Oct 192017
 

Some interesting recent developments:

Atlas Of The Underworld

When a continental plate is subducted beneath another, you’d imagine that it would melt away to liquid hot magma and become part of the gooey inner mess. but as it turns out, some of these plates can stick around in a more or less solid state for a *long* time, and they can be mapped using seismic tomography. Around a hundred of these slabs have been mapped out, down to several hundred kilometers in depth, still recognizably more-or-less solid chunks drifting in the mantle. This means that there *could* *maybe* *possibly* be some really interesting things down there where we’ll almost certainly never get to see them. Say, 300 million years ago something truly remarkable evolved on some small continent… say, an archosaur developed smarts *real* fast and became human-smart and built a whole civilization on their island, then they were wiped out by disease or aliens or a high tax rate married to an unwise nanny-state system of government. Their cities of concrete and stone were buried under dust and mud and ash, then fifty million years later the plate was subducted. Meaning that a hundred miles below your feet there might still be recognizable evidence of a far-pre-human terrestrial civilization, one you’ll almost certainly never get to hear about.

And…

Detection of intact lava tubes at Marius Hills on the Moon by SELENE (Kaguya) Lunar Radar Sounder

These aren’t piddly little tubes, either. These are big enough to plant whole cities within, inside where they’d be protected from radiation and thermal cycling.

 Posted by at 9:54 pm
  • Paul451

    Re: Lava tubes.

    As you go underground, the temperature tends to the average of the surface temperature. Not sure what that works out to for the moon, but if it’s below freezing, then any skylights or gaps in the tube will allow gas from comet impacts to condense inside the tube, as it does in permanently shadowed polar craters.

    So you could get radiation shielding, thermal stability, a handy heat-sink, and a billion years of accumulated water ice.

    • Scottlowther

      From: http://www.lunarpedia.org/index.php?title=Lunar_Temperature

      “so at the equator T is about 296 K, or a comfortable 23 degrees C
      if you bury yourself sufficiently. At 60 degrees that drops to 249 K or
      -24 degrees C. The average subsurface temperature near the poles (85
      degrees and higher) would be below 160 K or -110 degrees C”

      The Marius Hills are fairly close to the lunar equator so they’re probably too warm. But similar features elsewhere may have ice, assuming they have large openings. A lava tube that doesn’t open to the surface is probably filled with little more than vacuum and dust.

  • Poppa Smurf

    So let us assume there was some ancient civilization from 300 million years ago. Do we have any type of ground penetrating radar that can go deep below the mantle? And if so can it look for anomalies like straight angles or unnatural deposits of certain materials that would be signs of an artificial construct?

    • Gregory Wright

      Under the best conditions, low frequency ground penetrating radar can go through tens of meters of rock, so the mantle is inaccessible. The situation is different with ice; radar is used to map the ice-rock interface in Antarctica, which is two to three kilometers below the surface of the polar icecap.

      • sferrin

        If they can do that with ice, why not water? (As in mapping the ocean floor.) Is it due to salinity/conductance?

        • Scottlowther

          I can’t answer the “why not water” question, but I suspect the first customer for the system wouldn’t be going “hey, neato, I’ve mapped the seafloor,” but “hey, neato, look at this here metal tube slowly cruising around thinking it’s all stealthy n’shit.”

          • cygnus_darkstar

            A Sagan-esque number of dollars (and roubles) have been sunk into non-acoustic detection methods over the last 60 years or so. No luck so far.

          • FelixA9

            Yep. I knew about the ELF program (the antenna are huge), and that radar is pretty useless for trying to look under water. I just didn’t know why it would work with ice but not water.

        • robunos20

          Reflection at the surface, and absorption of the energy by the water. Have a look here . . .
          https://www.quora.com/Why-cant-radio-waves-transmit-through-water
          Extremely low frequencies(long wavelengths) penetrate water well, but of course, as wavelength gets longer, resolution gets worse . . .

