Composition of Inner Main Belt Planetesimals
Authors: Bourdelle de Micas Jules , Fornasier Sonia , Delbo Marco , Avdellidou Chrysa , Van Belle Gerard , Ochner Paolo 
Published: September 2022

&lt;p&gt;&lt;span&gt;Asteroids, along with other small bodies, are what is left &lt;/span&gt;&lt;span&gt;over &lt;/span&gt;&lt;span&gt;of the original planetesimal disk from the planet-formation era. &lt;/span&gt;&lt;span&gt;Therefore, these objects are consider&lt;/span&gt;&lt;span&gt;ed&lt;/span&gt; &lt;span&gt;the&lt;/span&gt;&lt;span&gt; best tracers for the processes that occurred during the earliest history of our Solar System. &lt;/span&gt;&lt;span&gt;However, the majority of&lt;/span&gt;&lt;span&gt; asteroids &lt;/span&gt;&lt;span&gt;are fragments generated by the collisional breakup of the planetesimals, the first ~100-km sized bodies &lt;/span&gt;&lt;span&gt;(&lt;/span&gt;&lt;span&gt;Morbidelli et al., 2008&lt;/span&gt;&lt;span&gt;, Delbo' et al., 2019)&lt;/span&gt;&lt;span&gt; that accreted in the protoplanetary disk of our Sun. &lt;/span&gt;&lt;/p&gt;&lt;p&gt;&lt;span&gt;Nevertheless, a small fraction &lt;/span&gt;&lt;span&gt;of the planetesimal population survived the col&lt;/span&gt;&lt;span&gt;l&lt;/span&gt;&lt;span&gt;isional evolution. &lt;/span&gt;&lt;span&gt;In order to study these objects, &lt;/span&gt;&lt;span&gt;the first step is to identify these surviving planetesimals among all the other (fragment) asteroids. To do so, we &amp;#8220;cleaned&amp;#8221; the inner part of the asteroid main belt (2,1 &lt; a &lt; 2,5) from all asteroid collisional family members (using the method of &lt;/span&gt;&lt;span&gt;Bolin et al., 2017, Delbo' et al., 2017, 2019&lt;/span&gt;&lt;span&gt;), &lt;/span&gt;&lt;span&gt;thus revealing those asteroids that are not fragments &lt;/span&gt;&lt;span&gt;that formed in the main belt. Thank&lt;/span&gt;&lt;span&gt;s&lt;/span&gt;&lt;span&gt; to this method, we revealed 64 surviving planetesimals &lt;/span&gt;&lt;span&gt;in the inner main belt.&lt;/span&gt;&lt;/p&gt;&lt;p&gt;&lt;span&gt;We &lt;/span&gt;&lt;span&gt;carried out a &lt;/span&gt;&lt;span&gt;spectroscopic survey of these identified IMB planetesimals, in order to constrain their composition and mineralogy. &lt;/span&gt;&lt;span&gt;In particular, we perfo&lt;/span&gt;&lt;span&gt;r&lt;/span&gt;&lt;span&gt;med&lt;/span&gt;&lt;span&gt; visible and near-infrared &lt;/span&gt;&lt;span&gt;spectroscopy using&lt;/span&gt;&lt;span&gt; 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 (&lt;/span&gt;&lt;span&gt;Hawaii&lt;/span&gt;&lt;span&gt;, 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, &lt;/span&gt;&lt;span&gt;as well as size and albedo information that we obtained from the Minor Planet Physical Properties Catalog (https://mp3c.oca.eu/)&lt;/span&gt;&lt;span&gt;.