Subsurface granite on the Moon? The anatomy of a lunar hot spot

Sarah Al-Ahmed: A decade's old mystery on the far side of the Moon gets an update, this week on Planetary Radio. I'm Sarah Al-Ahmed of The Planetary Society, with more of the human adventure across our Solar System and beyond. What's stranger than a hotspot on the far side of the Moon? What lies underneath? Matt Siegler from the Planetary Science Institute joins us today to talk about the Compton-Belkovich Thorium Anomaly and the surprising granite structure that might hide underneath its surface. Ambre Trujillo, our Planetary Society digital community manager, will also join us to talk about the updates to our space trivia contest as it moves into its new home and our member community app. And we'll close out our show with the great Bruce Betts, chief scientist of The Planetary Society as he joins me for what's up. In space news, the James Webb Space Telescope has found water near the center of a planet forming disc. The origin of Earth's water is still unknown, but new insights from the powerful Space Telescope may shed light on the mystery. Measurements by JWST's mid-infrared instrument have detected water vapor in a proto-planetary disc, specifically in the region where rocky terrestrial planets may be forming. This could suggest that planets like Earth form with water present from the beginning instead of having to be seated with water later in their planetary lives. And Chandrayaan-3's journey to the Moon is going well so far. India's moon rover mission has successfully raised its orbit around Earth through a series of maneuvers, which will get it to the speed and altitude it needs to reach the Moon. The rover is expected to attempt a landing on the Moon on August 23rd or 24th, so mark your calendars. Meanwhile, in the United States, the Artemis 2 moon mission just finished its first launch dress rehearsal. The launch team successfully completed a practice countdown, ensuring that all launch systems are working correctly and identifying any issues well ahead of the actual launch. NASA still expects the launch to take place as planned, no earlier than November, 2024. The Artemis 2 mission will carry four astronauts around the Moon and back. It's going to be the first time that humans have gone beyond low Earth orbit in over 50 years. You can learn more about these and other stories in our July 28th edition of our weekly newsletter, The Downlink. Read it or subscribe to have it sent your inbox for free every Friday at Planetary.org/downlink. Over the past several months, our team at The Planetary Society has been dreaming big in pondering all the ways that we can make our shared Planetary Radio adventure even more enjoyable and impactful. We have ambitions to connect even more of you, to share our love for space with all age groups from all walks of life. We're looking into integrating more videos into our work, allowing our members to interact more easily through our member community and creating content that you'll love to revisit years down the line. We've explored specific segments of the show that would truly shine in a visual format, and identified those timely pieces that would be more digestible as written content. For instance, our much love Space Trivia Contest will soon have a dedicated space in our member community. Now, Planetary Society members will be able to answer our trivia questions from their phones, adding a pinch of fun competition to our community. What's up in the night sky, is now a monthly article complete with imagery prepared by Bruce Betts, which makes star gazing even more accessible. And we've spun off the space news from our show into our popular weekly Downlink newsletter. In the coming weeks, you'll witness some of these tweaks in action. By reshuffling these elements, we free up valuable time to focus on what we've always aspired to do with Planetary Radio, including updating our recording studio for our upcoming videos. We've been working on these updates for a long time and we hope that you share our excitement. Thank you so much for all of your patience and your continued support. For a peek behind the scenes, here's Ambre Trujillo, our digital community manager at The Planetary Society to share some details on the new space trivia contest format in our member community app. Hey Ambre.

Ambre Trujillo: Hi.

Sarah Al-Ahmed: I'm really excited about this new update to the way that we're doing our trivia contest. I know that people have been loving the trivia contest in this show for literally decades, but behind the scenes we've been playing around with the tools as we move it into our new member community, and I think it's going to be a really cool experience.

Ambre Trujillo: Definitely. I agree. I think it's going to be an amazing experience.

Sarah Al-Ahmed: So for people who are just learning about this member community, what is our Planetary Society member community app and how can people join

Ambre Trujillo: The Planetary Society member community is really the place for our members worldwide to connect. You can enjoy so much complimentary online courses. You can engage in our book club hosted by former Planetary Radio host and creator Mat Kaplan, where he interviews authors of our monthly book selection. We just had Andrew in on to talk about Carl Sagan's Contact. This month, we have National Air and Space Museum curator, Matthew Shindell to talk about our July pick, For the Love of Mars We have a ton of live events, so many resources, a science journal club, and now trivia.

Sarah Al-Ahmed: We've been waiting for this community for so long, so it's really nice to finally get to do all these big moves we've been talking about for years and really get to interact in the community. And I think it's going to be a lot easier for people to join the trivia contest when they can just kind of boop the buttons on their phone and send in their answer.

Ambre Trujillo: Agreed.

Sarah Al-Ahmed: This is just for people who are members of The Planetary Society. So if you want to get into our member community, you're going to want to join The Planetary Society. But then you could just go to our our URL at community.planetary.org or download the app on your phone to participate. And then how do people actually get into this trivia contest and where can they find it in the community?

Ambre Trujillo: It will be a new "space," air quote, close air quote, which is a specific area within the platform where members can gather and interact around a particular topic. It's really easy to find. It's just like a little tab on the left-hand side. And trivia will be in the art culture and fun section of the app called Trivia, and it's going to be structured just like the Planetary Radio Space Trivia Contest was in the past. We will ask one question a week, and participants will have one week to turn in their answer before we pick our winner.

Sarah Al-Ahmed: I love that it's the same format that we all know and love. But are people going to be able to get the same kind of space prizes that we gave away in our Planetary Radio Contest?

Ambre Trujillo: Oh yeah. Yeah. It's still going to be the same very cool space swag. I would say that Planetary Society members should make sure their mailing address is up-to-date in the account center on our website so that we mail their prizes to the right location.

Sarah Al-Ahmed: Yeah, I wouldn't want to send some cool board game or something to people and then just have it get lost in space.

Ambre Trujillo: No, no, please don't do that.

Sarah Al-Ahmed: So when does the new trivia contest start?

Ambre Trujillo: So it starts next week on August 9th, and we have a really cool first prize.

Sarah Al-Ahmed: What is it?

Ambre Trujillo: Our first prize is, drum roll, a Star Trek cruise duffel bag signed by the one and only Bob Picardo, not only a member of our board of directors, but the actor behind the iconic emergency medical hologram from Star Trek: Voyager. And there's something else inside. One of the last Planetary Society Kick Asteroid rubber asteroid stress balls. So you're going to get the very last one on top of this super cool bag, so it's truly one of a kind.

