This transcript was generated automatically and lightly edited for readability.

Oliver Strimpel: This is Geology Bites with Oliver Strimpel. The rocky planets of the solar system have only three moons among them: our own Moon and the two much smaller moons of Mars, Phobos and Deimos. There have been no successful missions to either of the Martian moons, but now the Japanese space agency is leading a mission to Phobos to land and collect material from its surface and return it to Earth. But why Phobos, and what might we learn from such a mission?

Tomo Usui is a professor in the Department of Solar System Sciences at the Institute of Space and Astronautical Science of the Japan Aerospace Exploration Agency. He is leading the science team for the 2024 mission to Phobos. Tomo Usui, welcome to Geology Bites.

Tomo Usui: Thank you very much, Oliver. I'm very, very delighted to be here.

Oliver Strimpel: Japan has successfully mounted two sample return missions to asteroids. Why was Phobos chosen as the next target for a sample return mission?

Tomo Usui: Scientific reasons, and also programmatic reasons. For scientific reasons, we are going outward in the solar system. We chose the two asteroids, Ryugu and Itokawa, which are near-Earth asteroids, which orbit close to the Earth's orbit. But we are going further into outer space, and Phobos is a Martian satellite. So we go beyond the Earth's orbit, and we have more primitive, primordial information that we can get from the outer solar system materials. And also the programmatic reasons: we have tried to leverage each of the experiences of the asteroid sample return missions for the future missions. We chose Phobos because Phobos is a satellite of Mars, which means that we need to get the spacecraft into the Mars gravity field, which is a new challenge for us, and also to escape from the gravity to come back to the Earth. We are going to use this kind of technique for a future Mars mission. So MMX, the Martian Moons sample return mission, from a scientific point of view, gives us more primitive information about the solar system. But on the other hand, this is also a gateway for us to go on to Martian missions.

Oliver Strimpel: I can see that the reasons for going there make a great deal of sense, but when you get to Mars, you still have the choice of the two moons, Phobos and Deimos. Why did you pick Phobos?

Tomo Usui: We had a huge debate about that. Two reasons. One is that at this moment, we can see surface heterogeneity, I mean a variety of materials, on the surface of Phobos, whereas Deimos is pretty much homogeneous. Geologists like diversity; scientists like diversity. The other reason is that Phobos has an orbit inside that of Deimos, so Phobos is closer to Mars. And recent numerical simulations suggest that Martian ejecta, I mean material ejected from the Martian surface, come to the Phobos orbit, and then some of them could exist in the regolith of Phobos.

Oliver Strimpel: Which is the unconsolidated layer of loose dust and rocks on the surface.

Tomo Usui: So when you get the sample of the regolith of Phobos back to the Earth, those materials may include not only the Phobos materials but also the Martian particles. That is so exciting. So that's why we chose Phobos as our landing target.

Oliver Strimpel: Before we talk about the science aspects, I want to ask you about the mission itself. First, how was the launch date of late 2024 or early 2025 chosen?

Tomo Usui: We haven't had any successful Martian mission, so Japan needed a Martian mission in the 2020s. This is a request from our science community. Why 2024? So we have a launch window to go to Mars every two years, which means 2020, 2022, 2024, 2026. And MMX, the Martian Moons mission, is a five-year mission. So if we're going to launch the spacecraft in 2024, we can come back to the Earth with a sample in 2029. 2029 is programmatically very, very important, because NASA and ESA plan the Mars sample return mission, and the sample is planned to come back to the Earth in 2031. So JAXA and our community want to bring the sample back two years before that.

Oliver Strimpel: As you mentioned, Japan has already mounted two successful sample return missions so far, Hayabusa 1 and Hayabusa2. What has the Japanese space program learned from those, and how will this one be different? What's new about this one?

Tomo Usui: From the mission point of view, we learned a lot from the Hayabusa and Hayabusa2 missions: for example, the sampling devices, and also the sample capsule return logistics. Like last month, we successfully landed the capsule in the Australian desert. Also, we have to have collaboration with the Australian space agency and also the Australian government. We need their help, and also to keep close cooperation for the future missions. So we learned a lot, not only on the engineering and scientific side but also about the logistics, including the international partners.

Oliver Strimpel: What is the difference between the Hayabusa and Hayabusa2 and the MMX mission?

