Phobos may be more challenging than even Ryugu: Prof. Tomohiro Usui on preparing for MMX curation

The analysis of the samples brought back by Hayabusa2 from asteroid Ryugu is now underway. Journal papers have started to be published, and many scientific results are expected to be announced next spring. To keep the immensely precious samples that have been returned from space from being contaminated by the Earth’s environment, and to catalogue the characteristics of each grain in the sample prior to distribution, a process called “curation” is undertaken. Only NASA and Japan currently have the capabilities to curate extraterrestrial materials. Professor Tomohiro Usui from the Institute of Space and Astronautical Science (ISAS) at JAXA, oversees the Japanese curation team. Usui has studied water on Mars for many years and has previously been a member of NASA’s Mars Exploration Team. He is now preparing to receive the samples that the Martian Moon eXploration (MMX) mission will bring back from Mars’s moon, Phobos, in Japanese Fiscal Year (JFY) 2029. We asked Usui about the details of the project and about his own work.

(Interview and text by Kimiyo Hayashi, translated by Ayumu Tokaji with Elizabeth Tasker)

Video: Work in the Hayabusa2 clean chamber in the JAXA Extraterrestrial Sample Curation Center.

Professor Usui, you began working at the JAXA Institute of Space and Astronautical Science in 2018. That was the year that the Hayabusa2 mission arrived at Ryugu, is that right?

Tomohiro Usui (hereafter, Usui): Yes, I started working at ISAS in July, and everyone was very busy because we had to decide on a landing site for Hayabusa2 by August. There was no one to help me! (laughs).

Did you come to JAXA with the goal of overseeing the curation team?

Usui: No, that’s not how I started my time here. I was asked to work on exploration projects for celestial bodies with relatively high gravity, such as Mars and the Moon, including the MMX project. Originally, I was analyzing materials from meteorites that came from Mars. But because of my knowledge of Mars and contacts at NASA, I was asked, “Why don’t you take care of the whole curation program?” I’ve been in charge of the Extraterrestrial Materials Curation Center since 2020.

What can we learn from the sample from Phobos regarding how the moons formed?

I’m very interested to see what samples MMX will bring back to Earth and what these will reveal. One of the scientific objectives of MMX is to understand the origin of the Martian moons. There are two main theories about the origin of the Martian moons: one is the asteroid capture theory, which says that asteroids passed by Mars and were captured by Mars’s gravity. The other theory is that Phobos was formed after a giant impact with Mars and another celestial body, and fragments of both Mars and the impactor coalesced into the moons. What are the differences in the samples obtained from the two theories?

( → Hear more about the Martian moon formation theories.)

Usui: First of all, the asteroid capture theory is supported because the spectroscopic data of D-type asteroids and the surface of Phobos are very similar. D-type asteroids are thought to have formed in the outer Solar System, farther away and colder than C-type asteroids such as Ryugu. So they may yield materials that are richer in water-bearing minerals and organic matter.

( → Hear more about D-type asteroids.)

Tomohiro USUI is the Director of the Extraterrestrial Materials Research Group at the Institute of Space and Astronautical Science, JAXA. He was born in Fukuoka, Japan in 1976, and has worked as a researcher at Okayama University, Tennessee State University and the NASA Johnson Space Center in the U.S., and the Earth-Life Sciences Institute (ELSI) at Tokyo Institute of Technology. He has been in his current position since 2018.

The sample could be richer in water-bearing minerals and organic matter than Ryugu?! That would support the idea that asteroids may have been responsible for the movement of water and organic matter through the Solar System. What kind of materials would we get in the case of the celestial impact theory?

Usui: It will be a mixture of fragments of the colliding celestial body and material from the Martian surface. There are various theoretical studies, and the range is wide, but it is thought to be about 50% of each. However, it will not be a heterogeneous mixture of each type of matter. The temperature at the time of impact is about 1,800°C, which is above the melting point of rocks that sits at about 1,000°C. So the material must have been melted and then recrystallised. After experiencing high temperatures, if the rock cools down quickly, it would become something like volcanic glass. On the other hand, and if the rock cools slowly, minerals might develop. In either case, I don’t believe that the original material from the colliding asteroid will remain intact.

Which do you think is more likely, the asteroid capture theory or the celestial body collision theory?

