Moon Monday #219: Results from India’s Chandrayaan 3 experiment to benefit future missions eyeing lunar water

The Chandrayaan 3 lander touted a thermal probe called ChaSTE developed jointly by two ISRO-affiliated institutions SPL and PRL. The lander deployed and inserted ChaSTE almost 10 centimeters into the lunar soil to take pristine temperature measurements across the lunar day using ten spaced-out sensors. A new study published in Nature shows how the measurements are helping scientists learn exactly how the Sun’s heat propagates down from the Moon’s surface, which has notable implications for planning future polar missions scouting for water ice.

ChaSTE sensors noticed a rather large temperature difference of 50–80°C between the two ends of the probe, owing to the Moon’s soil being a poor conductor of heat. ChaSTE’s measurements from Chandrayaan 3’s near-polar landing site of 69°S are unique because heat flow experiments on earlier Apollo missions and most recently on Firefly’s Blue Ghost lander were in near-equatorial locations, thus tailoring to different objectives that aren’t related to understanding water ice deposits on the Moon’s poles. “ChaSTE is the first ever in-situ thermal profiling of the Moon’s near subsurface,” said PRL planetary scientist K. Durga Prasad who is one of the experiment leads.

A heater just above the probe’s tip later warmed up the subsurface soil, which let scientists determine its thermal conductivity, and from it infer its density and physical properties. Future missions operating at or near the Moon’s poles need to account for these characteristics. Likewise, even though the ChaSTE experiment did not directly study water ice, its results have relevant implications for future missions looking for said icy deposits.

Combining multiple measurements and extensive modeling that accounted for Chandrayaan 3’s landed geometry, local topography, and actual observational conditions, the ChaSTE study derived high-resolution surface and subsurface temperatures at the landed region that were more accurate than those inferred coarsely from orbit using the Diviner radiometer on NASA’s Lunar Reconnaissance Orbiter. This led to a conclusion relevant to future water-ice hunting missions on or near the Moon’s poles:

ChaSTE observations have shown that at high-latitudes, sunward slopes are considerably warmer and poleward slopes just about a meter apart could be much cooler, providing an environment conducive for the presence of water-ice within the shallow sub-surface.
[...]
A relationship between the local slope and expected surface peak temperature has been derived. The interesting outcome is that the high latitude sites with local slope greater than 14° in poleward direction might offer similar environment as polar sites for accumulating water ice at shallow depths.

The ChaSTE experiment thus suggests that the local terrain and slopes play an outsized role in the ability of near-polar and polar areas to “pump” and accumulate water ice deposits in their subsurfaces. “Temperatures dictate the presence, stability, and mobility of water on the Moon,” said Durga. More on that in the paper:

Modeling studies showed that surface temperatures ranging from 110 K to 114 K [-163°C to -159°C] are suitable conditions for cold-trapping of water ice, particularly at polar regions. It is also suggested that water-ice can migrate to the subsurface due to strong thermal pumping, if the diurnal maximum temperature (Tmax) is above 120 K [-153°C] and the mean temperature (Tmin + (Tmax-Tmin/π)) is below 105 K. [-168°C] Also, the presence of a dry regolith layer can reduce sublimation rates, potentially burying water ice deposits (due to any transport mechanisms) under a few centimeters thick layer. Such buried water-ice can remain stay put as long as the temperatures are maintained below cold-trapping limits, especially during lunar nights.

Considering that local surface temperatures at the Chandrayaan 3 landing site hovered around -168°C by nighttime, it’s possible that several larger poleward sloping regions around Chandrayaan 3’s landing site are hosting subsurface water ice. More crucially, the findings lend mission planners a broader area for water ice prospecting instead of being limited to the treacherously rocky regions of 86-90°S. From the paper:

High-latitude regions with larger poleward slopes can also be potential sites for water-ice prospecting for future exploration and in-situ resource utilization. Such sites would be technically less challenging for exploration in comparison to extreme polar sites, but scientifically equally interesting for enhancing our understanding about the water ice quantification, distribution and migration.

While exploring high-latitude lunar regions between 70–85°S is more challenging—in terms of terrain, slopes, temperature ranges, and access to the Sun and Earth—compared to the relatively benign near-equatorial and mid-latitude regions, the high-latitudes are still not as extreme as 86–90°S. The Moon’s peak polar areas demand 100-meter precision landings as well as operations during local summers to survive and enable full-duration missions. As such, ChaSTE’s results suggest that future missions with relatively lower budgets and lower risk appetites could still explore water ice in either the less extreme near-polar areas or in peak polar areas despite shorter mission durations.

“Future studies and even extraction operations of lunar soil will benefit from ChaSTE’s data,” said Durga. The data is publicly available on ISRO’s ISSDC portal. The datasets and model codes used in the study are available on PRL’s website.

India’s next Moon mission Chandrayaan 4 aims to launch by 2028 and bring lunar samples from a landing site somewhere between 84-85°S. And the follow-on joint ISRO-JAXA Chandrayaan 5/LUPEX rover mission is considering landing sites around 89°S to directly study the nature, accessibility, and abundance of lunar water ice.

Also see: Composition of high-latitude lunar soil by the Chandrayaan 3 rover and its importance

Many thanks to The Orbital Index, Astrolab and Open Lunar Foundation for sponsoring this week’s Moon Monday! If you too appreciate my efforts to bring you this curated community resource for free and without ads, support my independent writing. 🌙

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• As background work to realize advanced technologies in the 2030s or 2040s, China has been working on aspects of versatile mining robots and giant lunar farside telescopes while Japan is eyeing self-adapting multi-robot explorers.