by Marc Kaufman
All of a sudden, the undistinguished chunk of Murchison on display at the museum looked not so homely to me anymore. A gray-black, alternately smooth and craggy rock the size of a baked potato and mounted on a simple blue rod, it has an improbable and important story to tell. Murchison, it is commonly believed, is somewhere between 4 billion and 4.5 billion years old—meaning that it was formed around the time that Earth was. It was initially a part of the asteroid belt between Mars and Jupiter, and no doubt was spun in the direction of Earth after another asteroid crashed into it. At some point it was exposed to water. Ironically, it can tell us more about the early history of the universe than anything on Earth because the planet has been so altered by oceans, volcanoes, the cycling of elements like carbon, and, most important, by life.
Glavin and Dworkin walked me down to the museum’s meteorite collection, a treasure trove of gifts from outer space. Curator Linda Welzenbach got us gloved and began to take out more and more samples of Murchison, some in glass vials, some in plastic bags. In all, the museum has about sixty-five pounds of the meteorite, purchased by NASA from private collectors in 1971, two years after it fell. They sit in covered shelves inside a heavy-duty storage cabinet, hopefully not gathering dust. Welzenbach didn’t hesitate when asked if we could take a look at some of her collection.
Holding pieces of Murchison was an unexpectedly moving experience. To be in physical contact with something so profoundly and undeniably old was to be, for one transporting moment, connected to that near eternity, to be drawn back to the Big Bang, the great mystery that science can now describe but not really explain. No wonder meteoriticists, as they’re called, are a famously intense crew. They’re in daily touch with the concrete reality of deep space and deep time, and their job is to understand what the rocks are telling them about both.
“There is certainly a constant debate in the origins-of-life community about what fraction of the organic material on the early Earth was made in situ, locally, by reactions on Earth versus extraterrestrial input,” said Glavin. “A lot of people argue the amount coming in from [extraterrestrial] forces—meteorites, interplanetary dust—wouldn’t be enough… to get good chemistry to take place. You could argue it either way.”
“But the problem is the evidence is lost,” Dworkin interrupted. “You can infer things based on modern observations of meteorites and based on laboratory experiments of what the chemistry might have been like. But the record is gone. And the best you can do is make intelligent guesses and see what’s out there.” Glavin finds it paradoxical that no clear record exists of ancient, prebiotic chemistry on Earth, but that clues into the nature of those precursors to life arrive regularly from outer space. And so while trained and still immersed in the Miller-Urey tradition, Glavin and Dworkin nonetheless focus on meteorites and comets rather than the origins of the Earthly ingredients that might once have made up the primordial soup. A recent discovery of theirs, for instance, involved an Earth-bound meteorite that was tracked in space, landed in northern Sudan in 2008, and was collected in more than 600 pieces within weeks. The rock was initially formed when two asteroids collided and caused a heat shock of up to 2,000 degrees Fahrenheit, which is considered to be well beyond the point where all complex organic molecules (including amino acids) should have been destroyed. But Glavin and his team found the amino acids there anyway. Having determined that Earthly contamination was not involved, Glavin concluded the meteorite is telling us the very important news that there appear to be numerous ways to form the seemingly abundant amino acids in space, a dynamic that “increases the chance for finding life elsewhere in the universe.” Just as those amino acids fell on Earth and may well have played a role in the origin of life here, so too would they be falling on Mars, Europa, and planets and moons throughout the cosmos.
The initial Miller-Urey experiments set off an explosion of related research around the world, and scientists now know significantly more about the mechanics and logic of how nonlife might become life. But more than a half century after Miller-Urey, science remains without an explanation of the actual pathway, and many scientists no longer believe we will ever know the specifics of how life began on Earth. The original experiment was a spectacular proof of concept that was eagerly embraced by researchers, but the actual chemistry remains largely unraveled. Nonetheless, Miller-Ureyites have dominated, enhanced, annoyed, and frustrated the field ever since.
As described by British-born Mark Russell, a senior research fellow at the NASA Jet Propulsion Laboratory and a specialist in geochemistry, they have “commanded the high ground for fifty years” and, he argues, have hurt origin-of-life science. A brief 1988 critique of the prevailing Miller approach written by Russell and published in Nature resulted in a torrent of requests for reprints, and convinced him and colleagues that others bridled under the weight of the legacy. Russell’s own work has been focused on undersea origin-of-life chemistry, the field created when the powerful hydrothermal vents called black smokers were first found and explored on the floor of the Pacific in the late 1970s and ’80s. Surprising life-forms exist around the nutrient-rich heat of the smokers, leading to some theories that life started in their vicinity. Russell has long worked on a corollary to that notion, arguing that the smokers themselves were too intense to nurse life into being, but that the many subsidiary and less torrid undersea vents were ideal nurseries and provided the right kind of chemistry, geology, and potential energy. He was conducting a laboratory test of his theory when I met him at the Jet Propulsion Lab, located at the California Institute of Technology in Pasadena, California. But his position was temporary, and he wondered if the influence of the Miller-Urey progeny was part of the reason why.
