Humanity has been wondering about whether life exists beyond our little backwater planet for so long that we’ve developed a kind of cultural bias as to how the answer to this central question will be revealed. Most of us probably imagine that NASA or some other space agency will schedule a press conference, an assembled panel of scientific luminaries will announce the findings, and newspapers around the world will blare “WE ARE NOT ALONE!” headlines. We’ve all seen that movie before, so that’s the way it has to be, right?
Probably not. Short of an improbable event like an alien spacecraft landing while a Google Street View car was driving by or receiving an unambiguously intelligent radio message from the stars, the conclusion that life exists now or once did outside our particular gravity well is likely to be reached in a piecewise process, an accretion of evidence built up over a long time until on balance, the only reasonable conclusion is that we are not alone. And that’s exactly what the announcement at the end of last year that the Mars rover Perseverance had discovered evidence of organic molecules in the rocks of Jezero crater was — another piece of the puzzle, and another step toward answering the fundamental question of the uniqueness of life.
Discovering organic molecules on Mars is far from proof that life once existed there. But it’s a step on the way, as well as a great excuse to look into the scientific principles and engineering of the instruments that made this discovery possible — the whimsically named SHERLOC and WATSON.
Would You Like Some CHNOPS with That?
Defining what exactly constitutes biological life is difficult, and there are plenty of philosophical arguments that muddy the waters even when you reduce life to characteristics such as the transformation of energy or the ability to reproduce. But at the end of the day, such macroscale characteristics don’t help much when looking for microscopic life on other planets — especially when you suspect that you’re just looking for the remains of ancient microbial life, as is likely the case on Mars.
To explore the possibility that Mars once harbored life, the Mars 2020 mission’s Perseverance rover science payload includes a range of instruments designed to search for the smallest remains of past life. Chief among these instruments is SHERLOC, for “Scanning Habitable Environments with Raman and Luminescence for Organics and Chemicals” — a somewhat forced but impressively descriptive acronym.
At the heart of SHERLOC, which rides at the end of the rover’s two-meter robotic arm, is an ultraviolet laser Raman spectrometer, designed to identify the specific signatures of the so-called CHNOPS elements — carbon, hydrogen, nitrogen, oxygen, phosphorus, and sulfur. Something like 98% of the biomass on Earth is composed of these six elements; finding them on Mars will be pretty good evidence that life once existed there. But simply finding the CHNOPS elements doesn’t make a sample biologically relevant. It’s how those elements are organized and the structures they form that determine whether a sample might have the remains of ancient life, and figuring that out is what Raman spectroscopy is really good at.
Scattering Two Ways
Raman spectroscopy takes advantage of what’s known as inelastic scattering, or Raman scattering. Normally, electromagnetic waves interact with particles of matter by elastic, or Rayleigh, scattering. When incoming photons interact with molecules, they excite them from the ground state to a higher-energy virtual state. In Rayleigh scattering, the excited state quickly collapses and the particle returns to the ground state without any loss of the kinetic energy the incident photon had. It’s like a moving billiard ball that transfers all its kinetic energy into a motionless ball, which then goes on to move while the first ball stops dead.
But about one out of every 100 million scatterings results in dropping from the excited virtual state to a state different from where the molecule started. To stretch the earlier analogy, this would be like the moving billiard ball hitting a motionless ball with a crack in it. The cracked ball would still absorb the energy of the incoming ball, but the crack would attenuate some of it, sending the ball off at a different speed than the incoming ball, and perhaps even in a different direction than would occur in a purely elastic collision.
Just as the difference in speed and direction could reveal information about the characteristics of the cracked ball, so too can Raman scattering be used to probe the structure of a molecule. The difference in energy between the incident photons and the scattered photons depends on the vibrational and rotation states of the chemical bonds within the molecule. This results in a population of photons with different wavelengths that represent the different chemical bonds within a molecule. When spread out onto a detector with a diffraction grating, these photons create a fingerprint that’s characteristic of the molecules in the sample.
While Raman has been used for decades on Earth to analyze all sorts of chemical samples, SHERLOC is the first time the technique has been used on another world. And as you’d imagine, it takes some special engineering to package up all the optics and electronics and make it not only robust enough to survive the rigors of space travel, but also to operate autonomously.
Built to Perform
To accomplish all this, SHERLOC is divided into two major assemblies: the SHERLOC Body Assembly (SBA) and the SHERLOC Turret Assembly (STA). The STB is where all the command and data handling circuits are located, and where the power supply lives. The STA is the business end of SHERLOC, and lives at the end of Perseverance’s robotic arm. The heart of the STA is the deep-UV (DUV) laser, a heavily modified off-the-shelf neon-copper metal-vapor laser. It provides a highly stable 248.60 nm pulse and is expected to last long enough to deliver 3 million spectra, which is about seven times the design life of the rover.
As with any Raman spectroscope, the optics of SHERLOC are a complicated set of lenses, mirrors, beam splitters, and filters. Unlike most of its Earth-bound cousins, though, SHERLOC has to handle the “S” in its name: scanning. Rather than rely on fine control of the robotic arm to position its beam, SHERLOC has a scanner subsystem that’s quite similar to the galvanometers used for beam steering in laser shows. The scanner gives SHERLOC control of the beam over a 7 mm x 7 mm sample area with a step size of less than a micron in both dimensions, allowing it to gather data from the smallest of features without having to rely on robot arm moves.
Another way in which SHERLOC differs from other Raman instruments is in the need to correlate spectra with spatial information about a sample. It’s not enough to get the spectral fingerprint of a particular section of a sample; rather, SHERLOC must also determine the context of what that exact spot on the sample looks like in visible light. To accomplish this, SHERLOC requires the help of two cameras: the Autofocus and Context Imager (ACI), a high-resolution grayscale camera that shares the optical path of the Raman spectroscope, and WATSON, the Wide Angle Topographic Sensor for Operations and eNgineering camera. WATSON is a separate, full-color, high-resolution camera with a macro capability down to 1.78 cm focal length. WATSON and the ACI together are basically the equivalent of a geologist’s hand lens, allowing SHERLOC to overlay visible light images with Raman data over a wide range of operating distances.
Finally, SHERLOC’s Raman spectroscope is designed to survive the long trip to Mars, the high-energy landing, and the harsh conditions of the cold, dusty world. While the SBA is nestled safely inside the hull of Perseverance, the STA has to be exposed to the elements to do its job. SHERLOC is mounted on a hexapod arrangement of spring-loaded struts that dampen vibrations encountered both during spaceflight and rover operations. The STA is also equipped with a complex thermal management system, including survival heating elements that keep the electronics and optics warm enough to survive the worst-case Martian cold.
Context is Key
While most of the public’s attention to the Mars 2020 mission so far has understandably been drawn to the wildly successful Ingenuity helicopter, SHERLOC has been busily gathering data pretty much non-stop since Perseverance arrived on Mars back in March of 2021. The confirmation of organics in Jezero crater came from a series of samples analyzed back in September of 2021, and one rock in particular, which was dubbed “Garde.” The rover’s arm-mounted tool assembly was used to grind away some of the weathered rock before SHERLOC was swung into place to analyze the sample.
Thanks to the power of SHERLOC and its ability to overlay visible light images with Raman data, planetary scientists were able to determine that Garde contains both olivine minerals, which indicate an igneous history, and carbonate minerals, which suggest a past period of water reacting with the rock. This is consistent with what we already know about the Jezero crater and the river delta that once flowed into it. Finding organic materials in a rock with that kind of geological history is a tantalizing bit of data, and may someday prove to be part of the evidence that life once teemed on Mars.