The Gravity Assist Podcast is hosted by NASA’s Director of Planetary Science, Jim Green, who each week talks to some of the greatest planetary scientists on the planet, giving a guided tour through the Solar System and beyond in the process. This week, he is joined by lunar expert Sarah Noble to discuss how the Moon was formed, lava tubes and moonquakes, the “dark side of the Moon,” and mysteries we have yet to solve about Earth’s nearest neighbor.
Here’s a short teaser of this week’s podcast:
Jim Green: What do you think we have yet to learn about the Moon? Do we know everything we need to know?
Sarah Noble: No, we certainly don’t know everything we need to know. We’ve been to a few places on the Moon. It’s like if you were visiting Earth and you went to Iowa and North Dakota and then said you were done – “oh, we’ve seen the whole planet, right?” and you haven’t. The planet is big and the geology is varied. We know from remote sensing that there is geology on the Moon that we haven’t gotten to yet. There are rock types on the Moon that didn’t exist in the places where we went with Apollo that we haven’t yet sampled. So there’s still plenty we don’t yet understand about the Moon.
Jim Green: One of our missions now, the Lunar Reconnaissance Orbiter, is still operating at the Moon, making spectacular measurements. What additional things is that telling us?
Sarah Noble: We’ve learned a lot. The Lunar Reconnaissance Orbiter has been up there now for many years and it has been giving us a fantastic record [of the lunar surface]. Some of the coolest things I think it has taught us is about are things currently happening on the Moon. We can actually see impacts. We have imaged the Moon many times now and we can find new impacts that weren’t there the last time we went around. So we know they’re brand new things that just hit the Moon. That’s pretty exciting to be able to see the changes on the Moon in real time. And we can see down to incredible detail with these cameras. We can look at the Apollo landing sites and see the footprints that the astronauts left behind, which is just an amazing ability.
Jim Green: One of the ones I like are those of Apollo 17, where you can see the huge [areas] that they walked [across] and, of course, they had their neat little rover car, which is still on the Moon.
Sarah Noble: Yeah, you can actually see the flags. I like that. [At the] Apollo 11 [landing site], the flag got knocked over. But, [at the landing sites of] many of the other missions, you can actually still see the flags and the shadow that the flags are making on the ground.
Jim Green: It got knocked over because it was too close to the LAM, the Lunar Ascent Module, and the rockets blew it over.
Sarah Noble: After that, we learned to put [the flag] a little further away.
Jim Green: From Earth we see the Moon, but we only see one side of the Moon. No matter where you are on Earth and during the whole month that it takes for it to go around the Earth, we only see one side. Many people think that the back side of the Moon is the dark side of the Moon. How can we explain that better?
Sarah Noble: The back side of the Moon gets the exact same amount of sunlight as the near side of the Moon. As it’s traveling around the Earth, it is also seeing the Sun. Just like we have day and night on the Earth, the Moon has day and night, as well, although the Moon’s day is two weeks long, as is its night as it travels around.
Jim Green: When I was young what I really enjoyed when watching the video footage of the astronauts as they walked around the Moon is they were bouncing up and down and they were really kicking up the dust. What’s that all about?
Sarah Noble: Because the Moon is smaller than Earth, it has less gravity. It has about one-sixth the gravity that we have on Earth, so you’d weigh about one-sixth as much. And so, even though their spacesuits are actually big and heavy, the astronauts still weigh a lot less. So it’s pretty easy to bounce. It is a little tricky to walk, though, and it takes some time and effort to learn to get your Moon legs and figure out how to walk. If you watch the astronauts, particularly early in their missions, they’re stumbling around a lot. They took a lot of falls. It was not super easy. The [Apollo-era] suits didn’t help, either, since they didn’t have a center joint, so the astronauts could not bend forward, which made it very difficult to lean over and pick up rocks.
Jim Green: The Apollo astronauts brought back rocks from the Moon and other things like regolith [surface material]. Where do we store that material and what are we learning from it, and are those lunar samples still useful today?
Sarah Noble: Absolutely. The rocks are mostly kept in Houston. There’s a small percentage of them that we keep in a separate place in order to make sure that, if Houston [suffered] some sort of catastrophic loss, we would not lose all the rocks. They are stored in nitrogen. Most of the rocks have never been exposed to the Earth’s atmosphere to keep them clean and free of contamination. But, we do actually lend them out to researchers across the country and across the globe. Anybody who has a good idea can apply and ask to have some small bits of rocks to look at. This is actually what I did for my Ph.D. thesis, to look at Apollo samples to try to understand the effects of the space environment on the rocks and soils on the Moon. But we use them for all sorts of things. There are plenty of people still doing research on those rocks today. In fact, some of the big discoveries we’ve made about the Moon in the last decade have come from those samples. Even though we’ve had them now for 40-plus years, we have better equipment now, we have more detailed measurement techniques, so we’re still finding new things. For example, we found out a years ago that there’s water in lunar samples. We thought for years, because of the violent way that the Moon was created, that the Moon was bone dry and that there was no water left in those rocks. Now that we have the ability to sample at higher precision we found that, in fact, a lot of the Moon rocks still do have water in them.