&lt;/span&gt;&lt;/p&gt;&lt;p&gt;&amp;#160;&lt;/p&gt;&lt;p&gt;&lt;span&gt;W&lt;/span&gt;&lt;span&gt;e perform&lt;/span&gt;&lt;span&gt;ed&lt;/span&gt;&lt;span&gt; the taxonomic classification following the Bus-DeMeo taxonomy &lt;/span&gt;&lt;span&gt;(Bus et al., 2002; DeMeo et al., 2009)&lt;/span&gt;&lt;span&gt;,&lt;/span&gt; &lt;span&gt;and&lt;/span&gt; &lt;span&gt;using the &lt;/span&gt;&lt;span&gt;M4AST tool (&lt;/span&gt;http://m4ast.imcce.fr&lt;span&gt;) &lt;/span&gt;&lt;span&gt;(Popescu et al., 2012)&lt;/span&gt;&lt;span&gt;. A visual inspection to identif&lt;/span&gt;&lt;span&gt;y&lt;/span&gt;&lt;span&gt; the presence of absorption bands characteristic of some classes was performed to &lt;/span&gt;&lt;span&gt;check the robust&lt;/span&gt;&lt;span&gt;n&lt;/span&gt;&lt;span&gt;ess of the automatic &lt;/span&gt;&lt;span&gt;taxonomic classification. In addition, we compute for each planetesimal several spectral parameters, such as spectral slopes, and center, depth and &lt;/span&gt;&lt;span&gt;area&lt;/span&gt;&lt;span&gt; of absorption bands, when &lt;/span&gt;&lt;span&gt;these are&lt;/span&gt;&lt;span&gt; present. &lt;/span&gt;&lt;span&gt;We &lt;/span&gt;&lt;span&gt;also &lt;/span&gt;&lt;span&gt;performed &lt;/span&gt;&lt;span&gt;calculation of their mass, based on the method of &lt;/span&gt;&lt;span&gt;(&lt;/span&gt;&lt;span&gt;Carry, 2012&lt;/span&gt;&lt;span&gt;)&lt;/span&gt;&lt;span&gt;. Finally, we us&lt;/span&gt;&lt;span&gt;ed&lt;/span&gt;&lt;span&gt; the RELAB database &lt;/span&gt;&lt;span&gt;(Pieters, 1983)&lt;/span&gt;&lt;span&gt;, to look for meteorite analog&lt;/span&gt;&lt;span&gt;ues&lt;/span&gt;&lt;span&gt; of &lt;/span&gt;&lt;span&gt;each&lt;/span&gt;&lt;span&gt; planetesimal.&lt;/span&gt;&lt;/p&gt;&lt;p&gt;&amp;#160;&lt;/p&gt;&lt;p&gt;&lt;span&gt;We found that planetesimals &lt;/span&gt;&lt;span&gt;of the Inner Main Belt (IMB) &lt;/span&gt;&lt;span&gt;belong mainly to the S-complex (~&lt;/span&gt;&lt;span&gt;45&lt;/span&gt;&lt;span&gt;%), followed by C-complex (~25%) and X-complex (~17%). &lt;/span&gt;&lt;span&gt;Fu&lt;/span&gt;&lt;span&gt;r&lt;/span&gt;&lt;span&gt;ther investigations on S-complex planetesimals showed that, for a majority of them, they are &lt;/span&gt;&lt;span&gt;best matched by spectra of&lt;/span&gt; &lt;span&gt;o&lt;/span&gt;&lt;span&gt;rdinary &lt;/span&gt;&lt;span&gt;c&lt;/span&gt;&lt;span&gt;hondrites &lt;/span&gt;&lt;span&gt;meteorites&lt;/span&gt;&lt;span&gt;. We did not find any correlation with diameters, semimajor axis and the ratio of olivine/&lt;/span&gt;&lt;span&gt;pyroxene&lt;/span&gt;&lt;span&gt;. &lt;/span&gt;&lt;span&gt;Almost 60% of the C-complex planetesimals belong to the Ch/Cgh types, showing &lt;/span&gt;&lt;span&gt;spectroscopic &lt;/span&gt;&lt;span&gt;features associated &lt;/span&gt;&lt;span&gt;with&lt;/span&gt;&lt;span&gt; hydrated minerals, and consequently indicating the presence of liquid water &lt;/span&gt;&lt;span&gt;in the early formation phases of these objects&lt;/span&gt;&lt;span&gt;. &lt;/span&gt;&lt;span&gt;We also