reply to post by zorgon
So I am not sure why so many are having issues understanding this
The feeling is mutual. Here are the colors the Mars Express "sees":
Panchromatic (nm) 675±90 -Nadir, 2 stereo, 2 photometric
Near-IR (nm) 970±45
Red (nm) 750±20
Green (nm) 530±45
Blue (nm) 440±45
Here are the colors the Mars Rovers "see":
Empty slot (sharpest)
L1 750 nm (Near IR/ Red Stereo L)
L2 670 nm (Deep red - Geology)
L3 600 nm (CCD Pickup Red)
L4 530 nm (CCD Pickup Green)
L5 480 nm (CCD Pickup Blue)
L6 430 nm high pass (UV/ Blue Stereo L)
L7 440 nm + Solar Neutral Density
R1 430 nm high-pass (Near UV/ Blue Stereo R)
R2 750 nm (Near IR/ Red Stereo R)
R3 800 nm (Near IR - Geology)
R4 860 nm (IR - Geology)
R5 900 nm (IR - Geology)
R6 930 nm (Far IR - Geology)
R7 980 nm low pass (Far IR - Geology)
R8 880 nm + Solar Neutral Density
This is what the human eye sees:
Now which set of sensors most closely reproduces what the human eye sees? If you said "Mars Express," you would be correct. The color images the ESA
probe sends back are similar to the ones you would see on your television. These are not "true colors," they are images constructed by combining
black and white images taken at different wavelengths. Most digital imaging works this way.
The NASA probe is sensitive to "colors" the human eye cannot see and is "blind" in some areas that humans can see. The designers of the probe were
more interested in the regions we cannot see than the ones we can; they yield more important data for purely scientific purposes.
What color is the Martian sky? As you yourself have pointed out, Earth's sky is blue due to Rayleigh scattering, which scatters longer wavelengths
the larger the particles (or molecules). Knowing the exact composition of the Martian atmosphere, we can calculate the scattering using the equations
There is not enough coffee in the world for me to tackle those with a pocket
calculator on a Sunday morning to settle something on ATS, so let's look at this less rigorously.
The Earth's atmosphere is mostly nitrogen, which consists of two nitrogen atoms "stuck together." A nitrogen atom has a radius of about 65
picometers (pm), which means a single molecule of N2 would be about 260pm across. The Martian atmosphere is mostly carbon dioxide, CO2. A carbon atom
has a radius of 70 pm and an oxygen atom has a radius of 60 pm. (Here's the source
From some angles, the CO2 molecule would have the same width as an N2 molecule, whereas from others it would have about 30% more surface area exposed.
Based on this, I would suspect that the Martian atmosphere scatters a bit more red than the Earth's atmosphere.
Given that Mars' atmosphere is much thinner than Earth's, I would expect that it would be dark purple-blue overhead with a permanent reddish tinge
around the horizon where the light passes through more of the atmosphere. Think twilight in Los Angeles.
So why, then, do NASA's pictures look the way they do? As ArMap has explained, the process used in creating the photos is additive, There is some
light at every frequency. The blue filters used by the Rovers are at 430 nm and 480 nm. The human eye is most sensitive at 440 nm. The Rover is
"playing" F and G together, rather than F#. Consequently, all the different colors start to add together to make white, favoring red because the IR
filters tilt the sum in that direction.
So what is NASA "hiding?" Nothing. You can (and have) access all the raw images in their original format, which specifies which filter each one was
taken with. You can then combine any image with any other image, assigning color values at your own discretion. You can generate any number of false
color images in this way. The images we see in magazines and on the internet were created arbitrarily to approximate
what the human eye would
see. It is more of an art than a science, and NASA provides you with the materials to try it with. If you don't mind playing F and G together when
you need an F#.