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Can Abigail Allwood Find Life on Mars?


2018-5-15 06:03 Science 15 0 0
Can Abigail Allwood Find Life on Mars?

From 2003 to 2005, when Abigail Allwood was a graduate student in earth science at Macquarie University, in Australia, she made a series of remarkable discoveries. She was doing fieldwork in the country’s Pilbara region, where she was charged with studying fossilized stromatolites, or columns of sedimentary rock originally created by layers of microbes—some of the planet’s first known life.

The area, a 196,000-square-mile expanse of rust-colored desert populated with rock formations dating back more than 2 billion years, is more or less what you might picture when someone says “the ends of the Earth.” Parts of it remain virtually untouched by humans. Allwood recalled for me recently how one day she and Ian Burch, then her research partner (now her husband), hiked the length of a high, narrow ridge some 10 miles long. “I’m pretty sure we were the only people that had been there for thousands of years,” she told me. “I remember a northern quoll [a ratlike marsupial native to Australia] coming right up to us to take a closer look. It had never seen a human being before, so it wasn’t afraid.”

Even more exciting than the native fauna, however, were the geologic formations Allwood and Burch had come to study. They resembled stacks of upside-down ice-cream cones—a pattern typical of stromatolites, which usually form in shallow water. Their presence suggested that this part of the desert was once a very wet area, perhaps an ocean reef. Before long, Allwood would establish that the rocks constituted the oldest known evidence of life on Earth.

At the time, the scientific community was embroiled in a tense debate about evidence of early life. It had recently been established that hydrothermal environments could produce formations that looked like fossils of microbes, despite never having contained life. As a result, specimens that had previously been accepted as the earliest evidence of Earth’s life were cast aside. A race was now on to find fossils that had definitely formed in “fluvial” settings—cold, wet environments that could almost certainly have supported life.

Allwood knew that the fossils were old. Sample analysis dated them to the ripe old age of 3.45 billion years. But could she determine that the cluster had formed in a wet setting—and was biological in origin? After months of taking pictures, samples, and measurements of her find, Allwood became convinced that it had formed on an ancient microbial reef, in the middle of an ocean. “Most of [the formation] is a dog’s breakfast,” explained Allwood, who likes to pepper her otherwise technical scientific observations with Aussie slang. “Which is to be expected when you leave something sitting around for 3.5 billion years. But there are quite a few spots where the rocks aren’t so chewed up. And it’s there, in those little windows to the past, that the treasure lies.”

Taken together, Allwood’s findings demonstrated that there was life on our planet at least 3.45 billion years ago. This discovery landed her work on the cover of Nature. It also got the attention of nasa, which was looking for people with a talent for uncovering life in remote places.

And that’s how Allwood came to work at nasa’s Jet Propulsion Laboratory (JPL), in California, where she is today a principal investigator on the next Mars rover. Her mission: Search for signs of extraterrestrial life.

The scientific consensus is that Mars could have supported life. Like Earth, it is a little more than 4.5 billion years old. Also like Earth, it was once warm and wet, a pair of conditions under which life is known to thrive. It’s no longer either of these things, of course—over time, the Martian atmosphere has been eroded by solar winds and has grown too thin to support liquid water.

Back when Allwood’s microbes lived, however, the two planets were similar. If life had taken hold on Earth, why couldn’t it also have taken hold on Mars? “Life is not a fussy, reluctant and unlikely thing,” Allwood wrote in Nature in 2016, reflecting on Earth’s early conditions and what they might reveal about life’s chances elsewhere. “Give life half an opportunity, and it’ll run with it.”

nasa has been conducting Mars missions for more than 50 years, beginning with the Mariner 4 spacecraft (which in 1965 supplied the agency with its first close-up pictures of the planet) and continuing through three successful rover missions over the past 21 years. But while the search for life has always been one of the Mars program’s high-level goals—along with characterizing the planet’s climate and geology, and assessing its suitability for human exploration—no evidence of life has ever been found.

Each rover mission has inched closer to that goal, however. In 2013, the most recent rover, Curiosity, made headlines after identifying some of the building blocks of life—carbon, hydrogen, nitrogen, oxygen, phosphorus, and sulfur—near an ancient streambed. By demonstrating that Mars could have been a habitable environment, this discovery has had a galvanizing effect on the upcoming mission. The rover, which is currently known as “Mars 2020” (it has not yet been named), will be the first to identify and cache rock samples for future analysis, in hopes that one of them will include evidence of past life. A later rover mission will pick these specimens up and carry them home to Earth.

