The Color of Mars

author: Holger Isenberg, [email protected]
, http://mars news.de

created: March 1999
last update: 14.June 2000

The Red Planet, this name has to appear in every article of the main-stream press on Mars. The same importance play the little green men in contributions of renowned daily German papers such as Frankfurter Allgemeine Zeitung or Sueddeutsche Zeitung published on the Mars meteorite ALH84001.

Without doubt, the planet appears red with the naked eye in the night sky, whereby our earth viewed from Mars, would appear clearly blue, due to the 70% water coverage in connection with refraction of light in the atmosphere. With a reverse relation of the water land distribution however, rather a brown-green planet would be to be seen from space.

The color of the atmosphere, caused by Rayleigh Scattering^[4] at gas molecules, determines thus only in very small amount the color of a planet as seen from space and also directly on the surface!

Why then should the Mars sky, as NASA/JPL PR-department spreads it, should appear red? This coloring is justified if at all, only with the refraction of light at atmospheric dust. But such masses of dust in the atmosphere do not prevail over years on a planet, which has large water-clouds, fog and ground frost, since water would wash these away after short time.

Astronomers at the Hubble Spacetelescope and amateur-astronomers^[8] are observing, since long time now, white water-clouds and blueish atmosphere.

Pic. A, Viking 1, Nr. 12b069, 29. August 1976, 12.65 locale Mars time
This picture was created with color-correction derived from the filter response data. (click on picture to view it in original size)
All Viking and Pathfinder images courtesy of JPL/NASA/Caltech.
original data without correction

Indeed, when the first color picture from Viking 1 was received on Earth, the Mars soil was red-brown and the sky was blue, a landscape comparable with the desert of Arizona^[3] (fig. A and B).

These are original pictures^[5] of the two probes, which are only slightly color corrected to match the filter response values of the camera system. However, the original data without correction, you see on the right of each image, has almost no detectable difference in color.

The Viking cameras operated according to the principle of a color scanner, whereby for different light wave lengths different sensors with separate data channels are used.

To create a colorful picture from this scanner data, a color calibration table is necessary. These tables can be seen near the mast of the parabolic antenna in Pic. A and B and show among grayscales the three basic colours (RGB) of a color monitor.

Already with the naked eye it can be detected there that the colours are correctly shown. Also in Pic.E this can be acknowledged, since the white ground frost (water ice!) supplies a natural color calibration to the white alignment.

Pic. B, Viking 2, Nr. 22a158, 25. September 1976, 11.96 locale Mars time
This picture was created with color-correction derived from the filter response data.
original data without correction

Pic.C shows a typical (NASA-)red Mars picture in the color, taken 18 months later. How does it come to this color change, although the sharply bordered shadows and the otherwise clear colours suggest no atmosphere dust?

The solution of the mystery appears, when using an image processing program: By rising the color-values of blue and green about 50% and 25% one gets to Pic.D, which shows the well known true coloring from Pic.A and B.

Pic. C, Viking 1, 12h016,
11 February 1978, 15.56

Pic. D
Blue amplified by 50%, green around 25%

Pic. E, Viking 2, 21i093,
18.May 1979, 14.24

With the same method^[1] we can get true color picture from Mars Pathfinder (Pic. F and G). Note, the sharpness of the Pathfinder images is by far not that good as 20 years ago on Viking as during the Pathfinder mission an information-reducing picture compression algorithm (comparable to JPEG) was used.

Pic. F, Pathfinder,
August1997, source:^[6]
Pic. G,
Blue amplified by approx. 50%,
green by approx. 25%

Pic. H, 12e018
03.Jul 1977, 15:20
Pic. I, 12b166
6.Oct 1976, 7:48 Temporary, the surface illumination is really red, caused by dust-storms, darkening the sky. The image on the left was taken shortly after or during such a storm and the diffuse light with almost no shadows is visible. In contrast to this, the image on the right, shows sharp shadows and clear blue sky, the normal condition on Mars.
On the image-data of the Viking- and Pathfinder-Missions, this diffuse illumination is a very rare condition and not the normal state, as NASA seems to publish it with their dull-red pictures.
(click on images for unfiltered original data)

And now the "official" true color view of the Pathfinder landing site published by NASA:

Pic. J, Source: http://mars.jpl.nasa.gov/MPF/science/PDS/pds-jun99.html

Original Caption Released with Image:
The true color of Mars based upon three filters with the sky set to a luminance of 60. The color of the Pathfinder landing site is yellowish brown with only subtle variations. These colors are identical to the measured colors of the Viking landing sites reported by Huck et al. [1977]. This image was taken near local noon on Sol 10. A description of the techniques used to generate this color image from IMP data can be found in Maki et al., 1999. Note: a calibrated output device is required accurately reproduce the correct colors.

I don't know why you should calibrate your output device to view this funny bad-colored picture.

Literature

[1]

Pic. H, 12e018 03.Jul 1977, 15:20	Pic. I, 12b166 6.Oct 1976, 7:48	Temporary, the surface illumination is really red, caused by dust-storms, darkening the sky. The image on the left was taken shortly after or during such a storm and the diffuse light with almost no shadows is visible. In contrast to this, the image on the right, shows sharp shadows and clear blue sky, the normal condition on Mars. On the image-data of the Viking- and Pathfinder-Missions, this diffuse illumination is a very rare condition and not the normal state, as NASA seems to publish it with their dull-red pictures.
(click on images for unfiltered original data)