Tomo Usui: So Hayabusa and Hayabusa2 are sample return missions, but they're not landing on the surface of an asteroid. We call it touch-and-go: grab the sample during the touch-and-go. This is not a physical, steady-state landing. We can make it as long as it's a small, very, very small body, like Ryugu or like Itokawa. But we need to land on the surface with four legs onto a larger body, a relatively large body like Phobos, 20 kilometers in diameter. Landing is so challenging, at least for us. This is totally different. Touch-and-go is a challenge, but it's relatively easier than a physical landing. That's why this is different.

Oliver Strimpel: Obviously, one of the challenges of landing is that you don't really know what the surface is going to be like, as there's never been a landing on Phobos. So what can you do before you land to try and manage the risk of the landing?

Tomo Usui: First of all, we are still compiling the existing data from the previous missions. ESA and NASA have orbiters for Martian exploration, but other missions also look at Phobos sometimes and get data. But it's not good enough for us. The previous existing data are more like reflectance spectra, or very rough topography information. So before landing, we're going to orbit around Phobos for several months to get detailed topography and geological mapping. But we still do not have physical property information at this moment, which is like how fluffy it is, or how sticky the surface is. So we deploy a small rover onto the Phobos surface before the mothership lands. We deploy the small rover, which is provided by a France and Germany collaboration. This is so awesome. We're going to see how the rover wheels work, or how the rover lands or sinks in, to see the physical properties.

Oliver Strimpel: How can you deploy a rover without landing?

Tomo Usui: Well, we're going to just throw it. The gravity is so small, like 0.2% of the Earth's gravity, so we can throw it, in free fall, from about tens of meters above the surface, and then it's going to land safely on the Phobos surface.

Oliver Strimpel: OK. So will this landing procedure of the mothership be carried out just in a single location on Phobos?

Tomo Usui: We plan to have at least one, but we have the ability to land twice, and more than twice is better, because, as I said, the Phobos surface is so heterogeneous.

Oliver Strimpel: What is the limiting factor, then, on the number of landings you could do? I mean, why not three or four or five?

Tomo Usui: Fuel, and also sample containers. We do not have enough of the sample canisters, because the spacecraft is so small. So the number of the sample canisters, which is probably three or four-ish, is also a limitation. But also the fuel, if you will, is always a limitation, because we need fuel for coming back and forth.

Oliver Strimpel: So what instruments will the spacecraft carry?

Tomo Usui: We have many instruments, seven, eight. We have two cameras, visible cameras: one is telescopic; the other is a kind of wide-angle camera. So two cameras. And also we have an infrared spectrometer, which is provided by France; a gamma-ray and neutron spectrometer that would give us elemental information, which is provided by the US. And also we have a dust monitor, we have a mass spectrometer, and we have a lidar, which is the ranging system. Also we have the rover, as I said. The rover is a very, very unique instrument, provided by the cooperation of CNES in France and also DLR in Germany.

Oliver Strimpel: Here on Earth, a mass spectrometer is a huge room-size machine with vacuum chambers and big magnets. And how could you get the spectrometer onto a small spacecraft?

Tomo Usui: That mass spectrometer is just very small, because that spectrometer gets the already ionized materials. So the huge mass spec, as you mention, needs to be equipped with an ionization package: you need ablation lasers, you need the drilling system, you need the plasma stuff. But for us, at least, we're going to measure the already ionized pieces, elements. So we need just very small mass detector counters.

Oliver Strimpel: You said that the spacecraft will spend three years out of the five-year mission in orbit around Mars, around Phobos. Will it be surveying the surface to identify the possible landing site? And what kind of criteria will you be using to pick the landing site?

Tomo Usui: The most important criterion for us in picking the landing site is safety: safely landing and also coming back to the Earth. So we still do not know the surface roughness. We have landing gear, but the height is like less than 1 meter from the bottom to the deck of the spacecraft. So we should avoid big boulders, and also we should avoid fluffy, fluffy materials. A smooth and rigid surface is very important. But from a scientific point of view, we are going to pick the landing sites to maximize the surface heterogeneity, or variety of the materials that we can get. So safety first, but the scientific reasons are also important.

Oliver Strimpel: When you say heterogeneity of the surface, is it in albedo? What makes it heterogeneous?

