Composition of Inner Main Belt Planetesimals

Asteroids, along with other small bodies, are what is left over of the original planetesimal disk from the planet-formation era. Therefore, these objects are considered the best tracers for the processes that occurred during the earliest history of our Solar System. However, the majority of asteroids are fragments generated by the collisional breakup of the planetesimals, the first ~100-km sized bodies (Morbidelli et al., 2008, Delbo' et al., 2019) that accreted in the protoplanetary disk of our Sun.

Nevertheless, a small fraction of the planetesimal population survived the collisional evolution. In order to study these objects, the first step is to identify these surviving planetesimals among all the other (fragment) asteroids. To do so, we “cleaned” the inner part of the asteroid main belt (2,1 < a < 2,5) from all asteroid collisional family members (using the method of Bolin et al., 2017, Delbo' et al., 2017, 2019), thus revealing those asteroids that are not fragments that formed in the main belt. Thanks to this method, we revealed 64 surviving planetesimals in the inner main belt.

We carried out a spectroscopic survey of these identified IMB planetesimals, in order to constrain their composition and mineralogy. In particular, we performed visible and near-infrared spectroscopy using several telescopes such as the 1.82m Copernico Telescopio (Asiago, Italy) for the visible spectroscopy and the 4.2 Lowell Discovery Telescope (Flagstaff, USA); the 3.2 NASA Infrared Telescope Facility (Hawaii, USA) and the Telescopio Nazionale Galileo (La Palma, Spain) for the near-infrared part. To complete our survey, we also used spectra in the visible and near-infrared published in the literature, as well as size and albedo information that we obtained from the Minor Planet Physical Properties Catalog (https://mp3c.oca.eu/).

 

We performed the taxonomic classification following the Bus-DeMeo taxonomy (Bus et al., 2002; DeMeo et al., 2009), and using the M4AST tool (http://m4ast.imcce.fr) (Popescu et al., 2012). A visual inspection to identify the presence of absorption bands characteristic of some classes was performed to check the robustness of the automatic taxonomic classification. In addition, we compute for each planetesimal several spectral parameters, such as spectral slopes, and center, depth and area of absorption bands, when these are present. We also performed calculation of their mass, based on the method of (Carry, 2012). Finally, we used the RELAB database (Pieters, 1983), to look for meteorite analogues of each planetesimal.

 

We found that planetesimals of the Inner Main Belt (IMB) belong mainly to the S-complex (~45%), followed by C-complex (~25%) and X-complex (~17%). Further investigations on S-complex planetesimals showed that, for a majority of them, they are best matched by spectra of ordinary chondrites meteorites. We did not find any correlation with diameters, semimajor axis and the ratio of olivine/pyroxene. Almost 60% of the C-complex planetesimals belong to the Ch/Cgh types, showing spectroscopic features associated with hydrated minerals, and consequently indicating the presence of liquid water in the early formation phases of these objects. We also found that almost 5% of the IMB planetesimals belong to the D/T types with a diameter greater than 25 km. As this taxonomical class, as well as Ch/Cgh types, are likely to have formed in the outer part of the Solar System (at 3-7 au), their presence in the IMB (2,1-2,5 au) can be explained by dynamical models invoking large semimajor axis migration of these objects (e.g., Grand Tack of Walsh et al., 2011; low-mass asteroid belt of Raymond and Izidoro, 2017b for the C-complex and Vokrouhlický et al., 2016 for the D/T types).

Here, we will present the spectroscopic, physical and compositional results of our IMB planetesimals survey as well as the implications for planetary formation models.

Acknowledgements: We acknowledge support from the ANR ORIGINS (ANR-18-CE31-13-0014).

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