47th Lunar and Planetary Science Conference (2016)
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DETECTION OF OH/H2O OF POSSIBLE MAGMATIC ORIGIN ALONG THE INNER FLANKS OF
CRATER PROCLUS. Sriram Saran1, Sumit Pathak1, Mamta Chauhan1, 2, Satadru Bhattacharya1, Anup Das1, Prakash Chauhan1, 1Space Applications Centre, Indian Space Research Organisation, Ahmedabad – 380 015, India
([email protected]; [email protected]); 2School of Earth Sciences, Banasthali Women's University, Banasthali Vidyapith, Rajasthan-304022, India.
Introduction: Remote spectral study of lunar surface based on measurements from Chandrayaan-1 Moon
Mineralogy Mapper (M3) instrument has revealed widespread hydration in the lunar surface, strength of which
increase towards the cooler higher latitudes, thereby implying that the Moon is not completely anhydrous [1-3].
Remote observations and direct impact experiments also
discovered the presence of water and water-ice in the
permanently shadowed regions (PSRs) of lunar poles [4,
5]. Moreover, in-situ measurements of lunar melt inclusions and lunar apatites revealed some parts of the deep
lunar interior to be hydrated [6-8]. Recent discovery of
enhanced hydration associated with endogenic water
from Compton Belkovich Volcanic Complex (CBVC)
[9, 10] and central peaks of Bullialdus [11], Theophilus
[12] and Jackson [13] craters hint towards the presence
of a hydrous lunar mantle. Also, [14] has reported the
detection of magmatic water in association with olivine
of possible mantle origin from the inner ring of Sinus
Iridium recently. Here we report the detection of water/hydroxyl of possible magmatic origin along the inner
flanks of crater Proclus in association with crystalline
plagioclase-bearing PAN rocks, olivine-bearing dunitic/troctolitic lithologies and low-Ca pyroxene-bearing
noritic rocks.
Geological setting: Proclus is a 28-km diameter
crater (26.4°E, 11.4°S) situated on the northwestern rim
of Crisium basin in the near side of the Moon. The rays
from Proclus crater can be distinctly seen and rays from
other craters are mostly overlain by Proclus ejecta. This
Copernican crater is characterized by a diverse mienralogy as described by [15] exposed along the steep inner
flanks, which could be attributed to the complex geologic setting being situated on the north-western edge of
Crisium basin.
Data used: Moon Mineralogy Mapper (M3) data
from Chandrayaan-1, having 85 spectral channels spanning over 460-3000 nm at a spatial resolution of ~140 m
[16], have been utilized that are photometrically and
thermally corrected [17]. We have also used the 12.6
cm-wavelength Mini-RF data obtained from the LRO
(zoom mode, 15×30 m resolution) to examine the relationship between physical characteristics and composi-
tional diversity of the Proclus crater region. Radar can
penetrate to a depth of about 10-times the wavelength
and is sensitive to blocks that are one-tenth to 10 times
the wavelength collected.
Results and discussions: Figure 1 shows the IBDAlbedo based FCC of the study area clearly depicting the
first-order mineralogical diversity that exist in the studied region. Spectral analysis of the central and eastern
part of the inner flank region reveals the association of
OH/H2O absorption features with crystalline plagioclasebearing PAN lithology, olivine-rich dunitic exposures
and also with olivine-crystalline plagioclase-bearing
troctolitic lithologies and low-Ca pyroxene-bearing noritic lithologies. Interestingly, the eastern, northern and
southern inner flanks exhibit a strong OH/H2O feature
near 2800-3000 nm. The features are sharp, narrow and
consistent with that of the hydration features associated
with nominally anhydrous minerals as observed by [11].
Also, the hydration feature is found to be localized along
the southwestern, southeastern, northern, northeastern
and eastern inner flanks of the crater, whereas the western flank does not show such enhanced hydration feature. The equatorial location and localized nature of the
hydration feature possibly suggest a magmatic/endogenic source.
Fig. 1. IBD-FCC of crater Proclus. Pink indicating PAN
rock exposures, orange highlights noritic lithologies and
purple regions indicate dunitic/troctolitic exposures.
47th Lunar and Planetary Science Conference (2016)
Fig. 2. Locations of the ROIs within the Proclus crater
and their corresponding mean spectra. A strong OH/H2O
feature is seen in association with olivine- (green spectra), crystalline plagioclase- (red spectra) and low-Ca
pyroxene spectra (blue). Black vertical line indicates the
position of the OH/H2O feature near 2800 nm. Spectra
of olivine (maroon), crystalline plagioclase (sea green)
and norite and/or troctolite (magenta) exposed along the
western inner flank of crater Prolcus do not show any
hydration feature.
Among the more readily apparent features in Mini-RF Sband circular polarization data (CPR) image are the radar-bright features associated with impact structures and
the presence of coherent bodies of impact melt towards
the northeast and northwest region of the crater. These
impact melt deposits typically have CPR values up to
and exceeding unity, making them relatively easy to distinguish from the background terrain. We have generated
an m-chi decomposition image of the Proclus crater region (Fig. 3) to understand the dominant scattering mechanism near the hydration features observed from the
M3 data as discussed above. While the pink regions in
the IBD FCC (Fig.1) are observed to have randomlypolarized backscatter constituents in Fig.3, we observe
portions of surface covered by orange in Fig. 1 that
shows a mixture of Bragg scattering and volumetric
backscatter which is somewhat surprising. This is not
expected for inner flank material of the crater and is instead more indicative of the surrounding mature lunar
regolith.
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Fig. 3. An m-chi decomposition image of the Proclus
crater region generated from the Mini-RF data. Here,
Blue indicates single-bounce (and Bragg) backscattering,
Red corresponds to double-bounce, and Green represents
the randomly polarized constituent.
Future work: Future work will include quantifying
mean CPR values for melts and ejecta at Proclus crater
region relative to background values. Additionally, employing more band depth and integrated band depth RGB
color composites with the M3 data sets will help us to
further discern the composition of these melts and if certain compositions can be related to physical property
variations seen in the images obtained at radar and optical wavelengths.
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