Detection of NH-rich compounds in Bennu pristine samples via IR characterization at JAXA Curation Center and comparison with Ryugu

NH-rich compounds have been detected on Ceres (Sanctis et al., 2015), comet 67P (Poch et al., 2020) and possibly other asteroids (Rivikin et al., 2022), based on the 3.1 µm absorption in their infrared reflectance spectrum. This feature has never been found among the meteorite collections but recently reported on the returned Ryugu sample (Pilorget et al., 2022). Studying these compounds can help us understand the parent bodies evolution and the volatiles delivery within the Solar System. In 2023, NASA’s OSIRIS-REx mission returned ~120 g sample from the primitive asteroid Bennu (Lauretta et al., 2024). Preliminary analyses showed that Bennu samples share many chemical and mineralogical similarities with Ryugu samples. Here we search for the NH-rich compounds in the Bennu samples and compare them with Ryugu samples.

Thanks to an agreement between NASA and JAXA, ~0.6 g of Bennu samples was delivered to the Extraterrestrial Sample Curation Center (ESCuC) of JAXA (Sagamihara, Japan). They are stored and measured under controlled environment (N₂ purged) to avoid terrestrial contamination and alteration (Fukai et al., 2023). Samples are composed of 5 bulks, from which aggregate samples and mm-sized grains were extracted after a first campaign of analyses. Here we combined observations performed by the infrared-hyperspectral microscope MicrOmega (22.5 µm pixel size, 256x256 pixel, FOV 5x5 mm, spectral range 0.99-3.65 µm), a micro-FTIR spectrometer (single pixel, ~50-100 µm FOV, spectral range 2-13 µm) and a LEICA optical microscope (Bibring et al., 2017; Pilorget et al., 2022; Fukai et al., 2023). We have specifically searched for regions of interest (ROIs) with strong absorption around both 2.7 µm (OH) and 3.06 µm (NH).

We have detected 8 NH-bearing ROIs (by 23^rd December 2024), relatively rare compared to other minor phases such as carbonates. Their sizes vary from ~100 µm to ~1 mm, with different morphology, from rounded to elongated/irregular. For example, one of the ROIs shows a vein-liked shape, another one looks like a thin layer coating on the matrix material. The corresponding area in visible microscope images is often yellowish in color. In terms of brightness, they are generally much brighter in reflectance (~5-15%) than the surrounding matrix materials (2~3%). All the ROIs show much deeper (~40%) absorption around 2.7 µm than that of matrix materials (~10 %), which is similar to that of Ryugu ROIs if we average all the ROIs spectra for both datasets. Their band position is ~2.72 µm, similar to the matrix materials and those of Ryugu sample. All the ROIs also show deep absorption around 3.06 µm (~15%). The FTIR spectra are consistent with MicrOmega data in the overlapping wavelength region (2.0-3.6 µm). In the longer wavelength, these ROIs generally show bands around 6 µm (~1650 cm^-1, water molecule), around 7 µm (~1430 cm^-1, NH₄⁺), and around 10 µm (~1020 cm^-1, Si-O).

These results show that ammonium (NH₄⁺) is present in Bennu samples, with a possible connection with phyllosilicates. Importantly, we also observed similarities with NH-rich areas detected in Ryugu samples, thanks to the measurements on both collections by MicrOmega.