To identify the elements – and, more importantly, determining whether they may have once harbored life – you need even more color. Some of these colors are even more invisible. This is 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 – seeks to combine the elemental recipe of minerals with fine-grained textures. This is how you find stromatolites, 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 very first living things here; Scientists at Perseverance hope they will do the same on Mars.
The leader of the PIXL team, an astrobiologist and field geologist at the Jet Propulsion Laboratory named Abigail Allwood, has already done it. She used this technology in conjunction with high resolution images of sediment to find signs of first known life on Earth in Australia – and to determine that similar sediments in Greenland 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 light that humans see, but at a much lower wavelength – even more ultra than ultraviolet. It is ionizing radiation, a color only if you are a Kryptonian. X-rays cause different types of atoms to fluoresce, emitting light, characteristically. “We create the x-rays to bathe rocks, and then detect that signal to study elemental chemistry,” explains Allwood. And PIXL and the arm also have a bright white flashlight on the end. “The lighting on the front started out as a simple way to make rocks easier to see, to tie chemistry to visible textures, which hasn’t been done before on Mars,” says Allwood. The color was a bit annoying at first; heat and cold affected the blisters. “We tried white LEDs first, but with the changes in temperature it didn’t produce the same shade of white,” she says. “So the guys in Denmark who provided us with the camera, they provided us with colored LEDs.” These were red, green and blue – and ultraviolet. This combination of colors added together to create a better and more consistent white light.
This combination could make it possible to find Martian stromatolites. After locating probable targets – perhaps thanks to Mastcam-Z’s pans across the crater – the rover will straighten up and extend its arm, and PIXL will begin to lash. The tiniest strokes, grains and veins, can tell if the rock is igneous or sedimentary, melted like a stew, or layered like a sandwich. The colors of the layers on top of the other characteristics will give a clue to the age of each. Ideally, the visible color and texture map will line up with the invisible, digital-only map that the x-ray results generate. When the right structures line up with the right minerals, Allwood can tell if she has Australian type signs of life or a Greenlandic type bust. “What we’ve found that’s really interesting with PIXL is that it shows you things that you don’t see, through chemistry,” says Allwood. “That would be the key.”
Allwood is hoping the tiny PIXL scans will yield huge results – a map inferred of 6,000 individual points on the postage stamp-sized field of view of the instrument, with multiple spectral results for each. She calls it a “hyperspectral datacube”.
Of course, Perseverance has other cameras and instruments, other scanners looking for other clues of meaning in pieces of rock and regolith. Adjacent to PIXL is a device that looks at rocks in a whole new way, projecting a laser at them to vibrate their molecules – it’s Raman spectroscopy. The data Perseverance collects will be hyperspectral, but also multifaceted – almost philosophically. This is what happens when you send a robot to another planet. A human mission or rocks sent home by sample return would yield the best ground truth data, as one exoplanet researcher told me. A little behind that is X-ray and Raman spectroscopy, then moving cameras, then orbital cameras. And of course, all of these things work together on Mars.
“Finding life on Mars will not be:” such and such an instrument sees something “. It will be, “All instruments have seen this, that, and the other, and interpretation makes life reasonable,” says Allwood. “There is no smoking gun. It is a complicated tapestry. And like a good tapestry, the complete image emerges only from a warp and weft of color, carefully threaded together.
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