A fresh look at Apollo-era images combined with recent spectral data leads researchers to re-examine conventional wisdom about the Earth’s moon. Several lines of evidence suggest that the moon may have seen eruptions of interior gasses as recently as 1 million years ago, rather than 3 billion years ago – the date that had been most widely accepted.
Conventional wisdom suggests
that the Earth’s moon has seen no widespread volcanic activity for at
least the last 3 billion years. Now, a fresh look at existing data points to
much more recent release of lunar gasses.
The study, published in the journal Nature by geologists
Peter Schulz and Carlé Pieters of Brown University and Matthew Staid of
the Planetary Science Institute, uses three distinct lines of evidence to
support the assertion that volcanic gas has been released from the moon’s
surface within the last 1 to 10 million years. The researchers focus on a
D-shaped area called the Ina structure that was first recognized in images from
Apollo missions.
Millions of years – not billions
Apollo images of the D-shaped Ina structure indicate that gas releases may have exposed fresh surfaces on the moon. The main image shows very few craters within the depression. Low-angle illumination (inset) reveals sharp features with little evidence of weathering.
Image: NASA
The unusual sharpness of the features first called
Schultz’s attention to the area. “Something that razor-sharp
shouldn’t stay around long. It ought to be destroyed within 50 million
years,” said Schulz. On Earth, wind and water quickly wear down freshly
exposed surface features. On the airless moon, constant bombardment with tiny
space debris accomplishes a similar result. By comparing the fine-scale surface
features within the Ina structure to other areas on the moon with known ages,
the team was able to place its age at closer to 2 million years.
The scarcity of asteroid impact craters on the surface within
Ina provided a second line of evidence for the feature’s relative youth.
The researchers identified only two clear impact craters larger than 30 meters
on the 8 square kilometers of the structure’s floor. This frequency is
about the same as at South Ray Crater, near the Apollo 16 landing site. The
surface material ejected from South Ray Crater has long been used as a benchmark
for dating other features on the moon’s surface and most lunar scientists
studying these rocks agree on a date of approximately 2 million years,
based on cosmic ray exposure.
The third piece of support for the authors’ hypothesis
comes from comparing the spectral signatures of deposits in the Ina depression
to those from very fresh craters. As lunar surface deposits weather, the
wavelengths of light they reflect change in predictable ways. Overall
reflectance, or albedo, gets less bright and the ratio of light at 1,000 nm
wavelengths to 750 nm wavelengths increases. Based on these color ratios, the
deposits on Ina’s floor are exceptionally young – and possibly even
newly exposed.
Far from mature
This false color composite image indicates age and composition of lunar surface features. Titanium basalts (blue) are exposed on the floor of Ina structure and in the ‘fresh’ impact crater. Less mature soils (based on spectral ratios) appear in green.
Image: NASA
The appearance of the surface at Ina does not indicate an
explosive release of magma, which would result in visible rays of ejecta
surrounding a central crater. Rather, it suggests a rapid release of gasses,
which would have blown off the surface deposits, exposing less weathered
materials. This interpretation is particularly appealing because Ina is located
at the intersection of two linear valleys or rilles
– like many geologically active areas on Earth.
Ina also does not appear to be alone. The authors identify at least four similar features associated with the same system of rilles, as well as others in neighboring rille systems. Although several kinds of evidence support the authors’ conclusion that the moon is more geologically active than previously thought, the only sure way to resolve the question would be to collect samples at such sites. “Ina and other similar features are great targets for future exploration, by people or robots,” said G. Jeffrey Taylor, a lunar researcher at the University of Hawaii. “They might be the best place to get a good look at the interface between the powdery regolith and the consolidated rock beneath.”
Over the years, says Schultz, amateur astronomers have seen
puffs or flashes of light coming from the moon’s surface. Although most
professional observers have upheld the conclusion that the moon was inactive,
such sightings have kept open a window of doubt. A coordinated observation
campaign, including both professional and amateur astronomers, would be one way
to build additional evidence for activity, says Schultz. A gas release itself
would not be visible for more than a second or so, but the dust it kicked up
might stay suspended for up to 30 seconds. With modern alert networks,
that’s long enough to move a professional telescope into position to see
what’s happening.
NASA’s Planetary Geology and Geophysics Program supported
this research. Peter Schultz and Carlé Pieters are professors of
geological science at Brown University. Matthew Staid is a research scientist at
the Planetary Science Institute.
