Visible Near-infrared Spectral Evolution of Irradiated Mixed Ices and Application to Kuiper Belt Objects and Jupiter Trojans

Abstract

Understanding the history of Kuiper Belt Objects and Jupiter Trojans will help to constrain models of solar system formation and dynamical evolution. Laboratory simulations of a possible thermal and irradiation history of these bodies were conducted on ice mixtures while monitoring their spectral properties. These simulations tested the hypothesis that the presence or absence of sulfur explains the two distinct visible near-infrared spectral groups observed in each population and that Trojans and KBOs share a common formation location. Mixed ices consisting of water, methanol, and ammonia, in mixtures both with and without hydrogen sulfide, were deposited and irradiated with 10 keV electrons. Deposition and initial irradiation were performed at 50 K to simulate formation at 20 au in the early solar system, then heated to Trojan-like temperatures and irradiated further. Finally, irradiation was concluded and resulting samples were observed during heating to room temperature. Results indicated that the presence of sulfur resulted in steeper spectral slopes. Heating through the 140-200 K range decreased the slopes and total reflectance for both mixtures. In addition, absorption features at 410, 620, and 900 nm appeared under irradiation, but only in the H 2 S-containing mixture. These features were lost with heating once irradiation was concluded. While the results reported here are consistent with the hypothesis, additional work is needed to address uncertainties and to simulate conditions not included in the present work.

Effect of H2S on the Near-infrared Spectrum of Irradiation Residue and Applications to the Kuiper Belt Object (486958) Arrokoth

“…Two residue samples were produced by irradiation of ice films under vacuum at 50 K. A tholin film "without sulfur" was produced from a starting ice of CH 3 OH:NH 3 :H 2 O (3:3:1), and a tholin film "with sulfur" was produced from a starting ice mixture of CH 3 OH:NH 3 :H 2 S:H 2 O (3:3:3:1). These samples are similar to our previous studies investigating spectroscopic and chemical properties of H 2 S containing ices (Mahjoub et al 2016(Mahjoub et al , 2017Poston et al 2018). Electron irradiation experiments were carried out using the Icy Worlds Simulation laboratory.…”

“…The model-fitting coefficients are: [0.6, 3.77, 1.46] for [methanol, "with sulfur," "with sulfur] and intercept = −0.94. irradiation of ices with and without H 2 S supported the different NIR spectral reddening after irradiation with high fluence of electron irradiation: reddening slope is more pronounced in samples containing H 2 S (Poston et al 2018) and these samples have the 1.8 μm feature observed on Arrokoth. Further rigorous comparison to telescopic and spacecraft spectra will need laboratory measurements of optical constants for ice tholins with and without sulfur, requiring additional experimental work.…”

“…Also, H 2 S residues have not been directly detected on solar system bodies to date, although a recent study has shown evidence for sulfur-bearing species on Callisto's leading hemisphere (Cartwright et al 2020). We recently studied the role that H 2 S could play in the chemistry (Mahjoub et al 2016(Mahjoub et al , 2017, spectroscopy (Poston et al 2018), and astrobiology (Mahjoub & Hodyss 2018) of small icy bodies-particularly KBOs and Jupiter Trojans (Wong et al 2019)-and apply these results to Arrokoth.…”

Organosulfur Compounds Formed by Sulfur Ion Bombardment of Astrophysical Ice Analogs: Implications for Moons, Comets, and Kuiper Belt Objects

“…It should be noted that due to their formation beyond the H 2 S snowline, the aforementioned objects (TNOs/KBOs) are likely to include H 2 S, another putative source of sulfur chemistry. The presence of H 2 S is supported by comparisons between laboratory experiments and IR spectra of KBOs and Trojan asteroids (Poston et al 2018).…”

JWST/NIRSpec Prospects on Transneptunian Objects

“…-Irradiation (Brunetto et al, 2006;Bennett et al, 2013;Poston et al, 2018), -Volatile retention (Schaller and Brown, 2007a;Brown et al, 2011b;Wong and Brown, 2017), -Differentiation and surface renewal (McKinnon et al, 2008;Guilbert-Lepoutre et al, 2011;Shchuko et al, 2014;Malamud and Prialnik, 2015), and -Collisions (possibly disruptive; Barr and Schwamb, 2016), There is a consensus over the fact that Cold Classicals stand out from the rest of TNOs due to their physical properties: redder colors and in general a specific surface type (Tegler and Romanishin, 2000;Pike et al, 2017), a large number of binaries with large separations (Noll et al, 2008a,b;Parker et al, 2011), higher albedos (Brucker et al, 2009;Vilenius et al, 2014), and a different size distribution (Bernstein et al, 2004;Fraser et al, , 2014 lacking large objects (Levison and Stern, 2001). These TNOs are thought to have formed in place from material distinct from other TNOs in the disk, which were implanted from regions closer to the Sun than their current location.…”