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

TL;DR

This study uses JWST/NIRSpec spectra of midsized KBOs Salacia-Actaea and Máni to investigate surface composition trends among water-ice-rich bodies. Both targets exhibit strong H2O-ice and CO2-ice absorption, classifying them as H2O-type KBOs, with Máni showing stronger H2O features than Salacia. An ensemble analysis across 33 H2O-type KBOs reveals a size-correlated increase in the 3 $\mu$m H2O-band depth and a bimodal 2 $\mu$m H2O-band distribution, alongside a transition in CO2-band depth around $H_V \sim 5-6$ and size-dependent changes in CO2-band shapes; these results point toward interior heating, differentiation, and possible cryovolcanic resurfacing shaping midsized KBO surfaces. The findings support a formation-and-eevolutionary framework where interior and surface processes drive compositional diversity, and they highlight the need for larger, higher-quality datasets and radiative-transfer modeling to disentangle ice abundance, grain size, and layering effects.

Abstract

We present observations of the midsized Kuiper Belt objects (KBOs) Salacia$-$Actaea and Máni, 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 $μ$m reflectance spectra of Salacia$-$Actaea and Máni display prominent water-ice absorption bands at 1.5, 2, 3, and 4$-$5 $μ$m. The $ν_{3}$ fundamental vibrational band of carbon dioxide ice at 4.25 $μ$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 $μ$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.

Figures (5)

• Figure 1: Wavelength-averaged IFU images from the first dithered exposures of Salacia and Máni. A logarithmic stretch was applied to the flux scaling to highlight the PSF shapes and background variations. The black dots and circles denote the best-fit centroid positions and the spectral extraction apertures, respectively. In the case of Máni, the masked pixels across the field of view correspond to relatively bright background sources that were flagged prior to spectral extraction (see text for details).
• Figure 2: The reflectance spectra of Salacia (top) and Máni (bottom), normalized to unity at 2.5 $\mu$m. The detected absorption bands of H2O and CO2 ice are labeled. The red triangles correspond to the location of the 1.65 $\mu$m H2O-ice spectral feature that likely indicates the crystalline phase, which is clearly discernible in the spectrum of Máni. The inverted orange triangles mark the 3.1 $\mu$m Fresnel reflection peak of H2O ice.
• Figure 3: The reflectance spectra of Salacia (blue) and Máni (green) alongside the sample of H2O-type KBOs (black) from pinillaalonso2025, illustrating the broad similarities in overall continuum shape and absorption bands. The average CO2-type KBO (orange) and organics-type KBO (red) spectra are also included for comparison. All spectra have been normalized to unity at 2.5 $\mu$m.
• Figure 4: Plot of the measured band depths of the 2 $\mu$m H2O (top), 3 $\mu$m H2O (middle), and $\sim$4.25 $\mu$m fundamental CO2 absorption features as a function of $V$-band absolute magnitude $H_{V}$ for H2O-type objects. The points are color coded by dynamical class. The band depths for Salacia and Máni are highlighted by the larger markers and labeled "S" and "M," respectively. The three brightest/largest H2O-type members of the resonant KBO population (orange squares) are, in order, Charon, Orcus, and Achlys. The band-depth values, absolute magnitudes, and dynamical classifications shown in this figure are available as a machine-readable data behind the figure file.
• Figure 5: A selection of 4.0--4.5 $\mu$m reflectance spectra of H2O-type KBOs, normalized to unity at 4.3 $\mu$m. The spectra are arranged from top to bottom in order of increasing object size. The vertical dotted line at 4.27 $\mu$m denotes the expected band center for crystalline CO2 ice. The larger objects display distinctly broader and blue-shifted absorption features than the smaller KBOs. The spectrum of Charon, obtained with the high-spectral-resolution G395H grating of NIRSpec, reveals complex fine structure within the CO2 band. The black curve shows the same spectrum convolved to the lower spectral resolution of the PRISM grating.