Composition of Organics on Asteroid Bennu.
Most science enthusiasts would have by now at least heard about the asteroid Bennu. Astrophysicists have now moved a step closer to studying the asteroid. They have been successful in describing its composition of organic matter which was detected owing to the absorption at 3.4 micrometers, using the OSIRIS-REx mission.
They believe that most of the Carbon compounds there would be from the early solar system in the Interstellar medium (ISM) or in the sun's protoplanetary disk.
Observations
OSIRIS-REx collects data in the infra-red region, between 0.4–4.3 micrometers wavelength range. But, the particular region of interest was the 3.2–3.6 micrometer band which showed at least four absorption characteristics of Carbon ranging from -CH₃ to -CH groups. The analysis focuses on spectra collected by OSIRIS-REx, including the Nightingale site, from where the only rock sample taken by OSIRIS-REx will be flying back to Earth in 2023.
Using laboratory spectra, they compared the spectrum from the OVIRS in the above range of 3.2–3.6 micrometers. The laboratory components from which the laboratory spectra was taken consisted of candidates namely:
Material captured from comet 81P.
Laboratory made irradiated ice tholin to estimate cometary organic matter.
Terrestrial Aslphite.
These materials were chosen because they were thought of being the most likely organic components to be observed on Bennu.
Results
Some of the OVIRS data shows absorption around 3.3 micrometers, consistent with -CH stretches. A strong absorption band has also been observed around the same region, on the Comet 67P by the Rosetta mission.
The organic-rich spectra typically have strong absorption at 3.42 µm which is consistent with the position of the -CH₂ stretch.
Entire absorption spans from 3.2 to 3.5 µm covering the full set of aliphatic CH stretches.
The organic-rich regions of Ceres, on the other hand, have a stronger absorption feature at 3.4 µm than any carbonaceous chondrite. Absorption features due to C-C, C=C, and C=O bonds result from aromatic structures, ketones, and carboxyls, prominent in meteorite IOM are absent in the wavelength range covered by OVIRS spectra (0.4 to 4.3 µm) suggesting that there is an additional unknown amount of carbon in aromatic structure.
Conclusion
By comparing the OVIRS spectral data with other asteroids like 67P, they found that carbonaceous chondrite IOM is the closest laboratory analog to Bennu’s organics. Such finding strengthens the previously described connection between Bennu and carbonaceous chondrite meteorites.
The potential for the rapid breakdown of aliphatic organics would indicate the surface has been exposed for a relatively short period of time, where these compounds are observed on Bennu.
Spectral features near 3.4 µm including a strong absorption minimum between 3.38 and 3.42 µm and a possible shoulder at 3.3 µm are indicative of aliphatic and aromatic CH with a maximum H/C of 0.6 and a minimum H/C of 0.3.
So, to summarise, organics have been detected by the data from OVIRS showing absorption in the 3.2–3.6 micrometer range. More light would be shed on the matter when OSIRIS-REx arrives on Earth with the sample in 2023.
Reference:
The paper “Composition of Organics on asteroid (101955) Bennu” was published on 02 September 2021 on Astronomy & Astrophysics.
