JWST/NIRSpec observations of a blue binary KBO: Implications for planetesimal formation and dynamical evolution

The Kuiper belt is characterized by a complex orbital architecture that manifests the cumulative effects of dynamical processes that have shaped the outer Solar System. This complexity has been a driving force behind recent theories of Solar System evolution, and current models are able to reproduce much of the observed dynamical landscape of Kuiper belt objects (KBOs) through simulations of Neptune’s outward migration in the early Solar System and subsequent scattering of the primordial trans-Neptunian planetesimal disk [e.g.,1-2]. Contemporaneously, a concerted effort has been levied toward understanding the surface properties of KBOs. These previous studies have revealed a stark bimodality in visible and near-infrared spectral slopes, attested across all dynamical groups within the Kuiper belt, which separates the population into the so-called blue and red subpopulations [e.g.,3-5]. This color bimodality suggests that a sharp gradient in surface composition must have existed somewhere within the outer primordial planetesimal disk. Various models have attributed the color bimodality to irradiation chemistry that occurred on two sides of a volatile ice sublimation line [e.g.,6-7].

The cold classical KBOs challenge our current understanding of Kuiper belt formation and evolution. These objects are distinguished by their tight inclination distribution (i < 5°), near-circular orbits, and dense clustering between 42 and 47 AU, suggesting a significantly more quiescent dynamical history than the rest of the Kuiper belt. Notably, this population contains a high fraction of near-equal-mass binaries, including many with wide separations [8]. Dynamical studies have demonstrated that a majority of these binary systems would have been lost had they been scattered into their present-day orbits during the outward migration of Neptune [e.g.,9]. Instead, cold classicals may have formed in situ and therefore comprise a unique representative body of planetesimals from the outermost reaches of the protoplanetary disk [10]. Within the aforementioned compositional gradient hypothesis, we would expect in situ formation of cold classicals to produce exclusively red objects. While red objects indeed dominate the cold classical population, there is a significant admixture of blue objects in this region; moreover, these bluer objects are predominantly binaries [11-12]. Current models of KBO dynamical evolution struggle to reconcile the presence of blue binaries and absence of blue singletons among the cold classicals with predicted locations of the red-blue compositional gradient. It follows that the properties of blue binaries serve as a critical empirical test for assessing our fundamental understanding of Kuiper belt formation and evolution.

The blue binary 2016 BP81 was observed as part of Cycle 3 JWST Program #5940 (PI: Ian Wong). This system consists of two components with diameters of 188 and 170 km, respectively, at a mutual separation of ~11300 km. The NIRSpec integral field unit (IFU) observations were obtained with the PRISM grating, yielding a continuous reflectance spectrum spanning 0.6-5.3 μm at a spectral resolution of R~100. The binary components are well-separated on the IFU field of view, and we were able to extract the spectrum of each component individually. The two component spectra are statistically identical and reveal a deep 3-μm H₂O ice absorption band, weaker features from CO₂ ice at 2.7 and 4.25 μm, and a minor contribution from aliphatic organics between 3.4 and 3.6 μm. See Figure below.

Our interpretation of 2016 BP81’s surface composition is greatly facilitated by previously published JWST observations of dozens of KBOs [13], which enable detailed ensemble analyses. From both qualitative and quantitative spectral characterization, we find broad similarities between the blue binary and other H₂O-rich objects from other dynamical classes within the Kuiper belt, including resonant and scattered disk objects. This indicates that the blue binary likely shared a common formation environment with the other blue objects. However, 2016 BP81 also differs significantly from other H₂O-rich KBOs in several ways, including more abundant aliphatic organics, stronger CO₂ ice absorptions, and an absent H₂O Fresnel peak. Together, these findings suggest differences in the size of H₂O grains and CO₂/H₂O abundance ratio on the surface of 2016 BP81 relative to other blue objects in the Kuiper belt. Notably, we find close correspondence in spectral features between the blue binary and a few individual objects --- the extreme KBO 2016 QV89 and the Neptune trojan 2011 SO277 --- which indicates the possible presence of a distinct sub-class of blue KBOs with systematically different surface properties. In this talk, we present the results of our study of 2016 BP81 and discuss its broader implications for our understanding of the Kuiper belt and Solar System history.

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