EVIDENCE FOR SPATIAL VARIATIONS IN THE DUST-TO-GAS RATIO OF ENCELADUS’ PLUME FROM SOLAR OCCULTATION DATA. M.M. Hedman1, P. D. Nicholson2, D. Dhingra1 C.J. Hansen3 1University of Idaho, 875 Perimeter Dr. MS 0903, Moscow ID 83844, 2CRSR, Cornell University, Ithaca NY 14853. 3Planetary Science Institute, 1700 East Fort Lowell, Suite 106, Tucson, AZ 85719-2395. (mhedman@uidaho.edu, nicholso@astro.cornell.edu, ddhingra@uidaho.edu, cjhansen@psi.edu )

Encealdus’ plume of water vapor and ice grains is

one of the Cassini mission’s most exciting discoveries, and data from multiple instruments onboard the Cassi-ni spacecraft have provided important information aboue the various aspects of the moon’s geological activity [1]. However, some fundamental aspects of this activity are still not well constrained. In particular, the plume’s dust-to-gas ratio is a critical parameter for understanding how material is accelerated beneath the moon’s surface [2-8]. However, it has been difficult to measure this ratio reliably because different instru-ments observe the dust and vapor components of the plume and because the plume's properties vary with both space and time [9-15].

On day 138 of 2010 there was a unique opportunity to observe the dust and gas content of the plume at the same location and the same time. On this day, Encela-dus’ plume passed between the Sun and the Cassini spacecraft, and both the Ultraviolet Imaging Spectro-graph (UVIS) and Visual and Infrared Mapping Spec-trometer (VIMS) were able to observe the extinction of light from the Sun by the material in the plume. The cross sections of water vapor molecules and dust grains depend on wavelength, and it turns out that UVIS is more sensitive to the vapor component of the plume, while VIMS is more sensitive to the particles.

DamascusS

ulcus

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Figure 1: Optical depths of the plume as measured by UVIS and VIMS, overlaid on an image of Enceladus’ South Polar Terrain (from ciclops.org). The dashed line indicates the track of the point where the line-of-sight to the Sun was closest to the moon. Note that UVIS detects a signal over a longer timespan than VIMS does, suggesting that there are significant variations in the plume’s dust-to-gas ratio.

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Hansen et al. [16] found a significant reduction in sunlight observed by UVIS due to absorption by water vapor. VIMS did not observe a comparably strong sig-nal because plume vapor is less efficient at scattering light at near-infrared wavelengths. However, careful analysis of the VIMS data has revealed a very slight reduction in the solar flux between 1 and 4 microns. This signal not only appeared when the Sun was pass-ing behind the core of the plume, it also has a spectral slope consistent with the expected obscuration by the plumes’ particle population.

Comparisons between the UVIS and VIMS data reveal significant differences in the plume’s structure, with the inferred dust-to-gas ratio varying by roughly an order of magnitude across the plume (see Figure 1). These spatial variations are not strongly correlated with the Sun’s altitude above Enceladus’ surface, and so they cannot simply reflect the different launch ve-locities of the particles and the gas. Instead, these vari-ations may reflect variations in the dust-to-gas ratio of different sources of the plume (namely the tiger stripe fissures). In particular, it appears that some of the ma-terial emerging around Baghdad Sulcus is much more particle-rich thant that emerging from the vicinity of Cairo and Alexandria sulci These spatial variations in the dust-to-gas ratio for different fissures could there-fore be related to previously-identified varitations in the composition and spectral properties of the material

emerging from different fissures [17-22], and may con-tain important clues to the subsurface processes operat-ing beneath Enceladus’ South Polar Terrain.

References: [1], Spencer, J. and F. Nimmo, (2013) AREPS 41:693-717 [2] Porco, C.C. et al. (2006) Science 311: 1393-1401 [3] Schmidt, J. et al. (2008), Nature 451:683-688 [4] Keiffer, S. et al. (2009) Icarus 203:238-241 [5] Ingersoll, I. and S. Ewald (2011) Icarus 216: 492-506 [6] Degruyter, W. and Manga, M. (2011) GRL 38:L16201 [7] Yeoh, S.K. et al. (2015) Icarus 253:205-222 [8] Gao, P. et al. (2016) Icarus 264:227-238 [9] Saur et al. (2008), GRL 35:20105 [10] Smith, H.T. et al. (2009), JGR 115:10252 [11] Dong, Y. et al. (2011), JGR 116:A10204 [12] Hedman, M. et al. (2013), Nature 500:182-184 [13] Nimmo, F. et al. (2014) AJ 148:46 [14] Ingersoll, I. and S. Ewald (2017) Icarus 282:260-275 [15] Yeoh, S.K. et al. (2017) Icarus 281:357-378 [16] Hansen, C.J. et al. (2011) GRl, 38,11202. [17] Brown, R.H. et al. (2006) Science 311:1425-1428, [18] Jaumann, R. et al. (2008), 193: 372-386. [19] Postberg, F. et al. (2011) Nature 474:602-2, [20] Dhingra, D. et al. (2015a) 46th LPSC, Abst# 1648, [21] Dhingra, D. et al. (2016) 47th LPSC, Abst# 2638, [22] Dhingra, D. et al. (2017) Ica-rus, Under Review, .

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