CHANDRAYAAN DISCOVERS WATER

Chandrayaan's M3 discovers new lunar rock type

The Moon Minerology Mapper (M3) on Chandrayaan1,

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. The rocktype

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

ironmagnesiumadmixture of the form (Mg, Fe)(Al, Cr){2}O{4}.

Theserocks are usually found along with magnesiumironsilicate (olivine) andcalciumrich

aluminium silicate (pyroxene).

Unique feature

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 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 ApolloLuna

missions and meteorite samples. The crust is described as a rocky accumulation,

basically rich in calciumaluminium 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 Chandrayaan1

from its initial 100km 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 4603000

nanometreswith a spectral resolution of 2040 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. Interestingly, no unusual feature or any compositional boundary

was seen for any of these areas.

Calciumrich 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 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 magnesiumrich

spinel region

has been identified. "The clear interpretation of these spectra is that the

surfaces represent a new rock type dominated by magnesiumrich

spinel

with no other detectable mafic minerals.

No easy explanation

There does not seem to be any easy explanation for the occurrence

of these spinel formations. Since magnesiumspinels

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.

Lunar crust origin

But that still does not explain the localised nature of the anomalous

regions that extend only about a few kilometres across.Creating foreign

deposits without a trace of their origin is hard to do. But even that

interpretation is not entirely satisfactory.