Was there ever life on Mars? Perseverance’s SHERLOC laser sniffs for microscopic clues
The ultraviolet “imager” is a life-hunting machine more than two decades in the making.
The chassis of NASA’s Perseverance rover bristles with more than a dozen cameras (and, for the first time, a microphone), but the Martian explorer’s business end is a block of sensors engineers lovingly refer to as “The Turret.” Using a brawny seven-foot robotic arm, Perseverance can extend and swivel the nearly 100-pound instrumentation hub to park its discerning eyes just inches away from rocks of interest. There, it can spot gritty features just dozens of micrometers across, too tiny for human perception.
The turret’s main scientific instruments, PIXL and SHERLOC, act together as a yin and yang for rock analysis, collecting data in distinct yet complementary ways, according to Luther Beegle, SHERLOC’s principal investigator at NASA’s Jet Propulsion Laboratory (JPL). Together, the sensors will search for what NASA has, after decades of brainstorming, determined are the most universal “biosignatures,” or signs of life.
“If there are potential biosignatures in Jezero Crater, [where Perseverance landed], we’ll find them,” Beegle says. “We’ll bring them back to Earth.”
At the highest level, PIXL and SHERLOC work in similar ways. They sweep back and forth over a stone’s surface with beams of energetic light as thin as a strand of human hair and use the light that comes back to generate a sort of image of the rock. Where PIXL harnesses X-rays to see simple atomic elements, like iron or nickel, SHERLOC uses an ultraviolet laser to map out a rock’s more complex components, i.e., its minerals and organic molecules.
[Read more: Perseverance’s giant ‘hand lens’ will scour Mars for signs of ancient life]
SHERLOC’s laser interacts with a rock’s bits in two ways. In a phenomenon called fluorescence, during ultraviolet exposure some molecules soak up certain light frequencies, and emit a light of a different frequency back. The difference between the two tells researchers what type of molecule they’re looking at.
The instrument can also recognize specific chemical links, such as a carbon atom stuck to an oxygen atom, using what’s known as Raman spectroscopy. Some molecules vibrate or stretch more than others. When SHERLOC shines its ultraviolet laser at a rock, a few light particles (just one per billion) will plow into a quivering molecule, lose some energy, and bounce right back to SHERLOC. Depending on the specific amount of energy lost, researchers can distinguish various molecules.
Between the two techniques, SHERLOC will be able to ferret out specific organic molecules—that is, molecules rich in carbon. Geological and chemical processes can make these compounds, but they’re also essential for building proteins, DNA, and cells in general. “Basically, life is little bags of carbon,” Beegle says.
SHERLOC’s Raman capabilities (a first for any instrument on Mars), can also pick out minerals and chemical bonds that will help illuminate the backstory of particularly intriguing rocks. A bunch of minerals with hydrogen, for instance, would hint that those molecules formed in the presence of water (H2O), making that sample more likely to contain signs of past life—as opposed to a rock formed in a scorching volcano, which would contain far less hydrogen.
The designing and building of the current version of SHERLOC began in 2012, but Beegle traces the instrument’s origins back to a 1996 discovery that shook the nascent field of astrobiology. While pouring over a Martian meteorite discovered in Antarctica, researchers came across what looked like fossilized microbes. Alternative, non-biological explanations for the odd shapes later emerged, but the initial difficulty of telling life from non-life served as a wake-up call.
“It was the first time when the community said, ‘We don’t know if we really know how to do this,’” Beegle says.
Internally, JPL asked for proposals for instruments capable of what the organization called the “grand challenge” of finding life on another planet. One of the competition’s winning designs was for SHERLOC.
The key, JPL astrobiologists concluded, was to focus—not on specific shapes or chemical reactions common among terrestrial species—but on one general trait shared by all known organisms: the tendency to gather.
The non-biological forces that shape planets—such as deep-sea vents, volcanoes, meteorite impacts, cosmic rays, wind, and erosion—break things down and spread them around. But life concentrates resources—on multiple levels. There is a higher density of carbon-rich amino acids inside your body than outside, for instance. And many more humans live in New York City than in Antarctica. Everywhere researchers look on Earth, they find organisms thronging around resources, fighting the powers that would disperse them.
“Life wants to clump where it can eat and survive,” Beegle says, “and that’s what we think would happen on Mars.”
No one knows what ancient organisms on Mars might have looked like or how they might have behaved, if they ever existed at all. But JPL researchers suspect that otherwise hard-to-explain patterns of minerals, organic molecules, and elements would be promising samples for further study.
If Perseverance discovers hints of life, interpreting them will take a team effort among both instruments and researchers.
The Mastcam-Z will take in panoramic vistas that will help researchers get oriented, so they can tell if they’re looking at stones in a dried-up stream, a long-defunct volcanic vent, or a desiccated river delta.
Then the turret will bring in SHERLOC and PIXL for the close-up maps of a rock’s elements, minerals, and organic molecules. The turret’s dual instruments can discern features as small as 30 to 40 micrometers. That’s enough resolution to recognize biologically assembled clumps of molecules, but not sufficient to see fossils of actual microbial bodies, which might be as small as a single micrometer.
Even if it did, researchers wouldn’t want to waste days of Perseverance’s time scanning a single rock micrometer by micrometer for potentially rare fossils. “That’s a needle in a haystack on a planet full of haystacks,” Beegle says.
That level of analysis will come in the early 2030s. When Perseverance comes across a rock that SHERLOC and other instruments deem particularly promising, the rover can stash a sample in a tube for retrieval by a future mission. Back on Earth, keen-eyed researchers with mighty microscopes will be able to deliver more definitive answers as to whether Mars ever hosted life.
For Beegle, those studies will be worth the wait, whatever they find. “It’s a wonderful question to ask. It will help us figure out our place in the universe,” he says. “Is life everywhere or is life really rare?”