Mysterious purple coating found on Mars rocks

The red dust paints Mars in red hues, from the surface to the sky. But NASA’s Perseverance rover spotted bands of another color among the rusty hues. At nearly every site visited by the robotic geologist, the Martian Palace includes purple.

The color forms a thin, smooth layer on some stones and leaves paint-like drops on others. Other rocks appear to have been partially frosted in a magenta glaze, says anne ollila, a Los Alamos National Laboratory geochemist who presented a first analysis of the coatings at a recent American Geophysical Union (AGU) conference.

The color hits rocks of all different shapes and sizes – even tiny pebbles haven’t escaped the splashes of purple. But how, exactly, did these coatings form? “I don’t really have a good answer for you,” says Ollila.

Scientists are eager to learn more. “There’s a lot to look forward to as we continue to do analysis,” says Nina Lanza, the team leader for space and planetary exploration at Los Alamos National Laboratory, who studied the coatings alongside Ollila.

The origin of the mysterious spots could help reveal clues to Mars’ past, including whether it could have harbored ancient life. As the coatings formed, they could have encoded information about surrounding conditions into their chemical and mineral makeup, helping scientists reconstruct environments long gone. They could also contain more direct evidence of life: on Earth, microbes help make many similar stone veneers.

Studying these types of crusts can also help scientists better understand how other worlds work. “How universal are geological processes and how do they change from planet to planet?” wonders Cassandra Marnocha, an environmental microbiologist at Niagara University in New York.

painted purple

The purple Martian coatings have been found in Jezero Crater, a 28-mile-wide pockmark destroyed by a meteorite impact billions of years ago that once housed an ancient lake. Perseverance landed in the crater in February 2021 and has been roaming through it ever since. At nearly every stop along the rover’s route, purple flashes appeared in its footage.

While rock coverings aren’t a new discovery on Mars, Jezero’s frequent purple spots have left scientists scratching their heads.

“These particular purple spots that we haven’t really seen with past rover missions,” says Bradley Garczynski from Purdue University, which also presented an analysis of the coatings at the recent AGU conference. Garczynski studies coatings using images from a pair of cameras called Mastcam Z, which are “essentially the scientific eyes of the rover,” he says. By capturing images using various filters that block certain wavelengths of light, scientists can get a rough idea of ​​the composition of the rock.

Ollila and his colleagues take a closer look at the coatings using the rover’s SuperCam, who can fire a laser at a rock to vaporize a small piece of material and determine its elemental composition. Each laser shot also digs a small pit in the surface and produces a snapping sound. A microphone on the rover picks up the noise, allowing scientists to hear when the laser cuts through the liner 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 analyzes show that the purple color appears to be a softer, chemically distinct layer of the rock below. Mastcam-Z images suggest the coatings may contain types of iron oxide, Garczynski says. And SuperCam analyzes suggest they are enriched with hydrogen and sometimes magnesium, says Ollila.

The presence of hydrogen suggests that water played a role in the formation of the purple spots. Iron oxide also points to water, similar to the rust that forms on a bike left out in the rain. Further study could unlock a wealth of information about the Red Planet’s wet past, including how long water has been in Jezero Crater and, perhaps, the chemistry of the lake itself. “The presence of coatings could be a key part of this story,” says Garczynski.

However, the location of the purple spots presents a bit of a mystery. The current course of Perseverance does not cross lake sediments, but rather rocks which formed from cooling magma. How the rocks got to their current location on the crater floor – and when and how the water made contact – remains unclear. “If I had to guess where to find coatings in all this material set in Jezero, it probably wouldn’t be here,” says Lanza.

So far, the team has only analyzed a few samples and still faces many challenges. SuperCam’s different chemical readings and changes in laser sounds don’t always seem to line up, Ollila notes. Separating the chemical signatures from the coatings, the underlying rock, and the ubiquitous dust on the Martian surface is complex. And Mars’ strong winds limit when scientists can hear the laser clicks.

“Mars doesn’t make it easy for us,” says Lanza.

Microbial sunscreen

On Earth, these veneers are often linked to life, which means Martian rock coverings could be a major boon for astrobiologists.

The nooks and crannies of a rock can create a tiny haven for microbes in harsh environments, Marnocha says, providing nutrients, a shield from the sun or moisture in otherwise dry landscapes. Some of these microbes help make coatings by metabolizing metals mined from the surface of rock or dissolved in water. On Mars, coatings could even help preserve evidence of ancient microbes long after they died by preventing intense solar radiation from Mars from degrading delicate organic structures.

The first hints of rock coverings on Mars were spotted during the viking missions, which landed on the Red Planet in the mid-1970s. But it took a lot more ground wheels to identify the dark spots as coatings rather than spots on the surface, Marnocha says.

Of particular excitement was the discovery of manganese-rich dark coatings in Gale Crater, where the Curiosity rover is currently scanning the Martian surface. The discovery is tantalizingly reminiscent of a particular type of rock covering on Earth known as varnish, which tends to be filled with tiny life forms. In a recent survey of nail polishes across the United States, scientists uncovered “the who’s who of known radiation-resistant bacteria,” says Chris Yeager, environmental microbiologist at Los Alamos National Laboratory.

Yeager and his colleagues found that a particular type of cyanobacteria appears to be key to the manganese content of varnish, concentrate the metal to protect against the sun’s harmful rays, much like sunscreen.

The new coatings found at Jezero lack the manganese required to be considered varnish, but that doesn’t mean they can’t be associated with ancient microbial life, Lanza says. “Who knows what the Martian microbes are doing?

The team hopes to further unravel the chemistry of the coatings and search for organic matter associated with Martian crusts, which could hint at the presence of microbes. But bringing the rocks back to Earth for lab analysis is one of the few ways to definitively determine how the purple spots formed.

Perseverance has drilled samples of Martian rock as it passes through the crater, sealing them in tubes that will be cached on the surface of Mars for a future mission back to Earth. Although the purple coatings are fragile, Ollila hopes some can withstand the sampling process so scientists can examine them more closely in the future.

In the meantime, the team is excited to continue their work as Perseverance heads into the Jezero Delta, a sprawling fan of sediment deposited by ancient rivers flowing into the crater. “We’re just at the very beginning,” says Lanza. “It’s just one type of material that we’re likely to encounter, and I think there’s a lot of discoveries ahead of us.”

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