What color would Mars be if you were standing on it?
I have been following the photos taken on Mars by rovers and of Saturn from the Cassini satellite, and they are gorgeous and quite bright and colorful. However, if we were to send a manned mission to Mars, Jupiter or Saturn, would an astronaut looking out the window see these objects just as vibrantly? Is the difference in available light because of the increased distance from the sun negligible to the naked eye, or is the clarity of these photos not quite what we would see with our own eyes?
Contrast among colors, brightness of the surface, and the fogginess of the atmosphere may appear slightly different to an average human on Mars (or in the vicinity of Cassini) than the reported "true color" images. This is because the human eye is distinct from rover and satellite sensors in three key aspects:
(1) The human eye consists of "rods" and "cones" with very different operating principles from those of the charge coupled device (CCD) supported by color filter wheels on the Rover Panoramic Camera (Pancam) and on Cassini.
(2) The human eye responds differently under different lighting conditions, since rods and cones operate optimally under different conditions. Rods dominate under very low light (scotopic conditions) such as moonlight while cones dominate under bright light (photopic conditions) such as sunlight. Rods respond to a range of shorter wavelengths, and, since there is only one type, they do not yield color information like the three types of cones responding optimally to blue, green, and red. In addition, because rods are rare at the focal point of the eye (fovea), scotopic vision is often blurry. Pancam and Cassini sensors do not vary in this manner.
(3) The brain processes data from the retina in a very complex manner, and the gradations of color can be person-specific. In contrast, the Pancam data are processed using much more simplified standardized definitions.
These definitions yield a rendition of colors very close to what a human would see under appropriate lighting conditions. Therefore, as long as there is sufficient light on the surface of Mars, the Pancam "true color" images are quite likely to approximate the hue and saturation (think of it as vibrancy) a human on the surface would see with the naked eye. However, human explorers would most likely use visors for protection from ultraviolate light, which may render the naked eye comparison inappropriate.
So is the sun sufficiently bright on Mars for photopic conditions to be a valid assumption? The answer is yes. Mars is about 1.5 times more distant from the Sun than Earth, so the solar energy per unit area at Mars would be about 48% that on Earth, ignoring atmospheric effects. The fogginess of the Martian atmosphere, mostly due to dust, can reduce the brightness further.
In general, as long as the rover images are made during mid-day on Mars, the vibrancy of true color Pancam images would be very close to what the naked eye would see on Mars. More exotic images, such as the famous sunset are specifically marked as "false color." But even then, some of the color information would be similar to naked eye vision.
Things are slightly less tricky with Cassini images, since images are not taken inside an atmosphere. Once again, the actual astronaut would be using visors, which may render any discussion of naked eye vision meaningless. But assuming we could look at Saturn without protective gear, the key concern would again be, do photopic conditions exist? Surprisingly perhaps, the answer is still, yes! Saturn in about 9.5 times more distant from the Sun than Earth, so solar brightness is roughly 1% of that at Earth orbit. But as discussed above, this is comparable to the level of sunlight on a moderately cloudy day, enabling humans to have photopic vision at Saturn. So the vibrancy of the Saturnian colors in Cassini images is also pretty realistic.
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