Uranus System Imaging Options

[machi]

I finished my first good image of Uranus.
Planet is colorized from three filtered images (orange, green, blue).
Slightly brownish color of rings is entirely artificial.

Attached thumbnail(s)

 

[Bjorn Jonsson]

Shortly after finishing the flyby anniversary mosaic of Neptune I started thinking about Uranus, got an interesting idea, started processing the Voyager 2 Uranus images and came up with this (two versions of the same image):



I think these may be the most detailed Voyager 2 global images of Uranus I have ever seen (but they are far inferior to recent groundbased images).

The processing idea came from the description of the processing of these images of Uranus:

http://www.planetary.org/blogs/emily-lakda...eck-photos.html

In short, I used 6 orange and 6 green filtered images that I reprojected to simple cylindrical projection, removing (or at least greatly reducing) the effects of global illumination on the fly. This was sufficient to correct for Uranus' rotation but since the images were obtained over a period of ~5 hours this was not sufficient - it's necessary to correct for Uranus' fast east/west zonal winds as well. To do this I found a profile (and I think it's a very recent and up to date one) showing Uranus' zonal wind speed as a function of latitude. I used this to correct the individual cylindrical maps by shifting each image row in the east-west direction. I then stacked the 12 maps, rendered an image and then greatly increased the contrast and visibility of various features. In the left image I first enhanced the contrast and then sharpened the image with an unsharp mask. In the right one I first enhanced the contrast, then suppressed large scale brightness variations with a high pass filter and then further increased the contrast.

The source images were obtained on January 17, 1986 between 14:19:10 and 19:35:10. At this time Voyager 2 was ~9 million km from Uranus.

This is experimental work in progress and it is probably possible to get better results. A day earlier, a longer sequence of images was obtained but it took a longer time; I strongly suspect that there is a limit to how long the time can be if you want good stacking results but I don't know what this limit is. One interesting processing artifact that may occur if the zonal wind profile I'm using is inaccurate is that bright or dark features get too elongated in the east/west direction but I don't think this is a significant issue here. Interestingly, even if this happen the feature's longitudinal position can be accurately estimated as the average of the west and east edges of the feature, as pointed out in a very interesting article by Sromovsky et al. in Icarus 203 (1): 265–286 (2009). EDIT: It's actually in Fry et al., Astronomical Journal vol. 143, no. 6, June 2012 where this is pointed out (the Sromovsky et al. article is very interesting too though).

There is considerable noise in the above images but it should be fairly obvious which features are real and which ones are noise and/or image processing artifacts. All of the obvious bright spots are definitely real and all of the narrow bright/dark east-west cloud belts (there are several of these) should be real as well. The dark, "partial cloud belts" near the bright spots are real as well. Another faint "partial belt" to the right of the dark polar cap may be real but I'm not sure. Some of these features are completely new to me.

With the benefit of hindsight, it is apparent that the best way for Voyager 2 to image Uranus might have been to obtain a few big sequences of orange-only images (orange images show the biggest amount of detail) in as short a time interval as possible, maybe using the tape recorder to speed things up due to the low bit rate possible from Uranus. This would have made the stacking results better. But of course there was absolutely no way to know this back in 1986.

I think the images above might hint at what additional details Voyager 2 could have seen had it been able to carry a Galileo/Cassini-style imaging system with near infrared filters. And they may hint at what a future spacecraft might see if it arrives when the illumination geometry is comparable to what it was in 1986.

Uranus' is *not* an uninteresting planet, it's simply more difficult to observe than the other giant planets due to low contrast and hazes. In my opinion it's at least as interesting as e.g. Neptune - and significantly different from it.