08 January 2015

True Colors on Saturn's Icy Moons

One of the most frequently asked questions regarding planetary images is: "Are those the 'natural' colors?"  That's not so easy to answer.  Most imaging cameras, whether old style vidicon TV tubes or the more common CCD instruments have rather different light sensitivities than the human eye.  Thus its not so easy to replicate the apparent brightness we would see.  These cameras also tend to use color filters that don't line up very well with our R-G-B color sensitivity.  Nonetheless we can make an attempt to see what some of these bodies might look like to space travelers.  

Case in point, Cassini at Saturn.  In my previous post I showed some of the 'super' color maps that I recently released of Saturn's icy moons.  These are maps compiled from images acquired in IR, green, and UV filters.  They really bring out the color contrast between geologic materials, especially recently exposed materials like crater and fracture walls, which tend to have stronger UV signatures reflecting larger grains sizes.

Natural Color    -   DIONE   -   Super Color 
Natural Color - ENCELADUS - Super Color 
In this post I show some of the 'natural' versus 'super' color images that Cassini acquired.  These are not at very high resolution, however, mostly in the 1-5 kilometer range.  This is because when Cassini was close to these objects it was moving too fast to acquire the large number of images required to run through all the color filters, so it chose the minimum to accomplish its scientific tasks in the short time available, mostly the IR-Gr-UV sequences (~930 to 570 to 340 nanometers).  This gives the best geologic information.  These sequences were further away and Cassini acquired the full filter sequences.  It turns out that the centers of the R, G, and B filters (~700 to 400 nanometers) on Cassini are fairly close to optimal when attempting to simulate the human view of these bodies.

'Natural Color'

'Super Color'

300-meter-resolution color images of Enceladus, centered on longitude 180°W.

Most of the Saturnian icy satellites can be described as having a grayish tone, with a slight reddish or greenish cast.  This is because the strongest reflections from these surfaces tend to peak in the green-to-red portion of the spectrum.  Color variations on the surface would tend to be rather bland to our eye, though we would likely pick out the stronger features as subtle hue contrasts.  We would likely see the cliffs of Enceladus as pale bluish in tone, not unlike some terrestrial glaciers.  I will try to compile some additional shots in the coming weeks.  Enjoy!

No comments: