Martian Color Calibration with Mastcam-Z

"Are we alone? We came here to look for signs of life, and to collect samples of Mars for study on Earth.
 To those who follow, we wish a safe journey and the joy of discovery" 
- Inscription on the Perseverance rover

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I've been writing Astronomical Returns for more than two years now, and I still have visions of grandeur that one day I'll go viral, just like The Everyday Astronaut on YouTube or @BocaChicaGal on Twitter. Of course, the secret to success on social media is good photography, and the secret to good photography is, you guessed it... good lighting! My sister certainly has this skill down, her insanely popular food and dog pages owe their following to the great lengths she goes through to get the best lighting for her pictures: waking up at the crack of dawn for the perfect sunrise shot, expensive flash stands for a subtle white glow, and hours and hours of photo editing

Now ever since Perseverance landed on Mars last month, we've all been treated to a barrage of breathtaking HD photos from the Red Planet. Just like on Earth, lighting on Mars varies greatly depending on the time of day, the season of the year, and the presence of dust or other atmospheric phenomenon. But unfortunately for Percy, or any other robotic photographer, the rover doesn't have the luxury of a human eye on-site, which means there's no way to adjust for the constantly changing lighting conditions. All the rover can do is point and shoot, and leave post-processing to the scientists receiving the data back on Earth. So how do we solve this problem?

First full-color image from Perseverance, taking February 19, 2021

The solution is to bring along a little calibration target, a physical color palette whose brightness and hue is pre-measured on Earth before launch. Then whenever Perseverance's two Mastcam-Z scientific cameras snap an image of the Red Planet, we can also tell it to take a picture of the calibration targets. By comparing the live image of the calibration target against the predetermined standards, the Perseverance camera team will know exactly how to edit the images to reflect the actual lighting conditions on Mars!

Perseverance carries two calibration targets in different positions on the rover so that at least one of them isn't blocked by shadow. The primary target is a funny looking contraption: just 4 inches wide and weighing only 118 grams, it consists of 4 concentric grayscale rings that are used primarily to determine proper exposure. Outside of those rings, there are 8 colored dots used to calibrate image coloring, and in the center there's a small post called a gnomon that functions like a sundial to intentionally cast a shadow on the calibration target. Finally, the calibration target has heavy duty samarium-cobalt magnets mounted underneath it to repel Martian dust that would cover it up and render it useless

"Two Worlds, One Beginning" - A closer look at the calibration target in an actual image from Mars

The concept of carrying a color calibration target is not new; every mission NASA has sent to the Martian surface carried one, and even the manned Apollo missions brought along some calibration targets as they were exploring and photographing the lunar geology. One image from Apollo 17 was particularly memorable when the astronauts discovered surprisingly orange soil after digging just a few inches beneath the dull gray surface - you can see the color calibration target in the back left, which corroborates that the soil was indeed orange and not just the result of weird lighting or poor photo editing. Of course, the astronauts' own eyes were proof enough, as lunar module pilot and geologist Harrison Schmitt excitedly declared to mission control, "There is orange soil! It's all over! Orange!!"

As imaged by Apollo 17. The orange soil was the result of ancient volcanic processes when the Moon was still geologically active

More recently, when China's Chang'e 4 became the first spacecraft to land on the far side of the moon in 2019, it caught a lot of people by surprise when the images it returned depicted the surface as a deep reddish-orange. Has the far side always been so colorful, and we've just never seen it up close and personal? No, the real explanation is that Chang'e 4 didn't include a color calibration target, since the primary objective of the Yutu-2 rover it was carrying was navigation and exploration, not scientific photography. Getting the color exactly right wasn't a priority, so the Chinese simply released the unedited photo. By subsequently adjusting the RGB balance to weight green and blue more heavily, space fans online provided their best approximation of what the true color on the far side of the moon would have looked like

Raw photo on the left, as released by the Chinese space agency, vs approximate true color on the right

To learn more, check out the Planetary Society's detailed article on Perseverance's Mastcam-Z calibration targets here!




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