JWST/NIRSpec Observations of Salacia-Actaea and M\'ani: Exploring Population-level Trends among Water-ice-rich Kuiper Belt Objects

Abstract

We present observations of the midsized Kuiper Belt objects (KBOs) Salacia$-$Actaea and M\'ani, obtained with the Near-Infrared Spectrograph on JWST. The satellite Actaea was fully blended with Salacia at the spatial resolution of the integral field unit, and we extracted the combined spectrum. The 0.7$-$5.1 $\mu$m reflectance spectra of Salacia$-$Actaea and M\'ani display prominent water-ice absorption bands at 1.5, 2, 3, and 4$-$5 $\mu$m. The $\nu_{3}$ fundamental vibrational band of carbon dioxide ice at 4.25 $\mu$m is present in both spectra. From a quantitative band-depth analysis of the entire current JWST spectroscopic sample of water-ice-rich KBOs, we find strong evidence for a positive covariance between relative water-ice abundance and size, which may indicate the emergent impacts of internal differentiation and cryovolcanic production of surface water ice on midsized KBOs. A detailed look at the distribution of 2 and 3 $\mu$m band depths suggests additional sources of variability, such as different water-ice grain sizes. In addition, we report an apparent transition in the carbon dioxide band depth at object diameters of roughly 300$-$500 km, with larger objects showing systematically weaker absorptions, although selection effects within the sample do not allow us to confidently distinguish between a size-dependent phenomenon and a correlation with dynamical class.