Is CO₂ ice present on the Uranian moon Miranda?

The four largest Uranian satellites Ariel, Umbriel, Titania, and Oberon have been shown to possess deposits of CO₂ ice on their surfaces, discovered through ground-based near-IR spectroscopy. These detections rely on a distinctive triplet of narrow absorption features at 1.966, 2.012, and 2.070 μm.

The distribution of this CO₂ ice on the Uranian moons follows spatial trends; the CO₂ ice triplet is stronger on the moons’ trailing hemispheres, and stronger on the moons that are closer to Uranus. These spatial trends in CO₂ distribution are consistent with ongoing radiolytic production of CO₂ molecules via charged particle bombardment of the H₂O ice and carbonaceous compounds in the regoliths of the Uranian moons. Miranda is the closest classical satellite to Uranus and is embedded within the planet’s magnetosphere. Consequently, production of CO₂ molecules is expected to occur on Miranda, but previous observational studies have not identified CO₂ ice on this moon. The signal-to-noise in the 1.95-2.10 μm region was low (S/N ≲ 20) in spectra collected by these prior studies, potentially hiding low levels of CO₂ ice.

We therefore aimed to investigate the possible presence of CO₂ ice on Miranda with recently acquired higher S/N spectra (S/N ≳ 60) of its northern hemisphere. The CO₂ ice triplet was not visually apparent in any of our spectra of Miranda. We measured the integrated band areas and depths at the expected positions of the CO₂ ice bands, utilizing the same wavelength ranges defined by prior detection of CO₂ ice on the other Uranian moons, to search for low-level absorption. We found that none of the Miranda spectra showed evidence for the presence of CO₂ ice (>2σ detection).

Thus, cold trap deposits of CO₂ ice are unlikely to be present on Miranda’s surface, unlike the other classical Uranian moons. While radiolytic production of CO₂ molecules is likely to be occurring on Miranda, thermodynamical modeling studies indicate that retention of large CO₂ ice deposits is unlikely, as the average velocity of sublimated or sputtered CO₂ molecules from the surface is higher than Miranda’s escape velocity. While further observational studies (such as with JWST or a future Uranus orbiter) promise a wealth of insight into CO₂ on the Uranian satellites, detection of CO₂ on Miranda could be more challenging given its non-detection in ground-based telescope studies. We will (briefly) recap our prior results on the H₂O ice distribution on Miranda, then present our findings for (the lack of) CO₂ on Miranda and our preliminary assessment of weak absorption features in the 2.2-μm region that could result from NH₃- and NH₄-bearing species.