Sarah Al-Ahmed: It's really cool, too, because part of the reason why we actually have one of these leftover asteroids, because one of our members actually got two on accident a while back during our contest and was kind enough to send us the leftover ones so that we could contribute to another member. That just filled my heart with such joy when I got that in the mail with that beautiful letter.

Ambre Trujillo: That's really just a testament of our members. They're just all such amazing, honest people. And just being in the member community, you get to experience that. It's really, really cool to meet other people that not only love The Planetary Society, but love our cosmos and love protecting it and all of the science in between.

Sarah Al-Ahmed: Yeah, well just know Kay Gilbert out there in the universe, I got your letter. And thank you so much, and the next person is going to love that squishy asteroid. Well, thanks for joining me, Ambre. I know that people really wanted to know the details on this so that they can begin participating in the trivia contest again in its new format. I've been looking forward to this move for a while. I know it's a big change for people, but I hope that they enjoy this new contest format just as much as we do.

Ambre Trujillo: Agreed. I'm excited to see it.

Sarah Al-Ahmed: In 1998, NASA's Lunar Prospector analyzed an area on the far side of the Moon with a gamma-ray spectrometer and uncovered a mystery that's been fascinating scientists ever since. The spacecraft detected a thorium-rich hotspot in a region called Compton-Belkovich. Thorium is a radioactive metal, and this discovery suggested passed volcanic activity, but in an area on the Moon where we really didn't expect it. Our guest this week is Dr. Matt Siegler. He and his team have analyzed this region with new data from several space missions, including the Chinese Chang'e 1 and Chang'e 2 spacecraft. By peering beneath the surface of the Moon with microwave instruments, they've been able to reveal what might lie beneath this hotspot on the Moon, granite. Earth is the only world on which we've found granite until now. Matt Siegler is a research scientist at the Planetary Science Institute and an adjunct faculty research professor at the Southern Methodist University in University Park, Texas. Matt is an expert on planetary interiors, infrared and radio-remote sensing and thermal modeling among so many other things. His space education adventures took him from the halls of Cornell University to UCLA where he learned how to delve below the surfaces of celestial objects by studying subsurface ices on other worlds. His work has touched many spacecraft over the years, including NASA's Lunar Reconnaissance Orbiter, the Mars InSight mission and OSIRIS-REx. The results that we'll be talking about today are a stepping stone, helping to shape Matt's work on future lunar lander missions. Matt does the hard science, but he's also on a personal mission, not just to understand the mysteries of the worlds around us, but to make science accessible and exciting for everyone. His team's new paper called, Remote Detection of a Lunar Granitic Batholith at Compton-Belkovich was published on July 5th, 2023 in the journal, Nature. Hi, Matt.

Matt Siegler: Hi, how are you?

Sarah Al-Ahmed: It's wonderful to talk to you. I don't know if we've ever bumped into each other in person before, but we've both worked at Griffith Observatory in the past, so we're a observatory family.

Matt Siegler: Oh yeah. That family runs deep and it's always been a really fun part of my life in LA.

Sarah Al-Ahmed: It's funny, too, because a few years ago, I think it was the last excursion I went on before the COVID pandemic began, it was January, 2020 and I was at the Jet Propulsion Lab going to see the Perseverance rover for the first time, and I ran into one of your grad students and she spoke so highly of you that I still remember it to this day.

Matt Siegler: Okay, well, it's always good to know that your grad students don't hate you.

Sarah Al-Ahmed: No, that's really important. It's really important.

Matt Siegler: But yeah, that reminded me of why I was so excited about working at Griffith Observatory in grad school, was grad school is all about making you feel stupid, right? You're surrounded by a bunch of other people that you think are just the most brilliant people you ever met and you just feel so inferior all the time. But then going to Griffith Observatory, everyone was like, "Oh my God, you're the smartest person I've ever met." They're just so excited to hear every font of knowledge you bring of science that you really feel like, "Oh, I'm smart." And so that was part of the benefit, was that I didn't feel so dumb all the time because of public outreach, and I thought-

Sarah Al-Ahmed: Same.

Matt Siegler: That was the main benefit.

Sarah Al-Ahmed: I had that same feeling getting my degree in astrophysics because it challenges you in ways that you could not have seen coming. You do a lot of work to get to that point, and then it's like, "Oh, this is so, so hard." And then you step away from it a little bit and you start talking to people who are just beginning on their space journey, and you realize how much you've learned and how much that knowledge can mean to other people. It's really rewarding. So there are a lot of potential headlines that we could give this story. I've heard some people say that your team confirmed a hotspot on the far side of the Moon. But I think the headline for me is that you guys found evidence of a large subsurface granite patch on the far side of the Moon and that this indicates ancient volcanic activity. And I know that for some people, this at first might sound kind of unimportant because on Earth we have granite all over the place and we know that there's volcanic activity in the history of the Moon. But when you look into the details on this story, it gets weirder and weirder.

Matt Siegler: Yeah, and this is just, it's a neat culmination of a lot of different things that we've been working on for a long time. So I've worked on geothermal heat flow of the Moon. I've worked on subsurface temperatures and modeling ice at the poles of the Moon. And then a few years ago, I guess it's 10 years ago now, the Chinese sent this orbiter, actually two orbiters, both had the same instrument that were able to measure the Moon for the first time in microwave frequencies. So basically, infrared will measure surface temperature. We're all familiar with infrared cameras and all that. But with microwave, you can actually see temperatures below the surface. And so that gets pretty exciting because now you can see that there are certain locations on the Moon where it gets hotter as a function of depth faster than other places. And what that's from is that that area is producing more geothermal heat than another spot. We're used to this happening in on Earth like, "Oh, there's more geothermal heat coming out of Yellowstone than there is out of somewhere else." But does that mean we found Yellowstone on the Moon? And when we put all the pieces together, the best way to explain this high heat flux spot on the Moon was not some active vulcanism, which would've been even more exciting, certainly. This was just the fact that we had a large body that had radioactive heat production above the background crust of the Moon. And so to do that, you need to have a mineral or a rock that can contain more radioactive elements in it. And you get this in granites because you melt the crust a bunch of times over and over. They concentrate the elements like uranium and thorium that like to hang out in the melt. They're called incompatible elements. They don't play well with other minerals, and so they kind of get pushed out into the melt. And so when you melt the crust multiple times in one location, you're going to increase the uranium and thorium, and thus the geothermal heat production.

