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Is CO₂ ice present on the Uranian moon Miranda?

Presentation #106.03 in the session Unmeltable Me, verse 2: Other Icy Satellites.

Published onOct 20, 2022
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.

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