Laboratory perspectives on sample returns from
Hayabusa2 and OSIRIS-REx

Rosario Brunetto, Cateline Lantz

► To cite this version:

Rosario Brunetto, Cateline Lantz. Laboratory perspectives on sample returns from Hayabusa2 and OSIRIS-REx. Nature Astronomy, 2019, 3 (4), pp.290-292. 10.1038/s41550-019-0744-9. hal-02405347

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# Laboratory perspectives on sample returns from Hayabusa2 and OSIRIS-REx

The new generation of sample return missions from small bodies will deliver to us fresh witnesses from the early Solar System. In-depth laboratory analysis of retrieved samples will allow us to look in unprecedented detail at the formation and evolution of organic materials in asteroids.

Rosario Brunetto and Cateline Lantz

Our knowledge of the origin and evolution of Solar System solids has made great progress in the past decades. Yet we still know little about the formation and evolution of their organic components from the protoplanetary disk to planetesimals, and the composition of those small bodies that eventually reached telluric planets. In this context, the role played by water and hydrothermal processes in the evolution of the organic materials contained in the planetesimals is of particular interest. Small bodies such as asteroids and comets are privileged witnesses of this evolutionary state of the early Solar System.

Laboratory analyses of materials originating from asteroids and comets give us the opportunity to study directly the components that formed in the protoplanetary disk¹,². Extraterrestrial materials currently available in the laboratory include meteorites, interplanetary dust particles and micrometeorites. The laboratory study of meteorites and interplanetary dust, however, has intrinsic limits and biases. The samples may suffer modifications due to atmospheric entry and terrestrial weathering, with potential loss of the most fragile and volatile components. Additionally, the link between laboratory samples and their parent bodies is often ambiguous. Owing to various selection biases, many classes of asteroids and comets are probably absent from our collections. These limitations are overcome by sample return space missions.

Since the beginning of this century, we have witnessed renewed interest in sample return missions from several space agencies. Today, a new generation of missions is about to collect fresh materials from heretofore unsampled small Solar System bodies such as dark carbon-rich asteroids. Furthermore, some of these missions carry instrumentation for in situ measurements of their target bodies, which would complement and enlarge what can be obtained by ground-based observations³.

<figure><img src="image_2.png" /><figcaption>**Fig. 1** | Closing the loop thanks to retrieved samples. Sample return missions play a key role in linking laboratory analysis and experiments, remote sensing observations and in situ characterization of Solar System bodies, and in joining efforts among complementary scientific communities.</figcaption></figure>

Sample return missions also provide a great opportunity for bringing the scientific community together. Previous sample return missions (Apollo/NASA, Luna/Roscosmos, Stardust/NASA and Hayabusa/JAXA) have demonstrated that collaboration among astronomers, modellers, physicists, geologists, chemists and engineers is a key success factor.

The loop sketched in Fig. 1 illustrates the importance of bringing samples of traceable origin to the laboratory. As astrophysicists and astrochemists, we perform laboratory experiments reproducing astrophysical processes occurring in space (L1), supporting the interpretation of remote sensing observations (RS1 and RS2) and — when possible — the in situ characterization of Solar System bodies (IS1). Once we have obtained samples from astronomical bodies (IS2), they can be analysed in the laboratory using state-of-the-art techniques and apparatuses (L2). The results of these

measurements may therefore stimulate new laboratory experiments (L1), to deepen the understanding of what has been observed at the microscopic scale and further support the interpretation of remote sensing observations (RS1).

Hayabusa2/JAXA⁴ and OSIRIS-REx/NASA⁵ are the two sample return missions operating at the moment. They are currently orbiting two small and dark near-Earth objects: Ryugu and Bennu. Ground-based telescope observations classified these objects as C-type and B-type asteroids, respectively⁶,⁷. According to this classification, these asteroids are presumed to be small fragments of larger bodies that retained primitive units (grains and molecules) from the early Solar System. Thus, it is expected that samples from Ryugu and Bennu will provide valuable information about the main alteration processes of primitive solids (in particular organics), such as aqueous alteration⁸, heating⁹
<figure><img src="image_3.png" /></figure>

<figure><img src="image_4.png" /><figcaption>**Fig. 2** | An example of analysis of an extraterrestrial sample using synchrotron-based techniques. A micrometre-sized fragment of the Paris meteorite is mounted on a needle (bottom left) to be analysed at several synchrotron beamlines (top, the French synchrotron SOLEIL), with complementary non-destructive techniques such as infrared (centre — the setup is shown together with the result in the red frame) and X-ray 3D micro-tomography (right, purple frame)²². Credit: Top image, SOLEIL (https://www.synchrotron-soleil.fr/fr); bottom line, photo of instrument by authors, other images reproduced from ref. ²². Cambridge University Press</figcaption></figure>

(internal/external) and irradiation¹⁰. Both missions have delivered preliminary results showing spectral indications of hydrated and carbon-rich surfaces¹¹,¹². Further analyses of spacecraft data will indicate whether the objects have been heated and/or irradiated.