Sarah Al-Ahmed: But even that is really weird for a bunch of reasons. Usually when we find volcanic rock on other worlds, it's like a basalt or something that doesn't go through this really complex process. So that just seems a little strange to me.

Matt Siegler: Yeah, yeah. I mean, so that's what pretty much all vulcanism in the Solar System is kind of one and done. You have a material from the mantle, comes up to the surface, and that's basalt. And that's like what happens in Hawaii, is you have material from the mantle coming up to the surface and we have a big island made of basalt. But what also happens on Earth is sometimes you take that basalt, drag it down under the surface of the Earth with plate tectonics and water helps it melt at a lower temperature than it normally would, and so that stuff can then remelt several times, perhaps, and then bubble up to the surface as granites. And so that's where we get something like the Sierra Nevada mountain range is a big granite body that used to exist below some chain of volcanoes that is long eroded away. But here we're seeing that process on the Moon for the first time, and maybe it's always dangerous to say for the first time, people have found rocks with granite-like signatures from orbital data before. But here, we're finding a huge body below one of those small surface features that says that a large part of lunar crust has gone through this multiple remelting evolutionary process, somehow without water, without plate tectonics. How'd the Moon do it? And that's the mystery.

Sarah Al-Ahmed: That is the mystery. And who even knows what this means for the geologic history and the evolution of the Moon over time? Before we get into even more details on this, you were trying to understand what was going on underneath the surface of the Moon with its geothermal energy. Why is it so important for us to understand this geothermal gradient on the Moon or other worlds?

Matt Siegler: Geothermal heat flow has long been a tool of geophysicists to figure out what the inside of a planet is made of. So basically, we have lots of pictures of the upper 0.0001% of all the planets. We know what's going on at the very surface. And every once in a while there's some action, a crater digs a big hole or something that brings some of that material from below up. But what we're now wanting to know is what the other 99.99% of the planet is made of. And on Earth, we have figured that out with two main tools. I'm sure there are other tools that people will get mad at me for missing. But seismology, measuring Earthquakes because that will tell you about the density of different materials inside the Earth. Seismic wave has to travel across through the Earth and you go through the core and the mantle and the crust and those all have different seismic velocities, and so you can tell the net density of the interior of the Earth. Then you want to know the composition of those layers. Well, one way to do that is through geothermal heat flux. If I measure the heat coming out of the ground, that tells me how much radioactive material's below me. Things like uranium and thorium and potassium are the primary producers of geothermal heat. And it turns out uranium and thorium are what we call refractory elements. And there's a whole family of maybe 25 or so of these refractory elements. What they do special is they froze out first from the solar nebula. Even when the solar nebula was really hot in the early times, 2,000 degrees kelvin, that kind of temperature, these things would freeze out and they all freeze out with the same ratio. So what that means is if I can tell how much of one refractory element is there, I could tell how much of all the refractory elements are there. So now I measure how much uranium and thorium there are and I know how much calcium there are and magnesium, and you get a pretty good constraint of silicon. So then you have most of the composition of what a rocky planet like Earth or Mars or the Moon is made of. And so it's about learning about the bulk composition of what these bodies are and how similar they are to Earth. And so that's what we were trying to do with the geothermal heat flow probe on the InSight mission, and what we've done with the past measurements on the Moon with the Apollo, and then what we're going to do actually next spring on one of the new PRISM missions to the Moon, we're going to send the first geothermal heat flow probe in 50 years back to the Moon.

Sarah Al-Ahmed: And that's really cool because we attempted to do this kind of thing with the InSight mission and unfortunately, there was a probe that was supposed to hammer itself down into the Martian regolith and learn more about what was going on inside. It didn't exactly go according to plan, but was that the part of the InSight mission that you were involved with, trying to figure out that heat flow?

Matt Siegler: Yeah. And it's not my fault.

Sarah Al-Ahmed: It's no one's fault. I mean, space is hard. It's not like we have a bunch of Martian regolith that we can play around with.

Matt Siegler: Yeah. It's Mars's fault, and it's kind of based on our expectations that what really happened with InSight, I liken it to a matter of trying to scoop through sugar versus brown sugar.

Sarah Al-Ahmed: Right.

Matt Siegler: If you scoop through the brown sugar where it's all kind of cohesive together, you can scoop a hole where the walls don't collapse down in. And with that InSight design probe, it was basically like a jackhammer. And a jackhammer, you always envision, okay, there's the guy sitting holding the jackhammer and pushing it down, right? On Mars, we were expecting that the soil would fall back into the hole and kind of push that jackhammer down for us. And that was tested a lot with all our stimulants of what we expected on Mars, but the soil, when we got there, ended up being a lot more cohesive, a lot more like brown sugar. And so the hole didn't collapse into the level that we hoped and we kind of got stuck in place. Everyone tried for a real long time and it was really great. The new design for the Moon heat flow probe, it's kind of like maybe you've seen nature videos of these little fish that blow water out of their nose to dig a hole-

Sarah Al-Ahmed: Yeah.

Matt Siegler: ... into the mud. It's kind of like that, that it's air blowing out of the end of a tube to actually blow the soil out of the way and then drive down. And that's actually being built in Pasadena where you are at Honeybee Robotics and it's led out of Texas Tech.

Sarah Al-Ahmed: I think it's going to be a little easier, just because we actually have samples from the Moon. We have some idea of what that consistency is like, but to really understand what's going on on Mars, we've got to get samples back with the Mars sample-return mission. We could still try to send more heat probes and stuff like that, but it's proven very difficult.

Matt Siegler: Oh yeah. It's a big endeavor to get samples back. It's really exciting. Since you mentioned that, I'm also part of the OSIRIS-REx team. And we're, this September, going to get the samples back from asteroid Bennu, so that's going to be really exciting to see that. And I'm tangentially involved in the thermal properties measurements from that, but it's a big team that's doing all those.

Sarah Al-Ahmed: That's going to be so cool. We're already making plans here at The Planetary Society to send people out to see those samples come down, hopefully. We have samples from other asteroids, but honestly, every new body from across the Solar System that we can get that material back and compare, teaches us so much more than we could probably even anticipate. The things we've learned about the Moon with those lunar samples, it's just, it's key.

