Physics
Scientific paper
Dec 2010
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2010agufm.p23c..09h&link_type=abstract
American Geophysical Union, Fall Meeting 2010, abstract #P23C-09
Physics
[5480] Planetary Sciences: Solid Surface Planets / Volcanism, [6280] Planetary Sciences: Solar System Objects / Saturnian Satellites
Scientific paper
The Cassini Ultraviolet Imaging Spectrograph (UVIS) observed an occultation of the sun by Enceladus’ water vapor plume on May 18, 2010. UVIS used its extreme ultraviolet (EUV) channel for this new observation, to detect absorptions in the wavelength range 55 to 110 nm. Molecular nitrogen and water vapor have absorptions in this range. The N2 b(3,0) line is at 97.2 nm, extinguishing the solar H Lyman gamma emission. Cassini’s Ion and Neutral Mass Spectrometer (INMS) detected a species with an atomic mass of 28 amu, which could be CO, C2H4 or N2 [1, 2]. Definitive UVIS detection of N2 was important to clear up this ambiguity, and this was an important goal of the observation, as the presence or lack of N2 is key to models of the geochemistry in the interior [3, 4, 5]. UVIS did not detect N2 and we set an upper limit for the column density of 3 x 10^13 cm^-2. The absorption features in the spectrum are best fit by a water vapor column density of 0.9 x 10^16 cm^-2. This column density is in family with previous UVIS measurements from stellar occultations in 2005 and 2007 at far ultraviolet wavelengths, suggesting that Enceladus’ activity has been stable for the last 5 years [6, 7]. We used fluctuations in the signal to probe the structure of the gas jets again, as was analyzed in the 2007 occultation of zeta Orionis [7]. Gas jets are correlated to the dust jets detected by Cassini’s Imaging Science Subsystem [8]. The path of the sun cut through the jets horizontally at an altitude above the limb of ~15 km at the closest point. The resolution of the solar occultation is higher than the stellar occultation, and collimation of the gas jets observed in the solar occultation is greater than estimated in 2007. The observed collimation allows us to derive a mach number of ~4 for the ratio of the vertical velocity in the jet to the thermal velocity of the plume gas. The new opportunity afforded by this solar occultation is used to further model the structure and dynamics of the plume, allowing us to probe the source of the enigmatic activity below Enceladus’ surface. References: 1. Waite, J. H. et al, Science 311:1419-1422 (2006). 2. Waite, J. H. et al, Nature 460:487-490 (2009). 3. Niemann, H. B. et al, Nature 438:779-784 (2005). 4. Matson, D. L. et al, Icarus 187:569-573 (2007). 5. Glein, C. R. et al, LPSC XXXVIII abstract 1251 (2007). 6. Hansen, C. J. et al, Science 311:1423-1425 (2006). 7. Hansen, C. J. et al, Nature 456:477-479 (2008). 8. Spitale, J. N. and C. C. Porco, Nature 449:695-697 (2007). Acknowledgement: This work was partially supported by the Jet Propulsion Laboratory, California Institute of Technology, under a contract with the National Aeronautics and Space Administration
Esposito W. L. W. L.
Hansen Camilla Juul
Hendrix Amanda R.
Shemansky Don E.
Stewart Iain
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