Red dust paints Mars in ruddy hues, from the surface to the skies. But NASA's Perseverance rover has spotted swaths of another color among the rusty shades. At nearly every site the robotic geologist visits, the Martian palette includes purple.
The color forms a thin, smooth coating on some stones, and it leaves paint-like blobs on others. Still other rocks look as if they've been partially frosted in magenta icing, says Ann Ollila, a geochemist at Los Alamos National Laboratory who presented an early analysis of the coatings at a recent conference of the American Geophysical Union (AGU).
The color touches rocks of all different shapes and sizes—even tiny pebbles haven't escaped the pops of purple. But how, exactly, did these coatings form? "I don't really have a good answer for you," Ollila says.
Scientists are eager to know more. "There's a lot to look forward to as we continue to do analyses," says Nina Lanza, the team lead for Space and Planetary Exploration at Los Alamos National Laboratory, who has been studying the coatings alongside Ollila.
The origin of the mysterious splotches could help reveal clues about Mars's past, including whether it might have hosted ancient life. As the coatings formed, they could have encoded information about the surrounding conditions in their chemical and mineral makeup, helping scientists reconstruct environments now long gone. They also could hold more direct evidence of life: On Earth, microbes help craft many similar stony veneers.
Studying these types of crusts can also help scientists better understand how other worlds operate. "How universal are geologic processes, and how do they change planet to planet?" wonders Cassandra Marnocha, an environmental microbiologist at Niagara University in New York.
Painted in purple
The purple Martian coatings were found in Jezero crater, a 28-mile-wide pockmark blasted out by a meteorite impact billions of years ago that once hosted an ancient lake. Perseverance landed in the crater in February 2021 and has since been roaming across it. At almost every stop along the rover's route, flashes of purple have popped up in its images.
While rock coatings are not a new discovery on Mars, Jezero's frequent purple flecks have left scientists scratching their heads.
"These particular purple patches we haven't really seen with past rover missions," says Bradley Garczynski of Purdue University, who also presented an analysis of the coatings at the recent AGU conference. Garczynski is studying the coatings using imagery from a pair of cameras known as Mastcam-Z, which are "essentially the scientific eyes of the rover," he says. By capturing images using a variety of filters that block certain wavelengths of light, scientists can get a rough idea of the rock composition.
Ollila and her colleagues are taking an even closer look at the coatings using the rover’s SuperCam, which can shoot a laser at a rock to vaporize a small bit of material and suss out its elemental composition. Each laser shot also carves a tiny pit in the surface and makes a snap-like sound. A microphone on the rover picks up the noise, allowing the scientists to hear when the laser cuts through the coating and into the rock below. These sounds also reveal clues to some of the rock’s properties, such as its hardness.
Early results from these analyses show that the purple color seems to be a layer that is softer and chemically distinct from the rock below. Mastcam-Z images hint the coatings may contain types of iron oxide, Garczynski says. And SuperCam analyses suggest they are enriched in hydrogen and occasionally magnesium, Ollila says.
The presence of hydrogen suggests that water played a role in the formation of the purple patches. Iron oxide also points to water, akin to the rust that forms on a bike left in the rain. Further study could unlock a trove of information about the red planet's wet past—including how long water remained in Jezero crater and, perhaps, the chemistry of the lake itself. "The presence of coatings could be a key part of that story," Garczynski says.
However, the location of the purple patches presents a bit of a mystery. Perseverance's current route doesn't cut through lake sediments, but rather rocks that formed from cooling magma. How the rocks arrived at their current location on the crater floor—and when and how water came into contact—remains uncertain. "If I were to guess at where you would find coatings in this whole package of materials in Jezero, it wouldn't probably be here," Lanza says.
So far the team has only analyzed a few samples, and they are still contending with many challenges. SuperCam's different chemistry readings and the changes in the laser's sounds don't always seem to line up, Ollila notes. Separating the chemical signatures of the coatings, the underlying rock, and the ever-present dust on the Martian surface is complex. And Mars's strong winds limit when scientists can hear the laser's snaps.
"Mars doesn't make it easy for us," Lanza says.
On Earth, such veneers are often tied to life, which means the Martian rock coatings could be a major boon for astrobiologists.
A rock's nooks and crannies can create a tiny safe haven for microbes in harsh environments, Marnocha explains, providing nutrients, a shield from the sun, or moisture in otherwise dry landscapes. Some of these microbes help craft coatings by metabolizing metals scoured from the rock surface or dissolved in water. On Mars, coatings might even help preserve evidence of ancient microbes long after their death by preventing Mars's intense solar radiation from degrading delicate organic structures.
The first hints of rock coatings on Mars were spotted during the Viking missions, which landed on the red planet in the mid-1970s. But it's taken many more wheels on the ground to identify the dark splotches as coatings rather than stains on the surface, Marnocha says.
Of particular excitement was the discovery of dark manganese-rich coatings in Gale crater, where the Curiosity rover is currently scrutinizing the Martian surface. The find is tantalizingly reminiscent of a particular type of rock coating on Earth known as varnish, which tends to be rife with tiny lifeforms. In a recent survey of varnishes across the United States, scientists found "the who's who of known radiation-resistant bacteria," says Chris Yeager, an environmental microbiologist at Los Alamos National Laboratory.
Yeager and his colleagues found that one particular type of cyanobacteria seems to be key to the varnish's manganese content, concentrating the metal to protect against damaging solar radiation, kind of like sunscreen.
The newfound coatings in Jezero don't have the required manganese to be considered a varnish, but that doesn't mean they couldn't be associated with ancient microbial life, Lanza says. "Who knows what Martian microbes do?"
The team hopes to further untangle the chemistry of the coatings and search for organic matter associated with the Martian crusts, which could hint at the presence of microbes. But getting the rocks back to Earth for lab analysis is one of the few ways to definitively pin down how the purple patches formed.
Perseverance has been drilling samples of Mars rocks as it traverses the crater, sealing them in tubes that will be cached on Mars's surface for a future mission to return to Earth. While the purple coatings are fragile, Ollila hopes that some can withstand the sampling process so scientists can take a closer look in the future.
In the meantime, the team is excited to continue their work as Perseverance treks toward Jezero’s delta, a sprawling fan of sediments deposited by ancient rivers flowing into the crater. "We're still really early days," Lanza says. "This is only one type of material that we're likely to encounter, and I think that there's a lot of discovery ahead of us."