Sounds like a trick question, doesn’t it? Sort of like “Who is buried in Grant’s Tomb?” And yet the answer keeps confounding a lot of smart people. (DISCOVER even published a whole feature on the question.) Now the issue keeps coming up again in the latest images from the Curiosity rover. Blue skies on Mars? Can that be right? Which of these images shows what Mars really looks like?
The very first Viking images from the surface of Mars in July, 1976 showed blue skies, largely because that’s what people were expecting and so that is how the imaging experts initially set the color balance. They quickly realized their error and reissued the image with tangerine skies.
The Martian atmosphere is fully of orange tinted dust that dominates the color of the sky, it turns out. Changing the color balance to reflect that unexpected reality was a fairly straightforward adjustment, and yet even today images of the same scene on Mars often seem to take on different tones. Sometimes the landscape looks ruddy and rusty, sometimes tangerine, sometimes butterscotch, sometimes a drab brownish yellow.
Different magazines and web sites make their own color adjustments, of course, and there are plenty of amateurs out there doing their own corrections and distortions. But NASA itself still seems confused at times about what Mars really looks like. Consider three images of Mars from space. The first two are from the Hubble Space Telescope. The third is from the Mars Global Surveyor. You’ll notice that they don’t match up very well, even though Mars is very much the same planet and two of the images even come from the exact same telescope.
The short explanation is that color balance is a highly subjective thing. No two spacecraft use the exact same filters; even two instruments on the same telescope can use different filters and channels. Once the data come in from the digital detectors, then the imaging specialists process the data to make the most meaningful kind of picture. Sometimes they highlight what seems most scientifically significant. Sometimes they try to simulate what the human eye would see–what most people mean when they ask, “What does it really look like?” But even then, the color receptors of the human eye do not match up exactly with any of our instruments, so creating a color balance that resembles human vision is itself something of a subjective art.
And the problem of authenticity gets worse, because not all people perceive color the exact same way. You probably do not even perceive color exactly the same way in your two eyes. (Stare at a monochrome wall or a blue sky. Cover one eye, then the other. Is it exactly the same?) Perhaps the biggest challenge of all: The way the eye registers color changes depending on the amount of light. That is why deep-space photos of nebulas have all those amazing pinks and purples that you never see in the sky. You could look at the Orion Nebula through the best telescope in the world and you would only see gray-green. The human eye just cannot pick up the extremely faint reds.
Keep that in mind when looking at images from deep space. In fact, the nature of true color is a problem for any kind of unfamiliar imagery. You know exactly what to expect from local landscapes. But did the photographer bump up the greens in the Amazon rainforest, or make a sunset more colorful? Are Emma Stone’s eyes really that color? The confusions and the lies are as old as photography itself, and digital imaging only makes the manipulations easier and the subjectivity more evident.
At least there is a solid scientific motivation for NASA’s latest color manipulations. The white-balanced color adjustment that turns the sky blue in the latest Curiosity panoramas from Mars’s Gale Crater are designed to make the surface rocks look the way they would under normal terrestrial illumination. That makes life easier for planetary geologists who are used to looking at rock formation on Earth, and so simplifies the process of understanding the exotic setting where Curiosity is exploring.
And it is an exotic setting. The current location, called Yellowknife Bay, appears to be an ancient river or lakebed. It contains sedimentary rocks. It has a benign chemistry that would have been favorable to life, if it ever took hold there. It contains many of the elements used by microorganisms on Earth. From here, Curiosity will slowly make its way up Mount Sharp, testing chemicals and looking around as it goes. If you want to know whether Mars ever could have supported life–or might still today–this will be our best shot at getting an answer.
But if you want to know what color Mars really is, my advice to you is to wait until it comes into view later this year, take a look up, and decide for yourself.
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