PIXL, an instrument on the end of the Perseverance rover’s arm, will search for chemical fingerprints left by ancient microbes.
NASA’s Mars 2020 Perseverance rover has a challenging road
ahead: After having to make it through the harrowing entry, descent, and
landing phase of the mission on Feb. 18, 2021, it will begin searching for
traces of microscopic life from billions of years back. That’s why it’s packing
PIXL, a precision X-ray device powered by artificial intelligence (AI).
Short for Planetary
Instrument for X-ray Lithochemistry, PIXL is a lunchbox-size instrument located
on the end of Perseverance’s 7-foot-long (2-meter-long) robotic arm. The rover’s
most important samples will be collected by a coring drill on the end of the
arm, then stashed in metal tubes that Perseverance will deposit on the surface
for return
to Earth by a future mission.
Nearly every
mission that has successfully landed on Mars, from the Viking landers to the Curiosity
rover, has included an X-ray fluorescence spectrometer of some kind. One major
way PIXL differs from its predecessors is in its ability to scan rock using a
powerful, finely-focused X-ray beam to discover where – and in what quantity –
chemicals are distributed across the surface.
“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,” said Abigail Allwood, PIXL’s principal
investigator at NASA’s Jet Propulsion Laboratory in Southern California.
Rock textures will be an essential
clue when deciding which samples are worth returning to Earth. On our planet, distinctively
warped rocks called stromatolites were made from ancient layers of bacteria,
and they are just one example of fossilized ancient life that scientists
will be looking for.
A device with six mechanical legs, the hexapod is a critical part of the PIXL instrument aboard NASA’s Perseverance Mars rover. The hexapod allows PIXL to make slow, precise movements to get closer to and point at specific parts of a rock’s surface. This GIF has been considerably sped up to show how the hexapod moves. Credit: NASA/JPL-Caltech
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An AI-Powered Night
Owl
To help find the best targets, PIXL relies on more than a precision
X-ray beam alone. It also needs a hexapod – a device featuring six mechanical
legs connecting PIXL to the robotic arm and guided by artificial intelligence
to get the most accurate aim. After the rover’s arm is placed close to an
interesting rock, PIXL uses a camera and laser to calculate its distance. Then those
legs make tiny movements – on the order of just 100 microns, or about twice the
width of a human hair – so the device can scan the target, mapping the
chemicals found within a postage stamp-size area.
“The hexapod figures out on its own how to point and
extend its legs even closer to a rock target,” Allwood said. “It’s
kind of like a little robot who has made itself at home on the end of the
rover’s arm.”
Then PIXL measures X-rays in 10-second bursts from a
single point on a rock before the instrument tilts 100 microns and takes
another measurement. To produce one of those postage
stamp-size chemical maps, it may need to do this thousands of times over the
course of as many as eight or nine hours.
That timeframe is partly what makes PIXL’s microscopic
adjustments so critical: The temperature on Mars changes by more than 100
degrees Fahrenheit (38 degrees Celsius) over the course of a day, causing the
metal on Perseverance’s robotic arm to expand and contract by as much as a
half-inch (13 millimeters). To minimize the thermal contractions PIXL has to contend
with, the instrument will conduct its science after the Sun sets.
“PIXL is a night owl,” Allwood said. “The
temperature is more stable at night, and that also lets us work at a time when
there’s less activity on the rover.”
PIXL opens its dust cover during testing at NASA’s Jet Propulsion Laboratory. One of seven instruments on NASA’s Perseverance Mars rover, PIXL is located on the end of the rover’s robotic arm. Credit: NASA/JPL-Caltech
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X-rays for Art and
Science
Long before X-ray fluorescence got to Mars, it was used by
geologists and metallurgists to identify materials. It eventually became a
standard museum technique for discovering the origins of paintings or detecting
counterfeits.
“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,” said Chris Heirwegh, an X-ray
fluorescence expert on the PIXL team at JPL. “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 astrobiologists, X-ray fluorescence is a way to read
stories left by the ancient past. Allwood used it to determine that stromatolite
rocks found in her native country of Australia are some of the oldest microbial
fossils on Earth, dating back 3.5 billion years. Mapping out the chemistry in
rock textures with PIXL will offer scientists clues to interpret whether a
sample could be a fossilized microbe.
More About the Mission
A key objective for
Perseverance’s mission on Mars is astrobiology, including the search for signs of ancient microbial
life. The rover will also characterize the planet’s climate and geology, pave
the way for human exploration of the Red Planet, and be the first planetary
mission to collect and cache Martian rock and regolith (broken rock and dust).
Subsequent missions, currently under consideration by NASA in cooperation with
the European Space Agency, would send spacecraft to Mars to collect these
cached samples from the surface and return them to Earth for in-depth analysis.
The Mars 2020 mission is part of
a larger program that includes missions to the Moon as a way to prepare for
human exploration of the Red Planet. Charged with returning astronauts to the
Moon by 2024, NASA will establish a sustained human presence on and around the
Moon by 2028 through NASA’s Artemis
lunar exploration plans.
JPL, which is managed for NASA by Caltech in
Pasadena, California, built and manages operations of the Perseverance and
Curiosity rovers.
Learn more about the Mars 2020 mission at:
https://www.nasa.gov/perseverance
News Media Contact
Andrew Good
Jet Propulsion Laboratory, Pasadena, Calif.
818-393-2433
andrew.c.good@jpl.nasa.gov
Alana Johnson
NASA Headquarters, Washington
202-358-1501
alana.r.johnson@nasa.gov
2020-183
Source: Jet Propulsion Laboratory