Icy Giants of the Kuiper Belt: JWST Looks at Salacia and Máni
In this study, Wong and collaborators use the James Webb Space Telescope (JWST) to observe two mid-sized Kuiper Belt Objects (KBOs): the binary system Salacia–Actaea and the solitary body Máni. These are icy worlds orbiting far beyond Neptune. By analyzing their spectra, how they reflect sunlight at different infrared wavelengths, the team uncovers clues about their composition and what makes them different from both smaller KBOs and the largest dwarf planets like Pluto.
Introduction: A Complex Kuiper Belt
Over the past decades, astronomers have learned that the Kuiper Belt is far from uniform. The biggest KBOs retain exotic ices like methane and nitrogen, while smaller ones usually only show water or methanol ice. Mid-sized KBOs, roughly 800–1500 km across, are especially interesting because they may have enough internal heat for processes like differentiation (separating rock from ice inside) or even cryovolcanism (ice volcanism). Salacia and Máni, with diameters just under 1,000 km, fall right in this critical size range.
Observations and Methods
The team used JWST’s Near-Infrared Spectrograph (NIRSpec) to collect spectra from 0.7 to 5.1 microns. For Salacia, its moon Actaea was too close to separate, so both were observed together. The data processing involved careful corrections for background stars and instrument effects before producing clean reflectance spectra.
Spectral Results: Water and Carbon Dioxide
Both Salacia and Máni show clear absorption bands from water ice at several wavelengths (1.5, 2, 3, and 4–5 µm). They also reveal the fingerprint of carbon dioxide ice at 4.25 µm. Interestingly, hydrocarbons like methane or ethane, seen on slightly larger objects such as Quaoar or Sedna, are missing. This absence may mean that these smaller bodies cannot hold onto methane long enough for it to build up or be transformed into other molecules.
Population Trends Among Water-Ice-Rich KBOs
Wong and collaborators compare their results to dozens of other KBOs observed by JWST. They find that larger KBOs tend to show stronger water ice features, suggesting that internal heating and cryovolcanic processes may refresh their surfaces with new ice. At the same time, there seems to be a transition in carbon dioxide features: larger KBOs show weaker and differently shaped CO₂ bands compared to smaller ones. This shift might be tied to size, surface processes, or the region where these bodies formed.
Implications for the Kuiper Belt
Salacia and Máni fall into what astronomers now call the “Prominent Water” (PW-type) class of KBOs. These are bodies whose spectra are dominated by water ice but still show some CO₂. Their similarity to other PW-type objects suggests they likely formed in the same inner region of the primordial Kuiper Belt before being scattered outward. More broadly, the trends found in this work point toward size-dependent processes, like heating and differentiation, shaping the surfaces of icy worlds.
Summary and Outlook
By examining Salacia and Máni, the study strengthens the case that mid-sized KBOs are not just inert chunks of ice and rock. Instead, their compositions hint at internal activity and evolutionary histories that depend strongly on their size. Future higher-resolution JWST observations and lab experiments on ices will be crucial for teasing apart the roles of abundance, grain size, and surface layering in shaping the spectral signatures of these distant worlds.
Source: Wong
