Water plumes over Europa Options

[marsbug]

This seems like the relevant place to post this (could be wrong): Water plumes from Europa? Apologies if it's already been up. The link to the Science article at the bottom doesn't work for me, does anyone have a working link to the original? Cheers.

[Phil Stooke]

Not so fast! I was summarizing. The version of that image printed in the reference shows the plume area, much bigger than a reseau removal artifact (and we can see where the reseaux are, it's not there) - I'm not saying the Voyager observations are correct, but they are not disproven by this quick look.


Phil

[ollopa]

Bob Pappalardo and an assembled cast of the usual suspects addressed this directly in JGR in 1999. "Does Europa have a subsurface ocean?" (Circa Page 31):

our analysis shows that this is a false double-exposure rather than a real phenomenon, probably resulting from incomplete closing of the camera shutter blades in combination with an imaging mode that does not include a preexposure erasure of the CCD array.

I'm not saying the Voyager observations are correct, but they are not disproven by this quick look.


Phil

[vexgizmo]

Bob Pappalardo and an assembled cast of the usual suspects addressed this directly in JGR in 1999. "Does Europa have a subsurface ocean?" (Circa Page 31):

our analysis shows that this is a false double-exposure rather than a real phenomenon, probably resulting from incomplete closing of the camera shutter blades in combination with an imaging mode that does not include a preexposure erasure of the CCD array.

That was in response to the Galileo double-exposure false-plume. There are other statements in that paper about the Cook et al "plume":

The best Voyager evidence for the presence of anomalously fresh frost on Europa's surface is from the controversial, 143° crescent "plume" image of Cook et al. [1982, 1983] (Figure 17). In addition to showing an off-limb feature that was inter¬preted as a volcanic event [Cook et al., 1982, 1983], the image shows a conspicuous bright spot on the surface at 34°, 337°. Helfenstein and Cook [1984] measured the photometric con¬trast of the bright feature relative to surrounding terrains and compared it with that measured of the same geographic region viewed at 13° phase. They found that, relative to the surrounding features, the brightness of the anomalous spot increased as the phase angle increased from 13° to 143° phase by more than seven standard deviations above the average surface change. Although Helfenstein and Cook [1984] interpreted this brightness change to be due to active emplacement of surface materials, a more conservative interpretation would be that the feature represents relatively transparent frost that was deposited on Eu¬ropa's surface in recent geological history [cf. Verbiscer et al., 1990; Verbiscer and Veverka, 1990; Verbiscer and Helfenstein, 1998]. McEwen [1986a] suspected that this bright region was not actually anomalous compared to other bright regions on Europa which had not been seen at high phase angles by Voyager. Indeed, in low-phase global-scale Galileo images, this area appears similar to other bright plains regions.

Figure 17. Unprocessed Voyager 2 clear filter image (FDS 20767.37) at a resolution of ~44 km/pxl and phase angle of 143°, argued by Cook et al. [1982, 1983] to show an active plume along Europa's bright limb. Stretched inset image shows detail of the bright limb and putative plume, and an unusually bright area on the surface. The "plume" feature, with a signal level of just 5 DN, is not observed in subsequent images. Its location in the corner of the Voyager vidicon image, where noise and distortion are most severe, suggests that it is a camera artifact.

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