OT Interviews: Dr. Sarah Crowther on the Bennu Sample at the University of Manchester22nd Nov 2023
Orbital Today had the privilege to speak with Dr. Sarah Crowther of the University of Manchester. Dr. Crowther will study a Bennu asteroid sample sent from NASA to the UK. To do this, she will be using a unique xenon gas spectrometer called RELAX.
NASA made headlines after the OSIRIS-REx mission successfully dropped a capsule with a sample from Bennu into the desert in the state of Utah. A pleasant surprise followed as the capsule contained more material than expected. However, the recovery team has hit a snag as the OSIRIS-REx TAGSAM (Touch and Go Sample Acquisition Mechanism) head is stuck. Some screws cannot be removed in the manner planned, and the head can be opened only part-way.
So, to find out more about the state of affairs with the TAGSAM head, what the RELAX instrument will be doing (and why) – and just what RELAX is- we turned to Dr. Crowther.
OSIRIS-Rex and the stuck TAGSAM head
OT: Regarding the return capsule and the stuck screws, does NASA have a time frame for accessing the rest of the material?
Dr. Crowther: It’ll take however long it takes to figure out a new process and rehearse that. Once the team have worked out how to open the TAGSAM head, the curators and sample handlers will practice the processes, so they know exactly what they’re doing.
OT: So, it’s not just figuring out the tool to use and how to gain approval for its use, but also going through the processes so that their people are trained to do so beforehand.
Dr. Crowther: Yes, the curation team did lots of rehearsals before the samples came back. They had mock glove boxes, mockups of the sample, the container, and the TAGSAM head, so they could rehearse everything that they needed to do.
Sending samples out to the world
OT: Given the amount of material that’s already been collected, are there plans to start using some of it and send it out? There’s been news about preliminary findings, but are they thinking about sending some of that material out, perhaps to your team?
Dr. Crowther: The mission requirement was to collect 60g minimum of material. A little bit over 70g has been removed from the TAGSAM head so far. So yes, the samples should be starting to be made available to the sample analysis team in the quite near future.
OT: So, there’s enough collected to start. What are they going to do now that they have this overabundance? Do you know who else might be in line?
Dr. Crowther: Some of the material is going to international partners, which include JAXA, the Japanese space agency, and the Canadian Space Agency. A fraction will go into long-term storage so that it can be used in the future. The curation team will be making a sample catalog, which will hopefully be ready in about six months. And once the catalog is made, the samples will be opened up to the wider scientific community to apply for, in the same way, that other samples like the Apollo samples are available.
One of the great things about sample return missions is that we’re not limited by the questions we want to answer now, and the types of analytical techniques we have now. The samples will be carefully curated and stored so people who aren’t even born yet can analyse samples in the future using new analytical techniques that we haven’t even thought of yet to answer new questions, as has happened with the Apollo samples. So only a fraction of the material will be made available to researchers now, leaving plenty for future research.
RELAX and the hunt for xenon isotopes
OT: Regarding your own use, is this coming under the Isotope Geochemistry and Cosmochemistry group? What will you be doing with the sample?
Dr. Crowther: We’ll be looking at xenon isotopes in the samples.
For background, if we think about the periodic table of elements for a moment, the very right-hand column is what we call the noble gases – that’s helium, neon, argon, krypton and xenon. And these elements are particularly useful for studying the history of extraterrestrial materials for several reasons. They’re not very abundant in rocky materials, so it makes them difficult to measure. But it also means that any differences are very obvious, and I’ll come back to that in a second. They’re not reactive, so they just sit there minding their own business. They have lots of isotopes, which are different forms of the same element. Several different physical processes, such as the radioactive decay of other elements or interactions with cosmic rays, produce characteristic isotopic signatures of the noble gases.
It is the combination of these traits that make noble gases so useful as tracers of Solar System history. If, for example, we have a process, let’s say radioactive decay, that is producing a couple of thousand atoms of one noble gas isotope, then, because there’s not very much there to start with, that difference is really obvious. Whereas, if a thousand atoms of one isotope of oxygen were added to this room, we’d never notice the difference because there’s so much oxygen here to start.
