More from the AGU: Pink Moon, White Mountains

Friday, December 18th, 2009

My last two posts were about the American Geophysical Union meeting, held this week in San Francisco. The meeting is now over, but I’m not done writing about it yet — not by a long shot!

In 1971, Nick Drake recorded a song called “Pink Moon” that became a posthumous hit in 1999 when it appeared in a Volkswagen commercial. Now it turns out that he was really on to something. On Wednesday afternoon, Carle Pieters, the lead scientist for the Moon Mineralogy Mapper on Chandrayaan-1, the Indian spacecraft that circled the moon for ten months, talked about her discovery of a new, magnesium-rich pink spinel.

Well, okay, it isn’t really a pink moon. But if you go to the moon’s far side and land your spacecraft on the edge of the Moscoviense basin — a very attractive place, with one of the few maria on the moon’s far side — she says that you can dig up all the pink spinel that you want.

Because we have never landed anything on the moon’s far side — human, robot, or other — we’ll have to wait a while to find out if she is right. The evidence from the Moon Mineralogy Mapper (M-cubed for short) is a very distinctive spectrum, with no absorption at 1 micron and a “whopping” (Pieters’ technical term) absorption band at 2 microns, that is totally unlike any other known moon rocks but a very good match for pink spinel measured in the laboratory.

Jessica Sunshine also talked about spinel, but hers is very dark, not pink, and is probably mostly chromite. M-cubed found this deposit — also previously unknown — at only one place on the near side of the moon, a region called Sinus Aestuum. Why only there? Dunno. How much is there? Dunno. But this would be a great spot to explore once we start sending robots or astronauts back to the moon. (2015? 2020? 2100? Never?)

The good thing, and the bad thing, about both of these talks is that they were just good ol’ talks about rocks, the sort of things that geologists like to talk about when the press isn’t watching. Lest we forget, M-cubed was not really intended to look for water ice. The whole business about water was really an unexpected bonus. If they had really expected to see water, they would have made the spectrometer sensitive out to 3.6 microns. (3 microns is enough to see the peak absorption bands of hydroxyl and water at 2.7 and 2.8, but not the full spectra.)

Neither Pieters’ nor Sunshine’s findings are going to make headlines, but they are good examples of a scientific instrument doing what it was meant to do, and finding new stuff. The take-home message for non-geologists is just that the moon is not a homogeneous place; it has stuff we haven’t seen before and it most likely has stuff we haven’t even thought of yet. The other message is that all of these discoveries allow us to piece together a few more puzzle pieces to understand lunar geology.

In the case of the spinel, Larry Taylor explained in his talk how these deposits were probably formed by secondary intrusions of magma into the anorthosite layer at the top of the lunar magma ocean. In her final talk, Pieters also mentioned the magma ocean, which is believed to have encircled the moon immediately after its formation by a giant impact. She said that M-cubed found a “massive amount” of anorthosite in the Inner Rook Mountains in Mare Orientale, which she described as “very strong evidence for the magma ocean hypothesis.”

This comment made me sit up in surprise. I tend to think of the magma ocean as a done deal; I wrote about it in my book as one of the major discoveries, perhaps the major discovery, of the Apollo missions. Why would you care about proving something you already know?

The reason is that in science, nothing is ever really a done deal. A hypothesis may eventually become a theory, and it may become conventional wisdom, but you always want to collect new data and look for new evidence. The magma ocean hypothesis is based on a few anorthositic dust samples and anorthositic rocks collected by the Apollo astronauts. Until now, we hadn’t really seen any large-scale structures made of anorthosite. Now we have seen a whole mountain range of the stuff, or at least Pieters thinks we have, from orbit. The next thing to do is send a geologist there and see if she is right. Do we have any volunteers to go and look for Carle Pieters’s white mountain range?

Tags: anorthosite, Carle Pieters, Chandrayaan-1, Jessica Sunshine, Larry Taylor, lunar magma ocean, Mare Orientale, Moon Mineralogy Mapper, Moscoviense, Sinus Aestuum, spinel
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