I’m pretty sure the white stuff that appeared on the ground is frost. Looking at it from this angle and other angles you can see the frost slowly forming and then melting again when the water hits it.

I think this is best explained by the intense shock waves decompressing the air, which causes it to cool down and take heat from the ground (of course they also compress the air by a lot, but with shock waves the time interval that the air is below atmospheric pressure can be much longer than the time interval that it is above atmospheric pressure). interestingly, there are also thermoacoustic refrigerators that function in a similar way.

Edit: I now think that cold gases from engine startup are more likely to have caused this, because as the static fire just started in the video you can see spots freezing over as those gas clouds approach them.

Taken from the overhead drone shot https://twitter.com/SpaceX/status/1695159316670718231

  • @[email protected]English
    5•1 year ago

    decompressing the air, which causes it to cool down

    losing temperature and changing phase due to decompression is not the same as what we colloquially call “cooling down”. What we call cooling down is a slowing of molecular kinetic energy by direct transfer of that energy to somewhere else. Decompression inhibits that transfer entirely by dramatically increasing the space between molecules.

    • threelonmusketeersMEnglish
      4•1 year ago

      losing temperature and changing phase due to decompression is not the same as what we colloquially call “cooling down”

      How is it not the same? Unless the expansion is completely adiabatic into a vacuum, any gas will cool down as it expands by doing work on its surroundings.

      That being said, I’m still not entirely convinced that a shockwave could cause sufficient decompression to freeze a noticeable section of the ground.

      Edit: Confused “adiabatic” with “expansion into a vacuum”.

      • @m_rlnOPEnglish
        2•1 year ago

        I reckon your skepticism was warranted, looked at the video some more and now have a better explanation i think. i updated the post.

      • @m_rlnOPEnglish
        2•1 year ago

        yes, although adiabatic just means that there is no heat transfer between the gas and its environment, so the gas itself could still change temperature in an adiabatic process. but yeah if there’s no work done by the gas then it doesn’t change temperature.

        at this point i’m quite convinced but i’d love to hear an alternate explanation for the frost

        • threelonmusketeersMEnglish
          2•1 year ago

          Ah yes, you are correct. I was confusing “adiabatic” with “expansion into a vacuum”. I need to review my thermodynamics.

    • @m_rlnOPEnglish
      2•1 year ago

      as threelonmuseteers said, as long as the gas does some kind of work while expanding (which could just be exerting a force on the surrounding air), it will expend energy which will cause it to cool down.