since my early contributions to Celestia, I was always concerned with trying
to get things as close as possible to reality;-)...The true color
of Mars is one such issue that has puzzled me for quite a while.
After all that extensive texture work, I don't want it to be too far off...
So I did some fairly thorough research on this, and now I believe that I found
the right answer, at last! Thanks also to Jack Higgins, who pointed me
onto the right track a few days ago!
Here comes my little story:
For the Mars oppositions between 1995 and 2003, we have continuous,
high quality tricolor (RGB) imaging from HST's wide field camera. In view of the excellent color rendition of other planets by that HST camera over the last decade, I was always tempted to believe Hubble's reddish Mars color. Notably, since the earlier Mars missions were mainly restricted to two channel color imaging!
As an illustration of the great 'red color consistency', I have started off to
assemble a composite image of the official HST images from
Mars' oppositions in 1995, 1997 and 1999, below. I have only adjusted
slightly the brightness of these images to demonstrate clearly
that throughout these years, the HST Mars colors are perfectly
consistent and definitely redish orange.
So far so good, but here is the puzzle:
An amazing and systematic change of Mars' HST color rendition
happened somewhen around 2000, as you can see further below from the
excellent HST images of 2001 and this year's great opposition!!
Mars has now been definitely 'declared' yellowish brown!
What happended? Have a look first:
1995 - 1999 ==> only red Mars oppositions! Note also the clear and
persistent blue hue of the atmosphere....
The point is that the Mars Pathfinder mission has
apparently provided most credible new spectroscopic information both
of the sky color and that of the Mars surface!. This
new information was processed and published around 1999 and meant the
end of the red Mars;-)...
The most important scientific papers on this controversial matter are
these two that I have consulted also:
J.N. Maki et al., 'The Color of Mars: Spectrophotometric measurements
at the Pathfinder landing site', J Geophys Res 104, 8781 (1999)
N. Thomas et al., J Geophys Res 104, 8795 (1999)
The "Imager for Mars Pathfinder" (IMP) is a stereo imaging system that, in
its fully deployed configuration, stands 1.8 meters above the Martian
surface, and has color capability provided by 24 selectable filters --
twelve filters per 'eye'. Its red, green, and blue filters were used
to take the panorama at the top of my second image below.
The three color images were first digitally balanced according to the
transmittance capabilities of a specific high-definition TV device at
JPL, and then enhanced via changes to saturation and intensity while
retaining the hue. A threshold was applied to avoid changes to the
sky.
Maki et al., (1999) found that the IMP was stable during the mission
and were therefore able to confidently calculate the true color of the
Martian sky and surface by relying on the absolute calibration of the
IMP. Specifically, the authors categorically assert that,
notwithstanding the fact that Mars has historically been referred to
as the "Red Planet", the true color of the surface matches that of the
sky - yellowish brown, with differences attributed to brightness
levels. Moreover, although the surface color is not time dependent,
the sky color becomes red in the anti-Sun direction and is reddest at
noon. Blue, just enough to be scientifically measurable and only
present in enhanced imagery, becomes apparent only toward the Sun at
sunrise and sunset.
Moreover, Thomas et al., (1999) agreed with Maki et al., (1999) that
the reddening of the Martian sky, unlike Earth's blue sky color, is
highly variable and directly related to the angular separation from
the Sun.
So have a look at my next composite image, that again demonstrates the
overall consistency in the new 'official' True Mars color after
Pathfinder!. Again the blue hue of the atmosphere is clearly
visible.
Altogether, I am now convinced that the color below is the closest
approximation todate of the true color of Mars!;-):
Finally, a most revealing comparison of the sky color measurements from
Pathfinder with Celestia's present colors. There is definitely still some fine
tuning to be done, although the brown-bluish sky at sunset is very nice
already!
Bye Fridger
and being on vacation 