Usui: Both could be wrong, and there are both facts that fit the observations and facts that do not fit each theory. That the spectroscopic data of the D-type asteroid surface and Phobos surface are very similar strongly supports the asteroid capture theory. On the other hand, the orbits of Phobos and Deimos around Mars, that is, the orbits of the Martian moons, directly contradict the asteroid capture theory.

What do you mean?

Usui: Asteroids come toward Mars at all angles and speeds, some colliding and some passing by. They are gravitationally captured only when they pass by the very edge of Mars, and this should result in an inclined elliptical orbit for the captured objects. However, both Phobos and Deimos are in circular orbits on the equatorial plane of Mars. According to one researcher’s calculation, the probability of Phobos and Deimos orbiting in their current orbits is about one in 10 million if the asteroids are captured. That’s almost impossible. So the spectroscopic data supports the asteroid capture theory and the orbits support the celestial collision theory.

Are you saying that we need to bring the samples back from Phobos for analysis in order to determine which theory is correct?

Usui: We’ll know for sure if we bring back samples from Phobos, but the MMX spacecraft can do chemical analysis from above Phobos, which may be quite deterministic.

Preparations for MMX curation: The next major step forward from Hayabusa2

In the Hayabusa2 clean chamber, extra-large particles collected from the sample catcher are placed one by one in a special container. Photo taken at the Extraterrestrial Sample Curation Center.

I see. We are planning to bring back samples from Phobos in JFY2029, what is the status of the curation preparation now?

Usui: We are currently in a transition period. For the first three to four years, we were studying what kind of materials might be brought back from Phobos. From last year to this year, we’ve been working on the analysis protocol (procedures) to see what kind of analysis we can do on the returned materials to achieve the mission objectives. From next year, we will decide what kind of facility we need to build and the specifications for the design. This will continue for about three years.

You’re finally going to design the MMX curation facility. Is there anything that you inherited or learned from the Hayabusa2 curation method?

Usui: The technology for handling extraterrestrial materials without contamination has been cultivated since the first Hayabusa project. How to build a clean room, the materials used for the vacuum chamber, and the tweezers used within the chamber were all custom-made and carefully examined and will be inherited into our future missions. The good thing about the Hayabusa2 project is that we got back about 5g of sample, which was originally planned to be 0.1g. This is 50 times the minimum target amount! From 0.1g to 10g (the minimum target for MMX), it’s 100 times jump, but from 5g, it’s only twice as much. We are gaining more and more experience in handling large quantities of samples. I was thinking that we can improve curation technology in a hop, step, and jump fashion from Hayabusa to Hayabusa2 to MMX, and I think we took a pretty good step with Hayabusa2.

I heard that the sample brought back by Hayabusa2 was a large amount, 5g, and that it was difficult to procure sample dishes for the analysis?

Usui: Yes. But other than that, I think we were able to handle it. There was not 50 times the increase in the number of small particles, but there were many large particles that were relatively easy to handle. However, a key point when handling a large number of samples was the “blackness”. All of the samples from Ryugu were completely black, and it was hard to tell which ones to look at with the naked eye. But this time, by adding a near-infrared spectroscopic microscope, we were able to identify minerals that were invisible to the naked eye (visible light) but were clearly visible in the near-infrared. It was a great success. If MMX were to bring back a sample of a D-type asteroid, it would be even darker. I’m hoping that the near-infrared microscope will help us not to panic (laughs).

At a press conference on December 8, 2020, just after the Ryugu sample was delivered to ISAS. At that time, no one knew that 5g of the sample had been collected (credit: Kimiyo Hayashi).

Didn’t you use the near-infrared microscope for the Hayabusa project?

Usui: There was no near-infrared microscope at the time of the Hayabusa project. In fact, there are no other curation facilities anywhere the world that use near-infrared currently for extraterrestrial returned samples. Actually, we had a spare near-infrared microscope for the Hayabusa2 spacecraft, so we decided to include it in the curation facility. It’s a microscope, which visualizes particles in red that have traces of water .

When you start to see signs of water here in the grain, you’ll be excited to investigate it further!

Usui: That’s right. You cannot see it all instantly, as it takes time to produce an image in the microscope.

Do you do any of the curation work yourself? Or do you mainly supervise?

Usui: I’m like a manager. I know what we are doing, of course, but I don’t know much about what happens when I press any of the buttons (laughs).

