Basaltic material across the Main Asteroid Belt

 

The inner main belt asteroid (4) Vesta, with a diameter of about 525 km, is the largest differentiated asteroid showing a basaltic crust [1]. The collisional family of Vesta includes more than 15,000 known members [2] and it is considered the main source of basaltic material.  The results found by the Dawn mission support the hypothesis that this family is a result of cratering events. They revealed two remnant craters, Rheasilvia (the young crater retention age of this basin indicates that it was formed ≈ 1 Gy) and Veneneia (the crater counts suggest an age of 2.1 ± 0.2 Gyr ago) with diameters of 500 ± 25 and 400 ± 25 km respectively [3].

 

The discovery of V-types which can't be associated dynamically with the Vesta family shows that the basaltic material is common through the inner Solar System and suggests that other differentiated parent bodies once existed.  Furthermore, by using the data provided by the OSIRIS-REx instruments, [4] reported the presence of meter-scale exogenic boulders on the surface of (101955) Bennu which shows the spectral signature of basaltic material.

 

In this context, we aim to determine the distribution of basaltic asteroids based on large surveys data. We estimate their unbias distribution which provides us with an approximate calculation for the total volume of basaltic material present in the Main Belt. We compare this volume with the ejected volume from Vesta's Rheasilvia crater and find that the majority of this material has been lost or fragmented to smaller sized that go beyond detection capabilities.

 

For our work, we used the latest observations provided by VISTA -VHS survey  which observed the  southern hemisphere sky using four filters, Y, J, H, and Ks. The V-type candidates are selected using the following constraints  (Y − J) − (Y −J)err ≥ 0.45 and (J − Ks) + (J − Ks)err ≤ 0.35 were selected [5, 6]. 

 

In order to classify the asteroids according to HED typologies we used the K-Nearest Neighbor (KNN) algorithm. For the training set we selected the spectra of HED meteorites from the RELAB database. This set contains a number of 243 meteorites spectra out of which 42 are howardites, 160 eucrites and 41 are diogenites. The classification using the KNN algorithm shows that the majority of the basaltic candidates present an eucritic (≈ 39 %) and howarditic (≈ 37 %) type lithology while diogenitic material is less commmon with ≈ 24 %.

 

The majority of the basaltic candidates (≈95%) are located in the inner main belt while only ≈4 % and ≈ 1 % are located in the middle respectively outer main belt (Fig.1). However, we note that ~33% of these asteroids are linked with the Vesta family and a large fraction of them (64%) are not associated with any other collisional family [6]. There are only 14 bodies associated with other families, including four belonging to (15) Eunomia family, four to (135) Hertha family.

 

Fig. 1 The orbital distribution of the V-type candidates [6]. The fugitives are V-type asteroids with a < 2.3 A.U. (were a is the semi-major axis) and similar e (eccentricity) and i (inclination) as the Vesta family; the low inclination V-types are asteroids having i ≤ 6 deg. and 2.3 < a < 2.5 A.U. The remaining asteroids in the inner main belt are named inner-other (IO). The midlle main belt (MMB) are asteroids with 2.5 < a < 2.82. The  outer main belt (OMB) are asteroids with a > 2.82 A.U. The location of the most representative resonances with Jupiter are shown.

 

 

The data allow to find the unbiased distribution of basaltic asteroids across the main asteroid belt. The total unbiased volume for vestoids with the effective diameter of 0.5 - 8 km is V_Vestoids = 6.24 ± 0.54 x 10⁴ km³.  For comparison, [3] estimated that the minimum volume of excavated material from Rheasilvia is above ∼1 x 10⁶ km³ . Part of it, was retained on the surface and assuming an average ejecta thickness of 5 km over a range of 100 km they roughly estimated the volume of ejecta on the surface as 5 x 10⁵ km³. This shows that at least 80% of the ejected basaltic material from (4) Vesta is missing or is not yet detected because it is fragmented in sizes  smaller than ~1 km [6].

 

Acknowledgements

 

The article is based on observations acquired with VISTA. The observations were obtained as part of the VISTA Hemisphere Survey, ESO Program, 179.A-2010 (PI: McMahon). The work of J.A.M. and part of the work of M.P. was supported by a grant of the Romanian National Authority for Scientific Research – UEFISCDI, project number PN-III-P1-1.2-PCCDI-2017-0371. M.P., J.dL., and J.L. acknowledge support from the AYA2015- 67772-R (MINECO, Spain). M.P. and J.dL. also acknowledge financial support from projects SEV-2015-0548 and AYA2017- 89090-P (Spanish MINECO).

 

References

 

[1] McCord T. B., Adams J. B., Johnson T. V., 1970, Science, 168, 1445

 

[2] Nesvorny D., Bro ´ z M., Carruba V., 2015, in Asteroids, IV, Michel P., DeMeo ˇ F. E., Bottke W. F., eds, Identification and Dynamical Properties of Asteroid Families. Univ. Arizona Press, Tucson, AZ, p. 297

 

[3] Schenk P. et al., 2012, Science, 336, 694

 

[4] DellaGiustina et al., 2020, A&A, 637, L4

 

[5] Licandro J., Popescu M., Morate D., de Leon J., 2017, ´ A&A, 600, A126

 

[6] Mansour et al 2020, MNRAS, 491, 5966-5979