found that almost 5% of th&lt;/span&gt;&lt;span&gt;e&lt;/span&gt;&lt;span&gt; IMB planetesimals belong to the D/T types with a diameter greater than 25 km. &lt;/span&gt;&lt;span&gt;As this taxonomical class, as well as Ch/Cgh types, &lt;/span&gt;&lt;span&gt;are likely to have formed in the &lt;/span&gt;&lt;span&gt;outer part of the Solar System (at 3-7 au), t&lt;/span&gt;&lt;span&gt;heir presence &lt;/span&gt;&lt;span&gt;in the IMB&lt;/span&gt;&lt;span&gt; (2,1-2,5 au) can&lt;/span&gt;&lt;span&gt; be explained by &lt;/span&gt;&lt;span&gt;dynamical models &lt;/span&gt;&lt;span&gt;invoking large semimajor axis migration of these objects (e.g., Grand Tack of &lt;/span&gt;&lt;span&gt;Walsh et al., 2011&lt;/span&gt;&lt;span&gt;; low-mass asteroid belt of &lt;/span&gt;&lt;span&gt;Raymond and Izidoro, &lt;/span&gt;&lt;span&gt;2017b &lt;/span&gt;&lt;span&gt;for the C-complex and &lt;/span&gt;&lt;span&gt;Vokrouhlick&lt;/span&gt;&lt;span&gt;&amp;#253;&lt;/span&gt;&lt;span&gt; et al., 2016 &lt;/span&gt;&lt;span&gt;for the D/T types).&lt;/span&gt;&lt;/p&gt;&lt;p&gt;&lt;span&gt;Here, we will present the spectroscopic, &lt;/span&gt;&lt;span&gt;physical&lt;/span&gt;&lt;span&gt; and compositional results of &lt;/span&gt;&lt;span&gt;our&lt;/span&gt;&lt;span&gt; IMB planetesimals &lt;/span&gt;&lt;span&gt;survey &lt;/span&gt;&lt;span&gt;as well as the implications for planetary formation models.&lt;/span&gt;&lt;/p&gt;&lt;p&gt;&lt;span&gt;&lt;strong&gt;Acknowledgements&lt;/strong&gt;&lt;/span&gt;&lt;span&gt;: We acknowledge support from the ANR ORIGINS (ANR-18-CE31-13-0014).&lt;/span&gt;&lt;/p&gt;&lt;p&gt;&lt;span&gt;&lt;strong&gt;References&lt;/strong&gt;&lt;/span&gt;&lt;span&gt;&amp;#160;: &lt;/span&gt;&lt;span&gt;&lt;strong&gt;Morbidelli A. et al&lt;/strong&gt;&lt;/span&gt;&lt;span&gt;, &lt;/span&gt;&lt;span&gt;&lt;em&gt;Physica Scripta&lt;/em&gt;&lt;/span&gt;&lt;span&gt;, Volume 130, Issue, id 014028 (2008) ; &lt;/span&gt;&lt;span&gt;&lt;strong&gt; Delbo &lt;/strong&gt;&lt;/span&gt;&lt;span&gt;&lt;strong&gt;M.&lt;/strong&gt;&lt;/span&gt;&lt;span&gt;&lt;strong&gt; &lt;/strong&gt;&lt;/span&gt;&lt;span&gt;&lt;strong&gt;et al.,&lt;/strong&gt;&lt;/span&gt; &lt;span&gt;&lt;em&gt;A&amp;A, &lt;/em&gt;&lt;/span&gt; &lt;span&gt;Volume &lt;/span&gt;&lt;span&gt;624, &lt;/span&gt;&lt;span&gt;id.&lt;/span&gt;&lt;span&gt; A69 (2019)&amp;#160;; &lt;/span&gt;&lt;span&gt;&lt;strong&gt;Bolin &lt;/strong&gt;&lt;/span&gt;&lt;span&gt;&lt;strong&gt;B. T.&lt;/strong&gt;&lt;/span&gt;&lt;span&gt;&lt;strong&gt; &lt;/strong&gt;&lt;/span&gt;&lt;span&gt;&lt;strong&gt;et al.,&lt;/strong&gt;&lt;/span&gt; &lt;span&gt;&lt;em&gt;Icarus&lt;/em&gt;&lt;/span&gt;&lt;span&gt;, Volume 282, 2017, Pages 290-312&amp;#160;; &lt;/span&gt;&lt;span&gt;&lt;strong&gt;Delbo &lt;/strong&gt;&lt;/span&gt;&lt;span&gt;&lt;strong&gt;M.