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46th Lunar and Planetary Science Conference (2015) 1647.pdf

HEMISPHERICAL ASYMMETRIES IN H2O BANDS ON THE LARGE OF : EVIDENCE FOR SYSTEM-WIDE ALTERATION PROCESSES R. J. Cartwright1, J. P. Emery1, N. -Pinilla1, A. S. Rivkin2, D. E. Trilling3, 1University of Tennessee, 2John Hopkins University Applied Physics Laboratory, 3Northern Arizona University.

Introduction: Ground-based of the the amount of exposed H2O ice, and thus, reduce the Uranian satellites , , , and strength of H2O bands on the trailing hemispheres’ of indicates that their surfaces are dominated by H2O ice these moons, thereby enhancing leading/trailing asym- mixed with a dark, spectrally neutral constituent that is metries in H2O ice band strength. potentially -rich (e.g., [1-3]). More recent ob- The comparable relative velocities of IDPs and -1 servations of these moons have detected CO2 ice, pri- ISDPs (~10 and 30 km s , respectively) to the orbital mary on the trailing hemispheres of the satellites clos- velocities of these moons (~3 – 7 km s-1) should lead to est to Uranus [4-6]. Similarly, H2O ice bands on the preferential bombardment of their leading hemispheres large Uranian satellites also display leading/trailing as the moons intercept dust along their orbits [10], in asymmetries, with stronger bands on their leading hem- particular for the moons closest to Uranus where gravi- ispheres [4-6] (Figure 1). tational focusing greatly increases impactor fluxes (e.g., [11]). Over time, the preferential bombardment of these moons’ leading hemispheres by micrometeor- ites will promote regolith overturn (e.g., [12]), increas- ing the leading/trailing asymmetry in H2O bands. Un- like IDPs and IDSPs, irregular satellite dust should preferentially accumulate on the leading hemispheres of the regular moons furthest from Uranus due to their proximity to the irregular satellite zone [10]. Thus, charged particle bombardment and micro- impacts are likely enhancing leading/trailing asymmetries in the strength of H2O ice bands on the large Uranian satellites. Data and Methods: To test these different hy- potheses, we measured the H2O ice band areas and other parameters in 43 Uranian satellite spectra gath- ered by three different teams (Rivkin 2000, Grundy The presence of leading/trailing asymmetries in 2001 – 2006, Cartwright 2012 – 2013) using the SpeX the detected H2O ice bands on all four of these moons spectrograph at NASA’s Telescope Facility suggest that system-wide mechanisms drive the ob- [13]. H2O ice absorption bands centered near 1.04 and served differences, as opposed to native geologic pro- 1.25 μm in spectra of Saturnian satellites [14] are quite cesses operating independently on each . Two weak in the Uranian satellite spectra, and we therefore system-wide processes that could generate lead- focus our band parameter analysis on the two larger ing/trailing differences in the composition of these H2O ice band complexes centered near 1.52 and 2.02 moons are: magnetospheric charged particle bombard- μm (visibly apparent in all 43 spectra, Figure 1). ment, and impacts by interplane- Results: H2O ice band areas for both the 1.52 and tary/interstellar dust particles (IDPs and ISDPs, respec- 2.02 µm bands are at least one-sigma greater on the tively) or by intraplanetary dust in-falling from Uranus’ leading hemispheres of all four moons compared to irregular satellites. their trailing hemispheres (Figure 2). The measured Charged particles (electrons, protons, and heavy H2O band areas clearly trend with the geometric albe- ions) caught in Uranus’ magnetosphere should prefer- dos of the moons, with the strongest H2O bands on the entially bombard the trailing hemispheres of Ariel and leading hemisphere of the brightest moon, Ariel, and Umbriel due to the higher magnetic field the weakest on the trailing hemisphere of the faintest closer to Uranus [8]. Irradiation of H2O ice and dark, moon, Umbriel. One notable exception to this trend is potentially C-rich materials could drive radiolytic pro- the H2O bands on the trailing hemisphere of Ariel, duction of CO ice and other C-rich, oxidized species 2 which are weaker than the H2O bands on the trailing [9]. Such a radiolytic production cycle would reduce hemisphere of Titania. 46th Lunar and Planetary Science Conference (2015) 1647.pdf

