(65) Cybele: detection of small silicate grains, water-ice, and organics

Abstract

Context. (65) Cybele is the most representative member of a population of primitive asteroids in the outer edge of the main belt, the Cybele asteroids. Recent dynamical models suggest that a significant fraction of them originated in the primordial transneptunian disk, so the study of the physical properties of these asteroids is potentially a useful test of these models. Aims. Our aim is to obtain information on the surface composition of this asteroid. In particular we want to obtain information on the composition and properties of the regolith and the possible presence of ices and organic materials. Methods. We present 2-4 μm and 5-14 μm spectroscopy of (65) Cybele obtained with the NASA IRTF telescope and Spitzer Space Telescope respectively. We compare the results with spectra of Trojan asteroids and asteroid (24) Themis. We analyze the 2-4 μm spectrum using scattering models and we apply thermal models to the 5-14 μm data. Results. The 2-4 μm spectrum of (65) Cybele presents an absorption band centered at ∼3.1 μm and more weaker bands in the 3.2-3.6 μm region, very similar to those observed in (24) Themis. No hydrated silicates are detected. From the spectrum in the 5-14 μm region an effective diameter D = 290 ± 5 km, a beaming paramete η = 0.967 ± 0.014, and a geometric visible albedo p V = 0.05 ± 0.01 are derived using the NEATM thermal model. The emisivity spectrum in the 5-14 μm range exhibits an emission plateau at about 9 to 12 μm with an spectral contrast of ∼5%. This emission is similar to that of Trojan asteroids and active comets and may be due to small silicate grains being imbedded in a relatively transparent matrix, or to a very under-dense (fairy-castle) surface structure. The lower amplitude of the silicate emission in Cybele's spectrum with respect to that of Trojan asteroids could be attributed to larger dust particles and/or a slightly denser structure. Conclusions. The surface of (65) Cybele is covered by a fine anhydrous silicate grains mantle, with a small amount of water ice and complex organic solids. This is similar to comet surface where non-equilibrium phases coexist. The presence of water-ice and anhydrous silicates is indicative that hydration did not happened or is incomplete, suggesting that the temperatures were always sufficiently low.

The Active Asteroids

“…Spectral observations limited the production of the CN radical to Q CN 1.4 × 10 23 s −1 , again corresponding to a water production rate 1.5 kg s −1 assuming H 2 O/CN = 360. P/2008 R1 is located near the 8:3 mean-motion resonance Hsieh et al 2004Hsieh et al , 2009aHsieh et al , 2011b(8) Hsieh et al 2012;Licandro et al 2011;(9) Hsieh et al 2011c; (10) Moreno et al 2011a;Hsieh et al 2012;(11) Veeder et al 1984;Fernández et al 1997;Licandro et al 2009;Urakawa et al 2011. with Jupiter and is also affected by the ν 6 secular resonance. The dynamical lifetime in this orbit is short (20-30 Myr) compared with the age of the solar system, suggesting that P/2008 R1 was scattered into its present location from elsewhere in the asteroid belt.…”

The spectrum of Pluto, 0.40–0.93μm

“…Complex organics have been detected on the surface of asteroids (24) Themis and (65) Cybele (Rivkin & Emery 2010;Licandro et al 2011) and the Saturnian moons Iapetus and Phoebe (Cruikshank et al 2008, and their presence has been inferred for other solar system objects (e.g., TNOs and Titan, Cruikshank et al 2005). Recently a suite of 16 organic compounds, comprising numerous carbon and nitrogen-rich molecules, has been discovered on the surface of the comet 67P/Churyomov-Gerasimenko (Wright et al 2015;Goesmann et al 2015).…”

Physical properties of B-type asteroids from WISE data

“…The W1 band pass spans the 2.8 to 3.8 μm range (Wright et al 2010); therefore, the values of R p may also indicate the presence of the 3-μm absorption feature attributed to hydrated minerals or water ice detected on many asteroids (Rivkin et al 2000;Gaffey et al 2002;Campins et al 2010b;Rivkin & Emery 2010;Licandro et al 2011). Mainzer et al (2011c) (2012).…”