CO2-Driven Activity and Icy Coma Dust in the Active Centaur 450P/LONEOS

The Solar System’s Centaurs, sourced from trans-Neptunian object (TNOs) populations, contain some of the most primitive materials remaining from Solar System formation. They occupy dynamically unstable orbits between Neptune and Jupiter that generally evolve inward toward the Sun, with some eventually becoming Jupiter-family comets.

As descendants of TNOs, Centaurs likely contain abundant volatiles that may become activated during their inward migrations due to increased solar heating. However, the known active fraction of Centaurs is relatively small, hovering around 10-15% [1, 2], raising questions about what conditions are required for an ice-rich small body to become active and sustain mass loss. Because Centaurs reside at heliocentric distances that are generally too cold for vigorous water-ice sublimation, their activity is likely driven by other means, such as sublimation of more volatile species (e.g., CO, CH₄,  HCN, CO₂, etc.) or recent changes in orbital heating [3, 4]. Active Centaurs therefore provide important tests of how volatile inventories, thermal evolution, and dynamical history combine to drive comet-like activity at relatively large heliocentric distances. We present a multi-wavelength case study of the active Centaur 450P/LONEOS [5], whose orbit was significantly altered by a close encounter with Saturn in 1992, moving it inward from a long-term trans-Saturnian orbit toward a more strongly heated orbit with perihelion closer to Jupiter.

We used Gemini-N/GMOS imaging from 2019 to 2024 and JWST/NIRSpec IFU Prism-mode spectroscopy obtained in 2023 to characterize 450P's nucleus, dust and gas in its coma, and likely activity mechanism. Gemini observations recovered 450P after more than a decade without observations and likely captured the first views of its inactive nucleus. The inactive-state photometry implies a small effective nucleus radius of R_N = 1.8±0.5 km and a relatively red surface color of g' - i' = 1.15±0.09 mag. This color places 450P toward the red end of the neutral/gray active-Centaur population, suggesting that its surface may have experienced comparatively limited solar-driven processing since its recent inward migration.

Continued Gemini monitoring showed that 450P developed a faint dust coma as its heliocentric distance decreased from R_H = 7.83 au to R_H = 7.24 au. The coma was asymmetric and elongated in the tailward direction, consistent with dust grains being shaped by solar radiation pressure. From broadband optical photometry, we estimate a low dust production rate of approximately 4-8 kg/s, indicating weak but measurable activity at distances where water-ice sublimation from the nucleus should be inefficient.

JWST/NIRSpec observations obtained at R_H = 7.16 au revealed a coma containing both dust and CO₂ gas. The spectrum shows a strong CO₂ emission feature at 4.26 microns, while no H₂O or CO gas emission features were detected. We derive Q(CO₂) = (6.99±0.07)×10²⁴ molecules/s, with upper limits of Q(H₂O) ≤ 1.2×10²⁴ molecules/s and Q(CO) ≤ 5.2×10²⁴ molecules/s. The CO₂ morphology appears comparatively symmetric, in contrast to the elongated dust distribution, suggesting that CO₂ is being released from the nucleus directed in the sunward hemisphere while the dust is subsequently modified by solar radiation pressure.

The reflectance spectrum also exhibits absorption features near 2.0 and 3.0 microns, consistent with water ice in the coma dust. Spectral modeling favors relatively large, intimately mixed grains composed of amorphous carbon and crystalline water ice, with an effective grain diameter of D_eff = 5.9 microns and a volumetric ice fraction of approximately 33%. A subtle feature near 3.1 microns is consistent with the crystalline water-ice Fresnel peak, providing evidence that crystalline ice is present in larger coma grains.

Finally, we interpret 450P's activity in the context of its well-constrained orbital history. A simple thermal model incorporating its evolution since approximately 1500 CE suggests that the observed onset of activity is plausibly explained by CO₂ release from sub-surface amorphous water ice undergoing crystallization at temperatures of roughly 140-160 K. Together, these results identify 450P as a recently activated Centaur whose present activity records the early thermal response of a small icy body after inward migration, providing a valuable link between primitive TNO-like material and the onset of cometary activity in the giant-planet region.

 

Acknowledgments: We gratefully acknowledge the support provided by the NASA SSO Program through award 80NSSC23K0678, the Space Telescope Science Institute through award JWST-GO-02416, and the Florida Space Research Initiative.

 

References: [1] Jewitt, D. (2009), The Active Centaurs, AJ, 137, 4296. [2] Bauer, J. M., Ivanova, O. V., McKay, A., and Sarid, G. (2025), Activity, Outbursts and Explosions. In: Volk, K., Womack, M., and Steckloff, J. I. (eds.), Centaurs. [3] Fernández, J. A., Helal, M., and Gallardo, T. (2018), Dynamical evolution and end states of active and inactive Centaurs, Planetary and Space Science, 158, 6–15. [4] Lilly, E., Jevčák, P., Schambeau, C. A., et al. (2024), Semi-major Axis Jumps as the Activity Trigger in Centaurs and High-Perihelion Jupiter Family Comets, ApJL, 960, L8. [5] Schambeau, C. A., Kelley, M. S. P., Womack, M., et al. (2026), JWST and Gemini Observations of the Active Centaur 450P/LONEOS: Nucleus and Coma Characterizations, PSJ, in press.