A Hypothesis for the Color Bimodality of Jupiter Trojans

Abstract

One of the most enigmatic and hitherto unexplained properties of Jupiter Trojans is their bimodal color distribution. This bimodality is indicative of two sub-populations within the Trojans, which have distinct size distributions. In this paper, we present a simple, plausible hypothesis for the origin and evolution of the two Trojan color sub-populations. In the framework of dynamical instability models of early solar system evolution, which suggest a common primordial progenitor population for both Trojans and Kuiper Belt objects, we use observational constraints to assert that the color bimodalities evident in both minor body populations developed within the primordial population prior to the onset of instability. We show that, beginning with an initial composition of rock and ices, location-dependent volatile loss through sublimation in this primordial population could have led to sharp changes in the surface composition with heliocentric distance. We propose that the depletion or retention of H 2 S ice on the surface of these objects was the key factor in creating an initial color bimodality. Objects that retained H 2 S on their surfaces developed characteristically redder colors upon irradiation than those that did not. After the bodies from the primordial population were scattered and emplaced into their current positions, they preserved this primordial color bimodality to the present day. We explore predictions of the volatile loss model-in particular, the effect of collisions within the Trojan population on the size distributions of the two sub-populations-and propose further experimental and observational tests of our hypothesis.

THE 3–4 μm SPECTRA OF JUPITER TROJAN ASTEROIDS

“…Wong & Brown (2016a) propose that the red Jupiter Trojans formed in the outer part of the original outer disk and would have had H 2 S stable on the surface during irradiation, leading to much redder colors. In this hypothesis sulfur must suppress some of the N-H chemistry, a suggestion that can be experimentally tested and, if verified, would provide strong evidence that all Jupiter Trojan asteroids formed beyond the giant planet region and that the differences in the two color populations are only due to surficial differences in ice irradiation.…”

“…The solid green line shows the average of the smoothed spectra from Figure 3. specifically investigate Jupiter Trojan asteroid physical conditions and no optical constants are available to facilitate more quantitative modeling, so the lack of a perfect match is unconcerning. In the context of dynamical instability models, nitrogen should indeed be present on the surface of small bodies beyond the giant planets in the form of NH 3 so it should be expected in the involatile surface mantles upon irradiation (Wong & Brown 2016a). If the H 2 S evaporation hypothesis of Wong & Brown (2016a) is correct, the lack of 3.1 μm absorption in most red Jupiter Trojan asteroids would mean that the presence of sulfur must disrupt much of the N-H chemistry.…”

“…In the context of dynamical instability models, nitrogen should indeed be present on the surface of small bodies beyond the giant planets in the form of NH 3 so it should be expected in the involatile surface mantles upon irradiation (Wong & Brown 2016a). If the H 2 S evaporation hypothesis of Wong & Brown (2016a) is correct, the lack of 3.1 μm absorption in most red Jupiter Trojan asteroids would mean that the presence of sulfur must disrupt much of the N-H chemistry. Such a possibility is amendable to laboratory exploration using experiments such as those performed in Mahjoub et al (2016).…”

“…It is not known whether these two color populations represent distinct source populations, distinct dynamical pathways, distinct collision histories, or some other set of mechanisms. To date, the only hypothesis for the two populations is that they are both from the Kuiper belt source region, but the less-red objects formed in the inner part of the source region where H 2 S evaporated from the surface, while the red objects retained their surface H 2 S (Wong & Brown 2016a). In this hypothesis, subsequent irradiation then caused significant chemical differences in the sulfur-free and sulfur-containing involatile mantles (Mahjoub et al 2016), leading to the bifurcation of not just Jupiter Trojan colors, but also to those of the Centaurs (Tegler et al 2008, pp.…”

The Bimodal Color Distribution of Small Kuiper Belt Objects*

“…In Wong & Brown (2016), we speculate that the bluer colors of the two Trojan subpopulations may be due to secondary processing of the irradiated surfaces following their placement in a higher-temperature environment at 5.2au.…”