Ingersoll, Andrew P. and Ewald, Shawn P. (2017) Decadal timescale variability of the Enceladus plumes inferred from Cassini images. Icarus, 282 . pp. 260-275. ISSN 0019-1035. http://resolver.caltech.edu/CaltechAUTHORS:20161104-102822857
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The brightness of the Enceladus plumes varies with position in the satellite's eccentric orbit, with altitude above the surface, and with time from one year to the next. Hedman et al. (2013, hereinafter H13) were the first to report these variations. They used data from Cassini's Visible and Infrared Mapping Spectrometer (VIMS). Here we present brightness observations from Cassini's Imaging Science Subsystem (ISS), which has 40 times higher spatial resolution than VIMS. Our unit of measure is slab density, the total mass of particles in a horizontal slab per unit thickness of the slab. Using slab density is one approach to correcting for the variation of brightness with wavelength and scattering angle. Approaches differ mainly by a multiplicative scaling factor that depends on particle density, which is uncertain. All approaches lead to the same qualitative conclusions and agree with the conclusions from VIMS. We summarize our conclusions as follows: At all altitudes between 50 and 200 km, the corrected brightness is 4–5 times greater when Enceladus is farther from Saturn (near apocenter) than when it is closer (near pericenter). A secondary maximum occurs after pericenter and before apocenter. Corrected brightness vs. altitude is best described as a power law whose negative exponent is greatest in magnitude at apocenter, indicating a slower launch speed for the particles at apocenter than at other points in the orbit. Corrected brightness decreased by roughly a factor of two during much of the period 2005–2015. The last is our principal result, and we offer three hypotheses to explain it. One is a long-period tide—the decreasing phase of an 11-year cycle in orbital eccentricity; another is buildup of ice at the throats of the vents; and the third is seasonal change—the end of summer at the south pole.
|Additional Information:||© 2016 Elsevier Inc. Received 30 January 2016; Revised 8 September 2016; Accepted 12 September 2016; Available online 22 September 2016. This work was supported by NASA partly through the Cassini Project and partly through NASA's Science Mission Directorate, Planetary Science, Grant/Contract/ Agreement No. NNX15AH08G. We thank Miki Nakajima, Linda Spilker, John Spencer, and an anonymous reviewer for useful suggestions. Author contributions: S.P.E. did most of the image analysis. A.P.I. did most of the writing. Both authors worked on developing algorithms, testing them, and interpreting the results.|
|Subject Keywords:||Enceladus; Saturn satellites; Satellites; Atmospheres|
|Official Citation:||Andrew P. Ingersoll, Shawn P. Ewald, Decadal timescale variability of the Enceladus plumes inferred from Cassini images, Icarus, Volume 282, 15 January 2017, Pages 260-275, ISSN 0019-1035, http://dx.doi.org/10.1016/j.icarus.2016.09.018. (http://www.sciencedirect.com/science/article/pii/S0019103516305905)|
|Usage Policy:||No commercial reproduction, distribution, display or performance rights in this work are provided.|
|Deposited By:||Tony Diaz|
|Deposited On:||04 Nov 2016 19:08|
|Last Modified:||15 Dec 2016 18:38|
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