moons closest to Uranus are consistent with higher charged particle and IDPs/IDSPs fluxes, which fall off with distance from Uranus. Although the reduction in hemispherical asymme- tries with increasing orbital radius appears to be incon- sistent with irregular satellite dust bombardment, the lower relatively velocity of intraplanetary dust sources compared to IDPs and IDSPs (e.g., [12]) likely reduces the effectiveness of regolith overturn by irregular satel- lite dust. Additionally, compositional differences be- tween IDPs/IDSPs and intraplanetary dust might help modify the leading/trailing asymmetries in H2O band strengths (for example, if impacting IDPs/IDSPs deliv- er more H2O ice than intraplanetary dust sources). Future work: We are gathering additional spectral and photometric data over visible wavelengths (VIS). Spectral reddening in the VIS has been attributed to irradiation of icy moons (e.g., [15]), as well as material delivery and/or radiolysis via micrometeorite impacts (e.g., [10]). For example, enhanced spectral reddening of Oberon’s leading hemisphere (e.g., [16]) is con- Leading/trailing ratios for the mean areas of the sistent with the accumulation of dust from Uranus’ retrograde irregular satellites [10], which are spectrally 1.52 and 2.02 μm H2O ice bands display a clear plan- etocentric gradient, with pronounced hemispherical redder than the large [17]. Therefore, asymmetries on Ariel and only subtle asymmetries on by characterizing the leading/trailing hemispherical the furthest regular satellite, Oberon (Figure 3). Addi- distribution of spectrally red material, we will be able tionally, the mean leading/trailing ratios for the 1.52 to further constrain the relative contribution of these µm bands decrease more rapidly than for the 2.02 µm processes to the strength of the detected H2O ice bands. bands with increasing distance from Uranus, with only We will also collect spectra at wavelengths > 2.5 µm in order to characterize the H O ice Fresnel peak minor differences between the 1.52 and 2.02 µm H2O 2 ice band ratios on Oberon. (centered near 3.1 µm) on the leading and trailing hem- ispheres of these moons. Finally, we will utilize numer- Discussion: The larger H2O band areas on the lead- ing hemispheres of all four moons are consistent with ical models to constrain the grain sizes and composi- both the micrometeorite and charged particle bom- tion of these moons’ surfaces over the entire wave- bardment hypotheses. Furthermore, the planetocentric length range of our datasets (~0.4 – 4.0 µm). gradient in the mean leading/trailing ratios for the areas References: [1] Brown, R.H. (1983) Icarus, 56, of the 1.52 and 2.02 µm bands supports both hypothe- 414-425. [2] Brown, R.H. and Clark, R.N. (1984) Ica- ses as well. Larger hemispherical asymmetries on the rus, 58, 288-292. [3] Brown, R.H. and Cruikshank D. (1983) Icarus, 55, 83-92. [4] Grundy, W.M., et al. (2003) Icarus, 162, 223-230. [5] Grundy, W.M., et al. (2006) Icarus, 184, 543-555. [6] Cartwright, R.J., et al. (in prep). [7] Karkoschka, E., (2001) Icarus, 151, 51- 68. [8] Ness, N.F., et al. (1991) Uranus, 739-779. [9] Johnson, R.E., et al. (2005) : The , Satel- lites, and Magnetosphere, 485-512. [10] Tamayo, D., et al. (2013) Icarus, 226, 655-662. [11] Zahnle, K., et al. (2003) Icarus, 163, 263-289. [12] Porter, S.B., et al. (2010) Icarus, 208, 492-498. [13] Rayner, J.T., et al. (2003) PASP, 115, 362-382. [14] Emery, J.P., et al. (2005) A&A, 435, 353-362. [15] Schenk, P., et al. (2011) Icarus, 211, 740-757. [16] Buratti and Mosher (1991) Icarus, 90, 1-13. [17] Maris, M., et al. (2007) A&A, 472, 311-319.