Matt Siegler: Yeah. And that's the name of the game is planetary comparison. For the entire history of humankind, we've had one, we're a doctor with one patient, and so we can only know so much about how diseases and the body works if you only have ever observed one patient. But now if you can go and observe many that have been through many different histories, then we can learn a lot more about how these processes really happen.

Sarah Al-Ahmed: And this study that you've done was primarily using information from the China National Space Administration's Chang'e missions, right?

Matt Siegler: Yeah. I mean it's kind of neat that we now have this amalgam of so many different missions that have gone to the Moon. And so we did also incorporate data from NASA missions Lunar Prospector, from Lunar Reconnaissance Orbiter and from the Chandrayaan Indian missions. So basically, we now have this critical mass of knowledge about the Moon that we then can take a weird dataset like this microwave instrument that had never been sent to the Moon or any other planet before, and have enough information to interpret the data. We kind of needed to know the density of the surface from another instrument, from the infrared instrument on Lunar Reconnaissance Orbiter, the Diviner Radiometer. And we needed to know about how bright the surface was and the albedo. And that we knew from cameras that are on LRO and other instruments. And then Chandrayaan told us about the water content of the soil. And that relates to whether water was important in the fact that this granite evolved in this location. That's one of the ideas of how this feature might have formed, was that this could have been a weird wet pocket in the early lunar crust, or it could have been an area that just had extra heat for some reason. And that's where you get to some idea that, oh, it might be like Yellowstone or something where you have this hotspot coming from the mantle and heating up that crust in that location for some odd reason. And we don't really even understand why we get hotspots like the one under Yellowstone or Hawaii on Earth, let alone how they would form on the Moon. But lots of people are working on these things.

Sarah Al-Ahmed: Yeah. And we're just at the edge of this new Artemis wave of missions to the Moon. We've got all these countries signed onto the Artemis Accords, and even as we speak, the Chandrayaan-3 mission is getting closer to the Moon and about to land. So there's a lot of cool stuff that's about to come down. Even that rover from Canada that's going to be exploring the permanently shadowed craters, the poles of the Moon, we're about to have so much more information about this that can help us piece together puzzles like this. A bunch of water on the Moon helping us create granite? That is so far outside of what I expected.

Matt Siegler: Well, and that's one of the cool things. There's this big drive for understanding ice at the poles of the Moon, and why we apparently don't have as much ice at the poles of the Moon as we have at the poles of Mercury, even though they should have kind of similar conditions today. Maybe there was some kind of change in the past. One of the ideas was, oh, maybe the water came from inside of Mercury and there's not much water inside the Moon. But then if we have these other findings like this granite production that requires the initial mantle of the Moon to be water rich, which is a quite surprising thing, we've normally thought the Moon after the giant-impact formation should be very dry, then that gives us a different starting condition to go from, that maybe the Moon was outgassing quite a bit of water over time, and that might be captured in the polar deposits.

Sarah Al-Ahmed: That would explain a lot. But again, we need more missions at Mercury to help us explain this because that too is very strange. The fact that there can be water at the poles of Mercury is so awesome.

Matt Siegler: That's really exciting. We got the Mercury BepiColombo-

Sarah Al-Ahmed: BepiColombo.

Matt Siegler: ... which is a European Space Agency and JAXA, Japanese Space Agency team up for that, and that's very exciting. It's done a couple flybys now and it will get into orbit. I think it's still a couple years away, but you have to check on that.

Sarah Al-Ahmed: Yeah, it's doing some flybys here and there, but it's getting closer.

Matt Siegler: It's really hard to fall towards the Sun, it happens. And to slow yourself down when the Sun will really pull you in. And so getting the orbit around Mercury is pretty hard. And it's actually funny. People are like, "Oh, well, why do we explore Mars so much before we really got back to this kind of modern exploration of the Moon?" And it's that when you look at all the aerodynamics of it, it's easier to land something on Mars than it is to land on the Moon, because on the Moon, you have to bring all this fuel to slow yourself down. And on Mars, you can use a parachute. So now we're seeing how challenging it really is. And we'll see with these PRISM Eclipse missions over the next year, how well the US commercial companies and NASA do with landing on the Moon again.

Sarah Al-Ahmed: And that's also cool, too, that we now have this whole commercial side of lunar exploration. There's a lot that can be done there that we couldn't just do with NASA alone or with space agencies across the world alone. We're bringing in whole new industries to help out with this, which makes it feel like it'll be really sustainable in the long run. Fingers crossed.

Matt Siegler: Yeah, I mean it's pretty exciting. Anything that ups the cadence of missions and hopefully, it doesn't up the failure rate of missions. But I guess that's really kind of what NASA's doing with this is hedging their bets on. The risk is on the companies to succeed. And if NASA is not going to pay them more if they don't succeed or if they do succeed or if they don't, then they'll have to try again, basically. And so hopefully our instruments all get there in one piece. But there's a lot of exciting missions coming up in the next few years and a lot of exciting measurements that we'll do.

Sarah Al-Ahmed: So where on the Moon is this hotspot that you found?

Matt Siegler: Yeah, so this hotspot, it's an area called Compton-Belkovich, which is a very bizarre thing. It's just between two random craters called Compton and Belkovich. It actually wasn't found until the late '90s when we had the Lunar Prospector mission orbit the Moon. It's the only real major vulcanism like this on the far side of the Moon. The near side of the Moon, we're used to the mare that kind of form whatever people call the man on the Moon. I don't know if I've ever seen it, but all the volcanism on the Moon happens in this one region called Procellarum. And there are a number of these highly silicic things that we might call volcanoes that aren't just the flood basalts, there's maybe eight of them or so on the near side of the Moon. But then Compton-Belkovich is the sole one that's on the far side of the Moon, and it's also a higher radiogenic concentration. We can measure that with an instrument called the gamma-ray spectrometer that's able to tell us how much thorium is in the upper meter or so of the soil. We knew this was a weird oddball feature and it's just gotten odder over time. The question is, does the far side location have something to do with why it was able to build up more granite or why it was able to go through more of an evolution or why it has more thorium at the surface? Basically, because the far side crust is almost twice as thick as the near side crust for in Procellarum. And so the fact that you had to work your way through more crust, gave you more time to evolve or the only thing that could get all the way up there was this highly evolved material.

