CICLOPS: Cassini Imaging Central Laboratory for OPerationS

Porco, C., Mitchell, C., Nimmo, F., Tiscareno, M. (2018). "Multiple long-period variations in the plume of Enceladus offer insights into eruption mechanism" AGU Abstracts bstract P43F-3819 presented at 2018 Fall Meeting, AGU, Washington DC., 10-14 Dec.

We study the temporal variations in the brightness of the Enceladus plume using Cassini ISS images that span
the longest baseline available … 12 years, nearly Cassini's entire time at Saturn. In previous work [1,2,3], we
found the brightness, and hence mass, of the plume is modulated with 3 periods -- diurnal, ~4-yrs and ~11-yrs –
all attributable to the cyclic variations in tensional stresses on the moon’s ice shell arising from its 2:1 orbital
resonant relationship with Dione. These open and close the surface fractures from which the plume material
erupts. The diurnal period arises from the cyclical stresses associated directly with the eccentricity of the
moon’s orbit; the observed phase lag in this component from the predicted response of a simple elastic ice-shell
to the induced stresses is 30º - 60º, or 2.8 hours – 5.5 hours. The 11-year period is the modulation of the moon’s
eccentricity arising from the libration of the 2:1 resonance; and the 4-year period is the eccentricity modulation
arising from circulation of the 2:1 corotation-eccentricity near-resonance [4].
Here, we report the presence of a fourth period of 2.9 years in the plume’s mass modulation, associated with
precession of the moon’s apsidal line, another cause of eccentricity variations. In total, Enceladus’ plume mass
variation consists of 1 short-period and 3 long-period components. We find that the phase lags of the bestsampled long-period components are comparable to that of the diurnal component.
For bodies with global oceans like Enceladus’, the tidal amplitude is expected to be fairly insensitive to the
forcing period (e.g., 5). But the constancy of the phase lag is a surprising result. Whether for a simple
(Maxwell) viscoelastic material or a more complicated solid rheology, we would expect a variation in phase lag
of at least a factor of 5 between diurnal and long-period responses, much larger than is observed. We interpret
this result as indicating that the delay in the eruptions in response to tidal stresses is likely not due to the
viscoelastic response of the ice shell. Instead it may be caused by a delay in the response of the eruptive
materials, either in the fractures leading from the ocean to the surface [e.g., 6] or in the ocean itself.

1. Nimmo, F., Porco, C., and Mitchell, C. (2014) Astron. J. 148, 46.
2. Nimmo, F. et al. (2016) AGU #P33A-2118.
3. Porco, C. et al. (2018) LPSC 49, #2083.
4. Vienne, A. and Duriez, L. (1995) Astron. Astrophys. 297, 588.
5. Wahr,J. et al. (2009) Icarus 200, 188.
6. Kite, E.S. and Rubin, A.M. (2016) Proc Natl Acad Sci USA 113, 3972.