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
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
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
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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