Blue Skies Over Gusev Crater

Spirit catches some beautiful weather on Mars

    These images are some excellent examples of the true color of the sky on Mars.  We are often shown muddy orange or pink skies, when we already have the approximate color calibration figures that we need to assemble realistic color images.  Here are the raw images from the NASA/JPL site, along with the mixture ratios that I used and why.  Judge for yourself when you see these images.

Let's start with the raw data.  These images are from Spirit, Sol 187. Spirit- Panoramic Camera- Sol 187 Spirit- Panoramic Camera- Sol 187 Spirit- Panoramic Camera- Sol 187
The left image is from L4, the 601 nm filter.  The center image is from L5, the 535 nm filter.  The right image is from L7, the 440 nm filter.    These correspond closely to red, green, and blue in our visual system.  Here the frames are converted to color.
I assembled the first picture using the bright illumination standards- ratios of 100:80:60 for the red, green, and blue components.  This is the resulting image.    I chose to try these values as they correspond to the white calibration values, and the sky is typically a bright object.  However, there are no true white images in the field, so this could be wrong.    Click the image for a full sized version.
The second image was assembled using the "ground" color references ratios - 100:60:30 for red, green and blue.  Take note of the fact that this image has half the blue that the upper image has, and the red are far more intense, yet the sky is still blue, and not red.    So even though this is calibrated for ground colors with no bright or white images, the sky colors are still very Earth-like, as if at the top of a high mountain.    This image is far more satisfying as being realistic to me.  As someone who has seen the look of the sky at great altitudes, this convinces me more readily of the presence of thin atmosphere.    Click the image for a full sized version.

   In conclusion, there is no reasonable set of ratios that can produce a red or orange sky in these images.  Any such image is no more than a fiction and should not be believed.  The only times Martian skies will be pink is when there is a significant amount of dust in the air.  But also take note of this important fact- when such dust levels occur, liquid water is far more likely to be present because the dust levels that can color the skies also add a few millibars of air pressure and also shield the ground from some of the direct sunlight, greatly reducing the likelihood of evaporation.

   In other words, dust storms actually cause more liquid water to be present for longer periods of time on Mars.  However, they can also reduce the amount of sunlight reaching the ground, leading to the formation of ice when liquid water does emerge.  Here is an excellent article on Martian air pressure.