Gaia reflectance spectra reveal unexpected abundance of asteroids potentially originating from the mantle of differentiated bodies.

We present our observational survey devoted to the search of asteroids with a spectral class that is consistent with an olivine-rich composition. We will present our findings and their implications for the "missing mantle problem" (Burbine et al., 1996). This issue is a long-standing question in planetary science that refers to the observed scarcity of olivine-rich asteroids in the main belt. This scarcity appears to contrast with the theories suggesting that differentiated asteroids, which contain substantial amounts of olivine, should have been abundant during the early stages of our solar system's formation (e.g. Kruijer et al., 2014).Our observational survey is based on reflectance spectra of solar system objects obtained in the visible light by ESAs Gaia space mission (Galluccio et al., 2022). We show that analysis of Gaia spectra reveals that asteroids with a spectral class that is consistent with an olivine-rich composition are overabundant of more than a factor of 10 compared to previous results of ground based telescopic surveys dedicated to detection these type of asteroids (DeMeo et al., 2019).We followed up several of these asteroids by obtaining near infrared spectra using the SpeX instrument (Rayner et al., 2003) at the NASA Infrared Telescope Facility (IRTF). We then combined Gaia visible and IRTF near-infrared spectra to obtain a more compositionally diagnostic description of the reflectance of these asteroids than that offered by the Gaia data alone.We will discuss how the abundance of Gaia olivine-rich asteroid is dependent on the heliocentric distance, possibly contrasting with previous findings (DeMeo et al., 2019). A first collisional family discovered to be mostly populated by potentially olivine-rich asteroids has been reported earlier this year (Galinier et al., 2024). Here, we will also discuss the existance of olivine rich asteroids in collisional families, which are logical to be expected to form from the break-up of a differentiated parent body.References:Burbine, T.H., Meibom, A., Binzel, R.P., 1996. Mantle material in the main belt: Battered to bits? Meteoritics and Planetary Science 31, 607620. https://doi.org/10.1111/j.1945-5100.1996.tb02033.xDeMeo, F.E., Polishook, D., Carry, B., Burt, B.J., Hsieh, H.H., Binzel, R.P., Moskovitz, N.A., Burbine, T.H., 2019. Olivine-dominated A-type asteroids in the main belt: Distribution, abundance and relation to families. Icarus 322, 1330. https://doi.org/10.1016/j.icarus.2018.12.016Galinier, M., Delbo, M., Avdellidou, C., Galluccio, L., 2024. Discovery of the first olivine-dominated A-type asteroid family. AA 683, L3. https://doi.org/10.1051/0004-6361/202349057Galluccio, L., Delbo, M., Angeli, F.D., Pauwels, T., Tanga, P., Mignard, F., Cellino, A., Brown, A., Muinonen, K., Penttil, A., Jordan, S., 2022. Gaia Data Release 3: Reflectance spectra of Solar System small bodies. AA. https://doi.org/10.1051/0004-6361/202243791Kruijer, T.S., Touboul, M., Fischer-Gdde, M., Bermingham, K.R., Walker, R.J., Kleine, T., 2014. Protracted core formation and rapid accretion of protoplanets. Science 344, 11501154. https://doi.org/10.1126/science.1251766Rayner, J.T., Toomey, D.W., Onaka, P.M., Denault, A.J., Stahlberger, W.E., Vacca, W.D., Cushing, M.C., Wang, S., 2003. SpeX: A MediumResolution 0.85.5 Micron Spectrograph and Imager for the NASA Infrared Telescope Facility. PUBL ASTRON SOC PAC 115, 362382. https://doi.org/10.1086/367745Acknowledgments:We acknowledge nancial support from CNES, the Action Specique Gaia, the Programme National de Planetologie, and the ANR ORIGINS (ANR-18-CE31-0014). This work has made use of data from the European Space Agency (ESA) mission Gaia (http://www. cosmos.esa.int/gaia), processed by the Gaia Data Processing and Anal- ysis Consortium (DPAC, https://www.cosmos.esa.int/web/gaia/dpac/ consortium). Funding for the DPAC has been provided by national institutions, in particular the institutions participating in the Gaia Multilateral Agreement. The authors made use of the great SpeX instrument at the Infrared Telescope Facility, which is operated by the University of Hawaii under contract 80HQTR19D0030 with the National Aeronautics and Space Administration. This work is based on data provided by the Minor Planet Physical Properties Catalogue (mp3c.oca.eu) of the Observatoire de la Cte dAzur.