Spectral variability on primitive asteroids of the Themis and Beagle families: Space weathering effects or parent body heterogeneity?

Abstract

Themis is an old and statistically robust asteroid family populating the outer main belt, and resulting from a catastrophic collision that took place 2.5±1.0 Gyr ago. Within the old Themis family a young sub-family, Beagle, formed less than 10 Myr ago, has been identified. We present the results of a spectroscopic survey in the visible and near infrared range of 22 Themis and 8 Beagle families members. The Themis members investigated exhibit a wide range of spectral behaviors, including asteroids with blue/neutral and moderately red spectra, while the younger Beagle family members look spectrally bluer than the Themis ones and they have a much smaller spectral slope variability. Four Themis members, including (24) Themis, have absorption bands centered at 0.68-0.73 µm indicating the presence of aqueously altered minerals. The best meteorite spectral analogues found for both Themis and Beagle families members are carbonaceous chondrites having experienced different degrees of aqueous alteration, prevalently CM2 but also CV3 and CI, and some of them are chondrite samples being unusual or heated. The presence of aqueous altered materials on the asteroids surfaces and the meteorite matches indicate that the parent body of the Themis family experienced mild thermal metamorphism in the past. We extended the spectral analysis including the data available in the literature on Themis and Beagle families members, and we looked for correlations between spectral behavior and physical parameters using the albedo and size values derived from the WISE data. The analysis of this larger sample confirm the spectral diversity within the Themis family and that Beagle members tend to be bluer and to have an higher albedo. The differences between the two family may be partially explained by space weathering processes, which act on these primitive surfaces in a similar way than on S-type asteroids, i.e. producing reddening and darkening. However we see several Themis members having albedos and spectral slopes similar to the young Beagle members. Alternative scenarios are proposed including heterogeneity in the parent body having a compositional gradient with depth, and/or the survival of projectile fragments having a different composition than the parent body.

Ancient and primordial collisional families as the main sources of X-type asteroids of the inner main belt

“…The low-albedo X-complex asteroids could be compositionally similar to those of the C-complex (DeMeo et al 2009(DeMeo et al , 2015. An indication of the composition similarities and origin between the low-albedo asteroids belonging to the X-and C-complex also comes from the fact that C-and X-complex asteroids have been found within the same families (Morate et al 2016;Fornasier et al 2016). In the near-infrared survey of Popescu et al (2018) asteroids belonging to the Xcomplex are denoted by the class Xt.…”

Compositional Diversity Among Primitive Asteroids

“…Initial studies of primitive asteroid families with different ages, such as the Themis, Beagle, and Veritas families, show spectral differences. However, the cause for these differences between families is not clear; SpWe may not be necessary to explain these spectral differences, but it cannot be ruled out (Nesvorný et al 2005;Ziffer et al 2011;Campins et al 2012;Fornasier et al 2016).…”

“…Significant contributions are also expected in the understanding of asteroids families; for example, spectra of main belt asteroids as small as 2 km in diameter will constrain or determine the composition and thermal evolution of a family's parent body. In the case of the Themis family, one of the largest and oldest primitive families, Gaia's spectra will also help establish the role of water and hydrated minerals in the puzzling nature of the ''activated asteroids'' (also known as main belt comets) within this family (Fornasier et al 2016;Campins et al 2012).…”

Asteroid Family Associations of Active Asteroids

“…Meanwhile, spectroscopic observations of samples of both Beagle and Themis family asteroids showed that Beagle family asteroids are spectrally bluer, have higher albedos, and exhibit smaller spectral slope variability than background Themis family asteroids, suggesting that the Beagle parent body could have been a particularly blue and bright interior fragment of the original Themis parent body (Fornasier et al 2016).…”