Chandrayaan-1 M3 mapper study reveals new lunar rock family

Carle Pieters
Professor of Geological Sciences
Brown University

R. Ramachandran
The Hindu

The Moon Minerology Mapper (M3) on Chandrayaan-1, which famously discovered the presence of water and hydroxyl molecules on the lunar surface material last year, has now identified a new lunar rock type on the far side of the moon. The M3 is a NASA instrument. This was reported here on Monday by Carle Pieters of Brown University, lead author of the present study, at the Sixth Chandrayaan-1 Science Meeting being held at the Physical Research Laboratory (PRL), a unit of the Indian Space Research Organisation (ISRO).

The rock-type is dominated by a mineral termed as ‘magnesium spinel.’ Spinel is a generic name given to a class of minerals having the chemical formula AB{-2}O{-4} and the usual spinel formations found in lunar rocks is an iron-magnesium admixture of the form (Mg, Fe)(Al, Cr){-2}O{-4}. These rocks are usually found along with magnesium-iron silicate (olivine) and calcium-rich aluminium silicate (pyroxene).

Unique feature

According to Professor Pieters, the interesting feature of the new rock type is that it is exclusively composed of magnesium-rich spinel “with no detectable pyroxene or olivine present.” This, she said, does not easily fit with current lunar crustal evolution models.

Rich in anorthosites

The generally accepted characterisation of the lunar crust is based principally on retrieved lunar material by the Apollo-Luna missions and meteorite samples. The crust is described as a rocky accumulation, basically rich in calcium-aluminium silicates (anorthosites) infused with a mix of compounds containing magnesium and iron (‘mafic’ minerals).

However, the western ring of the Moscoviense Basin of the moon appears to be one of the several discrete areas that exhibit unusual compositions relative to their surroundings, but without morphological evidence for separate geological processes leading to their exposure.

The findings are based on data acquired by M3 in January 2009 during the first observation period of Chandrayaan-1 from its initial 100 km altitude orbit over a 40 km wide strip field of view, with a spatial resolution of 140 m/pixel. The mapping was done using the emission spectrum of the surface over the wavelength region 460-3000 nanometres with a spectral resolution of 20-40 nm.

Five anomalous areas

The general composition of the area observed had a low abundance of mafic minerals and a high abundance of feldspathic minerals such as pyroxene. While this was consistent with earlier observations, five anomalous areas that are widely separated were seen along the lower elevations of the ring (see pic.). Interestingly, no unusual feature or any compositional boundary was seen for any of these areas.

Calcium-rich pyroxene is prominent in areas 2 and some parts of 3 and 4. Olivine is prominent across 5 and parts of 4. In contrast, the whole of region 1 and part of region 3 were exceptionally dark in the images. This, according to Professor Pieters, is because of the high absorption that the areas seem to have in the 2000 nm region, together with the near complete absence of pyroxene or olivine (less than 5 per cent) as indicated by the lack of any absorption around 1000 nm.

While regions rich in olivine or pyroxenes have been seen in other basins, this is the first time a magnesium-rich spinel region has been identified. “The clear interpretation of these spectra is that the surfaces represent a new rock type dominated by magnesium-rich spinel with no other detectable mafic minerals,” Professor Pieters said.

No easy explanation

There does not seem to be any easy explanation for the occurrence of these spinel formations. Since magnesium-spinels have been seen in some asteroids, one possible explanation is that the source is exogenous asteroid or comet impacts. However, there is no evidence of any impact or dispersion of rubble pile and the like from the impact’s aftermath.

An interesting feature of the Moscoviense Basin is that the crust in the region is much thinner, compared to other basins. This is indicative of a magma upturning over much recent time scales as compared to other regions. Also this offers one possible explanation for the occurrence of magnesium-rich minerals because these are very dense and would have been deposited right at the bottom during the cooling and crystallization of the crust. The recent upturning may have brought it up from the lunar deep crust during the basin formation, Professor Pieters pointed out.

Lunar crust origin

But that still does not explain the localised nature of the anomalous regions that extend only about a few kilometres across, she said. “Creating foreign deposits without a trace of their origin is hard to do. We, therefore, favour a lunar crust origin,” she said. “But even that interpretation is not entirely satisfactory. We need to fully characterise the morphology of the anomalous regions with high resolution data from TMC [ISRO’s Terrain Mapping Camera] images,” she added.

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