Distribution of CO2 ice on the large moons of Uranus and evidence for compositional stratification of their near-surfaces

Abstract

The surfaces of the large Uranian satellites are characterized by a mixture of H2O ice and a dark, potentially carbon-rich, constituent, along with CO2 ice. At the mean heliocentric distance of the Uranian system, native CO2 ice should be removed on timescales shorter than the age of the Solar System. Consequently, the detected CO2 ice might be actively produced. Analogous to irradiation of icy moons in the Jupiter and Saturn systems, we hypothesize that charged particles caught in Uranus' magnetic field bombard the surfaces of the Uranian satellites, driving a radiolytic CO2 production cycle. To test this hypothesis, we investigated the distribution of CO2 ice by analyzing near-infrared (NIR) spectra of these moons, gathered using the SpeX spectrograph at NASA's Infrared Telescope Facility (IRTF) (2000 -2013). Additionally, we made spectrophotometric measurements using images gathered by the Infrared Array Camera (IRAC) onboard the Spitzer Space Telescope (2003 -2005). We find that the detected CO2 ice is primarily on the trailing hemispheres of the satellites closest to Uranus, consistent with other observations of these moons. Our band parameter analysis indicates that the detected CO2 ice is pure and segregated from other constituents. Our spectrophotometric analysis indicates that IRAC is not sensitive to the CO2 ice detected by SpeX, potentially because CO2 is retained beneath a thin surface layer dominated by H2O ice that is opaque to photons over IRAC wavelengths. Thus, our combined SpeX and IRAC analyses suggest that the near-surfaces (i.e., top few 100 microns) of the Uranian satellites are compositionally stratified. We briefly compare the spectral characteristics of the CO2 ice detected on the Uranian moons to icy satellites elsewhere, and we also consider the most likely drivers of the observed distribution of CO2 ice.

Probing the regoliths of the classical Uranian satellites: Are their surfaces mantled by a layer of tiny H2O ice grains?

“…We analyzed new IRAC Ch.1 and Ch.2 geometric albedos to test the hypothesis that the surfaces of the classical Uranian satellites are mantled by tiny H2O ice grains. Radiative transfer modeling of these new data support the ubiquitous presence of tiny H2O ice grains (≤ 2 µm diameters), consistent with previous analyses of other long NIR datasets (~3 -5 µm; Cartwright et al, 2015Cartwright et al, , 2018. Furthermore, our results indicate that these moons have higher IRAC albedos on their trailing hemispheres compared to their leading hemispheres (except for Miranda), suggesting larger abundances of tiny H2O ice grains and/or less H2O ice is exposed on their trailing sides.…”

“…The spatial trends in brightness across Ariel's surface suggests that its trailing hemisphere is mantled by a reflective material, peaking in abundance near its antapex (longitude 270º). This trend in brightness is likely consistent with the spatial distribution of CO2 ice (Grundy et al, 2003(Grundy et al, , 2006Cartwright et al, 2015), suggesting that the presence of CO2 is enhancing the IRAC albedos of Ariel's trailing side. Supporting this interpretation, spectroscopic laboratory measurements demonstrate that CO2 ice (e.g., Hansen et al, 1997) is much brighter than H2O ice (e.g., Mastrapa et al, 2009) over the ~3 to 4 µm and ~4.5 to 5 µm wavelength ranges covered by IRAC.…”

“…Although our best fit synthetic spectra contain no CO2 ice, spectral observations made by SpeX in short cross-dispersed (SXD) mode clearly demonstrate that CO2 ice is present on the trailing hemispheres of these moons, in particular on Ariel (Grundy et al, 2003(Grundy et al, , 2006Cartwright et al, 2015). Given the clear evidence for CO2 ice at shorter wavelengths (27% of best fit spectral models), perhaps the high IRAC albedos for Ariel's trailing hemisphere are influenced by the presence of CO2 ice.…”

“…Additionally, we applied these scattered light removal routines to five previously unreported Program 71 observations of these moons (Table 2). After removing scattered light from all contaminated frames, we averaged the corrected fluxes and converted them into geometric albedos (Section 2.2 of Cartwright et al, 2015).…”

“…The detected H2O bands on these moons are much weaker than the same H2O features on ice-rich Saturnian moons (e.g., Cruikshank et al, 1977), demonstrating how the presence of dark material weakens the spectral signature of H2O ice over these wavelengths (Clark and Lucey, 1984). Overprinting these "dirty" H2O ice features, narrow CO2 ice bands have been detected (between 1.9 and 2.1 µm), primarily on the trailing hemispheres (longitudes 181 -360º) of the inner moons, Ariel andUmbriel (Grundy et al, 2003, 2006;Cartwright et al, 2015). The central wavelength positions, band shapes, and band strengths of these CO2 ice features are remarkably similar to the second order overtone and combination bands of "pure" CO2 ice (i.e., concentrated deposits of CO2, segregated from other constituents, with crystal structures dominated by CO2 molecules) (e.g., Hansen, 1997;Gerakines et al, 2005).…”