Seeing in a different light.

In my last post I talked about the fun I had making huge mosaics of Ganymede and showed off one that I made of Ganymede’s anti-jovian hemisphere taken by Voyager 2. Today I’m going show you the side of Ganymede that permanently faces Jupiter, taken by Voyager 1 when it passed through the Jupiter system in January 1979, six months before its sister ship arrived there.

So, without further ado, here it is – the Subjovian hemisphere of Ganymede, again mosaicked by me in 1999 (click the image to see the full-size mosaic in all its glory):
VGR1 Ganymede OBV subjovian

This mosaic shows Perrine Regio (the dark blocks of terrain at top left) and Barnard and Nicholson Regiones (the dark areas in the central part of the hemisphere). The bright ray crater Tros is visible in the bright terrain of Phrygia Sulcus, between Perrine and Barnard Regiones. You can also make out the “polar caps” of Ganymede, visible as the paler terrain at the north and south poles (at top and bottom of the image). If you keep going west around the satellite from Perrine Regio, you’ll come to the eastern edge of Galileo Regio, which was the large area of dark terrain seen at the top right of the Voyager 2 image in my previous post.

This mosaic was made using the same filters as the Voyager 2 mosaic, but if you look closely you’ll see that the top-left part of Ganymede looks blurry. This is actually due to the motion of the spacecraft as it was taking the pictures. Voyager was moving pretty fast as it travelled through the Jupiter system (I’m not sure what its exactly velocity was at the time, but right now Voyager 1 is travelling at about 17 km/s relative to the sun!), and that had to be compensated for when taking pictures by rotating the camera on its scan platform or by rotating the whole spacecraft.

Another issue is that the solar illumination drops as the inverse square of the distance from the sun. This means that if you go twice as far from the sun as the Earth then the illumination there will drop to a quarter (1/4) of what it is as the Earth’s distance (conversely, if you go towards the sun to half the Earth’s distance then the sun will appear four times brighter). At Jupiter’s distance from the sun – just over five times further from the sun than Earth – the sunlight is about 27 times dimmer than at Earth. I don’t think this would actually be all that noticeable to the human eye, and it’d still probably be sufficient to blind us if we looked directly at the sun without any protection, but the dimmer illumination does make a difference for cameras, requiring longer exposures – these particular images were taken with 360 second (6 minute) exposures.

Despite the fast-moving camera (and spacecraft) and a long exposure times, most of the images taken by the Voyagers actually came out really sharp, which is a testimony to the skill of the people who planned the images and engineered the spacecraft – but there were still a few cases (like this one) when things didn’t quite turn out according to plan and the compensation wasn’t enough, and that’s why some of the images are blurry. Unfortunately there’s no way to fix this after the fact.

What about other filter combinations though? One simple trick that I tried was to simply take the Orange mosaic and duplicate that (changing the brightness and contrast to more closely match the Blue mosaic) and put the modified Orange mosaic in the green channel of the image instead of the blurry blue one. The result is shown below:
VGR1 Ganymede OOV subjovian

While this does sharpen the mosaic (since we’ve got rid of the blurry parts of the image), this is purely for aesthetic purposes though – it’s not accurate or useful for scientific purposes at all (you might also notice that the overall colour of Ganymede appears more yellow-tinged than in the OBV mosaic).

What about true colour? You might recall from my previous post that Voyager’s cameras don’t have a Red filter, so the closest that we can get to true colour is an Orange/Green/Blue combination. It so happens that Voyager 1 did take some Green images in this sequence, but unfortunately they don’t cover the whole hemisphere. But at least we can see something close to what Ganymede might look like to human eyes, and that’s shown in the next mosaic:
VGR1 Ganymede OGB subjovian

We’re interested in the central part of the image – going from top to bottom – which is covered by Orange, Green, and Blue filters. The rest of the image looks tinted magenta and dark-green because the Green filter images don’t cover those areas, so we’re just seeing those parts of Ganymede through the red and blue channels of the image. The colour difference is quite dramatic – in the OBV mosaics Ganymede looks a lot browner in colour, whereas in the OGB mosaic it looks more greyish/beige colour. But that’s closer to the true colour of Ganymede that we would see with our own eyes if we were there.

It occurs to me that I might be able to write a program to interpolate between the filters and give a more-accurate-but-still-simulated “true colour” view – so as an example I could take an existing Green image and Violet image, and linearly interpolate a Blue filter image (since Blue is about 43% of the way between Violet and Green in the spectrum). Of course, it wouldn’t be accurate because the way that the surface reflects blue wavelengths probably isn’t on a nice straight line between Violet and Green, but it’d be better than just eyeballing and fiddling with an image like I did in the mosaic where I modified the Orange to replace the Blue. Hrm…

All of this goes to show that one has to be careful when looking at colour images taken using different filters, because they are false-colour – not what one would see with the naked eye. That said, there’s still obviously a lot of very useful science that can be done with these images (including subtracting one filter from the other, taking ratios etc) which can tell us a lot about the nature of the surface material that we’re looking at, which I might discuss in a later post. But for me, nothing’s quite as satisfying as being able to see something in as close to true colour as possible, because that’s the closest thing to being there and seeing it with my own eyes – which is essentially why I got into planetary science and astronomy in the first place!

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