I see (laughs). Are there any new facilities or methods that you are considering installing for MMX?

Usui: The near-infrared spectroscopic microscope I mentioned earlier was installed in our facility because we happened to have a spare for the flight, but for MMX, we would like to install a microscope optimized for curation. More organics are likely to be included in that sample, so we need to be able to identify them. The sample is likely to be at least 10g and possibly around 30g, so we need to be prepared for that as well.

How do you identify organic matter?

Usui: In addition to near-infrared, there is a wavelength range in which UV (ultraviolet) light can be used to identify organic materials, so I am planning to use those wavelengths to investigate. However, the shorter the wavelength, the higher the energy. It is not good if the sample gets sunburned in the same way that humans get sunburned. I’m still examining the procedure to see if we can use UV.

We don’t know the nature of Phobos any better than we knew Ryugu.

Hayabusa2 curation work in progress.

Hayabusa2 didn’t discover that the asteroid material was pitch black or porous until the spacecraft arrived at Ryugu. How much do we know about Phobos at this point?

Usui: There are even more things that we don’t know. It would be correct to think that Phobos is more difficult to curate than Ryugu. We certainly didn’t know the size and shape of Ryugu until the arrival of Hayabusa2. However, we knew that the mission would return materials from an asteroid, and scientists have a lot of knowledge about asteroids in general. On the other hand, in the case of Phobos, we have photographs of the moon so we know its shape, and we have spectroscopic data, but we don’t know whether it resembles an asteroid or is more like Mars, if it is some kind of material that we don’t know anything about, or if it is mixed or melted. Furthermore, Phobos is orbiting Mars, so the moon is also being influenced by Mars. We have to consider the curation procedure for many different cases, such as the asteroid origin theory and the celestial body collision origin theory.

Specifically, if the asteroid capture theory is correct, there will be a lot of organic matter, so we need to prepare for that?

Usui: Yes. Also, there are likely to be a lot of hydrous minerals that are stable at low temperatures, so we need to prepare a near-infrared microscope to be able to identify them.

That certainly sounds like a lot of work.

Usui: We initially expected to get only 0.1g of the Ryugu sample, so we decided not to remove it from the clean chamber until it was to be distributed to the different analysis teams. However, if we can collect 10-20g of Phobos sample, we are considering removing about 0.01g of the sample from the chamber for various analyses in advance of the whole curation processes.

Video: A year following the Hayabusa2 sample

What does it mean to remove the sample for analysis in advance?

Usui: In the clean chamber, we can only look at the sample with visible light or near-infrared light, but if we bring it outside the chamber, we can irradiate it with X-rays or electrons, or decompose it to analyze its chemical composition. Then, within a month of starting the analysis, we will be able to say, for example, “We found this organic material”. In the case of Hayabusa2, we took the sample out for the initial analysis about six months after the sample capsule was recovered, so even a year after the sample was collected, all we know is that there are “traces of organic matter” and we haven’t been able to identify what kind of organic matter it is.

That’s a very important point. Personally, I am interested in the possibility of finding traces of life (such as dead microorganisms) in the material that has travelled from Mars to Phobos. How are we going to find Martian particles in the Phobos sample in the first place?

Usui: One of the ways to find Martian grains is with a near-infrared microscope. The difficulty is that even if there are Martian particles in the sample brought back from Phobos, their quantity is low, less than 1%. So even if we were told in advance that “one particle is from Mars” among the pitch-black particles, we would not be able to pick it up effectively. However, Phobos has no atmosphere or water, while Mars has an atmosphere and water. It is therefore very likely that the material from Mars consists of minerals that have reacted with the Martian atmosphere and water. If there are carbonates, hydrous minerals, and organic matter, we may see carbon in the Phobos sample. If we can reveal these in images under a microscope, I think it will be relatively easy to find the Martian grains.

Extraterrestrial curation expertise in the U.S. (NASA) and Japan (JAXA)

Delivery of part of the Ryugu to NASA’s Johnson Space Center at the end of November 2021. “The only C-type asteroid sample is the one brought back by Hayabusa2. NASA can practice with real asteroid samples before the return of the NASA OSIRIS-REx mission. The Japanese and U.S. teams will exchange samples and jointly develop analysis techniques.”– Tomohiro Usui (credit: NASA/JAXA).

