New candidates for active asteroids: Main-belt (145) Adeona, (704) Interamnia, (779) Nina, (1474) Beira, and near-Earth (162,173) Ryugu

Abstract

For the first time, spectral signs of subtle coma activity were observed for four main-belt primitive asteroids (145) Adeona, (704) Interamnia, (779) Nina, and (1474) Beira around their perihelion distances in September 2012, which were interpreted as manifestations of the sublimation of H 2 O ice in/under the surface matter (Busarev et al., 2015a, 2015b). We confirm the phenomenon for Nina when it approached perihelion in September 2016. At the same time, based on results of spectral observations of near-Earth asteroid (162173) Ryugu (Vilas, 2008) being a target of Japan's Hayabusa 2 space mission, we suspected a periodic similar transient activity on the Cg-type asteroid. However, unlike the main-belt primitive asteroids demonstrating sublimation of ices close to their perihelion distances, the effect on Ryugu was apparently registered near aphelion. To explain the difference, we calculated the subsolar temperature depending on heliocentric distance of the asteroids, considered qualitative models of internal structure of main-belt and near-Earth primitive asteroids including ice and performed some analytical estimations. Presumed temporal sublimation/degassing activity of Ryugu is a sign of a residual frozen core in its interior. This could be an indication of a relatively recent transition of the asteroid from the main asteroid belt to the near-Earth area.

Expected spectral characteristics of (101955) Bennu and (162173) Ryugu, targets of the OSIRIS-REx and Hayabusa2 missions

“…In roughly half of the studied cases, asteroids showing an absorption band in the 3-μm region due to hydrated silicates do not show the corresponding band at 0.7 μm (Vilas 1994;Rivkin et al 2002Rivkin et al , 2015. A recent study made by Busarev et al (2018) based on the shape of visible spectra of Ryugu from Vilas (2008) and Sugita et al (2013) obtained a month after aphelion passage, suggests the existence of sublimation/degassing activity of Ryugu and the presence of an ice reservoir at small depths, indicative of a relatively short residence time in the near-Earth space. As in the case of Bennu, different studies indicate that Ryugu's most likely origin is in the primitive collisional families of the inner belt or the low-albedo and low-inclination population of background asteroids in the same region .…”

“…Recent laboratory experiments to simulate space weathering effects on low-albedo, primitive materials suggest that their visible spectra tend to get bluer and their albedo tend to get higher as exposure age increases . This result can explain the fact that the younger Clarissa family presents a lower fraction of B-types (blue) and a larger fraction of X-types (red) than the much older Polana-Eulalia complex (Campins et al 2018). Regarding the albedo, we cannot establish any conclusion, as the number of asteroids with good-quality albedo values in the Clarissa family is too low.…”

“…Regarding the origin of Bennu, spectroscopic and dynamical arguments suggest that it most likely originated in the primitive collisional families of the inner belt, in particular in the Polana-Eulalia complex (Campins et al 2010;. The absence of a 0.7 μm absorption band in the existing spectra of Bennu is additional evidence in favor of an origin in the Polana-Eulalia complex (Campins et al 2018). Nevertheless one should also consider the possibility that Bennu has lost the 0.7 μm band due to its proximity to the Sun as a near-Earth asteroid or that space weathering might have as well removed the signs of such band.…”

“…There are currently two sample-return missions on their way to two primitive NEAs: NASA's OSIRIS-REx mission to asteroid (101955) Bennu (Lauretta et al 2017) and JAXA's Hayabusa2 mission to asteroid (162173) Ryugu (Tsuda et al 2013). Asteroid Bennu is classified as a B-type asteroid from its visible spectrum (Clark et al 2011), and its most likely origin is the Polana-Eulalia family complex (Campins et al 2010;Walsh et al 2013;. Likewise, Ryugu, a C-type asteroid, most likely originated in the Polana-Eulalia complex or in the population of low-albedo and lowinclination background asteroids ), identified for the first time by Gayon-Markt et al (2012).…”

Small bodies science with the Twinkle space telescope

“…The largest (diameters 50 to 100 km) Themis family members are observable with high SNR for relatively short exposure times, making this group of asteroids targets of interest for Twinkle that could extend the number of detections of organics and our knowledge of organics in the asteroid belt. Additionally, the Themis family contains three of the newly discovered main-belt comets, 43,82,83 where a full 0.4-to 4.5-μm spectrum uncontaminated by atmospheric water would significantly benefit small bodies science.…”