&lt;/strong&gt;&lt;/span&gt;&lt;span&gt;&lt;strong&gt; &lt;/strong&gt;&lt;/span&gt;&lt;span&gt;&lt;strong&gt;et al.,&lt;/strong&gt;&lt;/span&gt; &lt;em&gt;&lt;span&gt;Science, &lt;/span&gt;&lt;/em&gt;&lt;em&gt;&lt;span&gt;Volume 357, Issue 6355, pages&lt;/span&gt;&lt;/em&gt;&lt;span&gt; 1026-1029, &lt;/span&gt;&lt;span&gt;(&lt;/span&gt;&lt;span&gt;2017&lt;/span&gt;&lt;span&gt;)&lt;/span&gt;&lt;span&gt;&amp;#160;; &lt;/span&gt; &lt;span&gt;&lt;strong&gt;Bus &lt;/strong&gt;&lt;/span&gt;&lt;span&gt;&lt;strong&gt;S. J.&lt;/strong&gt;&lt;/span&gt;&lt;span&gt;&lt;strong&gt; &lt;/strong&gt;&lt;/span&gt;&lt;span&gt;&lt;strong&gt;et al.,&lt;/strong&gt;&lt;/span&gt; &lt;span&gt;&lt;em&gt;Icarus&lt;/em&gt;&lt;/span&gt;&lt;span&gt;, Volume 158, Issue 1, Pages 146-177 &lt;/span&gt;&lt;span&gt;(2002)&lt;/span&gt;&lt;span&gt;&amp;#160;; &lt;/span&gt;&lt;span&gt;&lt;strong&gt;DeMeo &lt;/strong&gt;&lt;/span&gt;&lt;span&gt;&lt;strong&gt;F. E. &lt;/strong&gt;&lt;/span&gt;&lt;span&gt;&lt;strong&gt;et al.,&lt;/strong&gt;&lt;/span&gt; &lt;span&gt;&lt;em&gt;Icarus&lt;/em&gt;&lt;/span&gt;&lt;span&gt;, Volume 202, Issue 1, Pages 160-180&amp;#160;&lt;/span&gt;&lt;span&gt;(2009)&lt;/span&gt;&lt;span&gt;; &lt;/span&gt;&lt;span&gt;&lt;strong&gt;Carry, B. &lt;/strong&gt;&lt;/span&gt; &lt;span&gt;&lt;em&gt;Planetary and Space Science&lt;/em&gt;&lt;/span&gt;&lt;span&gt;, Volume 73, Issue 1, p. 98-118.&lt;/span&gt; &lt;span&gt;(2012) ; &lt;/span&gt;&lt;span&gt;&lt;strong&gt;Popescu &lt;/strong&gt;&lt;/span&gt;&lt;span&gt;&lt;strong&gt;M. &lt;/strong&gt;&lt;/span&gt;&lt;span&gt;&lt;strong&gt;et al.&lt;/strong&gt;&lt;/span&gt;&lt;span&gt;, &lt;/span&gt;&lt;span&gt;&lt;em&gt;A&amp;A,&lt;/em&gt;&lt;/span&gt; &lt;span&gt;Volume &lt;/span&gt;&lt;span&gt;544, &lt;/span&gt;&lt;span&gt;id. &lt;/span&gt;&lt;span&gt;A130 (2012)&amp;#160;; &lt;/span&gt;&lt;span&gt;&lt;strong&gt;Pieters, C. M.&lt;/strong&gt;&lt;/span&gt;&lt;span&gt;, &lt;/span&gt;&lt;span&gt;&lt;em&gt;J. Geophys. Res.&lt;/em&gt;&lt;/span&gt;&lt;span&gt;, &lt;/span&gt;&lt;span&gt;Volume &lt;/span&gt;&lt;span&gt;88, &lt;/span&gt;&lt;span&gt;pages &lt;/span&gt;&lt;span&gt;9534&amp;#8211; 9544 (1983) ; &lt;/span&gt;&lt;span&gt;&lt;strong&gt;Walsh K. J. &lt;/strong&gt;&lt;/span&gt;&lt;span&gt;&lt;strong&gt;et al.&lt;/strong&gt;&lt;/span&gt;&lt;span&gt;, &lt;/span&gt;&lt;span&gt;&lt;em&gt;Nature&lt;/em&gt;&lt;/span&gt;&lt;span&gt;, Volume 475, Issue 7355, Pages 206-209 (2011) ; &lt;/span&gt;&lt;span&gt;&lt;strong&gt;Raymond S. N. et al.&lt;/strong&gt;&lt;/span&gt;&lt;span&gt;, &lt;/span&gt;&lt;span&gt;&lt;em&gt;Science Advances&lt;/em&gt;&lt;/span&gt;&lt;span&gt;, Volume 3, Issue 9, Pages e1701138 (2017) ; &lt;/span&gt;&lt;span&gt;&lt;strong&gt;Vokrouhlick&lt;/strong&gt;&lt;/span&gt;&lt;span&gt;&lt;strong&gt;&amp;#253;&lt;/strong&gt;&lt;/span&gt;&lt;span&gt;&lt;strong&gt; D. et al.&lt;/strong&gt;&lt;/span&gt;&lt;span&gt;, &lt;/span&gt;&lt;span&gt;&lt;em&gt;The Astronomical Journal&lt;/em&gt;&lt;/span&gt;&lt;span&gt;, Volume 152, Issue 2, id. 39 (2016). &lt;/span&gt;&lt;/p&gt;