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To identify real elements and, more importantly, figuring out if they may have ever hosted 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 of minerals with fine-grained textures. This is how stromatolites are found, layers of sediment with tiny domes and cones that can only come from mats of living microbes. Stromatolites on Earth provide some of the evidence for the earliest living things here; Perseverance scientists hope they will do the same on Mars.
The PIXL team leader, an astrobiologist and field geologist at the Jet Propulsion Laboratory named Abigail Allwood, has done this before. He used that technology in conjunction with high-resolution sediment imaging to find signs of the oldest known life on Earth in Australia, and to determine that similar sediments in Greenland they were not evidence of ancient life there. It is not easy to do in Greenland; it will be even more difficult 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 ultraviolet than ultraviolet. It’s ionizing radiation, just a color if you’re Kryptonian. X-rays cause different types of atoms to emit fluorescent light, characteristically. “We create the X-rays to bathe the rocks, and then we detect that signal to study elemental chemistry,” Allwood says. And PIXL and the arm also have a bright white flashlight on the end. “The lighting in the front began as a way to make the rocks easier to see, to link the chemistry to the visible textures, something that had not been done before on Mars,” says Allwood. The color was a bit irritating at first; heat and cold affected the bulbs. “Initially we tested the white LEDs, but with changes in temperature it did not produce the same shade of white,” he says. “So the guys from Denmark who provided us with the camera, they provided us with colored LEDs.” They were red, green, and blue, and ultraviolet. That color combination adds up to create a better, more consistent white light.
That combination could find Martian stromatolites. After locating potential targets, perhaps thanks to Mastcam-Z pans through the crater, the rover will glide up and extend its arm, and PIXL will begin to ping. The tiniest features, grains and veins, can tell if the rock is igneous or sedimentary, fused like a stew or layered like a sandwich. The colors of the layers on other features will give a clue to the age of each one. Ideally, the map of visible colors and textures will align with the invisible map of only numbers that the X-ray results generate. When the correct structures are lined up with the correct minerals, Allman can tell if he has Australian-type life signs or a Greenland-type bust. “What we’ve found to be really cool with PIXL is that it shows you things that you don’t see through chemistry,” says Allwood. “That would be the key.”
Allwood expects the tiny PIXL scans to produce great results: an inferred map of 6,000 individual points in the instrument’s postage stamp-sized field of view, with multiple spectral results for each. She calls this a “hyperspectral data cube.”
Of course, Perseverance has other cameras and instruments, other scanners looking for other hints of meaning in bits of rock and regolith. Along with PIXL there is a device that looks at rocks in a different way, firing a laser at them to vibrate their molecules, that is Raman spectroscopy. The data Perseverance collects will be hyperspectral, but also multi-faceted, almost philosophically. 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 terrestrial truth data, as one exoplanet researcher told me. Something behind that is X-rays and Raman spectroscopy, then moving cameras, then orbiting cameras. And of course all those things work together on Mars.
“Finding life on Mars will not mean ‘this or that instrument sees something.’ It will be, ‘All the instruments saw this, that and the other, and the performance makes life reasonable, “says Allwood. “There is no smoking gun. It’s a complicated tapestry. ”And like a good tapestry, the whole image only emerges from a warp and weft of color, carefully threaded.
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