Color Systematics of Comets and Related Bodies

Abstract

Most comets are volatile-rich bodies that have recently entered the inner solar system following long-term storage in the Kuiper belt and the Oort cloud reservoirs. These reservoirs feed several distinct, short-lived "small body" populations. Here, we present new measurements of the optical colors of cometary and cometrelated bodies including long-period (Oort cloud) comets, Damocloids (probable inactive nuclei of long-period comets) and Centaurs (recent escapees from the Kuiper belt and precursors to the Jupiter family comets). We combine the new measurements with published data on short-period comets, Jovian Trojans and Kuiper belt objects to examine the color systematics of the comet-related populations. We find that the mean optical colors of the dust in short-period and long-period comets are identical within the uncertainties of measurement, as are the colors of the dust and of the underlying nuclei. These populations show no evidence for scattering by optically-small particles or for compositional gradients, even at the largest distances from the Sun, and no evidence for ultrared matter. Consistent with earlier work, ultrared surfaces are common in the Kuiper belt and on the Centaurs, but not in other small body populations, suggesting that this material is hidden or destroyed upon entry to the inner solar system. The onset of activity in the Centaurs and the disappearance of the ultrared matter in this population begin at about the same perihelion distance (∼10 AU), suggesting that the two are related. Blanketing of primordial surface materials by the fallback of sub-orbital ejecta, for which we calculate a very short timescale, is the likely mechanism. The same process should operate on any mass-losing body, explaining the absence of ultrared surface material in the entire comet population.

Nucleus of the active Centaur C/2011 P2 (PANSTARRS)

“…The extreme case of 167P/CINEOS, which has the largest perihelion distance (q = 11.8 au) among the active Centaurs, has been used as a case study for numerical simulations of the survival of amorphous water ice against crystallisation, which is considered the most probable trigger of comet-like activity at such heliocentric distances (Guilbert-Lepoutre 2012). Jewitt (2015) has recently shown, based on a sample of 37 targets (12 of which with evidence of activity), that active Centaurs tend to be those with smaller perihelion distances, as is expected for comet-like activity driven by a thermal process.…”

“…4, where the diameter D of 42 Centaurs is shown as function of the perihelion distance q, and in Fig. 5, where the diameter of 40 Centaurs is given as a function of the semi-major axis a (Jewitt 2009(Jewitt , 2015Stansberry et al 2008;Shi & Ma 2015;Pál et al 2015;Mazzotta Epifani 2011, 2014aLin et al 2014;Duffard et al 2014). For clarity, two values of a outside the Centaur region (between Jupiter's and Neptune's orbits) have been removed from the sample (31 inactive targets and 11 active targets) in Fig.…”

“…Also, using the photometry reported by Jewitt (2015) for P2 observed in October 2014, when the Centaur was at r = 8.39 au (m R = 22.04 ± 0.05 in ρ = 2.2 × 10 4 km), and adopting the same parameters used above (β = 0.11 mag/…”

Interstellar Interloper 1I/2017 U1: Observations from the NOT and WIYN Telescopes

“…Figure 5. B -V vs.V -R color-color plot adapted from Jewitt (2015) to show the location of U1 relative to other solar system small-body groups. The red circles denote sub-populations of the Kuiper Belt; the blue circles show the mean colors of inner solar system populations.…”

Properties of the dust in the coma of 67P/Churyumov-Gerasimenko observed with VIRTIS- M

“…They found that the reflectivity gradient decreases with wavelength from 5-18 per cent per 100 nm at wavelengths 0.35-0.65 μm to 0-2 per cent per 0.1 nm at 1.6-2.2 μm. An interesting and extensive overview has been done by Jewitt (2015). He find that the mean optical colours of the dust in short-period and long-period comets are identical within the uncertainties of measurement, as are the colours of the dust and of the underlying nuclei.…”