Not only did the science of the Miller lab command the “high ground,” Russell said, but it and its graduates also “commanded a lot of the NASA think tanks and commanded where their ex-students went to in terms of different universities, and generally kept control of origin-of-life science and evolution of biosphere.” He said that as editors and peer reviewers for the major science journals, they also had inordinate control over what got published in those most prestigious publications. But fortunately there are other journals eager for his work, so the debate continues.
But even if the Miller-Urey details and chemical pathways remain debated, the basic validity of “abiogenesis” does remain in place—the assertion that the building blocks of life, and so life itself, can be formed from entirely nonbiological sources. This logic is still being used and expanded, and in some unusual places. There is probably no better example of this than the experiment commonly called “Miller-Urey in space.”
The presence of organic compounds in space was first detected eight decades ago, but an understanding of their ubiquity is relatively new. It turns out they are most everywhere in space—carbon dioxide, more complex amino acids, and even formaldehyde. The most frequently found are polycyclic aromatic hydrocarbons (PAHs), an early phase of chemical evolution of carbon and hydrogen into the more complex molecules that are useful to life. Just as Miller and Urey wanted to learn if complex amino acids could be formed in the lab by sparking certain gases and water, a team of largely European researchers proposed a while back doing the same thing in space. The experiment was quickly approved for the orbiting International Space Station or ISS, though not so quickly designed, built, or scheduled for delivery. But it is now far along in development and remains a priority for the European Space Agency, and so “Miller-Urey in space” will very likely one day join Miller-Urey in Chicago as an important pathfinder in the origins of life.
But what a difference fifty years make. While Miller-Urey was famous for both its results and its remarkable simplicity, the effort to test similar dynamics in space is anything but simple. It involves shipping the experiment up to the space station and installing it into the Microgravity Science Glovebox in a pressureless, weightless room on board the European-built Columbus module of the station. The Glovebox will simulate low-temperature
conditions at the solar nebula, the disk that formed after the collapse of an enormous cloud of dust and gas formed our solar system. Astronauts will place two sets of vials in the chamber with ice-crusted grains of silicon minerals and two different mixes of gases: one corresponding to the initial Miller-Urey experiment and the other taking into account current thinking about what was present in the solar nebula. Each vial is equipped with two electrodes that produce an electric charge, and the ice-covered particles float through the gaseous and electric mix, just like in the solar nebula. After being exposed for several weeks, the vials have to be returned to Earth in –36 degree Fahrenheit containment for analysis using instruments Stanley Miller could hardly dream of back in 1953.
The person heading the effort is Pascale Ehrenfreund, one of the stars of international astrobiology. She has been principal investigator or co-investigator on many NASA and ESA experiments, including satellites, planetary probes, and experiments on the International Space Station. The basic logic of the Miller-Urey experiment remains the same: to see whether combinations of gases, including water, can produce the nonliving but essential building blocks of life when hit with an electric charge. The difference is that Miller-Urey in space will do the experiment in zero gravity, and will be looking to mimic the formation of complex organic molecules in the solar nebula rather than the oceans of Earth.
Ehrenfreund, an astrochemist, is an expert in polycyclic aromatic hydrocarbons, which are known on Earth as a pollutant formed by burning fossil fuels but in astrobiology are important, much-studied players. In recent years PAHs have been detected across the universe, and are now known to be ubiquitous and plentiful, perhaps the most common complex chemical compound in interstellar space. They also make up about 70 percent of the organic carbon that Glavin and Dworkin study in carbon-based meteorites, micrometeorites, and interplanetary, or interstellar, dust that falls to Earth.
“So for us, an important goal of the experiment is also to see about the formation of polyaromatic hydrocarbons,” Ehrenfreund told me. “They were falling onto Earth in large amounts when life was being formed, so obviously we want to know if they were somehow involved.” At the very beginnings of the process that later resulted in life, she said, “everything had to be extremely simple and extremely robust to withstand the hostile conditions.” PAHs are both, and also have a structure not too different from that of a cell wall, leading some to speculate that PAHs played a role in organizing cell structure, too.
Miller-Urey in space is an important experiment for Ehrenfreund, but it isn’t her only work now flying on the ISS. She is a co-investigator as well on an experiment that exposes spores, bacteria, fungi, and other organic material to the harsh space environment by hanging them out of the Russian and European laboratories attached to the station—research designed in large part to test the theory of “panspermia,” that life on Earth arrived from Mars via space. Results so far show that some microbes can indeed withstand the radiation, desiccation, and cold of space.