Curation was first started in the world by NASA when they brought back Moon rocks with the Apollo program. So the US and Japan are the only two countries in the world that currently can curate extraterrestrial material. How difficult is curation?

Usui: It is a difficult and time consuming task to achieve the goal of creating a catalog under controlled conditions without contaminating the objects. Even if only a pair of tweezers and gloves are used to handle the material, we have to check whether they are OK to use with extraterrestrial materials. Accumulated experience is the key. In this sense, Japan has a slight advantage because of our experience with the first Hayabusa project. It is difficult to catch up, even if you have a large budget.

NASA has a track record in curation dating back to the Apollo era, bringing back solar wind samples for analysis in the Genesis program and samples from comets for analysis in the Stardust program. What fields are unique to Japan?

Usui: What is unique to Japan is the curation of asteroids. This involves techniques for handling small objects in a vacuum environment. We have a lot of experience in using items like ESD (electrostatic discharge) tweezers that are used for grabbing small things.

If MMX samples are returned in JFY 2029, it will be the world’s first sample return from the Mars system. What are your expectations and what are you most looking forward to?

Usui: I have been studying Mars for a long time. The material from Phobos that MMX will bring back is expected to contain material from Mars. I’m hoping that Japan will therefore be the first country in the world to return Martian material. Even if we do not find living organisms, if the sample contains organic matter, I think the material will be very interesting. In planetary science, which has a strong element of natural history science, new evidence rewrote theories by “going where no one has ever gone before” and “discovering things that no one has ever seen before”. Even geniuses become silent when faced with evidence. I am very much looking forward to planetary exploration and sample return because it has such an appeal.

Thoughts on Hayabusa’s return to Earth

Photograph of the fireball taken by Woomera on June 13, 2010, during the return of the asteroid explorer Hayabusa (credit: JAXA / National Central University of Taiwan).

By the way, I heard that you were at NASA when the first Hayabusa returned to Earth in June 2010. How did you feel?

Usui: At the time, I was working as a researcher at NASA’s Johnson Space Center, and we were all watching Hayabusa entering the Earth’s atmosphere like a fireball. My American colleagues were excited, saying things like “Beautiful” and “Fantastic!”, but I was more sentimental about what my senior colleagues had accomplished in the country where I grew up. At that stage, I had never been involved with Hayabusa and was only familiar with the mission name.

Did you also have a desire to contribute to Japan?

Usui: The precursor to this was when I was part of NASA’s Mars exploration team, and enjoying being recognized as a fellow team member. However, to NASA, I was a foreigner and did not have the same rights as my American colleagues. At one point, there was a science meeting about the NASA Mars Rover, and when the topic turned to engineering, the leader asked us to leave the room unless we were a U.S. citizen or permanent resident. Out of a hundred or so people, I was the only one to leave. My colleagues were apologetic, and I thought that was the way it just had to be, but then Hayabusa returned. I had gone to the U.S. to pursue my research out of pure scientific interest, but there was also more to my aspirations than this. Then I had a chance to find a job in Japan. I wanted to contribute to the country where I was born, grew up, and received my education, and where my parents, siblings, and friends were.

You have been studying water on Mars now for many years. What are your goals for the future?

Usui: Can I talk about my big ambitions? (laughs) My three goals have been set since I arrived at JAXA: the first is to conduct strategic and continuous space exploration in Japan. The second is to create an academic research environment that allows for free and fair discussions regardless of age or position. The third is to improve my own research performance over the previous year. These three things are all connected as ends and means. In improving research achievements, I also explore further and junior students will join our group, and then their research achievements will increase and exploration increases still more.

Are you now so busy that you have less time to devote to your research?

Usui: I have less time for my own research. So, I’ve been working with students and postdoctoral researchers, asking them, “This theme sounds interesting, why don’t you try it?” (laughs).

What kind of missions are planned for this continuous space exploration?

Usui: JAXA is participating in the study for the Mars Ice Mapper project, which aims to learn about the distribution of ice beneath Mars. It’s a difficult and interesting mission, and JAXA is thinking of going beyond that to a landing mission on Mars. But if we limit ourselves to Mars exploration, we’ll only be able to do run a mission once every five to ten years. We have the SLIM project, which will land on the Moon to acquire the technology for landing exploration. I would like to explore lunar and asteroid exploration so that junior researchers can be involved in exploration missions several times in their research life.

Further information:

The Astromaterials Science Research Group