Sarah Al-Ahmed: I love that you brought this point up. I think a lot of people listening to this have probably noted that if you look at pictures of the near side of the Moon, it's really interesting, lots of variation in what's going on there, but on the far side of the Moon, it's really boring by comparison. I'm sure a lot of people are wondering why that is.

Matt Siegler: There are some exciting things on the far side of the Moon, the South Pole-Aitken basin is mostly on the far side of the Moon, and a lot of people are very excited about that. It's basically one of the biggest impacts in the Solar System. You've recorded from very early history of the Moon and may have even punched down into the mantle of the Moon. And so there are some interesting features that people have spent their careers on, on the far side of the Moon as well. But I wish I had a clear explanation for you of why the near side is different. But this near side, far side dichotomy is a big, big mystery of the Moon. And then the near side, the main differences are that there's a large amount of thorium and uranium in the near surface crust on the near side. It seems to be in a mineral that we kind of playfully call KREEP, which just stands for, K is for potassium, and then R-E-E is for rare-earth elements, and then P is for phosphorus. So it's just that the rocks that we found from there that had more of those elements in them, and so they've been called that. And then in the 1990s with the Lunar Prospector mission and the gamma-ray spectrometer, they were able to measure that this whole region of the Moon where all the vulcanism on the Moon is, where all these mare basalts are, had much higher concentrations of this KREEP than the far side of the Moon. Somehow we ended up with all this heat-producing stuff stuck on one side of the Moon. I liken it almost to the magic eight ball. You have that bubble in there and you shake it up. And the idea is that that bubble of material kind of ended up on the near side, roughly facing the Earth, and for some reason, this odd pocket of materials stayed behind at Compton-Belkovich. We're not quite sure why it would've gotten to that location then.

Sarah Al-Ahmed: It drove to the wrong location for the party.

Matt Siegler: Yeah, yeah. Yeah.

Sarah Al-Ahmed: So it's this volcanic activity that's dredging up this subsurface radioactive material and kind of depositing it on the surface?

Matt Siegler: Yeah. And so I mean basically the idea is that the whole Moon was just a ball of molten rock, right? And then you what's called a magma ocean where it's just magma all the way to the surface. And then you can cool it two ways. You can crystallize from below as you cool. And so the mantle kind of is becoming solid from below. And then you crystallize from above because you're cooling out to space, and you're left with the sandwich where you get all the residual liquid is kind of trapped in that area. And so elements, again, like uranium and thorium that are these incompatible elements, they have their atomic radius is bigger than most, and so they don't fit in many minerals and they get stuck in this last liquid. And so that last liquid that's sandwiched between the crust and the mantle should be very rich in these radioactive elements. That bubble of that material sandwiched between the early lunar crust and the early lunar mantle somehow seems to all pool on the near side of the Moon. And then that can occasionally break up to the surface and come up. And then sometimes it just goes straight up to the surface, and sometimes it has to remelt multiple times and form these more granite-like materials.

Sarah Al-Ahmed: We'll be right back with the rest of my interview with Matt Siegler after this short break.

Jack Kiraly: I have an urgent message for all American Planetary Radio listeners. Space science needs your help. Congress has proposed the first cut to NASA's budget in a decade, and missions like Dragonfly, VERITAS and Mars sample-return are facing shrinking budgets, delays and even cancellation. Our elected leaders in Washington need to hear from you, reverse course and fund NASA's innovative science programs. On September 17th and 18th, join The Planetary Society in Washington DC for the annual Day of Action. This is our return to in-person, and the timing couldn't be better. While partisan politics today sows division, space is something that can truly unite us all. As a part of the Day of Action, you'll receive an all-inclusive advocacy training, briefings from experts, a minimum of three scheduled meetings with your lawmakers, and the opportunity to shape the future of space exploration. Go to Planetary.org/dayofaction to learn more and sign up. With your voice, you can speak up for space science. With your passion, you can inspire others to take action. Join us.

Sarah Al-Ahmed: In this case, were you targeting Compton-Belkovich to learn more about it? Or were you looking more at the broader kind of geothermal things going on the Moon and then it kind of led you to Compton-Belkovich?

Matt Siegler: Compton-Belkovich has always been the oddball. It's the highest concentration of the element thorium on the Moon. We basically have spent a couple years looking at this Chang'e data and trying to figure out what it was really telling us. And what we saw was that there was a good correlation with our expectations of geothermal heat flux in the Moon. And we were seeing higher microwave brightness temperatures in areas where there should be higher heat flux. And so the best place to look is, well, we're going to look where the most radioactive material on the Moon is. Let's see if that also has the highest heat flux on the Moon, and it most certainly did. So I think we can clearly say the Compton-Belkovich is probably the highest heat production on the Moon as well. It was the low-hanging fruit in that, is the term we always use. But then also, these microwave instruments really depend on the surface composition to how deep you can see. So the mare have a lot of material called ilmenite, which this mineral will actually absorb microwaves. And so we can't see very deep into the mare compared to how deep we can see into highlands material. Because we can see deeper in the highlands, we can get a better constraint on the geothermal gradient. And so Compton-Belkovich being completely in the highlands versus most of these other features being either in the mare or a mix between mare and highlands, we just got a much sharper signal as well. And so it had the right composition, it had the glowing hot thorium signature, it was the right place to start. We do see similar high heat fluxes in other areas of the Moon, and that's kind of what we're working on next, but it's more complicated part of the data to work through. And so it's going to be a little more model dependent, which people always hate.

Sarah Al-Ahmed: How hot is this spot compared to the average temperature around that area?

Matt Siegler: So I usually think of it in terms of geothermal heat flux. The Apollo geothermal heat flux is measured something like 15 to 20 milliwatts per meter squared. So that's how much heat is coming out per square meter on the Moon. And the background Moon, the highlands is probably around five to 10 milliwatts per meter squared. So the Apollo values are maybe a little higher because they're in that Procellarum region. But this spot we're constraining it getting up to something like 180 milliwatts per meter squared, so something like 20 times higher than what we'd expect for the background Moon. And we'll see when we actually land a heat flow probe on the surface in that location in some time in the future, how correct we were because that is dependent on how we modeled the antenna pattern of this instrument and all that. But I think it's in that ballpark of something 150 plus milliwatts per meter squared. So it's definitely producing a lot more heat than other places on the Moon. The surface itself wouldn't feel any hotter. We have infrared measurements of the surface even at night, and it's still around 80 kelvin at night in this location, so it's very cold. But you go down five meters down, it's going to be 50 kelvin hotter or something than whereas other places wouldn't be.

