Post-impact self-gardening: a poorly known mechanism with consequences on the properties of asteroid surfaces

Asteroid families are groups of asteroids sharing similar orbits and surface properties resulting from the disruption of a parent body. Asteroid families can be identified as overdensities in the phase space of the proper elements [1] or from their spectral properties [2]. The latter method assumes that families are homogeneous in composition. However, according to the theories of the Solar System’s evolution and the presence of metallic and basaltic asteroids in the Belt, there should have been differentiated planetesimals, some of which should have undergone fragmentation. Therefore, there should be asteroid families characterized by an inhomogeneous composition. Still, such structures have never been observed, or at least not in a sufficient quantity compared to the number of predicted differentiated objects [3]. This implies that there could be a mechanism that intervenes in the first phases after the formation of an asteroid family acting on the surface compositions of its members, homogenizing them.

We investigate the possible role of collisions internal to a family, among its members, occurring in the first thousand years after the disruption of a differentiated parent body. Fragments with different compositions coming from different regions within the parent body (i.e, core, mantle, crust) would collide, leading to a gradual compositional mixing.

This mechanism could be efficient at the beginning of the evolution of the family, before the randomization of the anomalies and orbital nodes. In such conditions, the usual statistical approaches cannot be used [5], and numerical simulations are required to test this hypothesis. The fragments are assigned to an ejection velocity field [4] and integrated over time under gravitational and non-gravitational perturbations. Collisions are recorded during the simulation and are then converted into collisional probabilities [5], from which the results can be generalized into size distributions approximating real families. Figure 1 reports the collision probability numerically determined in a system of 25,000 particles, each with a radius of 5 km, integrated up to the randomization of the true anomalies. In this regime, the collision probability aligns with that predicted by the statistical approach [5].

Various methods can be implemented to estimate the degree of mutual surface contamination within the family. One approach involves assigning each fragment a specific mineralogical composition based on its original location within the parent body. The compositions would then evolve as a result of collisions. Since the surface composition of asteroids is not directly observable, it cannot be directly compared with real data. Instead, reflectance spectra linked to the mineralogical compositions can be derived using empirical models such as those developed by Hapke [6] and Cloutis [7].
An alternative method is to assign to each fragment a meteorite spectrum representing its initial region within the parent body. The final spectrum for each fragment would be computed as a weighted linear combination of the initial spectra, where the weights reflect the number and nature of collisions the fragment has undergone.
Other alternative methods could be explored as well.

The spectra of the family members can finally be compared with spectroscopic observations, such as Gaia DR3 data, which have proven effective in identifying asteroid families without additional data [2] [8]. This comparison will be crucial to assess whether intrafamilial collisions significantly contribute to homogenization, offering insights into the distribution and abundance of differentiated planetesimals in the early Solar System.

In this contribution, we present the first results obtained from this ongoing investigation, most notably the collision probabilities at the very beginning of the evolution of the families after their breakup. We also test their evolution towards the results presented in [5] for later stages, and show a preliminary evaluation of the degree of intra-family spectral contamination after the breakup of a completely differentiated asteroid.

Figure 1: The collision probability determined for a system of 25,000 particles, each with a radius of 5 km, integrated up to the randomization of the true anomalies.

 

References

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[2] Balossi, R., Tanga, P., Sergeyev, A., Cellino, A., Spoto, F. (2024), Gaia DR3 asteroid reflectance spectra: L-type families, memberships, and ages, A&A, 688, A221.
[3] Burbine, T. H., DeMeo, F. E., Rivkin, A. S., Reddy, V. (2017). Evidence for Differentiation among Asteroid Families. In L. T. Elkins-Tanton & B. P. Weiss (Eds.), Planetesimals (pp. 298–320). Cambridge University Press.
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[8] Balossi, R., Tanga, P., Delbo, M., Cellino, A., (in prep.), An ancient L-type family associated to (460) Scania in the Middle Main Belt as revealed by Gaia DR3 spectra. Manuscript in preparation