New Insight about Harbouring Water-ice on the Moon

By R Anil Kumar

Bengaluru. India bags the credit for the first-ever in-situ measurement of the temperature of the Moon’s surface, down to a depth of ten centimetres, at the Southern higher latitude.

So far, the temperature measurements from the subsurface of the Moon by the Apollo 15 and 17 missions focussed to a larger depth of few meters, but direct measurement of temperatures within the topmost fluffy layer is not available.

However, in order to assess the propagation of the solar heat flux down to the lower layers of the Moon’s soil, one needs to probe first few centimetres down the Moon’s surface (the epi-layer), which is exactly the unique feat achieved by the ChaSTE payload in Chandrayaan 3, said V. Narayanan, Chairman ISRO, Speaking exclusively to R. Anil Kumar, India Strategic, on the sidelines of Nano Electronics Roadshow and Conference.

That too, strategically measured near a high latitude in the Southern polar region of the Moon for the first time and marks a major progress in lunar science, as well as harnessing of lunar resources at those latitudes in future, Narayanan said.

ChaSTE observations from Chandrayaan-3, carried out for a significant fraction of a lunar day, revealed that the Moon’s surface temperatures show a significant spatial variability at metre scales at high latitudes, unlike at the equatorial regions.

To elaborate on the scientific importance of this discovery, only Apollo 15 and 17 missions have provided the in-situ data and merely for the equatorial regions of the Moon, while global surface temperatures have been mapped through remote sensing.

No in-situ measurement was, so far, available from polar regions of the Moon, until the Chandra’s Surface Thermophysical Experiment (ChaSTE) experiment onboard Chandrayaan-3’s Vikram lander investigated the temperature profile and thermophysical properties within the top 10 cm of the lunar surface at a high latitude south polar landing location.

After the Vikram lander has achieved soft landing at the Shiv Shakti point on the Moon, ChaSTE probe was deployed and successfully penetrated into the lunar soil to carryout measurements for the entire duration of the mission. ChaSTE in-situ measurements were carried out for a significant fraction of a lunar day (~8 AM – 4 PM Local time at the Moon) i.e. approximately 10 Earth days (i.e. from 24 Aug. – 2 Sept. 2023) at an interval of about a second.

A team of scientists from Physical Research Laboratory, Ahmedabad; Space Physics Laboratory-Vikram Sarabhai Space Centre (SPL-VSSC), and ISRO, have reported these first-ever in-situ temperatures up to a depth of 10 cm inside the lunar surface near the southern polar region of the Moon at 69.37o S. These temperatures were measured with ten numbers of temperature sensors, mounted along the length of a probe, at different distances.

The following figure shows the ChaSTE thermal probe and the locations of the temperature sensors mounted along it.

The peak surface temperature at Shiv Shakti landing site was measured to be 355 K (± 0.5 K), a temperature relatively higher than ~330 K (±3K), predicted by earlier observations.

This unexpected higher temperature is due to penetration of ChaSTE on the Sun-ward (equator-ward) facing surface with a slope of ~6o. Temperatures estimated using 3-D thermophysical model developed by PRL, and appropriately applied for the Chandrayaan-3 landing conditions, is consistent with ChaSTE in-situ measurements.

Lunar surface temperature measured from a flat surface using an independent sensor, about a metre away from ChaSTE location, was found to be ~332K (±1K), which is consistent with orbiter based remote sensing observation (~330 K).

Therefore, ChaSTE observations indicate that the lunar surface temperatures show a significant spatial variability at metre scales at high latitudes, unlike at the equatorial regions. These effects become prominent as we move towards poles, an important aspect that should be considered for future exploration.

The Moon’s surface thermal environment is among the most extreme of any planetary body in the solar system. Lunar near surface temperature and thermophysics are essential parameters that not only dictate the stability of water-ice/volatiles, but also important for lunar geology and geophysics, resource exploration, mission safety and establishing sustainable long-term habitats on the Moon.

Using 3-D Model calculations based on ChaSTE measurements, a relationship between the local slope and expected surface peak temperature was derived to assess the possibility of water-ice stability within the region. An important outcome of these simulations is that the high latitude sites with local slope higher than 14o towards pole might offer similar environment as polar sites for accumulating water ice at shallow depths of few 10s of centimetres. These locations could be promising sites for future lunar exploration and habitation. Such sites are not only scientifically interesting but also pose less technical challenges for exploration in comparison with regions closer to the poles of the Moon.

ChaSTE experiment is jointly developed by Space Physics Laboratory (SPL) and Physical Research Laboratory (PRL), Ahmedabad with the help of various entities of Vikram Sarabhai Space Centre (VSSC), Thiruvananthapuram and Space Applications Centre, Ahmedabad.

This result has both scientific and technological significance. Knowing how heat propagates through the Moon’s top layer of soil, just within the first few centimeters, is really important for understanding the depth of influence of surface heat. Because the Moon has no air, this layer gets super-hot and super-cold, which changes how it acts.

By understanding how well the surface layer conducts heat and how much heat it can hold, as done by ChaSTE, scientists can figure out how heat moves around, predict temperatures below the surface, and see how sunlight interacts with the Moon. This would also help engineers to find subsurface locations with benign thermal environment and design safe places to plan future trips and live on the Moon. In addition to this, knowing how the soil responds to heat tells us how meteoroids and solar wind changed the Moon’s surface over time, giving us a better idea of its unique environment.

These measurements will also complement and supplement the remote sensing based global temperature measurements, while the finer in situ probing helps to look for any embedded water-ice within the Moon’s subsurface from the local temperature signature. This important result will also help refining our understanding about the energy balance at the Moon, by considering the solar heat forcing, re-radiation of the heat by the Moon’s surface, and absorption of the heat. This is another important aspect that help planning scientific measurements for the future lunar missions.