I caught many of the lunar talks but I spent some time in Mars, terrestrial impact, asteroid, and Mercury MESSENGER sessions, too. Regarding the last, there are few new data and not much new interpretation since I wrote about Mercurian volcanism and crustal composition in November, so the session was a bit of a tease. The Mercury MESSENGER orbital insertion is scheduled for March 18, 2011, and there should be reams of great data soon after.
Of the lunar sites discussed, the South Pole-Aitken basin (SPA) received a good dose of attention. SPA is on the lunar farside, extending ~2500 km from the South Pole to Aitken crater. It’s the largest and deepest impact structure on the moon and one of the largest in the solar system. The highest mountains on the moon are remnants of SPA’s uplifted rim and the lowest points lie near its center. The impact excavated so deeply that the bottom of the basin contains melt sheets from lower lunar crust or even the upper mantle. Because of this, SPA represents an entirely different lunar composition from the highlands or mare samples.
The South Pole-Aitken basin is the roughly circular anomaly on the southern far side.
Sasaki et al. studied crustal structure of SPA with Kaguya (Selene) data. They used laser altimetry to refine the topography and the first precise gravity data to model depth to the Moho (the crust-mantle interface of a rocky planet is known as the Mohorovocic discontinuity, or Moho). When crust is thin, dense mantle will rise to compensate and maintain isostatic equilibrium. The Sasaki study’s model interprets the Moho to be at a depth of 60-80 km around the basin and around 35 km in the basin. Thus, they suggest that that lower crust, but not mantle, is exposed in SPA.
Even if the Moho is well below the surface, mantle melting may have contributed to the impact melt sheet in the basin center, as suggested by Potter et al. based on computer impact modeling with a resolution of 30 km/cell. Depending on the degree of mantle melting, the melt composition will vary from near-mantle compositions to basaltic, and the commensurate density could mimic a range of mantle-lower crust-crust mixtures. Since gravity models only "see" density and thickness, they can't constrain composition in such cases.
(Both the Sasaki et al., and Potter et al. studies compliment the past work in their fields, and I’ll leave it to the interested reader to follow up on the references in their abstracts.)
One of the papers I found most interesting was the Petro et al. study on the Apollo Basin, a 480 km crater in the northeast section of SPA. They investigated compositions in Apollo using spectral data from the Moon Mineralogy Mapper (M3) – a NASA instrument aboard Chandrayaan-1 – and compared the Apollo lithologies to the rest of SPA and to the far side highlands outside of SPA. The high resolution (1.4 km/pixel) and quality of the data allowed them to recognize: (1) “anorthositic” rocks in the uplifted center rim of Apollo (anorthosite is a rock that composes the upper lunar crust and contains more than 90% plagioclase feldspar, and Petro et al. call the Apollo rocks “anorthositic” because they approach the composition of anorthosite but contain more mafic [Mg-Fe] minerals); and (2) noritic materials exposed in some of the deeper craters within Apollo (norite has a composition intermediate between anorthosite and a mafic or a ultramafic [mantle] composition). The crust in Apollo is very thin, and because it lies near the edge of SPA the overlying impact melt sheet is relatively thin as well. Petro et al. interpret the anorthositic material to be a layer in the crust below the true anorthosite higher in the section, and the noritic lithology to be an even deeper level of preserved lunar crust.
Apollo crater from Petro et al.'s Figure 1. (A) Albedo image with dashed lines showing inner (240 km) and outer (480 km) crater rings. (B) Long wavelength image showing topography. (C) False color image: deep blue, as in the southwest inner crater rim, is anorthositic material; deep green, as in crater between the NW rims, is noritic. Yellows are basaltic impact melt.
If Petro et al. are correct, this adds to the reasons why SPA is an ideal site for a sample return mission. Not only might it have preserved crustal stratigraphy not exposed anywhere else, but it is far from the nearside mare basalts and impact ejecta that are mixed into our previous samples (U.S. Apollo 11-17 and Russian Luna missions). The other primary reason to target SPA is its age. It is known to be the oldest lunar impact basin based on mapping of ejecta blankets and on crater counts, but its absolute age is unknown. A precise radiometric age would be an anchor to all of the other relative lunar ages and would place an upper limit on basin-forming impacts in the inner solar system. An SPA impact melt age would constrain the early history of the Moon and, to a significant extent, of the Earth.
One of the three proposed New Frontiers NASA missions is a sample return from SPA called MoonRise. I personally hope it goes through, though all three missions are worthwhile. In the near-term, there are some exciting developments in lunar meteorites that might allow for study of SPA without a mission. Based on its composition and mineralogy, the group at Washington University have identified meteorite Dhofar 961 as possibly coming from SPA. They presented some of this work at the 2009 LPSC and, this year, Korotev et al. and Zeigler et al. both presented evidence that several other lunar meteorites are directly related to Dho 961. It’s an exciting hypothesis and they’re building a good case. Many scientists (including some in my group) are waiting for pieces of these samples to date them.
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