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Tuesday, September 21, 2021

Scientists reveal ‘unambiguous presence of water ice’ at permanently shadowed regions of Moon

The ability to combine polarimetric radar images from two wavelengths has also brought forth subsurface features and the polarimetric data helps in identifying the distribution of impact melts.

Written by Sohini Ghosh | Ahmedabad |
Updated: September 8, 2021 9:36:19 am
Permanently shadowed regions (PSR) have largely remained inaccessible as no sunlight reaches these regions, making it difficult to get images. (PTI)

One of the eight payloads of India’s second Moon mission, Chandrayaan-2, developed at Space Applications Centre (SAC) at Ahmedabad, has detected an unambiguous presence of water ice at the permanently shadowed regions of the Moon, revealed scientists of the Indian Space Research Organisation (ISRO) on Tuesday, the second day of a two-day lunar science workshop.

Permanently shadowed regions (PSR) have largely remained inaccessible as no sunlight reaches these regions, making it difficult to get images.

The Dual Frequency Synthetic Aperture Radar (DFSAR), one of the eight payloads on board Chandrayaan-2, is the only full polarimetric radar sent on a planetary mission in the world so far and its capability to combine radar images from two wavelengths, allows it to differentiate surface roughness properties from water ice properties.

Earlier studies using hybrid-polarimetric SAR data led to ambiguous detection of water ice regions as it had similar sensitivity to surface roughness and water ice. However, full polarimetric DFSAR, which uses measurements of electrical properties of materials, can decouple the effect of water ice and surface roughness, “leading to encouraging results on unambiguous detection of water ice in some PSRs”, as stated in the public documents released by ISRO chairperson K Sivan on Monday.

Potential patches of “dirty ice” within the Cabeaus crater on the lunar south pole, were also detected by the radar instrument. Patchy dirty ice involves ice crystals mixed with the lunar regolith, unlike continuous sheets of ice. Regolith is the top surface of the moon extending upto three to four metres, consisting of loose deposits.

The ability to combine polarimetric radar images from two wavelengths has also brought forth subsurface features and the polarimetric data helps in identifying the distribution of impact melts.

“This is very very essential to get information on what kind of impact cratering took place and how the impact melts distributed around the craters,” said Anup Das of Ahmedabad’s SAC and part of the DFSAR science team, during his presentation.

“One benefit (of DFSAR) is, we are seeing better resolution so we are seeing more number of smaller craters and scattering mechanism is more prominent here… this data gets finer details of smaller craters compared to Mini-RF (miniature radio frequency on the lunar reconnaissance orbiter launched
by NASA in 2009),” added Das.

Freshness of a crater indicates that it has not been exposed enough to space weathering and in the polar regions of the moon, the findings can be expanded to further estimate the age and impact processes that the craters or boulders or other subsurface structures underwent over the years.

The discussions also promised a better imaging of the PSRs by combining data and image results from orbiter high resolution camera (OHRC), one of the payloads designed for imaging in very low Sun illumination conditions, along with DFSAR results.

Presenting the science results from OHRC, scientist at SAC, Ahmedabad, Aditya Kumar Dagar said that while OHRC imaging has “high constraints”, the data can be used to understand boulder distributions around a fresh crater. Boulder is important to understand regolith formation and also to determine future landing sites so that the lander is not jeopardised.

Chandrayaan-2 mission director Ritu Karidhal said that after two years of operation, “the propellant is enough to support more than four (more) years of life”.

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