Voyager and Galileo Images of Ganymede, The Ganymede images and mosaics thread Options

[Bjorn Jonsson]

I've been processing some of the high resolution Galileo Ganymede images recently. As far as I know the two mosaics below have not appeared at the official websites (at least not in this form) so in a sense they are 'new'.

The first one was obtained during the G1 flyby in 1996. It covers a part of Memphis Facula which is centered at roughly 15°N, 132°W. The images were obtained at a distance of approximately 5000 km from Ganymede's center.


The second one was obtained during the G28 flyby in May 2000. It is centered near 14.5°S, 319.7°W. The images making up the mosaic were obtained at a distance of roughly 4500 km from Ganymede's center.


I will probably post more Ganymede mosaics later this month or next month.

[Bjorn Jonsson]

Awesome mosaics. The huge global mosaic is probably a bit distorted though but is a great overview of Ganymede's anti-Jupiter hemisphere. It's easy to see from these mosaics that the Voyager 2 Ganymede coverage is really good and the data quality is much higher than in the Voyager 1 data set (in particular there are no smeared images). The only 'missing' thing is really hi-res images - Voyager 2 got no closer to Ganymede than about 60,000 km, corresponding to a resolution of 600 m/pixel.

Here is a new 4000x3800 pixel color mosaic of a large area around Osiris from Voyager 2 images:



North is up and the color should be fairly accurate - possibly a bit too reddish and saturated but Ganymede's color from Voyager images is a complicated subject and I'm still working on it. This mosaic is composed of 21 clear filter images obtained by Voyager 2 at a range of 91,000 to 118,000 km (resolution 0.9 to 1.2 km/pixel). Its northern half has been colorized using a three frame color mosaic obtained at a range of a little over 300,000 km (resolution 3 km/pixel). The southern half (starting from just south of Osiris) has been colorized using a single wide angle color composite obtained when Voyager 2 was ~75,000 km from Ganymede (resolution ~5.6 km/pixel). Some of the lower resolution color coverage is included in the mosaic outside of the area covered by the higher-res clear filter images. I mosaicked everything in simple cylindrical projection and also did all of the color processing there and then rendered the resulting map using the viewing geometry Voyager 2 had when it obtained its best image of Osiris (image C2063759).

Ganymede exhibits big albedo differences and this can complicate the image processing. I took great care not to overexpose/saturate any parts of the brightest terrain (Osiris in particular). This preserves every detail but makes the terminator area a bit dark.

A huge variety of interesting terrain can be seen in the mosaic. With the benefit of hindsight I can safely say that there are clear hints of the extreme roughness of some terrain types covered by some of the higher resolution Galileo images but there is also terrain that looks smooth in the Voyager mosaic. I look forward to seeing Osiris in hi-res JUICE images and also the bright smooth-looking terrain around the bright crater SSW of Osiris.

I'm going to end this with yet another version of a well known global Voyager 2 image. This version should be of slightly higher quality than my earlier versions of it.