Hayabusa2 and OSIRIS-REx are expected to return samples to Earth in late 2020 and 2023 respectively. After the capsule recovery, the samples will be stored in their curation facilities to be sorted and catalogued. A preliminary examination phase will follow. The JAXA initial analysis involves a number of international sub-teams organized by scientific goals and techniques¹³, focused on, for example, the chemistry of samples (elementary and isotopic content), the mineralogy/petrology for coarse and fine grain sizes, the volatiles (extracted before capsule opening), and soluble and insoluble organic matter. The analyses are expected to elucidate the formation and evolution of Ryugu. After completion of the first analyses, the samples will be made available for the broader scientific community. A fraction of the samples will be preserved for posterity, with the hope that future technological advances will allow for more sophisticated inspections. A similar procedural chain is being implemented for OSIRIS-REx.

Considering the limited amount of material that will be retrieved by these missions (a few grams at most), there is a

need to optimize the analytical strategy, maximize the scientific outcome and minimize sample loss. A multi-analytical sequence from less destructive (for example spectroscopy¹⁴) to more destructive techniques (such as nanoscale secondary ion mass spectrometry¹⁵ or transmission electron microscopy¹⁶) needs to be established. Synchrotron-based studies (imaging and spectroscopy through scanning transmission X-ray microscopy using X-ray absorption near edge structure (STXM-XANES)¹⁷, X-ray diffraction¹⁸ and microtomography¹⁹), which started about 15–20 years ago, can play an important part in maximizing the information obtained from micrometre- and sub-micrometre-sized samples.

Among the possible techniques, infrared spectroscopy is important in being totally non-destructive and comparable to remote sensing observations of small bodies²⁰. Thanks to infrared imaging microspectroscopy, we are able to detect and study the spatial distribution of molecular bonds associated to minerals, water and organics²¹. We consider infrared three-dimensional (3D) microtomography²² an excellent starting point in a multi-analytical sequence to be applied on returned samples (Fig. 2). It will provide a first quick look at the composition, abundance and 3D distribution of carbonaceous materials at the scale of a few micrometres within

individual grains up to several tens of micrometres in size. Once the organics are revealed by infrared measurements, thin sliced sections of the grains can be analysed by more destructive techniques to retrieve the structure and the elemental and isotopic composition of the carbonaceous component and its mineral host, down to the nanometre scale. This top-down sequence will build a bridge between the observations at macroscopic scale and the chemical and physical processes operating at the nanoscale.

Many scientific questions will be addressed by laboratory analysis of returned samples, based around wide topics of broad interest. Some questions will deal with 'formation and origins': what was the galactic environment in which the Solar System formed? What processes took place in the natal molecular cloud? How were solids formed in the protoplanetary disk? Other issues will be centred on 'evolution': which processes modified the primordial grains, before and after their incorporation in the first planetesimals? Which secondary processes altered the primitive asteroids and determined the formation of the present-day Ryugu and Bennu? Finally, we expect that many studies will focus on topics relevant to 'astrobiology': how much water is contained in these bodies? How much carbon? What are the prebiotic molecules that asteroids like Ryugu and Bennu may have delivered to the telluric planets?

Among these topics, we consider it particularly interesting to study the small-scale association between water, organics and minerals. Samples retrieved by Hayabusa2 and OSIRIS-REx are expected to bring information about the hydrothermal environment in the parent bodies of Ryugu and Bennu and its effect on minerals and organics. It will also allow us to gain insights into how the organic content and nature of these bodies differ from those of primitive meteorites and interplanetary dust particles. Multi-analytical laboratory studies²³ will be key to address these issues, supported by experiments to simulate the formation and evolution of primitive organics²⁴, or the hydrothermal evolution inside parent bodies²⁵.

Laboratory analysis of Ryugu and Bennu grains will also allow direct detection of space weathering processes, induced by irradiation and solar heating, similar to the case of the S-type asteroid Itokawa (mission Hayabusa). Understanding surface space weathering is necessary to de-bias the remote sensing observations and have access to the subsurface composition of the objects. The detection of space weathering on the grains will be possible by studying
the alteration of their surface composition and structure, both spectroscopically<sup>14</sup> and microscopically using transmission electron microscopy<sup>26</sup>. In addition, by performing irradiation experiments on relevant asteroid analogues, we can simulate the energetic processes affecting asteroid surfaces<sup>10</sup>. The results of these experiments will be used both to support the analysis of retrieved samples and to formulate predictions about the trends that should be measured on asteroid families by remote sensing spectroscopy<sup>27</sup> (L1-RS1 in Fig. 1).

We anticipate that the return and laboratory analysis of Hayabusa2 and OSIRIS-REx samples will trigger a new wave of experimental studies, which in turn will support future observations of small Solar System bodies. In the context of these new missions, collaborations are being created to guarantee the establishment of the beneficial loop depicted in Fig. 1. The positive feedback between complementary scientific

communities and forthcoming technological developments such as the next generation of synchrotrons will also pave the way towards future sample return from comets, Mars and its moons<sup>28</sup>.

Rosario Brunetto* and Cateline Lantz

Institut d'Astrophysique Spatiale, CNRS/Université Paris-Saclay, Université Paris-Sud, Orsay, France.

*e-mail: rosario.brunetto@ias.u-psud.fr

Published online: 5 April 2019

https://doi.org/10.1038/s41550-019-0744-9

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