Moon Gas Indicates Recent Geologic Activity
It’s aliv-v-v-v-e! Evidence for recent geological activity on our moon has been reported in Nature.1 Katharine Sanderson introduced the findings in News@Nature, in an item titled, “The moon has gas: Eruptions confound the idea that our nearest neighbour is a geological dead zone” –
Some think the Moon has been geologically dead for billions of years. But Peter Schultz of Brown University, Rhode Island, is not one of them. His results show that some lunar craters were formed as recently as 10 million years ago by gas eruptions, suggesting that there’s still something bubbling away beneath the Moon’s surface.
In terms of lunar geology, 10 million years is yesterday. It was thought that all volcanic activity stopped 3.2 billion years ago, and that any young craters were caused by meteor impacts. Time to think again, say Schultz and his colleagues in this week’s Nature1.
Here is the abstract from the paper by Schults et al.1 It hints at a serious intellectual tension between the evidence for recent activity and the belief that this activity should have ceased billions of years ago:
Samples of material returned from the Moon have established that widespread lunar volcanism ceased about 3.2 Gyr ago.2 Crater statistics and degradation models indicate that last-gasp eruptions of thin basalt flows continued until less than 1.0 Gyr ago, but the Moon is now considered to be unaffected by internal processes today, other than weak tidally driven moonquakes and young fault systems. It is therefore widely assumed that only impact craters have reshaped the lunar landscape over the past billion years. Here we report that patches of the lunar regolith in the Ina structure were recently removed. The preservation state of relief, the number of superimposed small craters, and the ’freshness’ (spectral maturity) of the regolith together indicate that features within this structure must be as young as 10 Myr, and perhaps are still forming today. We propose that these features result from recent, episodic out-gassing from deep within the Moon. Such out-gassing probably contributed to the radiogenic gases detected during past lunar missions. Future monitoring (including Earth-based observations) should reveal the composition of the gas, yielding important clues to volatiles archived at great depth over the past 4-4.5 Gyr.
This tension continues in the body of the article. “Both morphological and spectral criteria indicate that the exposed surfaces within Ina are exceedingly young,” the authors say. “In fact, our observations do not preclude the possibility that it is still in the process of formation.” The 10 Myr figure, clearly, was intended as an upper limit.
Moreover, Ina is just one of four such features they identified. Since these four trend along a rille around the Imbrium basin, “These occurrences in similar structural settings indicate that volatiles (for example, juvenile CO2 and even H2O) trapped deep within the Moon episodically escape along crustal weaknesses, thereby continually freshening the regolith.”
Another clue that something very recent is going on comes from the identification of Polonium-210 alpha particles by the alpha particle spectrometer on Lunar Prospector. These short-lived radioactive decay products (half-life = 138 days) “indicate radon release within the past 60 yr.”
If these features indicate recent geological activity, it could mean that at least some craters were formed by gas explosions, News@Nature explained. It has long been assumed that all craters formed since the ancient volcanism stopped were from impacts. Although these findings are “suggestive, rather than definitive,” the implications take lunar scientists back to square one, Sanderson concluded:
Lunar scientists will now be forced to rethink assumptions made about the Moon. “There is more to the Moon than we had previously discovered,” says Schultz. “This shows there’s still some [geological] life. We just have to learn how to take its pulse.”
1Schultz, Staid and Peters, “Lunar Activity from Recent Gas Release,” Nature 444, 184-186 (9 November 2006) | doi:10.1038/nature05303.
2Gyr = billions (giga) of years; Myr = millions of years.
No sooner had we speculated that the Egyptian anomalous craters (11/01/2006) might have been endogenic, leading to new hypotheses about other solar system craters, than this report appears in Nature suggesting the same thing. Imagine: some craters on the moon forming from the inside out, in recent times. That is a startling change in thinking. Undoubtedly many, if not most, are still impact craters (the authors did not consider secondaries; see 06/08/2006), but it’s hard to understate the surprise that geologists must feel finding something this young on a moon their minds have been trained to think as old, old, old.
The authors, for sure, considered details that might distinguish the young features from the presumably old ones, and do not doubt the 4.5-Gyr accepted age. (How could they and keep their jobs?) But then they had to admit that the new findings are forcing lunar scientists to “rethink assumptions made about the Moon.” It’s been almost heresy to consider the moon being geologically alive. Only a few “last gasp” episodes from isolated craters like Aristarchus, location of numerous lunar transient phenomena sightings, have been considered as interesting but anomalous. The moon was supposed to have cooled and quieted down three billion years ago. You can almost sense the astonishment as they first talk glibly about billions of years, then 10 million, then 60 years, then Yikes, it might be even active today!
Isn’t it just like secular scientists to sidestep the most serious implications of their discoveries? At the end, the scientists focused on how this finding might help future astronauts understand what resources are available to exploit. Come on; this is a story with big epistemological ramifications. The lesson can be stated as a question you should train children to ask when they hear things about the unseen past taught as accepted fact: “How do you know that?”