On a summer morning on Mars, NASA's Curiosity rover woke in its usual crater, opened its shutter, and snapped a sprawling view carpeted in blues and grays.
The panorama subverts the usual perception of the Red Planet, a reputation that dates to ancient civilizations. Romans called it Mars, the god of war, because its color reminded them of blood. Egyptians called it Her Desher — "the red one." Even people today associate Mars with rust: dry, desolate, and caked with iron minerals that oxidize red.
Curiosity captured this scene on Aug. 11, around 8:30 a.m. local time with its Mastcam instrument. A pale custard sky hangs above slate-colored terrain.
What makes the image strange is what the U.S. space agency didn't do. The team almost always adjusts the colors in Mars photos before releasing them. This time, it skipped that step and kept the dim morning light as the camera recorded it. In a news release, NASA called it "an artistic choice."
Which brings up a surprisingly complex question. No human has ever stood on Mars, so what would the planet actually look like to someone who did? The answer depends on several factors: the weaker sunlight to its farther distance, the dust suspended in the air, the minerals on the ground, the camera's sensor, the editing software, and, not to be underestimated, the eye of the beholder.
When mission teams tweak the photos, scientists say the goal is to make the rocks look as they would in sunlight on Earth. Martian dust tints everything the sun touches, so removing its influence shows the rocks' intrinsic colors.
In photography, that correction is called white balancing. The phone in your pocket already does it, shifting every picture so the scene looks lit by ordinary daylight.
"The surface of Mars is covered with pink-red dust that disguises the surface geology," Mike Malin, principal investigator for Curiosity's Mastcam instrument, told Mashable. "That dust is also suspended in the atmosphere, and sunlight passes through it to illuminate the surface. If you have ever been near a forest fire or something like that, the coloring of the sunlight is not unlike the coloring of the sky on Mars most of the time."
Correcting for that haze gives geologists a stable reference point, whether it's a dusty or clear day, said Mark Lemmon, a co-investigator on the Perseverance rover's Mastcam-Z, who wasn't involved in processing this particular image. It also lets scientists compare the terrain to Mars-like places on Earth where they have done much of their fieldwork. Without it, everything takes on the same dusty cast, and the color differences that reveal detail fade away. When a global dust storm engulfed the planet in 2018, the uncorrected view turned "virtually monochrome," he said.
So is the editing tricking us? In a sense, yes. But the unedited image can't claim to show the real Mars, either. True color, meaning what a person would actually see, is hard to pin down. The planet orbits about 1 1/2 times farther from the sun than Earth, so it gets less than half as much sunlight, Malin said. A visitor standing beside Curiosity that morning probably would peer into a murky landscape and see distant cliffs of light tan. The shadows would reveal little color or detail.
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But the visitor's own eyes would complicate things further because they don't hold a fixed setting. Given time, a person's brain would start doing what software does.
"If you spend time in a very colored environment, the brain starts to white balance the scene," Lemmon told Mashable. "So there is also something human in the white balancing."
Is the camera tricking us, then? Mostly not. Lemmon traces the blue to the ground itself. Rocks scoured clean of dust and patches of wind-driven sand look gray to blue-gray, and against the surrounding reds, oranges, and yellows, they tend to look especially blue, he said.
But the camera can deepen the effect, Malin said. A quarter of the sensor's light-detecting elements are designed to catch blue light, yet Martian sunlight carries very little of it. When the team calibrates the camera to compensate for that weak signal, the boost can overshoot and give gray rocks and dark shadows an extra blue tint. Even a well-calibrated camera records Mars in its dust-tinted light. Images that stop at calibration come out looking orange, he said, as the Viking landers' did in the 1970s.
For research, both scientists say they prefer the corrected view. Neither could recall a case where the raw image actually revealed something the balanced one hid. But Malin cautioned that it's possible to go too far. Some people process images with harsh contrast that turns the colors "garish," and that can distract other scientists trying to interpret the rocks.
Inspecting rocks is what Curiosity has done since 2014, slowly climbing Mount Sharp, a three-mile-high mountain in Gale Crater stacked in layers, each one recording a different chapter of Martian history. Those layers tell of a world that once held lakes and streams, then dried out billions of years ago and left salty minerals behind. In August, the rover passed one kilometer — about 0.6 miles — of elevation gain, the highest any rover has climbed on Mars.
The cliffs on the panorama's horizon are Curiosity's next major target. Scientists call them yardangs — ridges the wind carves over long stretches of time that form in sedimentary rock of a specific grain size, Malin said. They stretch about 10 miles across the mountain's northwest flank, and they puzzle the mission team.
"The yardang layer looks out of place," said Ashwin Vasavada, Curiosity's project scientist at NASA's Jet Propulsion Laboratory, in a statement, adding that its color is distinct from its surroundings, and the layer almost looks plastered on. One possibility is that ancient volcanoes blanketed the area in ash, but there are other hypotheses.
Color processing does somewhat shape how scientists read a feature like this, Lemmon said, and mostly for the better. But the yardang layer's prominence doesn't depend on editing.
"The yardang unit stands out in images from orbit, too," he told Mashable.
Scientists hope Curiosity will reach the base of the yardangs sometime in 2027, and the rover's robotic arm can then gather close-up evidence of what they are.
In the meantime, the panorama gives the public rare insight into whether the Red Planet lives up to its name. People tend to picture Mars redder than it is, Lemmon said: "It is frequently much more butterscotch."