        • Gregory Wright

          Fresh water is lossy because of the polarizability of the water molecule.
          (Ice is mostly transparent at RF because, when frozen into the ice crystal lattice, the water molecules can’t bend.) Salt water is very lossy at RF because of the ions from salt, which make it slightly conductive.

          Typical RF penetration into salt water is just centimeters. However, there was a Extremely Low Frequency (ELF) system for communicating with submerged submarines developed in the 1960s through the early 80s. It was named ‘Project Sanguine’ and could penetrate several hundred meters into salt water. To do this required working at low frequency – 45 or 76 Hz (that’s Hertz, not kiloHertz or megaHertz). The data rate was correspondingly slow: 15 minutes to transmit a three letter code word.

          Frequencies that low are useless for radar. The wavelength at 100 Hz is 3000 km, so continents or oceans could be resolved but it would be impossible to, say, map the seafloor.

    • Scottlowther

      Ground penetrating radar can penetrate *yards* deep, so that’s no good. The way the slabs were located was basically by listening to the Earth.An earthquake or explosion goes off somewhere, the sound waves pass through the Earth. the speed of the waves travel varies based on what it passes through, so the difference in travel times can give you a crude idea of the layout of things. Do it often enough, in enough locations, and that crude idea can crystallize into a clearer image. To build up enough resolution to spot, from 100 km away, something regular the size of, say, the pyramids would doubtless require a vastly more sensitize and far broader system than exists now. There is undoubtedly a theoretical limit to what could be seen, but I imagine a city of regular pyramids could theoretically be detected. Whether there’ll ever be a way to say “this is a city of artificial pyramids” rather than “this here is a wacky collection of giant regular crystal points,” I couldn’t guess.

  • robunos20

    So if you’re going to build your lunar city in this cavern open to the surface, what’s the best way to do it? Leave the cavern open to vacuum, and build a pressurised city as it would be on the surface, or seal the cavern and fill it with an atmosphere?

    • Scottlowther

      Sans a detailed structural analysis of the lave tube, I would say that the safest approach would be to build a self-contained “city” that does not rely on the lava tube for anything but shielding. If you pressurize the tube, you risk adding stresses to it that the tube cannot handle. Additionally, you will be pressurizing a lot of wasted volume, and you’ll use a lot of your pressurant gas to diffusion through the rock and leaks through cracks unless you seal the entire surface. And there could be unfortunate chemical reactions… some of the regolith will happily rapidly oxidize if you introduce “air” to it, as the Apollo astronauts found, with the dust on their suits smelling like burnt gunpowder after reacting with the LEM atmosphere.

      These tubes might be great for initial colonies, but long term the colonists will need to learn to build either on the surface and shield, or dig under the surface and bury. There almost certainly won’t be enough of the tubes to cover the whole of lunar colonization potential. It’d be like pilgrims landing in Massachusetts and finding a Home Depot… great to help you get started, but you’ll still need to learn to get along without it.

      • robunos20

        That’s about what I thought.So, for the early phases use Bigelow-type inflatable habitats, protected by the lava tubes, these would be easy to transport in large numbers, when deflated, then, when the ability to smelt metals from the lunar rocks is established, switch to Convair Atlas-style metal ‘balloons’ made on the Moon, still inside the tubes, then extend these cities by tunneling, still using the metal ‘balloons’ . . .

  • Jeffersonian

    I seem to recall the subduction angle being a major plot point in David Brin’s Uplift series.

    As for the lava tubes, that doesn’t sound too far removed from Heinlein’s The Moon is a Harsh Mistress.

  • sferrin

    “Say, 300 million years ago something truly remarkable evolved on some small continent… say, an archosaur developed smarts *real* fast and became human-smart and built a whole civilization on their island”

    They bailed millions of years ago and now fly Stardestroyer-sized ships in the Delta Quadrant.

    • Scottlowther

      That episode irked me. the *idea* of the episode was fairly cool, but so much of the execution was just… irksome.

      “Computer: show me a parasaurolophus and then project what it would look like after 75million years of evolution.” Yeah… no.

      • publiusr

        Should have been Troodon