On February 18th, 2021, NASA’s Perseverance rover will reach Mars, make a harrowing descent through the planet’s atmosphere, and put down in the Jezero Crater. Like it’s sister-rover, Curiosity, this robotic explorer will explore a region that once supported flowing water and poke around with an advanced suite of scientific instruments for signs of microscopic life that may have existed there billions of years ago.
This is no small feat, which is why the rover will be bringing along its Planetary Instrument for X-ray Lithochemistry (PIXL). This precision instrument is located at the end of its 2-meter (7 foot) long arm and is powered by artificial intelligence (AI). The PIXL will play a pivotal role in Perseverance‘s mission, using a coring drill to obtain samples that will be cached on the surface that will be returned to Earth by a future mission.
Every mission from the Viking lander to the Curiosity rover has been equipped with an X-ray fluorescence spectrometer, which is used to study the composition of soil and rock samples. However, PIXL improves upon all these previous instruments thanks to its powerful, finely-focused X-ray beam to determine the quantity and distribution of chemicals across the surface.
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To help this precision X-ray beam find the best targets to study, the instrument also comes with a device featuring six mechanical legs (a hexapod) that connects PIXL to the robotic arm. This is where artificial intelligence comes into play, which assists with the aiming of the arm to make sure it gets the most accurate scan possible.
Abigail Allwood, the principal investigator for the PIXL instrument at NASA’s Jet Propulsion Laboratory (JPL), explained in a NASA press statement:
“PIXL’s X-ray beam is so narrow that it can pinpoint features as small as a grain of salt. That allows us to very accurately tie chemicals we detect to specific textures in a rock. The hexapod figures out on its own how to point and extend its legs even closer to a rock target. It’s kind of like a little robot who has made itself at home on the end of the rover’s arm.”
The ability to assess particular rock textures is essential to determining which samples are worth sending back to Earth. On Earth, there are many distinctive types of shaped rocks that are the result of ancient layers of fossilized bacteria. Stromatolites are a good example, a type of layered sedimentary rock created by photosynthetic cyanobacteria, usually in shallow water.
Once a rock sample is selected, the rover’s arm will be placed close to it and PIXL will gauge the distance using a laser and a camera. Then hexapod’s legs will then make tiny movements (in the micrometer range) so the device can scan a target no larger than the size of a postage stamp. This will consist of bombarding the target area with 10-second bursts of X-ray pulses, then tilting 100 microns to take another measurement.
Over the course of about eight or nine hours, the PIXL instrument will do this thousands of times to produce a chemical map of a rock’s surface. This will tell the science teams back on Earth what chemicals are found within this tiny area of the rock. The timeframe of these microscopic adjustments is critical due to the extreme temperature changes the Red Planet goes through in the course of a day.
During the day, the temperature on Mars changes by more than 38 °C (100 °F), which will cause the metal on Perseverance’s robotic arm to expand and contract by as much as 13 mm (half an inch). To minimize thermal contractions, PIXL will conduct its science operations only at night when temperatures are more stable. “PIXL is a night owl,” Allwood summarized
Long before missions were using X-ray fluorescence to determine the composition on rocks on other planets, geologists and metallurgists were using it to identify certain materials. One place where it became a standard technique was in museums, where experts would use it to determine the origins of paintings and detect counterfeits. As Chris Heirwegh, an X-ray fluorescence expert on the PIXL team at JPL, explained:
“If you know that an artist typically used a certain titanium white with a unique chemical signature of heavy metals, this evidence might help authenticate a painting. Or you can determine if a particular kind of paint originated in Italy rather than France, linking it to a specific artistic group from the time period.”
For the sake of NASA’s Perseverance mission, X-ray fluorescence is an integral part of its astrobiology operations. As a burgeoning field of study, astrobiology is concerned with finding the various compounds that are associated with life (aka. biosignatures) on other planets and asteroids, thus allowing scientists to trace how they were distributed throughout our Solar System billions of years ago.
When used on Mars, it will be a means of unlocking the secrets of Mars’ ancient past. Allwood is well-versed in the process, having used X-ray fluorescence to determine the existence of stromatolite rocks in her native country of Australia. This discovery made waves internationally since the stromatolites contained some of the oldest fossilized microbes on Earth (ca. 3.5 billion years old).
In addition to searching for signs of past life (and maybe even present!), Perseverance will also characterize the planet’s climate and geology and pave the way for human exploration. It will also be the first mission in history to leave samples in a cache that will be returned to Earth by a future mission likely consisting of a lander, rover, and an orbiter (see the video above for more details).
With any luck, Perseverance will be the closing act for astrobiological studies on Mars, a process that began almost fifty years ago. However, the more likely possibility is that it will obtain data that raises more questions than it answers, which will make human missions to Mars all the more necessary. Regardless, whatever we do find will provide more insight into the evolution of the Solar System and the origins of life itself.
Further Reading: NASA