Enceladus Plume Search, Nov. 27 Options

[jmknapp]

Interesting item in the science plan kernel (S16) just released to the NAIF website:

OBSERVATION_ID: S1629

SEQUENCE: S16

OBSERVATION_TITLE: Plume Search

SCIENCE_OBJECTIVE: Hope to detect/observe plumes, whether from volcanic activity or geysers.

OBS_DESCRIPTION: Point and stare.

SUBSYSTEM: ISS

PRIMARY_POINTING: ISS_NAC to Enceladus (0.0,5.0,0.0 deg. offset)

REQUEST_ID: ISS_018EN_PLUMES001_PRIME

REQUEST_TITLE: ENCELADUS Geyser/Plume Search

REQ_DESCRIPTION: 1;ENCELADUS Geyser/Plume Search 1x1xNPp -- 3 different exposures

BEGIN_TIME: 2005 NOV 27 19:00:00 UTC

END_TIME: 2005 NOV 27 20:00:00 UTC

[edstrick]

Various comments and responses:
Looking at jmknapp's side-by-side posting of images from 144000 and 174000 km, there are quite strong differences in plume apperances. I really doubt that the plumes are "chugging"--varying rapidly with time over an hour or whatever, so I expect that the effect is parallax. Since multiple observation were taken over a moderate range of viewing azimuths, if the plumes were in fact time-invarient, it will be possible to do computed tomographic reconstructions of their 3-D structure. Not perfect, since it would be "limited angle tomography", but extremely useful.

Regarding flyby's optimized for gravity measurements, once the pole is in winter night, imagery will be relatively useless, unless the plumes continue to vent into sunlight well the shadowed surface. Thermal imaging would still be useful, but nighttime passes would be most useful to "divert" to gravity measurements. Gravity mapping from flyby's is best done with passes at different latitudes and different longitudes, so (besides searching for gravitational anomalies) they can measure the triaxial ellipsoid shape of the gravity field to compare with the triaxial ellipsoid shape of the surface, which tells you a *LOT* about concentration of mass toward the center: Core vs No-Core, etc. Magnetic data from such flybys also tells you about interactions with Saturn's mag field, which at Jupiter revealed electically conductive, presumably fluid, layers inside moons.

Regarding scattered light problems, while I'd seen fogging and some blotching beyond the limb of earlier moon images, I never saw anything that really looked plume-like, and never got pointed toward specific comparison images. What really pushed my buttons in the January images were that the features progressively increased in contrast, narrowed, and sharpened toward the limb, and there were no plumelike features at all on the terminator side of the overexposed and saturated crescent, just the usual trace of camera-fog.

Anyway, I'm not claiming credit.. that goes to the team, paricularly for a very nicely designed imaging sequence that covered all bases and seems to have provided far better information on the venting than can be extracted from the previous images. I'll be very interested in color results and phase angle dependent photometry. Does the December sequence go to higher phase angles?.. the graphic suggests it may..... the results might be even more spectacular if so.

jmknap: If the venting is from exposed ice, the hottest ice will "retreat" relative to colder ice. Everythign I see in the images of Enceladus suggests an extremely high thermal gradient below the surface. We can arm-wavingly-imagine venting pits retreating down into the surface, forming volcano-like or dry gyser-like pipes, with hot sublimating ice tens to hundreds of meters below the local surface.

Marz: The E-ring is very very faint, except at very high phase angles, where foreward scattering of sunlight by the fine dust-like ice grains makes it relatively bright. This scattering is probably mostly by diffraction or "Mie scattering" (which makes colorless and sometimes colored aureoles around the sun in our sky), and probably has only weak internally-scattered contributions that yield composition data. Certainly, there will be attempts as the mission proceds to get VIMS spectra of the plumes as close to the sources as possible. Cross fingers that they see anything beyond water ice.

I don't know the last decade's post-Voyager science on the E-ring, much less any of Cassini's beyond press release data. One model of the ring based on earthbased and Voyager photometry (and maybe polarimitry?) data stated that the ring had a narrow size range of particles and appeared to consist of more or less spherical water "droplets": flash frozen ice-sphere grains, rather than snow-like or crushed ice grains. This was one thing that suggested long before Cassini got there that Enceladus might actually be venting, rather than that we were seeing something like a torus of ejected ice from a recent impact or something.

[The Messenger]

What really pushed my buttons in the January images were that the features progressively increased in contrast, narrowed, and sharpened toward the limb, and there were no plumelike features at all on the terminator side of the overexposed and saturated crescent, just the usual trace of camera-fog.

Anyway, I'm not claiming credit.. that goes to the team, paricularly for a very nicely designed imaging sequence that covered all bases and seems to have provided far better information on the venting than can be extracted from the previous images. 

None-the-less, it was a gutzy call at a time when everyone seems to be stuck scratching their heads...including Bruce.