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The Vikram lander on the lunar surface with the ChaSTE probe deployed. ISRO (Indian Space Research Organisation)ShareWe tend to think of the moon as a cold, dusty rock, but in fact, its surface can get pretty hot during a lunar day. How hot? It turns out that the answer can vary dramatically over a very short distance.When the Chandrayaan-3 mission touched down on the moon in August 2023, one of the first things it did was measure the surface temperature. The resultsdetailed in a study published March 6 in the journal Communications Earth and Environmentwere both unexpected and intriguing. They also raise tantalizing implications for the biggest lunar question of all: how much water is up there? Get the Popular Science newsletter Breakthroughs, discoveries, and DIY tips sent every weekday. By signing up you agree to our Terms of Service and Privacy Policy.During the Chandras Surface Thermophysical Experiment (ChaSTE), the spacecraft took the first measurements of this kind since those taken by the Apollo missions over half a century ago. While Apollo landed near the moons equator, Chandrayaan touched down at a latitude of 69 degrees northwell into the moons previously unexplored polar regions.Scientists expected the surface temperature on this part of the moon to be in the region of 330 degrees Kelvin (around 57 degrees Celsius or 134 degrees Fahrenheit.) The landing site was significantly hotter: a scorching 355K (82C or just under 180F), at least 20K higher than predicted. However, barely a meter away, the surface temperature was only 332K (59C or 138F). Thats still hot, but much less so than the landing site and more in line with what scientists expected.Such a dramatic difference in a distance of just over three feet turns out to be explained by one simple fact. Chandrayaan-3 ended up perched on a shallow slope, at an angle of about 6 toward the equator. The cooler region, meanwhile, was essentially flat. It seems difficult to believe that this relatively small difference in slope could explain such a large difference in temperature. According to K. Durga Prasada study co-author and a Faculty Member at the Planetary Sciences Division of the Indian Governments Department of Space, the measurements actually illustrate two important facts about the moon.The first is that the lunar surface is covered by a thin layer of dust and rock fragments, known as the fluff layer. This layer is characterised by its very low thermal conductivity and high porosity, Prasad tells Popular Science. In other words, its exceptionally bad at conducting heat. This poor conductivity means that heat does not diffuse across the surfaceif a piece of ground is hot, it stays hot, even if its right next to a relatively cold area.Whether a piece of ground is hot, Prasad says, depends on the incident solar flux, or how much sunlight falls on it. Given the lack of trees or other sources of shade on the moon, the key factor here is topography. If the rest of the terrain is on an equal level, a piece of ground that slopes away from the sun will be colder than one that slopes toward the sun. Just as importantly, as the angle of the slope increases, so does its effect on the incident solar flux.Crucially, this last effect depends on latitude. As you move away from the equator, the angle at which the suns rays strike the moon becomes more obliqueand so does the degree to which a given variation in the angle of a slope will have on the amount of sunlight that slope receives. At the moons poles, even small variations in topography can make for significant differences in surface temperatureas demonstrated neatly by Chandrayaan-3s landing site measurements.But how does all this relate to the amount of water that may or may not be present on the moon? According to Prasad, there are two factors that created the large temperature in those measurementspoor surface thermal conductivity and increasing sensitivity to topographic variation away from the equator. Both of these also affect the moons subsurface temperature, which plays a large role in determining the moons ability to accumulate and retain water.The fluff layers poor thermal conductivity also means that heat has a hard time propagating downward into the lunar regolith. This lack of heat creates a significant difference between the temperature of the surface and the temperature of rock thats only a few inches down. The thickness of the fluff layer at a given point is the dominant factor in determining the difference between the surface and subsurface temperatures.Due to the low conductive nature of the surficial layer, heatpropagates very slowly to the subsurface, [and] a large temperature difference is seen between the surface and subsurface temperatures, Prasad says.On the other hand, [where] the thermal conductivity is high, the heat propagates faster into the subsurface, resulting in [a relatively small] temperature difference.The subsurface temperature is important when it comes to water. Its one of several factors that affect the accumulation of water ice, but according to Prasad, the main point is not just accumulation of water, but [the] migration [of water[ to the subsurface and [it] staying put for longer periods of time. These processes, he says, are largely dependent on temperature, both at the surface and below.Scientists have long predicted that whatever water might be present on the moon would most likely be found at the lunar poles. However, this new study uses the Chandrayaan-3 data to suggest that there might also be more water in the regions surrounding the poles. The terrain in these regions that has a large enough slope away from the sunabout 14, according to the calculations in the papermight be just as good places for water as the actual poles themselves.The present study indicates that [such locations] offer a similar environment as polar sites for accumulating water ice at shallow depths, says Prasad.So how much extra water might be present? Thats difficult to say and its certainly not as simple as multiplying the extra depth by the surface area of suitable slopes.The whole process of accumulation and storage of water-ice, Prasad says, is quite complex.Nevertheless, the data gathered by the Chandrayaan-3 mission is already allowing for improved models of the migration and stability of the water-ice for different representative locations on the moon. Ultimately,the paper demonstrates just how important it is for astronomers to gather data first-hand.Based on the temperature profiles from ChaSTE as ground truth, [we] can model the migration and stability of the water ice for different representative locations on the moon, says Prasad. [The aim is] to obtain a comprehensive understanding and distribution of water ice on the moon.