Soutenance de thèse d’Alice PRAET le vendredi 10 décembre 2021

Title

"Hydrogen abundance estimation on asteroids (101955) Bennu and (162173) Ryugu, targets of the OSIRIS-REx and Hayabusa2 missions, using spectral data analysis"

Abstract

In the last decades, several space missions were dedicated to the study of asteroids to investigate the building blocks of our Solar System. Two asteroid sample return missions are currently ongoing, the NASA mission OSIRIS-REx (Origins, Spectral Interpretation, Resource Identification, and Security–Regolith Explorer) and the JAXA mission Hayabusa2. Both missions targeted a primitive (low-albedo) near-Earth asteroid : the B-type asteroid (101955) Bennu and C-type asteroid (162173) Ryugu, respectively. Hayabusa2 delivered to Earth 5.4 g of regolith sampled from Ryugu’s surface on December 5th 2020 (Japan time) while the OSIRIS-REx sample from Bennu’s surface (about 400g collected estimated) is scheduled to land on Earth on September 24th 2023.
Trough the detailed global and local study of the two primitive asteroids, both missions aim to better characterize the early Solar System environment, alongside the transfer and mixing processes in the protoplanetary disk with a focus in water (hydrated minerals) and organic matter which asteroids are believed to be a major source to early Earth. Both missions acquired a vast quantity of data during asteroid proximity operations, in particular, the visible and near-infrared spectrometers, OVIRS (OSIRIS-REx Visible and InfraRed Spectrometer) as well as NIRS3 (Near-InfraRed Spectrometer), which revealed the presence of hydrated phyllosilicates across the surface of both asteroids. During my thesis, I had the chance to participate and contribute to these two NASA and JAXA space missions. In particular, I analyzed the spatially resolved visible–near-infrared spectra of Bennu and Ryugu, with a focus on the hydrated phyllosilicate absorption band centered at 2.74 μm and 2.72 μm respectively. My goal is to investigate the hydrogen content of the water (H2O) and hydroxyl (OH−) groups in hydrated phyllosilicates (i.e. H content) on the surface of both asteroids. I applied different methods, namely the normalized optical path length (NOPL) and the effective single-particle absorption thickness (ESPAT) to the hydrated phyllosilicate absorption band of both asteroids, as well as Gaussian modeling of the absorption band in the case of Bennu. I compared the obtained spectral parameters with those obtained (with the same methods) on carbonaceous chondrite meteorites whose H content was determined in laboratory. From the comparison, I derived a correlation between the selected meteorite H contents and their respective ESPAT and NOPL parameters and thus quantified the average value of the H content of the two asteroids’ surface with its relative variations.

The estimation of the global average H contents of Bennu and Ryugu is in agreement with those of several aqueously altered carbonaceous chondrite meteorites measured in laboratory and is most similar to heated CMs’ H contents and also C2 Tagish Lake’s in Bennu’s case. The obtained results and the study of phyllosilicate H2O and OH− group hydrogen content on a larger number of objects, will allow the better understanding of the formation and evolution of the Solar System. The exponential function correlation I defined could be applied to other observed primitive asteroids that exhibit a 3-μm region absorption band in order to estimate their average H content.
Finally, the laboratory analysis of the returned samples from both missions will validate the described methods and hydration quantification results with higher precision. The quantification of the asteroids’ hydration is essential to constrain Solar System formation and evolution models as well as providing insight on the origin of Life on Earth.