The future of space exploration

A city-size lava tube has been discovered on the moon. This tube could provide shelter for astronauts that could potentially allow them to live on the moon. Professor in the department of earth, atmospheric and planetary sciences at Purdue University, Jay Melosh, joins Hari Sreenivasan via google hangout to discuss the future of space exploration.


A city-size lava tube has been discovered on the moon.

This tube could provide shelter for astronauts and could, potentially, allow them to live on the moon.

Professor in the Department of Earth, Atmospheric, and Planetary Sciences at Purdue University, Jay Melosh, joins us via Google Hangouts to discuss the future of space exploration.

All right, this is fascinating to me.

First of all, how do we know that there's a tube inside the moon?

Well, we're not entirely certain.

The idea that there are lava tubes came from an experiment that NASA ran in 2012.

We had two satellites in orbit around the moon, very, very low, and we measured the distance between them with very high precision.

The idea behind that experiment was to measure the attraction of gravity in the moon's surface.

What we discovered is that, underneath the vast lava plains on the moon's surface, there were kind of sinuous or river-shaped deficits of mass, so that the attraction of gravity wasn't as big in these kind of sweeping, curved riverlike valleys.

We know of features like that on the Earth, much smaller.

They're called lava tubes.

And what they are, are feeder channels for the lava flows that ran empty.

And as the lava froze, they remained as tunnels.

We see those on the surface of the Earth, too.

But on Earth, they're not that big.

They're maybe 50 or 100 feet across, at most.

On the moon, they seem to be almost a mile in diameter.


There are two reasons for that.

One, of course, is the lower gravity of the moon, so you can support a bigger cave without collapsing.

But the other reason is that the eruption rate of lunar lava is much higher than that of the Earth.

We know that because we see open feeder channels, and they're vastly bigger than anything on Earth.

So, when the lava came up to the surface of the moon -- and that was about 4 billion years ago -- the lavas have had plenty of time to solidify, to cool off.

But when that lava came up, if flooded the near side of the moon, and the channels that fed the big flows then ran empty after the lava stopped welling up from the moon.

And as they emptied out, the surface at the top of these channels was able to freeze solid and to maintain a big tunnel.

So, how do we know the size of these tunnels?

You said that you can kind of extrapolate from the valleys that are on the surface, and we can say that they're probably underground, as well.

And I know you've done some math on the gravity.

But how do we know that... Let's say, it might be dome-shaped or it might be, you know, in a different shape in a different area.

And what kind of holes are kind of poking in and leading to it?

Well, you're exactly right.

We don't know the shape.

We do know the volume, though, because we know how much mass is missing, and that's a huge amount of material.

So it could be dome-shaped.

They could be round.

They could be almost any shape.

And we don't also know the depth below the surface.

For that reason, we really need to do some kind of a ground-penetrating radar.

And the thing that happened after we had announced the finding of these tubes is that a group with the Japanese Kaguya mission, which had a radar like that aboard, reanalyzed the data with a new point of view, realizing that they might be looking for caves, and they, indeed, found them.

One of our candidates and one of their data tracks cross each other.

They looked at the data.

They had to reanalyze it.

They did that, and they found reflections from both the top and the bottom of the tube, which tells us that it's at least, oh, about 150 feet between the top and the bottom.

That's a little smaller than what we inferred from the gravity deficit, so they may be picking up one of the smaller tubes.

At the moment, we don't know.

We really need to go back to the moon with a designed radar, that is intended to look at these things, to find exactly their depth below the surface and their shape.

So, if we have a station on the moon, is it more likely that it would be inside one of these tubes, versus on the surface, given that there's no atmosphere there and the astronauts are probably in danger of the kind of radiation of the sun?

Well, the whole rationale for thinking about living in caves on the moon is, in fact, the hazards.

The big one is radiation.

But, in addition, the temperature on the surface of the moon ranges from about minus-220 degrees to plus-230 or 240 degrees during the lunar day-to-night cycle.

So that kind of temperature variation can cause real problems.

And there are other hazards that lava caves would really allow us to get away from.

It's maybe a little inconvenient to get into these caves, but we know that there are several entrances on the surface of the moon.

They're called skylights.

That name derives from similar features on the lava flows on the Earth.

But we know that there are openings probably created by ancient meteorite impacts on the top of the tube.

But we could imagine lowering astronauts or supplies into there with a bunch of wire networks -- we're very familiar with doing that, from terrestrial mining operations -- and get the astronauts away from the severe surface hazards, particularly the radiation.

All right, Jay Melosh from Purdue University, thanks so much for your time.

It's a pleasure.