OT: And so you’ll be using RELAX for that?
Dr. Crowther: The RELAX mass spectrometer is specifically designed to analyse xenon isotopes. And there’s nine isotopes of xenon. A lot of people say that’s too many, that xenon is too hard to understand! But actually, there’s so many different contributing processes and components that we need that all those isotopes to deconvolute all those processes and components
The RELAX mass spectrometer is a bit different from conventional noble gas mass spectrometers in the way that we create the ions. We use what we call a resonance ionization process. We have a laser that’s tuned to a specific wavelength to excite a xenon atom. And then we ionize the xenon atom from that excited level. That’s a very efficient process, and that excitation wavelength is characteristic of xenon. The same process of exciting an atom of carbon, oxygen, neon, argon or whatever, requires a different wavelength of light. This means that we can ionize the xenon very efficiently and we don’t ionize the other stuff that’s there.
Combining this with the other key features of RELAX, we can analyze really small samples. The mass spectrometer incorporates a cryogenic sample concentrator,and the overall volume of the mass spectrometer is smaller than a conventional instrument, both of which contribute to making RELAX a very sensitive instrument for analysing xenon isotopes.
The cryogenic sample concentrator is a spot inside the mass spectrometer that’s at a cold temperature. The gas condenses on that spot in the same way as water is condensing on the window when you get up in the morning on a cold day. Once the gas condenses on this cold spot, we use a laser to release the gas from the cold spot, which creates a concentrated plume of gas in the ion source, rather than the gas being everywhere. And then we will fire the ionizing lasers through that concentrated plume of gas.
RELAX and doing more with less
OT: How many other machines are there like that out in the world?
Dr. Crowther: RELAX is unique. It’s the only one for xenon. There is a similar instrument for analysing krypton samples, though. We can do analyses of smaller amounts of material than is possible with conventional noble gas mass spectrometers, and we can do higher resolution analysis. We’ve recently done work on the Japanese Hayabusa2 samples. There were some other groups looking at noble gases in the Hayabusa2 samples as well, and some of the analyses have now been published.
We use a technique we call step heating. Very simply, we heat a sample with a laser, and analyse the gas that comes out. Then we heat it a bit hotter again; look at the gas that comes out, and keep repeating that process. The increments in heating steps depend very much on how much gas is coming out. You want enough to make a good measurement, but not too much. Because RELAX is so sensitive, we could do more heating steps than could be done with the same mass of material using a conventional noble gas mass spectrometer. This means that if there are different components of xenon that are hosted in different minerals, for example, that might be released at different temperatures, there’s greater chance of separating those components out in more heating steps.
OT: What do you what do you expect to be able to determine from the amounts of xenon produced. What are you looking for?
Dr. Crowther: We will be able to look at the different components that contribute to the measured composition of the xenon. The xenon that’s in the sample could contain xenon from different sources, for example, it could contain solar wind xenon from the solar wind coming off the Sun. It could also contain xenon produced by radioactive decay, or xenon produced by cosmic rays interacting with elements in the sample. We will be able to see these different components if they are present in the samples.
We will also try to get an age for the sample using something we called iodine-xenon dating, which is a radioactive chronometer. It’s similar to carbon dating, many people have probably heard of carbon dating in relation to things like archaeological finds. Instead, we’re using radioactive isotopes that were around in the beginning of the solar system and have since decayed, but we can see the decay products.
OSIRIS-REx, Hayabusa2, and meteorites
OT: You mentioned that you’d worked with the Hayabusa2 samples. Do you expect to see any real differences between them? Is there any difference in the way you handle them?
Dr. Crowther: I don’t know yet because I haven’t got my hands on any OSIRIS-REx samples yet! The asteroids Bennu and Ryugu are both what we call carbonaceous chondrite material, so we might expect some similarities, but there will likely be some differences too.
OT: So, is it ‘a sample is a sample is a sample’?
Dr. Crowther: Yes and no! Some material can be easily pick up with a pair of tweezers. Other materials are very friable, they disintegrate very easily. I haven’t got my hands on the OSIRIS-Rex samples yet, so I’m not sure how it will behave.