Tomo Usui: At this moment, albedo and the infrared are the only information we have. We call it reddish or bluish, which means these are reflectance features.

Oliver Strimpel: OK, let's talk about the scientific aspects now. What are the main scientific questions that we hope to shed light on?

Tomo Usui: The major goal of the MMX mission is to determine the origin of Phobos and Deimos. We still do not know how these moons formed. We have at least two competing hypotheses. One is the capture of an asteroid; the other is in situ formation by a giant impact. So the capture-of-an-asteroid scenario is that Phobos and Deimos were originally asteroids. They were from the main asteroid belt, or farther than that, came into the Mars gravity field and were captured by Mars, and then became satellites of Mars. On the other hand, the giant impact scenario is something like the formation scenario for our own Moon as well. So a large body impacted Mars, and most of the ejecta got together at one or two locations to make the satellites. So two competing hypotheses, but these hypotheses still have positive and negative evidence to support them, so it's so confusing right now. So the MMX mission will definitely give fingerprints to test those hypotheses.

Oliver Strimpel: Tell me about this fingerprint. Let's say we're in 2029 and you have your sample back from Phobos. What analysis will you perform that will help you distinguish between these two different theories of the origin?

Tomo Usui: The fingerprints are the isotopes, isotopes of the constituent elements of the building blocks, like oxygen and also titanium, chromium, and also heavier elements, maybe. But the chromium, titanium and oxygen isotopes are good indicators to show the origin of planetary, solar system materials. So we have a lot of information, based on the Apollo samples and meteorites, to make a kind of classification diagram using the oxygen and also the titanium and chromium isotopes. So we're going to use them to identify the origins: whether it is like an asteroid or comet, or the impact scenario, which could be a mixture of the Martian material plus the impactor.

But for me, I'm very, very interested in the Martian ejecta, the samples coming from the surface of Mars. Because we still have a very limited number of Martian materials, which are called the Martian meteorites. But the Martian meteorites are so limited in terms of age, in terms of geological setting. But Phobos materials in the regolith could include lots of materials coming from the whole history of Mars, so that would give us a continuous history of the evolution of the Martian surface. So that is so unique.

Oliver Strimpel: Presumably the Martian meteorites we can collect on Earth were delivered relatively recently, and I expect Phobos, on the other hand, has been collecting material from Mars for many millions of years.

Tomo Usui: The meteorites that we have landed on the Earth within, let's say, 10,000 years. Old meteorites that came to the Earth very early are gone. We didn't have any chance to get such old meteorites that landed on the Earth very early, whereas Phobos captured the ancient stuff. So for the Martian meteorites, we have about 150 meteorites so far, but the 150 meteorites come from just about 10 geological locations, not 150 locations, and tens of them, or 20 of them, come from one place. And also, Martian meteorites are all igneous rocks, crystallized from magma, which means that Martian meteorites experienced very high-temperature magmatic processes. But we do not have any sedimentary rocks, sedimentary Martian meteorites. Igneous rocks are also very important, but sediments, like lake-bed sediments, or sea and ocean sediments, are very important for us, to give us information about the paleoclimate and also the environment, and also, hopefully, habitable conditions. Whereas the igneous ones give us information about the deep interior. So we'd like information about the surface conditions: the Martian environment, habitability, climate, weathering. Martian particles in the Phobos regolith are hopefully a mix of everything, because the ejecta from Mars come from all over the entire Martian surface, and the ejection velocity is much, much lower than for the Martian meteorites, because these ejecta do not have to escape the Mars gravity: just popping out and being caught by Phobos.

Oliver Strimpel: For my last question, I want to ask you, if you could choose the destination for the next sample return mission after Phobos, where would you go?

Tomo Usui: Oh, two ways. One goes to the outer solar system, like Jovian asteroids, or also Saturnian moons. The other is a landing mission, landing on Mars. MMX is a kind of gateway for the Mars gravity field, for Mars exploration. So we're going to use MMX as a stepping stone to a more comprehensive Mars landing mission tomorrow.

Oliver Strimpel: Tomo Usui, thank you very much.

Tomo Usui: Thank you.

Oliver Strimpel: For more about Geology Bites, as well as pictures and diagrams that illustrate this podcast, you can go to geologybites.com.