For identifying actual elements—and, more importantly, figuring out if they might have once harbored life—you need even more colors. Some of those colors are even more invisible. That’s where x-ray spectroscopy comes in.
Specifically, the team running one of the sensors on Perseverance’s arm—the Planetary Instrument for X-ray Lithochemistry, or PIXL—is looking to combine the elemental recipe for minerals with fine-grained textures. That’s how you find stromatolites, sediment layers with teeny tiny domes and cones that can only come from mats of living microbes. Stromatolites on Earth provide some of the evidence of the earliest living things here; Perseverance’s scientists hope they’ll do the same on Mars.
The PIXL team’s leader, an astrobiologist and field geologist at the Jet Propulsion Laboratory named Abigail Allwood, has done this before. She used that technology in conjunction with high-resolution pictures of sediments to find signs of the earliest known life on Earth in Australia—and to determine that similar sediments in Greenland weren’t evidence of ancient life there. It’s not easy to do in Greenland; it’ll be even tougher on Mars.
X-rays are part of the same electromagnetic spectrum as the light that humans see, but at a much lower wavelength—even more ultra than ultraviolet. It’s ionizing radiation, only a color if you’re Kryptonian. X-rays cause different kinds of atoms to fluoresce, to give off light, in characteristic ways. “We create the x-rays to bathe the rocks in, and then detect that signal to study the elemental chemistry,” Allwood says. And PIXL and the arm also have a bright-white flashlight on the end. “The illumination on the front started out as just a way of making the rocks easier to see, to tie the chemistry to visible textures, which hasn’t been done before on Mars,” Allwood says. The color was a little vexing at first; heat and cold affected the bulbs. “We initially tried white LEDs, but with temperature changes it wasn’t producing the same shade of white,” she says. “So the guys in Denmark who supplied us with the camera, they provided us with colored LEDs.” Those were red, green, and blue—and ultraviolet. That combination of colors added together to make a better and more consistent white light.
That combination might be able to find Martian stromatolites. After locating likely targets—perhaps thanks to Mastcam-Z pans across the crater—the rover will sidle up and extend its arm, and PIXL will start pinging. The tiniest features, grains and veins, can say whether the rock is igneous or sedimentary, melted together like stew or layered like a sandwich. Colors of layers on top of other features will give a clue about the age of each. Ideally, the map of visible colors and textures will line up with the invisible, numbers-only map that the x-ray results generate. When the right structures line up with the right minerals, Allman can tell whether she’s got Australia-type life signs or a Greenland-type bust. “What we’ve found that’s really interesting with PIXL is that it shows you stuff you don’t see, through the chemistry,” Allwood says. “That would be the key.”
Allwood is hoping PIXL’s tiny scans will yield huge results—an inferred map of 6,000 individual points on the instrument’s postage stamp-sized field of view, with multiple spectral results for each. She calls this a “hyperspectral datacube.”
Of course, Perseverance has other cameras and instruments, other scanners looking for other hints of meaning in bits of rock and regolith. Adjacent to PIXL is a device that looks at rocks a whole other way, shooting a laser at them to vibrate their molecules—that’s Raman spectroscopy. The data Perseverance collects will be hyperspectral, but also multifaceted—almost philosophically so. That’s what happens when you send a robot to another planet. A human mission or rocks sent home via sample return would produce the best, ground truth data, as one exoplanet researcher told me. Somewhat behind that are x-ray and Raman spectroscopy, then rover cameras, then orbiter cameras. And of course all those things are working together on Mars.
“Finding life on Mars will not be, ‘Such and such an instrument sees something.’ It’ll be, ‘All the instruments saw this, that, and the other thing, and the interpretation makes life reasonable,” Allwood says. “There’s no smoking gun. It’s a complicated tapestry.” And like a good tapestry, the full image only emerges from a warp and weft of color, carefully threaded together.
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