OT: Lastly, with all the meteorites in the world, why go through the trouble of doing something like OSIRIS-REx? Why not just heat up one of them?
Dr. Crowther: We want a pristine sample. A lot of our work is on meteorites, but they’ve always been contaminated by our atmosphere. Even if they’re seen to fall and picked up very quickly, they’ve still been contaminated by our atmosphere in some way. One of the great things about the sample return missions like OSIRIS-Rex and Hayabusa2 is that we can ensure these samples never see the Earth’s atmosphere. If you’ve seen any pictures of the curation team working in at the Johnson Space Center, you’ll have noticed that they’re working in glove boxes, and those glove boxes are full of nitrogen. The samples aren’t exposed to Earth’s atmosphere. The samples should be shipped in containers full of nitrogen. We will handle them in a smaller nitrogen glove box until they get into our spectrometer, at which point they’ll be under vacuum. So they will never be exposed to Earth’s atmosphere. That can be particularly important for people looking at things like organic molecules because it’s so easy for samples to get contaminated by organic material on Earth. But if the samples are never exposed to Earth’s atmosphere, we can be fairly sure that organic compounds found in the samples actually are from the asteroid.
Not just another data point
OT: You mentioned that you were going to look at the radioactive decay and try to get an estimate of the dating. Given all the other data points that we have for dating bits of the solar system, what would you expect to find here that would make it worthwhile to do that?
Dr. Crowther: I think the question might be, what process or event will we be dating? It may not be when these rocks actually crystallized. We can see evidence of water on Bennu, not liquid water, but water trapped in the minerals. The asteroid has been altered by water, and a radiochronometer might date when that alteration process occurred. We won’t really know what the data will tell us until we’ve got the data. and combine that with other data about the nature of the sample and maybe from other chronometers and techniques. So it’ll be kind of a case of putting the data together and then seeing what’s what.
Once we’ve got data from different chronometers, we can build a timeline for processes happening in the early solar system. We know that most asteroids formed about 4.6 billion years ago, but what physical processes were happening then? Some asteroids evolved relatively undisturbed, whereas others broken up and then reassembled in what we call a rubble pile. By dating different types of materials from different types of asteroids, we can learn about the physical processes that were happening in the early solar system.
OT: When people hear or read ‘4.6 billion years ago’, they might think in static terms, but it’s not like that, is it?
Dr. Crowther: We think the carbonaceous chondrite asteroids may have delivered water and volatile materials to Earth. Those are elements and compounds that are essential to life. I’m not saying these asteroids delivered life to Earth, but they may have delivered some of the ingredients essential to life. Understanding more about them helps us understand more about how life might have evolved on Earth. And also, these asteroids are the building blocks of Earth itself. Earth is geologically active, there are volcanoes, earthquakes, plate tectonics, rivers, on Earth, and the surface of Earth is constantly changing. The record from when Earth first formed 4.5 billion years ago has been overwritten. But some asteroids have barely changed since they first formed, and still contain the building blocks of planets like Earth. I’ve heard people describe asteroids like this essentially as time capsules from the beginning of the solar system, so we can look back to see what the solar system was like then, what Earth and the other planets were made from.
OT: Are all of the UK researchers working on the OSIRIS-REx samples connected with RELAX?
Dr. Crowther: There are several teams here in the UK who are part of the OSIRIS-Rex Sample Analysis Team, different people are working on different things. Here in Manchester, there are people looking at halogens and mineralogy and petrology, as well as our xenon analyses. The team at the Natural History Museum will be looking at the mineralogy and petrology. The team at the Open University will be looking at oxygen and carbon and nitrogen. And there’s a team at the University of Oxford who are doing something a little bit different. They look at the spectra of asteroids and samples in the lab and try and match up the different types of meteorites with asteroids, so they will be working on the spectra of the samples in the lab to see if that matches up with the data from the spacecraft. And then going forward, that would help with future missions or future studies of asteroids from Earth. Everybody is doing something a bit different, everybody has different expertise, and we need all those different expertise and analyses to fully understand the samples and learn as must as we can from them.
Orbital Today would like to thank Dr. Sarah Crowther for taking the time to speak with us about RELAX and the upcoming research involving the sample from Bennu.