Sarah Al-Ahmed: That's wacky. But speaking of the instrument, how do these microwave radiometers work? How do they allow you to pierce beneath the surface?

Matt Siegler: I mean, so you're kind of using a microwave radiometer right now in that your computer is talking with the wifi. It kind of works in the same range in the gigahertz frequency range. And why we use that for our wifi is because it can penetrate through walls and through materials and such. And so people are already kind of familiar with this idea that, yeah, microwaves can pass through deeper material. It's not the same frequency as your microwave oven, it's a longer wavelength than that is what we're dealing with here. The Chang'e instrument was from 37 gigahertz to three gigahertz, which is about one centimeter wavelength to about 10 centimeters wavelength. And so with that, you can penetrate to, depending on your material, if it's mare or highlands or the wall of your apartment, it's going to penetrate to different depths. And so on the Moon, we think that lowest frequency, the 10 centimeter or three gigahertz channel is seeing temperatures down to five meters that deep or so in the lunar surface. So we can see that extra geothermal heat that we were talking about.

Sarah Al-Ahmed: And if people try to go read your paper, they're going to encounter this term, batholith. This is a granitic batholith. What is a batholith?

Matt Siegler: This is where I had to fall on my... My wife happens to be a igneous geochemist. So she did her PhD thesis and does her research on all this. And so that's what was exciting about this project from personal level, was that here we found this high signature that could only be explained by the thing that my wife happens to study for her career. So it was really great to be able to team up. Essentially, these magma bodies or whatever, when they come up to the surface or near the surface and solidify, we call that a pluton, the one bubble that comes up. People long thought that there was a pluton below Compton-Belkovich based on geometry of the surface. There was some hills and there was a depression in the middle. And you get that when you get this bubble comes up to the surface and then deflates at some point. Either that magma then flows off to the side or just cools off and deflates. People thought maybe there's like a 10 to 15 kilometer pluton below this. But what a batholith is, is essentially hundreds and hundreds of those plutons kind of all amalgamating together over time, over millions of years, likely, and they all build together. So a pluton is a single eruption or whatever you might... I don't know if that's the right way to describe it, whereas a batholith is something that's made of many of these plutons. So something like the whole Sierra Nevada mountain range is the Sierra Nevada batholith.

Sarah Al-Ahmed: So the structure of this thing underneath, it's not just one chunky bit of granite, it's a way more complex structure?

Matt Siegler: Yeah. Something we estimate because it was kind of cool because there were two Chang'e missions and they had these four different frequencies that we could look at the Moon with, and all of them have different angular resolution. And then when they fly at different altitudes, you also change how big your spot on the surface is and all that. And using all that, we could estimate how wide this feature was. And from that, we could estimate that it's probably around 50 kilometers in diameter or so, which is pretty big batholith even for Earth standards.

Sarah Al-Ahmed: That is kind of surprising because any kind of batholith at all, this much amount of granite is weird. But 50 kilometers across, that's pretty huge. How long ago did we think this thing formed?

Matt Siegler: So like I said, we're not the first ones to study this feature. It's been known about for about 25 years or that kind of ballpark, and have people looked at it with every dataset they could get. Groups counting the craters from Stony Brook University, they were able to see, okay, yeah, based on how many craters per square kilometer, we think that this is a layer of ash and that that ash from this volcano having erupted is about three and a half billion years old. And so the last time it probably had a big eruption is three and a half billion years ago. There's no way it's active today at that rate. It's long cooled off from a volcanic heat production stance, and so we think it's really just producing heat from the radioactive materials that are in this batholith below.

Sarah Al-Ahmed: So it formed maybe a billion years after the Moon itself formed?

Matt Siegler: Yeah, yeah. So we're talking early in the Moon's history. And most of the vulcanism in the Moon is happening in that first billion years. There are some of these basalts happening 3 billion years or so after the Moon formed, but there's not too many young volcanic areas on the Moon. There are some weird features we call IMPs. They're called irregular mare patches, and just last week, actually, the most recent selection of a PRISM mission to the Moon was to land in one of these places, because the big question is, are these incredibly young vulcanism or are they old vulcanism that just kind of has a weird property that makes it not retain as many craters? The Dimple mission is the name of the mission that was just selected, that will actually go and explore to try to find was there really such young vulcanism on the Moon or is it all kind of old, the Compton-Belkovich kind of age? Just because it's old doesn't mean it's not cool, right?

Sarah Al-Ahmed: That's what I tell my kids. Well, I mean these things are so old that we don't even know what's going on at that point in the Moon's history, and that's why it's so important to study these things because it's not like we can just step in a time machine and totter ourselves back to the beginning of the Moon to understand this.

Matt Siegler: Well, that's what's kind of cool about the Moon is that it kind of can in that it froze itself in time back then and very little has changed in the past 3 billion years on the Moon. Occasionally, you'll get hit by a rock falling from the sky. But as far as early processes in a planet's history, they're all recorded on the Moon. Whereas on Earth, all those things are gone because we have erosion and plate tectonics that takes materials from the surface and shreds them up and eats and spits them out again. Whereas the Moon is really this great block that tells us about that early history of how a planet forms.

Sarah Al-Ahmed: I just keep coming back to this in my brain. How did we get this giant chunk of granite without the regular conditions we expect on Earth? Would it have to require water in order to form, or is there some other way that we can explain it?

