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Vikram finds temperatures of lunar soil varies significantly, depending on depth

Temperature range of soil at Vikram landing site

Based on data from one of the instruments on India’s lunar lander Vikram, scientists have found that the temperatures of the lunar soil at the landing site vary significantly, depending on depth. The temperature dropped from 55 degrees Celsuis to -10 degrees Celsuis when going from about 10 millimeters above the surface to about 82 millimeters below the surface, as shown in the graph to the right.

That’s equivalent from going from a summer day in Death Valley of 131 degrees Fahrenheit to a winter day in Minnesota of 14 degrees Fahrenheit, in a distance of only about 3.5 inches. While it was expected that there would a temperature drop, it appears the quick temperature drop just below the surface was faster than expected.

Genesis cover

On Christmas Eve 1968 three Americans became the first humans to visit another world. What they did to celebrate was unexpected and profound, and will be remembered throughout all human history. Genesis: the Story of Apollo 8, Robert Zimmerman's classic history of humanity's first journey to another world, tells that story, and it is now available as both an ebook and an audiobook, both with a foreword by Valerie Anders and a new introduction by Robert Zimmerman.


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  • Scott M.

    I’m tickled pink that India has earned the opportunity to do this. While I suspect that our host and I are united in thinking that their space program is far too top-down and government driven, the fact that they can do this is still worth celebrating.

  • pawn


    Check the graph again.

  • pawn: Oy. Thank you. I switched axis without noticing. Now corrected.

  • Kyle

    So you wont have to dig very far to find temperature conditions that favor ice formation.

  • Jhon B

    Ideal for a Geothermal Cooling System😁

  • J Fincannon

    The depth of 80 millimeters is 0.08 meter (3 inches or 8 cm).

    During Apollo experiments, temperatures were measured in drill holes down to 2.3 meters. It showed a gradual INCREASE in temperature from a depth of 70 cm to 2.3 meters. In fact, for Apollo 15, 16, 17, the slope is from 0.8 K/meter to 2.52 K/meter, depending on the site. So, for 1.35K/meter and a surface temperature of 253K, you need to go down 35 meters to get 300K (80 F). That would be comfortable. Do you believe it? You shouldn’t. Extrapolating data is always dangerous. But it does show we need more data. 35 meters is pretty far down to go anyway, but wouldn’t it be interesting for liquid water to be there?

    As to the Vikram results showing lower temperature, it suggests that there is a lower thermal conductivity in the few centimeters (the Apollo probes found this). Thus, the cold interior (cold because it was in darkness for 15 days) is holding its cold pretty well like a Thermos. It is not perfect and will heat up over 15 days of light. Apollo results stated that 30cm of regolith is enough to maintain a stable temperature for the thing it covers (also get some radiation shielding).

  • john hare

    Enough temperature differentiation to be a useful compact power source??

  • Jeff Wright

    Thinking Sterling engine?

    Cox’s timepiece/barometer clock below ground…

  • J Fincannon

    >Enough temperature differentiation to be a useful compact power source??

    No, the temperature difference is not the whole story in getting power. It is how much heat flux. There is not enough to power more than a milliwatt system. Apollo measured 3.0E-6 W/cm2 in the first few centimeters. 3E-6 =.000003. Even if you are able to spread out your absorber to 1 meter by 1 meter (100cm by 100cm) or 10000 cm^2, that it still .03 W. Using both sides of the absorber is .06W. Stirling is 50% efficient at best, so back to .03 W. Plus the bigger the size the more losses. It is just not the “best” (mass effective) solution for power. Also, there is a question of whether you draw out enough heat (or put it in) to use up the reservoir since the thermal conductivity is very poor.

  • John S.

    “No, the temperature difference is not the whole story in getting power. It is how much heat flux.”

    True dat, but drilling 30 meters down might be a different story, as poster above suggests. “Geo” internal temperatures, and the mechanism for sustaining such, are critical for the Moon, Mars, and even Earth, long term or even midterm. How deep have the Ruskies drilled over 30,000 ft? Suggesting a no-brainer for exploitation.
    As I commonly tell those NA admirals the “light” side of the Moon can pinpoint to the gnat’sA those Carrier Strike Forces, 24/7, or, at least every periodic 24/7.

    John S.

  • pzatchok

    I do not think dry lunar soil could possibly absorb enough heat to make a TEG generation possible.
    But a huge ice column could possibly do it.
    Sunlight for heat and deep core ice for cooling.

    But a sealed turbine nuclear generator would work far better. It doesn’t even need to run on water steam. As long as we can get enough thermal exchange with a geo system to cool the “steam”.

  • J Fincannon

    John S.,

    “True dat, but drilling 30 meters down might be a different story, as poster above suggests. “Geo” internal temperatures, and the mechanism for sustaining such, are critical for the Moon, Mars, and even Earth, long term or even midterm. How deep have the Ruskies drilled over 30,000 ft? Suggesting a no-brainer for exploitation.”

    Its a little bit (pun) challenging for drill 30 meters. Russians drilled deep on Earth. Again, you need the right heat flux. Geothermal vents on Earth do. The heat on Moon from deep underground comes from nuclear decay and cooling of the core, not the plate tectonics of Earth.

    So, while we have looked at it, it is not practical. A nuclear reactor (or radioisotope power system) is more so.

  • J Fincannon

    Yes, proper modification of the regolith can make it absorb more heat. There was an idea researched called “lunar thermal wadis” which looked into that. Using a solar concentrator aimed at the modified regolith helped. Adding other thermal mass as you suggest could help.

    But the nuclear approach is best and you do not need to cool the steam with anything other than a radiator pointing upward.

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