This is issue # 756 of the Bad Astronomy Newsletters, and as it happens 756 is an interesting number: it’s the sum of six consecutive prime numbers! 109 + 113 + 127 + 131 + 137 + 139 = 756 (the first such number is 41). If I’m thinking of it correctly, though, there are an infinite number of such numbers, so maybe it’s not really all that special. I just like numbers, is all. No apologies here.

Some exciting news about our one natural satellite: Re-analyzing data from 2010, a team of scientists has found strong evidence that there are underground caverns on the Moon. This is potentially a very big deal, because these chambers would be very useful for human habitation.

So what are these caverns? They’re not carved by erosion, since there’s no flowing water on the Moon. Instead, they were carved by flowing rock. In volcanic areas on Earth, lava tubes are common: leftover voids under the surface after lava flows create a channel, solidify at the top to create a “roof”, and then the lava flows on to leave an empty tube behind. I hiked in one in Oregon once, and it was extremely cool.

We see rilles all over the Moon; channels carved by lava. So it makes sense there should be lava tubes as well. We do see lots of evidence for them. Scattered across the lunar surface are pits, deep holes that look to be where the roof of such a lava tube has partially collapsed (sometimes also called a skylight, for obvious enough reasons). One of the best is the Mare Tranquilitatis Pit, a hole about 100 meters across; in images taken by the Lunar Reconnaissance Orbiter (LRO) it looks to be about 100 meters deep leading to a floor with boulders scattered around it, probably pieces of the collapsed roof.

Using new processing techniques on older LRO radar data, the scientists made a map of the chamber underground and it appears to extend for at least some tens of meters, but could be much longer [link to journal article]. To be honest the news isn’t surprising, since the pit really, really looks like a skylight in a lava tube. But there was no direct evidence for the tube itself (maybe the pit leads to a small bubble under the surface only a bit wider than the hole itself). This radar data seems to show it extends much farther. Even this evidence doesn’t clinch the deal, but it’s very promising.

Living on the Moon would be hard; conditions on the surface suck, to say the least. There are huge temperature swings due to the lack of atmosphere, and while the radiation environment is bad under normal conditions, it’s deadly during a big solar storm. For future lunar explorers, lava tubes are extremely handy places to live. The temperature is more constant, and the many meters of rock in the roof is a natural shield from radiation (fans of the show For All Mankind are already familiar with this idea; in it NASA astronauts explore and mine ice in a pit near the Moon’s south pole).

There are issues, of course. Getting down to the bottom could be tricky. The roof is likely pretty stable — the Mare Tranquilitatis Pit is probably millions of years old, at the very least — but having a lot of people and equipment stomping around on it might not be the best idea. However, there are lots of other pits and other places to look for extant lava tubes (ground penetrating radar on the surface should reveal them easily enough once people are there). Also, if you live down below you won’t see the sky unless you go for a walk on the surface.

I wonder if, in the future, we’ll build habitats on the surface and cover them with rock for shielding, and use the lava tubes for storage or some other purpose. Living in in one isn’t ideal, but I can guarantee one way or another they’ll prove very useful.

• The last mirror blank has been cast for the European Southern Observatory’s massive Extremely Large Telescope, a 39-meter wide behemoth currently being built in Chile. The primary mirror will be made up of 798 smaller hexagonal mirror segments, each about 1.5 meters across but only 5 centimeters thick. They’re made of a material that’s extremely durable and resists expanding and contracting due to thermal changes. The blank still needs to be ground and polished, and will travel around the world to be completed. The ‘scope won’t be done until 2028, but when it is, well, wow. It’ll be by far the biggest visible light/infrared telescope in existence. I’m very excited to see what it will do.
  

• A big mystery in astronomy right now is whether globular clusters (spherical clusters of hundreds of thousands of stars) can host an intermediate black hole in their cores. These black holes are a few hundred to a few hundred thousand times the Sun’s mass, intermediate between ones with a few solar masses and a few million. Lots of tantalizing evidence shows they exist, but no smoking gun has been found (though Omega Centauri looks like it has one, and 47 Tuc may have one as well, and I have info at that link about these kinds of black holes). Simulations show they have a hard time forming; as a smaller black hole eats stars and other black holes, it emits off-center gravitational waves that can act like a rocket, propelling it out of the cluster. But new work simulating how stars form in a globular cluster shows these black holes can exist there! [link to journal paper] The new sims are very sophisticated and can track the formation and motions of millions of stars, something that was too computer-intensive to do before. If this holds up, it shows that globulars can indeed make and host intermediate black holes… and now all we need to do is find one. Easier written than done.

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