M-type (22) Kalliope: A tiny Mercury

Abstract

Context. Asteroid (22) Kalliope is the second largest M-type asteroid in the main belt and is orbited by a satellite, Linus. Whereas the mass of Kalliope is already well constrained thanks to the presence of a moon, its volume is still poorly known, leading to uncertainties on its bulk density and internal structure. Aims. We aim to refine the shape of (22) Kalliope and thus its diameter and bulk density, as well as the orbit of its moon to better constrain its mass, hence density and internal structure. Methods. We acquired disk-resolved observations of (22) Kalliope using the VLT/SPHERE/ZIMPOL instrument to reconstruct its three-dimensional (3D) shape using three different modeling techniques. These images were also used together with new speckle observations at the C2PU/PISCO instrument as well as archival images from other large ground-based telescopes to refine the orbit of Linus. Results. The volume of (22) Kalliope given by the shape models, corresponding to D = 150 ± 5 km, and the mass constrained by its satellite’s orbit yield a density of ρ = 4.40 ± 0.46 g cm−3. This high density potentially makes (22) Kalliope the densest known small body in the Solar System. A macroporosity in the 10–25% range (as expected for this mass and size), implies a grain density in the 4.8–5.9 g cm−3 range. Kalliope’s high bulk density, along with its silicate-rich surface implied by its low radar albedo, implies a differentiated interior with metal contributing to most of the mass of the body. Conclusions. Kalliope’s high metal content (40–60%) along with its metal-poor mantle makes it the smallest known Mercury-like body. A large impact at the origin of the formation of the moon Linus is likely the cause of its high metal content and density.

Speckle observations of the binary asteroid (22) Kalliope with C2PU/PISCO

“…Although the shape of ( 22) Kalliope is well modeled and highly non-spherical, the orbit found by these authors is compatible with a purely Keplerian motion, suggesting that E-mail:eric.aristidi@oca.eu Kalliope could have a differentiated interior (Ferrais et al, 2022). The orbit is also compatible with a multipole model of the shape of ( 22) Kalliope, suggesting a homogeneous composition (Ferrais et al, 2022). Since the internal structure of (22) Kalliope is currently unclear, monitoring the orbit of Linus is of high interest because the long-term dynamics of the system can reveal small perturbations due to the internal structure of Kalliope.…”

“…It has a small companion Linus, which was discovered in 2001 (Merline et al, 2001;Margot & Brown, 2001) by means of adaptive optics (AO) on large groundbased telescopes (3.6 m CFHT and 10 m Keck). Since 2001, astrometric measurements of Linus have been measured by several authors using different techniques (AO imaging, stellar occultations, speckle interferometry), leading to a set of 188 measurements (see Ferrais et al, 2022, and references therein for more details). The orbit of Linus has been studied by various authors and methods as the number and quality of available measurements increased (see Descamps et al, 2008;Marchis et al, 2008;Vachier et al, 2012).…”

“…The orbit of Linus has been studied by various authors and methods as the number and quality of available measurements increased (see Descamps et al, 2008;Marchis et al, 2008;Vachier et al, 2012). The latest analysis, on the largest set of observations, was published by Ferrais et al (2022), using two different orbit-fitting algorithms, Genoid (Vachier et al, 2012) and xitau (Brož et al, 2021). Although the shape of ( 22) Kalliope is well modeled and highly non-spherical, the orbit found by these authors is compatible with a purely Keplerian motion, suggesting that E-mail:eric.aristidi@oca.eu Kalliope could have a differentiated interior (Ferrais et al, 2022).…”

“…The latest analysis, on the largest set of observations, was published by Ferrais et al (2022), using two different orbit-fitting algorithms, Genoid (Vachier et al, 2012) and xitau (Brož et al, 2021). Although the shape of ( 22) Kalliope is well modeled and highly non-spherical, the orbit found by these authors is compatible with a purely Keplerian motion, suggesting that E-mail:eric.aristidi@oca.eu Kalliope could have a differentiated interior (Ferrais et al, 2022). The orbit is also compatible with a multipole model of the shape of ( 22) Kalliope, suggesting a homogeneous composition (Ferrais et al, 2022).…”

V-type Asteroids as the Origin of Mesosiderites

“…Such a level of macroporosity seems, however, a bit low for such a small body, which may hint at a differentiated interior (Vernazza et al 2011(Vernazza et al , 2021Weiss et al 2012). A differentiated interior is also the likely case of M-type (22) Kalliope characterized by a density of ρ = 4.40 ± 0.46 g cm −3 , so far the densest known small body of the solar system (Ferrais et al 2022).…”