that volcanism on the orb has ceased, its by-products are still bubbling up to the surface. Lowman hedges less on his belief, "The moon, although it's cold and dead on the outside," he says, "is still warm on the inside and probably still active."