Organic matter and functional chemistry in the asteroid Ryugu
By: Catherine Maggiori
Synopsis: A new Nature Communications paper discusses the functional chemistry of carbonaceous and organic matter within the asteroid Ryugu. This matter seems to be both more abundant and more diverse than other similar material, indicating that direct asteroidal material can offer more clues about the early Solar System than meteorites alone.
Author info: Dr. Catherine Maggiori is an astrobiologist and microbiologist. You can find her at the bench, lurking on Twitter, or at the climbing gym.
This is a colored view of the C-type asteroid 162173 Ryugu, seen by the ONC-T camera on board of Hayabusa2. Filters: vwx date:2018-07-12 08:01 Image level: 2b (Images after hardware correction and radiometric correction) Image Credit: JAXA Hayabusa 2
Last month at SAGANet, we talked about linking Martian meteorites with their source craters, and how this information yields important clues about the formation of our Solar System and inner planet evolution. Meteorites could also have delivered primordial carbon to primitive Earth as simple organic molecules or complex macromolecules, an important consideration for astrobiology and prebiotic chemistry.
Material from asteroids provides similar details, but with some extra upside: collecting material directly from its parent body keeps it relatively pristine, avoiding any transformation from ejection and entry into Earth’s atmosphere, as well as radiation exposure, heating or oxidation.
(And I maintain that asteroid material and meteorites are just inherently super cool; I repeat, they’re rocks from outer space!)
These principles were some of the reasons behind JAXA’s Hayabusa2 mission. Launched in 2014, Hayabusa2 (translating to “Peregrine Falcon 2”) is the successor to JAXA’s Hayabusa mission (2003 - 2010), the first ever asteroid sample return mission.
Figure 1. Hayabusa2 instrument inventory and configuration (from Kikuchi et al., 2021).
Hayabusa2 rendezvous-ed with the asteroid 162173 Ryugu (aka Ryugu) in 2018, with two touch-and-go landings for sample collection in 2019, and returning to Earth in 2020. Currently, Hayabusa2 is on its way to a 1998 KY26, a small, fast-rotating near-Earth asteroid, with an expected rendezvous in 2031.
Figure 2. Hayabusa2’s second touch-and-go touchdown with Ryugu on July 11, 2019. Image courtesy of JAXA via https://www.planetary.org/articles/hayabusa2-nailed-second-touchdown.
Ryugu (translating to “Dragon Palace” after the mythical undersea castle in Japanese folklore) is an ideal target for astrobiology and origin of life studies: as a carbonaceous Apollo asteroid, it contains unaltered material that could act as a benchmark for the origin of organic matter and water on Earth. Ryugu itself could even be the source of some terrestrial carbonaceous chondrites.
Previous data from Hayabusa2 showed that Ryugu probably formed from some kind of collision event involving larger parent bodies and is similar in composition to Ivuna-type (CI) carbonaceous chondrites. CI chondritic material is similar in composition to the solar photosphere, indicating Ryugu’s importance as a potential reference for the early solar nebula.
Ryugu also contains water, minerals, organic compounds like uracil and niacin, and presolar grains. In this new Nature Communications work entitled “Variations of organic functional chemistry in carbonaceous matter from the asteroid 162173 Ryugu”, De Gregorio et al. increase this knowledge by describing carbon populations and functional organic chemistry in Ryugu grains.
Using X-ray absorption near-edge structure (XANES) spectroscopy, transmission electron microscopy (TEM), and nanoscale secondary ion mass spectrometry (NanoSIMS), De Gregorio et al. were able to classify carbonaceous grains via their spectral shape.
3 different types of sample preparations were used: insoluble organic matter (IOM) residues, thin sections of particle fragments via ultramicrotomy, and thin sections extracted via focused ion beam (FIB). IOM residues represent the aggregate carbonaceous matter from Ryugu, while ultramicrotomy and FIB-prepared samples provide more context regarding Ryugu’s local petrology and soluble organic matter.
STXM and STEM imaging of discrete carbonaceous grains from Ryugu IOM show aromatic, ketone, and carboxyl functional groups (Figure 3A - C); indeed, XANES spectra of the regolith grains also show peaks typical of these 3 functional groups (Figure 3E), which are commonly present in carbonaceous chondrites.
About half of the IOM grains analyzed show an IOM-like (IL) pattern, with dominant aromatic, ketone, and carboxyl groups. ~30% of the grains have highly aromatic (HA) spectra, with a broad C=C peak and minimal ketone and carboxyl signatures. The remaining 20% of the grains have a unique alkyl-aromatic (AA) spectral signature, with single or double benzene rings possessing carboxyl functional groups (Figure 3E).
The IOM residues thus indicate that Ryugu grains are fairly functionally diverse and this diversity is supported with the microtome and FIB-prepared samples. The HA:AA:IL spectral shape ratio of the microtome and FIB-prepared samples is ~20:25:55, similar to that of the IOM samples.
Figure 3. Adapted from Fig. 1 in De Gregorio et al. showing scanning transmission x-ray microscopy (STXM) and scanning transmission electron microscopy (STEM) imaging (A - C), dendrogram of fitted XANES spectra from Ryugu IOM, and XANES spectroscopy of Ryugu IOM (E).
Most of the discrete carbonaceous grains from Ryugu have C and N isotopic compositions identical to that of bulk Ryugu organic matter, but larger Ryugu grains deviate, with 15N and 13C enrichments in grains with IL functional group chemistry and depletions in 15N and 13C in grains with HA functional group chemistry (Figure 4). This finding suggests that distinct functional chemistries may also indicate distinct isotopic compositions in Ryugu’s carbonaceous grains.
Figure 4. Adapted from Fig. 3E in De Gregorio et al. showing C and N isotope compositions of carbonaceous grains. Dashed lines are bulk Ryugu values and gray points have not been correlated with STXM.
Diffuse organic matter is also abundant in Ryugu and is dominated by carbonate functional groups associated with phyllosilicate minerals. STEM images show that these carbonate groups are associated with clay sheet surfaces (Figure 5).
Figure 5. Adapted from Fig. 4C in De Gregorio et al. showing STEM images of diffuse carbonaceous matter and phyllosilicate sheets from Ryugu. Arrows signify carbonate-rich organic matter.
Ryugu OM is similar to CI and other carbonaceous chondrites, but distinct in that it is more aromatic, and contains more CO3 functional groups and AA spectral shapes. The AA material could indicate unique, low temperature (<150 °C) aqueous processing from Ryugu’s parent body, but these differences could also be the result of how Ryugu samples were collected: directly from Ryugu.
Material from asteroid sample return missions like Hayabusa2 and OSIRIS-REx are exceptionally valuable, providing untainted and unaltered pieces from their parent bodies, and may soon become the gold standard for studying carbonaceous matter in our Solar System.