Matt Siegler: People have found granites in the Apollo samples, but we're talking about grains, like millimeter size. Oh, we found this little bit of granite mixed in with the rest of the soil. And so people were always trying to do exotic ways to form these. The popular one was called silicate liquid immiscibility, which maybe is a way you could form larger bodies of granite, but this kind of stretches a little bit. It's basically just that things will kind of separate out over time on their own given enough time, is the basic idea. To form a body like this in the way that we do on Earth, you can remelt it by decompression. If you basically took a bunch of material off the surface and then you brought material from depth and bring it to the surface, that heats up in that process and can remelt. That might be what's happening in some of these silicic constructs on the near side of the Moon. The GRAIL mission found big cracks right in the same areas as those. Compton-Belkovich, we don't see that. There's no big impact crater here. There's no cracks. There's nothing that seems to have caused that kind of decompression melting. And then it could be that there was this wet pocket there, that the rock was able to melt at a lower temperature in this location because it had a little more water in it. How you get that bubble of water in the early lunar crust or upper mantle is definitely a mystery of its own. But there have been other measurements I said from the M3 instrument on the Chandrayaan Orbiter was able to measure that there was more hydrogen in this area or more signs of water molecules in the surface rocks in this area. There was estimates from the size of the volcanic ash deposit, that you needed a very explosive eruption sometime in the early history of the Moon from this location. And the best way to get an eruption to explode is to have water in the magma when it's coming out. And so there are evidences that this could be a area where you had wet materials to go from. Or the third idea is that you had some kind of long-lived heat source, and that can happen two ways. One is that you have some external heat like a mantle plume that's just coming up in this area from the mantle and baking this area from below and adding enough heat to melt and remelt. The other way you could do it is just melting itself. I said every time you go through this melting process, you concentrate the uranium and thorium in the material. You could imagine that if you get high enough concentrations, you develop the self-sustaining remelting where you concentrate enough uranium and thorium in the melt that it remelt itself, and then that liquid bubbles up because it's lower density, usually, than the liquid it came out of. And so then it rises and then that's more radiogenic and it heats itself up again and rises. And yeah, so you might have a column of these plutons coming up and they're each more radiogenic as you got to the surface.

Sarah Al-Ahmed: That's a wacky idea. Imagine if you could just drill down into this thing and see what's below. I know that we've detected thorium there with previous missions like Lunar Prospector, but have we detected uranium as well?

Matt Siegler: Yeah, I think the thorium measurement is a little easier to interpret than the uranium. The maps that I've seen of thorium are always a little sharper than the ones of uranium, but I don't know if I can argue the physics of why. Basically, a gamma-ray spectrometer is looking at peaks in the energy spectrum of gamma-rays, and I think the peak of thorium is just kind of sharper than the one for uranium, or there's no other elements that have a peak at the same energy range, and so it's easier to interpret. But I think, yeah, we have seen uranium there as well, and potassium and these other elements that can produce heat.

Sarah Al-Ahmed: This is really cool because we're about to enter the new human age of lunar exploration all over again, but this time with serious, serious face on and we're going to need energy to power human settlements if we go there. We got plenty of sunshine and things like that, but could we conceivably use these radioactive materials to create reactors that we can power human settlements?

Matt Siegler: Yeah, I mean, that's a good question of how high of a concentration do you need of any material on the Moon for it to become more viable to get it from the Moon than to bring it from Earth? This is always the question with the water on the Moon. We know there's some water there, but is it in a high enough concentrations, in a small enough spot that we can mine it and have water that's actually cheaper than bringing water from Earth? And so I don't know how concentrated of radioactive material we would need for it to be reactor grade or even to do geothermal heat production on the Moon. Normally, we do geothermal heat production on the Earth by pumping fluids like water down to the hot rock deep underneath. And so then you have this complication, well, you need to bring the water with for that. And so I don't know if you can get some energy production just from sticking something in the ground and using the fact that it's warmer underground than above, but I don't know. That's for the geothermal engineers to figure out now, is this a good source for putting our Moon base and powering it?

Sarah Al-Ahmed: Right. We'll start a pipeline from the water at the poles, we'll bring the water to this place and then use it to build some-

Matt Siegler: Which is exactly what Percival Lowell thought they were doing on Mars when he saw the telescope images of Mars in the early 1900s as well. "Oh, it looks like there's canals bringing the water from the poles down to the Equator where it's warm." And we're the aliens now doing it ourselves.

Sarah Al-Ahmed: Oh gosh, how wacky is that going to be? And I know that the reason I bring up these water pipelines is because I know that some people are already working on it. And in fact, one of the presenters at the upcoming NASA Innovative Advanced Concepts Symposium, which I'm going to be going to, is actually thinking about these pipelines of water on the Moon. We're almost there. It's a weird time to be alive.

Matt Siegler: Right now, we're just hoping for at least some water. I'm part of the VIPER mission, which is going to land at the South Pole of the Moon or on Mons Mouton, which is not right at the South Pole. It's actually four degrees off of the South Pole or so, but we'll land in Mons Mouton in the fall of 2024. We launched right around Halloween, so it should be a fun time for VIPER's going during Halloween.

Sarah Al-Ahmed: I can already see the Halloween costumes among your friends.

Matt Siegler: Dress like a rover. But that, we're using models that my grad student and I have made of where ice could be stable underground, and then the rovers driving around using a drill made in Pasadena, California where you're based. It's going to drill down to about a meter depth and see if ice is in these places that we think there is and what kind of concentrations are there. If it turns out it's everywhere we think it is, then yeah, we can start building those pipelines. But if it isn't where we expect it to be or not in enough quantities to really use it to the level that we need a pipeline, then we're going to figure it out and figure out why it's more concentrated in some locations and less concentrated in others. So there's a mystery ahead, but it would be good to have the pipeline ready.

Sarah Al-Ahmed: Right. And I would love if you would be willing to come back on and talk to us about VIPER when that goes down because this is key to future exploration of space. We're using the Moon as a stepping stone to go out to other places in our Solar System, and if we could find water in an abundance that we can actually use to sustain humans, it would be pivotal. Not just for keeping humans alive, but we could use it as a fuel source even.

Matt Siegler: Yeah, I mean, that's what I always tell people is that you see this space shuttle launch and it's got this big smoke plume, but it's not smoke. It's actually taking hydrogen and oxygen and putting them together to make water vapor, but you get a lot of energy out of that. So ice is rocket fuel. If we can get ourselves ice on the Moon so we can have our fueling station and the astronauts can drink martinis.

Sarah Al-Ahmed: It's got to be so exciting to be a part of so many of these missions and to have so many of them to look forward to. And is there any specific other place on the Moon that you're hoping to explore more in this way? I mean, other than this one feature?

