A new study led by scientists at the central Physical Research Laboratory (PRL) discovered a “cake-like” multi-layered regolith on the lunar surface that possesses varying porosity, temperature, and density.
Regolith is a loose material covering the bedrock of the Earth, Moon, and other celestial bodies. Scientists have established that the lunar regolith layer at 2 to 6 cm is highly cohesive, hyper-porous, and can show properties of a thermal blanket.
“This layer is crucial for the storage of water-ice molecules in the subsurface and in the selection of sites for future scientific base and habitat construction on the Moon,” the Indian Space Research Organsiation (Isro) said in a statement.
This is a first-of-its-kind thermal profile at lunar twilight with density and stratigraphic information of the lunar regolith. This data could be useful when planning future moon walks or for building lunar bases.
On August 23, 2023, India scripted history when its Chandrayaan 3 successfully landed on the lunar south pole, making it the first country to achieve this feat.
The site of Chandrayaan 3 landing was named the Shiv Shakti point. After nine days of scientific operations, the Vikram lander made a brief hop to a new site located 50 cm away. This hop on September 2 aimed to validate the re-ignition capability of the lander propulsion system to and assess its ability to generate sufficient thrust for controlled liftoff, a critical requirement for future lunar ascent and sample-return missions, Isro said.
Just like a vacuum cleaner blows away dust, Vikram’s hop forced the top regolith layer, up to about 3 cm, to blow off. It later exposed the underlying layers. The findings regarding the lunar surface and subsurface temperature profile, regolith density, and porosity have been described in the latest study, published in the Astrophysical Journal.
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After the hop, as all the onboard subsystems operated nominally, Isro redeployed two of its onboard instruments post Vikram’s touchdown to the new site. These instruments were the Chandra’s Surface Thermophysical Experiment (ChaSTE) and Instrument for Lunar Seismic Activity (ILSA).
ChaSTE consists of a rod-shaped probe with a sharp tip. It is fixed with temperature sensors mounted along its length and a heater mounted at its tip. This thermal profiler can measure the temperature within the top 10 cm of the lunar surface.
In its second deployment at the new site, ChaSTE performed temperatures measurements for three days and gave scientists a unique opportunity to assess regolith characteristics and spatial variability at local scales that too at a secondary location other than the Shiv Shakti.
As the new site was located further inside a crater or along a depression with a relatively higher slope, scientists were able to deploy only the bottom five sensors inside the lunar regolith.
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ChaSTE performed thermal profiling of the lunar regolith layers for 57 minutes (16.20 – 171.17 Lunar Time) during lunar twilight, which lasts for a few hours, unlike the Earth twilight that lasts for a few minutes before sunset.
Regolith porosity
The study concluded that the moon’s surface was not just a uniform pile of dust but instead had a distinct two-layer structure sitting within the top few centimetres. The thickness of the uppermost porous layer was found to be 6 to 9 cm. This multi-layer organisation showed how the lunar surface has been getting hammered and crushed by the micrometeorite bombardments, since the solidification of the lunar crust, said researchers at Isro.
For erecting a new structure on any surface, having good knowledge of the ground, its elevation, density and stability, and soil or regolith composition are among the most vital parameters required by civil engineering and architecture teams.
The bulk density of the regolith changed from 750 to 1,600 kg per cubic metre from the top surface to about 6.5-cm depth. In such a scenario, the astronaut would feel like walking on a layer of dry flour, as even at a few inches deep, it behaves more like damp and stiff clay.
Regolith temperature profile
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It was found that the temperature at the new site ranged between 273 to 293 Kelvin within the 4 x 4 km area. After landing, the location’s temperature recorded was 330 (± 3) Kelvin at noon. However, the temperature at this site peaked to 355 (± 0.5) Kelvin due to penetration on a sunward slope.
“The temperatures at all the depths showed a small and steady decrease until 17:00 Lunar Time, beyond which a sharp fall was observed. Interestingly, there was a quick reversal of temperatures recorded by the top two sensors, indicating a surficial regolith of relatively higher density or thermal conductivity than the layers below,” the PRL researchers said in the statement.
These results indicate the local-scale layering and heterogeneity in temperature and geotechnical properties at the landing site within just 50 cm of the Shiv Shakti point on the lunar south pole.
