Water plumes over Europa |
Water plumes over Europa |
Dec 12 2013, 04:55 PM
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#1
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Member Group: Members Posts: 401 Joined: 5-January 07 From: Manchester England Member No.: 1563 |
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.
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Dec 16 2013, 08:37 PM
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#2
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IMG to PNG GOD Group: Moderator Posts: 2254 Joined: 19-February 04 From: Near fire and ice Member No.: 38 |
I agree, looking closely this looks like a double exposure. In addition to being brighter on the left side it's also parallel to the horizon. The exposure, viewing geometry and lighting geometry also isn't particularly favorable for detecting plumes. Here is a schematic view showing the context for image 484888253 at 30 times Galileo's field of view:
The small gray box at center shows the camera's field of view. ...But it's interesting that it's only in this image with so short exposition (6.25 ms). In one way this image is 'special': It's the last one in the sequence of 400x400 pixel short exposure image. A 4 minute pause followed before the next sequence of images started (800x800 pixel images). I don't know the details of how Galileo's camera was operated so I don't know if this is of significance - doubt it though. |
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Dec 16 2013, 09:24 PM
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#3
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Senior Member Group: Members Posts: 3516 Joined: 4-November 05 From: North Wales Member No.: 542 |
I agree, looking closely this looks like a double exposure. I agree too. In fact I think individual features in the ghost image can be matched with features on the limb. The displacement between the two is approximately parallel to the top of the image. It's just a fainter copy of the main image shifted to the left. How that might arise I have no idea. |
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Dec 18 2013, 02:02 PM
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#4
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Junior Member Group: Members Posts: 78 Joined: 29-December 05 Member No.: 623 |
I agree too. In fact I think individual features in the ghost image can be matched with features on the limb. The displacement between the two is approximately parallel to the top of the image. It's just a fainter copy of the main image shifted to the left. How that might arise I have no idea. Indeed! Phillips, C.B., A.S. McEwen, G.V. Hoppa, S.A. Fagents, R. Greeley, J.E. Klemaszewski, R.T. Pappalardo, K.K. Klaasen, and H.H. Breneman. The search for current geologic activity on Europa. J. Geophys. Res., 105, 22,579-22,598, 2000. "An interesting side note is that one of the images taken in another imaging sequence on orbit E19 had what appeared upon initial inspection to be a limb haze just off the bright limb of Europa. frame s0484R88253 is shown in its raw, unprocessed (just contrast-enhanced) form in Figure 2a, and a cutout of just the limb, with a hard stretch, is shown in Figure 2b. The potential limb haze is visible in figures 2a and 2b as a bright feature paralleling the limb ~100 km above the surface, at a brightness level -7% of the average surface brightness. There was originally much guarded excitement when this image was received on the ground, but the fact that the "haze" brightness seemed to exactly parallel the limb brightness, and that the "haze" was not visible in immediately adjacent images, led the Galileo engineering team to search tor another possibility. The match between haze and limb brightness patterns suggests the possibility of a double image or "ghost image." This possibility is demonstrated in Figure 2c, which shows a simulated ghost image constructed by offsetting and adding a dimmer version of the actual image in 2b, shifted 16 pixels to the left. "Examination of the imaging sequence and the operation of the SSI camera itself reveals a likely cause. The image was taken in the Al8 camera mode, which has a fast frame time and thus does not reset the charge-coupled device (CCD) detector by performing a full light flood and erasure cycle in between exposures [Klaasen eta!., 1997]. This mode also has a reset of the shutter blades 0.2 seconds before the exposure begins. In all other imaging modes, the light flood and erasure take place between the shutter reset and the exposure, but since this particular mode has no light flood, this docs not occur. The location and brightness of the offset "ghost image" in frame s0484888253 are consistent with a small light leak equivalent to about 0.5 ms of exposure during the shutter reset stage of image aquisition, which occurred during a slew from the position of the previous image to this position. The direction and speed of the slew are consistent with the position of the "ghost image.'' The light leak would not be noticeable unless the many conditions or this image were met, namely, the platform slewed from one position to the next; the exposure time was short enough that the slight light leak was visible next to the full image; and the image contained a high-contrast feature (the limb) against which the ghost image is obvious. The last two frames of the first swath of plume search images (s04R4889846 and s0484889849) also show a ghost image of the limb that is consistent with the shutter reset light leak theory. Only four other images taken during the Galileo orbital mission have the characteristics necessary (camera mode, high-contrast boundary, short shutter time, platform slewing) to detect ghost images produced during the shutter reset; of these, ghost artifacts consistent with this theory were detected in three of them." |
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