But which samples should be brought back? Even on Earth, it’s very hard to find fossils that are billions of years old. Given the immense cost and complexity of flying rocks, even tiny ones, the tens of millions of miles to Earth, each prospective sample will need to be scrutinized closely ahead of time to make sure it’s worth carrying back. In 2013, nasa announced a contest, open to the broader planetary-science community, to determine which instruments the Mars 2020 rover should take along for this purpose.

Competition was fierce, but Allwood, who was by then working at JPL, jumped at the opportunity. She had long been thinking about an instrument that could mimic some of the methodologies she had used to examine the stromatolites in Australia—only faster, more efficiently, and remotely. The instrument she entered in the competition, which she called pixl, would use X‑rays to identify chemical elements in rocks as small as a grain of salt.

pixl would be a couple of orders of magnitude more sensitive than any instrument previously sent to Mars. Moreover, it would be the first one to conduct petrology—that is, to try to determine rocks’ origins by studying their texture and composition. When previous rovers had sent back images, scientists could make out various geologic features, but couldn’t tell what they were made of. pixl, Allwood suggested, could measure and map the chemistry of even tiny features picked up by the cameras. Allwood told me that these capabilities have the potential to “make a huge difference to how much we can learn about the past.”

By measuring the abundance and spatial distribution of specific elements (so much calcium, say, to so much iron), pixl promised to infer the context in which a rock had formed. Was it volcanic or sedimentary? Had it once been a part of a river, a delta, or an ocean? The presence of specific elements and their distribution and proportions to one another could also suggest that microbes had been present when the rock formed.

Using all this information, nasa would be able to make an informed guess as to whether a given sample might contain fossilized microbes—and decide whether to haul it home for further tests.

nasa was sold. In 2014, the agency announced that pixl would be one of just seven instruments aboard the Mars 2020 rover, and it named Allwood a principal investigator on the mission. She is the first woman ever to serve in this capacity on a Mars mission.

Last November, Allwood gave me a tour of her labs, where she was busy assembling a fully working model of pixl, to be tested before the real thing is put together. The JPL campus, which straddles Pasadena and La Cañada Flintridge, California, and employs about 6,000 people, looks more like a run-down high school than a high-tech planetary-science facility—linoleum floors, blue doors, lots of beige. This hasn’t been lost on Allwood, who’s brightened up her otherwise functional office with some colorful paintings and an antique reading chair from eBay. “Beats the standard-issue JPL seat,” she said, pointing to a sad-looking black plastic chair in a corner.

As I drank some truly horrendous coffee from a courtyard cart (Allwood wisely declined a cup), she told me about her upbringing in Brisbane, a city better known for its proximity to the beach than its academic institutions. When high-school science proved uninspiring, she turned to David Attenborough and Carl Sagan. She remembers learning about the Voyager mission to Jupiter, Saturn, Uranus, and Neptune during an episode of Sagan’s Cosmos and, later, watching a televised interview with Carolyn Porco, a planetary scientist who worked on the mission in the 1980s. “She talked about sitting alone in her office one night and suddenly seeing the first close-up images of Saturn coming down from Voyager,” Allwood said in her usual, unhurried tempo, her pale-blue eyes scanning mine. “And feeling as if she was exploring the frontiers of the solar system—and I thought to myself, Wow, imagine doing that.”

In college, Allwood told me, she attempted to major in physics, but struggled with the math and dropped out; she eventually reenrolled, and at age 28 received a degree in geoscience. She went on to pursue a doctorate in earth science, working under Malcolm Walter, the founder of the Australian Centre for Astrobiology, which was then based at Macquarie University, in Sydney. Astrobiology is the study of life’s origin and evolution throughout the universe; Australian scientists have long enjoyed something of an edge in the field, because their continent’s relatively untouched deserts offer a perfect place to find records of how life on Earth formed. (Indeed, nasa describes regions like the Pilbara as a “stand-in” for Mars.) Allwood’s work with Walter would not only take her to the Pilbara to study stromatolites, it would teach her to think beyond Earth.

When I had given up on the coffee, Allwood and I headed downstairs to the main pixl lab, a cramped room in the same building as her office. In one corner, various wires and small motors lay jumbled on a workbench; in another stood an X-ray machine, encased in hand-me-down plexiglass from an old Curiosity instrument.

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.

Allwood led me into a viewing bay overlooking the assembly area, where mechanisms designed to bring the rover safely to its destination were being built.

A few minutes later, a school group entered the bay. “You can’t see the full rover right now,” a JPL employee told the kids, some of whom had pressed their face to the glass in awe. “We’ll start putting it together in about six months.”

Next to me, Allwood smiled. “I can’t wait,” she whispered. Her tone was measured, but when I looked over at her, she was bouncing slightly.


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