Jim Green: There was another mission called Chandrayaan-1. It was launched by the Indian Space Research Organization and NASA had an instrument on it, and that instrument looked at the mineralogy and therefore gave us an indication of water on the Moon. We found that the Moon had quite a variation in it. What was that variation all about?
Sarah Noble: Yeah, that’s yet a different kind of water on the Moon. We’ve talked about water at the poles, water in the rocks, and then there’s this third kind of water that the Chandrayaan M-cubed (or M3, for “Moon Mineralogy Mapper”) instrument found, which is sort of ephemeral water that shows up on the surface of rocks – I want to say dew, but that’s a terrible way to think of it because it’s far less water than you would have in dew. It’s a small amount of water that is created during interactions with the solar wind and it sits on the surface of the Moon in certain places.
Jim Green: The impacts on the Moon are just everywhere, and as you point out, there are still impacts going on today on the Moon, and we see them. What does this tell us about the environment around the Earth?
Sarah Noble: For everything that hits the Moon, things are hitting the Earth, too, as a matter of fact. We have an atmosphere, so small things get filtered out and burn up in our atmosphere. On the Moon, everything that comes through hits the surface, down things that are microscopic. Bigger things hit too, and they hit with some frequency, as we’ve seen from the Lunar Reconnaissance Orbiter. Fortunately, most of the Earth is ocean, and even of the part that’s land, most of it is uninhabited. So we actually don’t hear about all of the things that hit, but they are hitting regularly.
Jim Green: Those things that hit the Moon, the asteroids and meteoric material, bust things up and produce some of the regolith (surface material) we talk about, and that’s called the gardening process. What do we know about that?
Sarah Noble: As the Moon has no atmosphere to protect the surface, it is constantly being hit by things and [these impacts are] what form the dirt on the Moon, as opposed to on the Earth or Mars where there is wind and water and other things that are breaking things down. On the Moon, it’s almost entirely impacts. It is just billions and billions of years of impacts over and over again that turn over that soil and make it active.
Jim Green: The Apollo program brought back some of that regolith. What were some of the surprises that we found when we looked at it?
Sarah Noble: It turns out most of that soil is glass. Every time one of these tiny little things comes in and hits, it melts a very tiny amount of material. It turns out that 50, 60 percent of lunar soil is actually [made of] little tiny shards of glass, which is interesting. The astronauts found it actually very difficult to deal with because it’s very sharp. They stick to everything, and they get caught in your clothes, and they get caught in your eyes, and it’s not fun stuff to deal with.
Jim Green: In fact, one of the hazards of walking around on the Moon would be bringing those shards on your suit back into the habitat and then eventually breathing that and getting that in your lungs.
Sarah Noble: You have to make sure you engineer ways to prevent that sort of thing from happening.
Jim Green: One of the things I ask every one of my guests is what their Gravity Assist was. Sarah, what was the thing that happened to you that just propelled you into this field? What was your Gravity Assist?
Sarah Noble: I was always a space nerd. I was into space from the time I was very young. I absolutely love space. I headed to college and I started off as an aerospace engineer for real because it was the only major that had the word space in it, although it took me about a year to figure out that I was not an engineer, I was not destined to be an engineer, it was not my thing. I wandered around for a while, and I stumbled into geology, and I fell in love with geology. I had fantastic professors at Minnesota who knew I loved space and who steered me into planetary geology, who made sure that there were speakers that came to talk about planetary geology. They made sure that, in my petrology class, they got the lunar thin section rocks brought in and they told me to go to all the labs. Don’t just go to your section, Sarah. Go to all the labs. Spend as much time with the Moon rocks as you want. Apparently it worked, because here I am still in love with the Moon rocks.
Jim Green: That’s fantastic. Another thing that you do, Sarah, that I dearly love, is your artwork; you are really quite a talented artist. How did you get involved in that, and what are the kind of things that you like to do?
Sarah Noble: Much like space, I have always been in love with art. I actually minored in art as an undergraduate and have tried to find ways to work that into my life ever since. I’m so inspired by the Moon. The Moon is a very frequent motif in my work, as are all the planets just because I think they’re beautiful and amazing and I just want to share that with the world.
Jim Green: Thanks, Sarah. Join us next time as we continue our virtual tour of the Solar System. I’m Jim Green, and this is your Gravity Assist.