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NASA engineers have shipped an envoy, the Perseverance rover, nearly 300 million miles to read the secrets trapped in the stones of Mars. A seven-foot robotic arm is responsible for completing the journey, bringing a cluster of various mechanical eyes to peer at Martian rocks from just inches away. That will be just close enough—researchers hope—to spot subtle carvings possibly left by ancient life.
One such mechanical peeper is the Planetary Instrument for X-ray Lithochemistry, or PIXL, which serves a similar purpose to the 10x hand lenses many geologists carry in the field. But in addition to magnifying, PIXL will also analyze rocks in a way never before done on Mars (and not done all that commonly on Earth). The instrument will tell the small army of planetary scientists and astrobiologists directing it not only precisely what a rock is made of, but also how its composite elements are arranged—information essential to figuring out where an object came from and whether primitive microbes were involved in its formation.
PIXL works by beaming powerful X-rays at a rock and watching for what X-rays come back (since atoms of different elements interact with the X-rays in different ways). Every Mars lander since Viking in the 1970s has carried such an “X-ray fluorescence” instrument to distinguish, say, iron from copper. But PIXL applies the technique in a whole new way.
Instead of pointing a thick beam at a rock and getting back a simple list of the elements present at a single location, PIXL uses an X-ray beam as fine as a human hair to scan back and forth across a sample. As it covers the grid over the course of a few hours, it assembles a series of stackable images showing exact arrangements of nearly two dozen elements—such as sodium, potassium, and nickel—inside an area no bigger than a postage stamp.
These “elemental maps” will help researchers unlock each sample’s unique history. Most rocks carry scars from indignities suffered in the past, like being skewered or buried. Understanding the distributions of materials in the samples will help scientists reconstruct what was happening on Mars billions of years ago. “Younger things cross older things, and younger things sit on top of older things,” says Abigail Allwood, PIXL’s principal investigator.
And sometimes, at least on Earth, living things chisel inanimate things. Allwood and her colleagues plan to use PIXL to search for such carvings on Mars.
The treks of previous rovers have helped planetary scientists determine that Mars was once warm enough and wet enough for organisms (as we know them) to survive, but it probably didn’t stay that way for very long. If life managed to get a toehold, it likely wouldn’t have had time to evolve beyond an extremely basic form, and any traces it left behind would be similarly low-key.
But in aggregate, communities of even the simplest microbes may have etched marks into Martian rocks that PIXL and other instruments might help identify. On Earth, for example, the most ancient signs of life aren’t fossilized bodies, but rather odd stone patterns known as stromatolites.
Stromatolites are the remnants of microbes that once banded together in thin films, feasting on sunrays and inadvertently shaping the sand that fell around them. As fresh “microbial mats” grew on top of old ones, they formed a stack of sheets that survived as stone long after other traces of the organisms had vanished.
Researchers have little notion what motifs Martian microbes might have made, but the layered texture of stromatolites serves as a proof of concept. If PIXL’s maps of elements showed a pile of wavy layers, that rock would be worth sampling for further study on Earth (Perseverance will stash samples for a future mission to pick up).
In fact, Allwood developed PIXL in part to decipher the cryptic messages of stromatolites here on Earth. She was part of a team that investigated the oldest terrestrial stromatolites (Australian specimens dating back 3.5 billion years), first in 2006 and again in 2009, using a commercial X-ray fluorescence microscope—PIXL’s grandparent.
At the time, most researchers thought the key to figuring out whether a stromatolite-like pattern had been shaped by primitive microbes or geologic forces was to scrutinize a sample micrometer by micrometer. But to Allwood, zooming out and studying a wider patch of rock often told a clearer story. “I always find stepping back from a problem helps you understand it better,” she says.
She soon joined NASA’s Jet Propulsion Laboratory as a postdoctoral researcher, and by 2011 was working on the first PIXL prototype with an eye toward someday sending it to Mars. The instrument had developed further by 2016, when a team announced the discovery of an even more archaic clustering of stromatolites in Greenland, from 3.7 billion years ago.
But after Allwood and other colleagues used the PIXL prototype to analyze samples of the Greenlandic stromatolites, the microbial origin didn’t hold up. PIXL’s analysis showed that there was little difference in how elements were distributed inside and outside of the supposed stromatolites. They also couldn’t see any layers, or “lamination,” whatsoever—a sure sign that the patterns in the rocks had not been left by microbes. “Mapping with PIXL showed that there was not even a chemical ghost of a lamination.”
Given that researchers spend years debating the authenticity of fossilized microbial mats here on Earth using a menagerie of instruments large and small, PIXL’s lone beam of X-rays has almost no chance of finding clear evidence of past life on Mars on its own.
A convincing case that the Red Planet was once home to microbes will require additional lines of evidence from Perseverance’s other various instruments, and, ultimately, the retrieval of the most promising samples for a full analysis on Earth.
Astrobiologists will need a “rich tapestry of information involving both in situ instrument observation and return sample observation,” Allwood says. “There are no smoking guns.”