pixl may be the most complicated instrument aboard the 2020 rover, but not all of its technologies are new. While analyzing the Pilbara stromatolites, Allwood employed a similar tool known as a micro-XRF, which uses X-ray fluorescence to determine a material’s chemical makeup. (When exposed to X-rays, an atom of potassium behaves differently than, say, an atom of gold—making it easy to differentiate chemical elements.) At the time, micro-XRF instruments were popular mostly in archaeology and art restoration; the model Allwood used for her stromatolites had previously analyzed pigments on an ancient Nepalese manuscript.
Adapting the technology for use on Mars has presented unique challenges. The micro-XRF machine Allwood originally used was more than two feet wide and weighed more than 600 pounds. To fit pixl onto the Mars 2020 rover, she’s had to make it roughly the size of a Nintendo GameCube console.
The X‑ray technology has also proved difficult, she said. All XRF instruments need an X‑ray source, and pixl’s runs on 28,000 volts. “This is the stuff of nightmares on Mars,” Allwood told me. “To produce a voltage this high in Martian atmosphere, you’re looking at a spectacular breakdown.” In other words, she is sweating the question of how not to set the rover on fire. “We’re basically trying to prevent a Martian fireworks display.”
pixl will be mounted on the rover’s arm, which means that it will have to contend with the extreme temperature fluctuations on Mars (from highs of about 30 degrees Fahrenheit to lows of about 120 degrees below zero). Just heating pixl will require a significant portion of the rover’s available power. “The joke around here,” Allwood said, grinning, “is that pixl needs to be like Tahiti, always balmy.”
On Mars, pixl will work in tandem with several other instruments, including sherloc, which will also be mounted on the rover’s arm. While pixl focuses on detecting chemical elements, sherloc focuses on finding organic carbon (something left behind by all organic life). If pixl and sherloc jointly detect something worth examining, Allwood and other members of the JPL team down on Earth will have as few as five minutes to look at the incoming data and instruct the rover to either look more closely or move on.
Still, the instruments will be able to do only so much remotely. While pixl should be able to make very good guesses, it won’t be able to establish unequivocally whether a rock contains signs of past life. Instead, if a particular specimen seems promising, the rover’s robotic arm will drill a sample a few inches deep, seal it in a tube, and carefully stash that tube away for later. The rest will have to be done by powerful laboratory instruments back on Earth.
Before leaving JPL, I asked Allwood whether I could see the place where engineers will assemble the Mars 2020 rover. Her eyes lit up. “Oh, you want to see it? Let’s go.” We wandered over to the Spacecraft Assembly Facility, a large hangar on the edge of the campus. We couldn’t enter the assembly area; the entire floor is a designated clean room, meaning that anyone coming in has to undergo a robust decontamination process that involves donning what’s known as a “bunny suit.” Bunny suits play a crucial role in protecting the rover from human contamination. If one of the rover’s main goals is to search for life on another planet, it must be careful to avoid leaving this one with biological material inadvertently stowed on board.