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30th Anniversary Voyager 2 at Europa, a hint of things to come
volcanopele
post Jul 10 2009, 02:35 AM
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Coolness!

I uploaded a new post to my Io blog about this anniversary, and yes, I do talk about Europa with out saying anything derogatory:

http://gishbar.blogspot.com/


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The Gish Bar Times - A Blog all about Jupiter's Moon Io
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machi
post Nov 10 2009, 12:17 PM
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Second mosaic taken by Voyager 2. Its in lower resolution (2,23 km/pix) and monochrome, but it has slightly better coverage.
Attached thumbnail(s)
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tedstryk
post Mar 12 2010, 02:10 PM
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I posted a blog entry with my version of this mosaic.

Here are the two variants of the image, one with the color balance mixed between the actual data and an OGV overlay, the second weighing more heavily on the OGV overlay.

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Bjorn Jonsson
post May 20 2013, 11:17 PM
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Available color data for Europa is so severely limited that reprocessing the Voyager data is still rewarding after all these years. Here is my version of the Voyager 2 color mosaic from 240,000 km. It's a mosaic of 5 color frames, each of which is composed from orange, blue and violet images:

Attached Image


This is actually the *only* hi-res Voyager color observation of Europa (that is, if 2.4 km/pixel qualifies as "hi-res"). The second highest resolution Voyager color observation is this one by Voyager 2 from 1.2 million km - note the color difference between the dark terrain at left and the dark terrain at right:

Attached Image


Both of these images are shown slightly bigger than their original size to compensate for the slight loss of resolution due to resampling in some of the image processing steps. They have also been sharpened a bit for the same reason.

These images highlight the really poor color coverage of Europa (in particular, both Voyagers obtained better color coverage of both Ganymede and Callisto). Galileo obtained a 1.4 km/pixel IR756-GR-VI mosaic during its E14 orbit showing largely the same terrain as the Voyager mosaic. There are several versions of the E14 mosaic in circulation; the best known one is probably Ted's version. The Galileo E14 data is almost global but has some gaps. There is also a partial 1.6 km/pixel color image from G1 and a very noisy 1.4 km/pixel color image from E12. In addition there is near-global coverage at much lower resolutions (by far the best one is the 7 km/pixel global G2 image). In addition there are some high resolution color samples but these are extremely limited in coverage.

Some notes on the color processing in the mosaic above:

I used the calibrated and geometrically corrected Voyager images available at the PDS Rings Node as source data. The flatfielding in these images is exquisite; this is particularly important when mosaicking images showing rough terrain near the terminator. The only drawback is that in my opinion the reseau removal could be better in some cases - sometimes an excessive number of pixels gets smoothed but this is easy to fix in Photoshop once the color channels have been aligned.

Voyager 2 was imaging Europa using the orange, blue, violet and ultraviolet filters. At first glance this may not seem ideal since there are no green filter images but things are not that simple. The effective wavelength of the Voyager filters is OR=589 nm, GR=564, BL=475, VI=402 and UV=332. One important thing to note is that there are only 25 nm between the OR and GR filters (and lots of overlap.) This makes it rather difficult to create synthetic red from OR and GR by extrapolating - the results are usually bad (it should be noted though that the OR and GR filters are pretty different even though 25 nm isn't a big number). I usually get much better red images by using OR and BL (or even VI when BL isn't available). Luckily, Voyager 2 acquired test images of Jupiter using all of the filters at the start of its approach phase and these can be used for checking various filter combinations. C1838155 is the first one of these images. I used these as 'ground truth' when testing various ways of making synthetic images (red etc.) since I have a pretty good idea of what Jupiter's color should look like. Once I had acceptable color of Jupiter from OBV images I used identical color processing for Europa. I'm fairly happy with the resulting color - in particular I get very similar color when processing the Galileo E14 stuff even though the filters used are different. I also experimented with interpolating the data to get the entire visible spectrum and then converting it to sRGB. The results were similar (slightly stronger green color component though).

For the interested, the synthetic colors were created using these formulas:
R = 1.526315789 x OR - 0.52631579 x BL
G = 1.01 x (0.719298246 x OR + 0.280701754 x BL)
B = 1.07 x (0.726027397 x BL + 0.273972603 x VI)
This results in fairly realistic images of Jupiter with whitish zones etc. - the 1.01 and 1.07 values are there mainly to make the zones more whitish. The 'target wavelengths' here are R=649 nm (the Cassini red filter), G=557 nm (the Galileo green filter) and B=455 nm (the Cassini blue filter). The Cassini green filter is 569 nm which is too close to yellow for my taste. This approach might work fairly well for the entire Voyager 2 Jupiter data since the source data is calibrated but I need to check that more carefully. In particular, Io is a likely exception because of its weird spectrum.

The original data had some gaps but they aren't big and in all cases two colors are available, making it possible to fill the gaps using synthetic data. The location of the missing data can be seen in this ancient version that was released back in 1979:

http://photojournal.jpl.nasa.gov/catalog/PIA00366
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tedstryk
post May 21 2013, 02:29 AM
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Beautiful work! This is the same dataset as I used in the previous post in this thread.


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Bjorn Jonsson
post May 21 2013, 11:35 PM
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Yes, the same dataset with one exception: I didn't use any of the wide angle images for color, I only used narrow angle data. The drawback of this is the absence of green filter data but as mentioned above I don't think that's a big issue. But it's not always easy to know exactly how accurate the resulting color is.

One 'crazy' color processing idea I have is to somehow use Europa's entire visible spectrum together with the color information from spacecraft images to compute an entire synthetic, visible spectrum for each point in an image and use this to make synthetic R/G/B. I'm not sure exactly how I would do this but this would replace the linear interpolation that is usually used, both when mixing the color channels as I did above and also when the entire visible spectrum is created from (usually) three filters and then converted to sRGB. I doubt this will work very well though but I'm interested in trying it. One obvious complication is that the leading and trailing hemispheres have slightly different spectra and obviously the spectrum of a dark feature is different from the bright terrain. And there is no spectral information available in visible light that resolves these small (relative to Europa itself) features and also has high spectral resolution.
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tedstryk
post May 22 2013, 01:08 AM
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I focused mostly on stacking images to get the best feature clarity possible and, as a last minute "oh, that would be nice" figured I should have some color. I must say I am impressed...that's the most accurate color I've ever seen for Europa from Voyager filters.


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machi
post May 24 2013, 12:18 PM
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Excellent images!


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JRehling
post May 25 2013, 08:12 PM
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Bjorn, I share your crazy dream. I think Europa's entire surface can be represented fairly well as a pointwise blend of three or four surface components. Then the subtasks would be:

1) Determine the Hapke parameters of each of those components, based on areas where the surface is a relatively pure representative of that component.
2) Determine for as much of the surface as possible, what the local coefficients are that weight how much the various components contribute.

Once this process came along, it would probably be possible to make some pretty good estimates of (2) based even on single-filter images.

And then one could render realistic color images of much of the surface of Europa even where no such images have been captured.
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