Matt Siegler: I've certainly always been a buffer. The South Polar terrain and all these ice deposits, I mean, so that's what I've been most invested in. Compton-Belk, which is exciting from this geothermal heat production, that it's this odd place that if we just take the estimate of how much radiogenic material we think is in the crust based on the surface material, it's like one or 2% of all the radiogenic material in the lunar crust might have to be in this one body. So it gets a little crazy. And maybe that means that there's more hiding below the surface than we knew. And so we can measure that. And so we want to measure that in other areas like the Aristarchus crater is an area that also seems to have these signs of having silicic or evolved vulcanism like this and potentially has a lot of heat production below. We're actually going just north of there to the Gruithuisen Domes. The MoonRise mission is going there probably in 2027 now. So I'm part of the infrared instrument team for that. That's another one of these highly silicic, evolved vulcanism spots on the Moon. There's so many interesting places on the Moon and we haven't given it credit for the last 50 years that how interesting it really is and dynamic it is, and how there are these areas that are so different from each other. And we'd love to explore them all. And so hopefully this new era of exploration in the Moon lets us tick off a few of these

Sarah Al-Ahmed: Hear. Hear. Well, thanks for joining me, Matt, and the story, I've said it a few times, it was pretty surprising to me and I'm really looking forward to seeing what we can learn in the future because it clearly could be key to a lot of things that we don't understand and a lot of mysteries. So thanks, Matt. It's amazing that even our closest neighboring world, our Moon, still presents so many mysteries. Now let's check in with Bruce Betts, the chief scientist of The Planetary Society for what's up. Hey, Bruce.

Bruce Betts: Hey, Sarah. How you doing?

Sarah Al-Ahmed: So what is there to see in the night sky this week?

Bruce Betts: Well, we're closing in, I suppose it's just slightly past this week as the peak, but the Perseid meteor shower, usually one of the top meteor showers of the year peaks on August 12th and 13th, but is already increasing the meteor rate as we go to press, if that's what we do, with increased activity several days, actually even a couple weeks before and after. But the peak night is the night of the 12th to the 13th, and there will be little interference from moonlight this year. So that's exciting. Only a crescent moon that rises not long before dawn. So pretty much if you can get away from the clouds and get away from the city lights, preferably, you should see a lot. If you can get to a very dark site, you may see as many as 50 to 75 meters per hour at the peak. And in any case, from a urban environment, you still have a good chance of seeing a few and obviously everything in between. So go see those. Planets are shifting, that's what they do. Those silly wanderers, as they name them planets. Venus is pretty much decked below the horizon in the evening west, but I don't know, maybe you can at least imagine it. Maybe at certain places you can see it, but don't worry, Venus, it'll come back in a few months hanging out in the morning sky. Right now, we've got in the evening sky, Mars and below it, Mercury hanging on low, low, low in the west after sunset. Mars looking reddish and Mercury looking kind of whitish. Again, a tough thing to see. But good news, early evening, Saturn, yellowish coming up in the East, high in the sky for the rest of the night. And Jupiter, really bright. Jupiter not as bright as Venus, but still quite bright, will be coming up later in the evening in the East and being super bright object out there for the rest of the evening. And that's what we got going on in the sky land.

Sarah Al-Ahmed: Do you have any plans to go see the meteor shower?

Bruce Betts: No, that's my planning horizon usually operates on a few hours. And so no, I'll probably go out in my yard and typically use the suburban environment, urban, suburban, and see what I can see. But maybe, maybe, we'll see. We'll see whether... What about about you?

Sarah Al-Ahmed: I don't have any plans this time. Usually, I try to go out to Joshua Tree or a nice dark sky site so I can see them, but-

Bruce Betts: Oh nice.

Sarah Al-Ahmed: That does happen sometimes where a friend will just be like, "Hey, I'm heading out to this place. You want to hop in the car?" And then I'll just be out there no blanket or anything under the sky, but it's always worth it.

Bruce Betts: I've done that many a time, but probably not this year. We shall see. All right, we move on to this week in space history, all sorts of good launches and landings. I'll focus on two of them. Juno, amazingly, Juno launched in 2011 this week and is just still partying at Jupiter. And Curiosity rover landed in 2012 this week and obviously still going, still chugging along. So impressive, as always, how long these spacecraft tend to last.

Sarah Al-Ahmed: Really though, and the way that people have been just so smart about the way that they can extend the lifetime on some of these rovers. I mean, whether or not it's got a dead wheel and you got to drive it backwards or something, it's impressive.

Bruce Betts: Shall we move on to... [inaudible 01:00:45] Venus, surface lovely, wonderful [inaudible 01:00:53] vacation there. Venous surface, we haven't talked about the atmospheric pressure being so high, but not surprisingly, the atmospheric density also very high. It is 54 times denser than the atmosphere at sea level on Earth. So that puts it as a non-trivial, well, it's still a small fraction of the density of like, say water, but it's awfully dense. 65 kilograms per cubic meter for those playing the home game. Water's, what? A thousand.

Sarah Al-Ahmed: It's weird to think about because I mean, you'd have to put aside the literal melting temperatures, but that's got to be weird. Even just stepping out the door when it's humid in the air, I feel weird and sluggish. But imagine trying to go through air when it's just like way thicker than it should be.

Bruce Betts: Yeah, and you got all the other problems. But yeah, it'd be weird.

Sarah Al-Ahmed: But everyone, if you're enjoying the show, please send us your comments and poetry and all of your awesome insights. I love reading the things that surprised you or the things that you can teach us. I see people in the comments every once in a while talking to each other about extra stuff from the show that we didn't get a chance to mention because there's a lot that you can talk about when it comes to space. But if you'd like to send us your comments or poetry or anything, you can email us at [email protected], or you can leave your comments in our member community. If you're a member of The Planetary Society, each week we post an episode into our Planetary Radio section on our community, and then you can talk about all the stuff that you enjoyed. So we read all of those and I am looking forward to sharing some of those comments on the show.

Bruce Betts: It's always fun. We have amazing listeners, so thank you all.

Sarah Al-Ahmed: Yep.

Bruce Betts: All right, everybody, go out there, look up in the night sky, and think about if you or your brain or you made an AI, what would you call it?

Sarah Al-Ahmed: Nice one.

Bruce Betts: Thank you. And goodnight.

Sarah Al-Ahmed: We've reached the end of this week's episode of Planetary Radio, but we'll be back next week to talk about the proposed Mars Life Finder mission. Planetary Radio is produced by The Planetary Society in Pasadena, California, and is made possible by our members around the world. You can join us as we continue to work together to support missions to other worlds at planetary.org/join. Mark Hilverda and Rae Paoletta are our associate producers. Andrew Lucas is our audio editor. Josh Doyle composed our theme, which is arranged and performed by Pieter Schlosser. And until next week, ad astra.