Ehrenfreund also leads an ISS experiment that dangles a metal container filled with PAHs and other complex carbon molecules from the space lab. The goal of this “Expose” experiment is to understand the effects of zero gravity and ultraviolet and other cosmic radiation on PAHs, and it focuses especially on photochemistry, the changes that occur when different forms of light hit the compounds. The Miller-Urey breakthrough involved learning how amino acids could be formed in the early days of Earth; scientists hope that this Expose experiment, as well as Miller-Urey in space, will help show how the compounds ultimately found in life came to be formed in the early solar system, billions of years before and many light-years away.
If you step back for a moment, you can see that this work is all part of an interplanetary and interstellar forensic quest to tease apart how the building blocks of life came to be on Earth. Since those same building blocks are landing on planets and moons around the universe, the obvious question is whether they are being used as the chemical backbones to life on other planets and moons, too. It’s demanding, technically complex, and totally absorbing—solving mysteries that make crime forensics look, well, elementary. Since the Miller-Urey experiment and its primordial soup became household words a half century ago, nothing in the world of origin-of-life research has made anything like that kind of splash. Half joking, half serious, Ehrenfreund imagined how that might change.
“What we need is a movie or a TV show, something about how we’re trying to solve these big mysteries,” she said with a pleased laugh. “That’s it—a show all about the forensics of life in the universe. I think it could be a hit.”
5 ON THE TRAIL OF LIFE ON MARS
The scientist who first confirmed the presence of the gas methane on Mars at specific locations and at specific times—the best sign yet that the planet is, or was, alive—has been a rebel since he was a boy. Michael Mumma grew up in Amish country outside Philadelphia, Pennsylvania, in a family whose life was built around a farm and the fundamentalist Evangelical United Brethren Church. But Mumma had a strong independent streak—he liked to spend his days roaming the woods and fields and at ten he took on his own newspaper route to build an income stream of his own. At eleven, he confronted his pastor. The science teacher had told his class that the Earth was more than 4 billion years old, that humans were the product of evolution, and that a fossil record existed that supported both ideas. So the world was not created by God in seven days? “My minister had a simple response when I asked him about it,” Mumma recalls. “He said that to be part of the church, I had to believe what is taught here and not what the teacher says. He said the teacher at school was wrong.”
More than five decades later, you can pretty much draw a straight line from that encounter between Mumma and his minister and the announcement in early 2009 of one of the most significant discoveries in astrobiology: the appearance of methane gas on Mars at predictable spots and at regular intervals. Now Mumma is on a quest to understand how and why the gas is spewing out from the Martian underground, and what its origin might be. On Earth, some 90 percent of the methane in the atmosphere is a by-product of living creatures, and biology has to be considered a serious candidate for the production of methane on Mars as well. In other words, the methane could be the result of living or once-living extraterrestrial life. It is also true that methane can come out of a geological process. But the alternative to biology would be almost as significant: Mars has long been considered geologically dead—without volcanoes, earthquakes, and submerged moving plates—so the presence of geologically replenished methane would mean it is not.
That straight line running from Mumma’s early disenchantment with his minister to his Mars announcement is drawn like this: Mumma’s already strong independent streak was given a powerful boost by his encounter with the minister and the break with his church that followed. He later became the first in his family to leave Lancaster County and the fold since his ancestors arrived there in 1731. But rather than defeating him, his departure put him on the path to becoming a seasoned researcher known for his mantra of always following the scientific data and his hardwired habit of thinking outside the box. That willingness to buck the community—be it religious or scientific—was essential, because he could never have made the methane discovery had he stuck to tried-and-true strategies, and had he been swayed by the many people who told him his approach was misguided and his goal most likely unattainable.
Mumma first came to Goddard, in Greenbelt, Maryland, just outside Washington, D.C., in 1970, choosing it over six other job offers at a time when students with doctorates in atomic and molecular physics were not exactly in high demand. He set to work on improving—revolutionizing, really—the use of spectroscopy in astronomy, the gathering of photons from afar and breaking them down into distinguishable component parts. His particular job was to build a spectrometer that could better detect and identify the molecules in comets, using the signatures of particles coming off the nuclei of atoms. He and collea
gues at NASA Goddard came up with the technology to take readings of distant molecules in the previously little-used infrared band, a high-energy form of radiation on the electromagnetic spectrum. Using the infrared allowed Mumma to collect more subtle data that could much better identify and define distant molecules and compounds, a process akin to building a telescope with a bigger mirror that can see farther and with more precision. Without the ever-increasing detection power of infrared spectroscopy, Mumma could never have identified those seasonal releases of methane on Mars, nor would he have had the tools needed to determine whether the methane comes, at least in part, from some living, or once-living, creatures.
The great excitement within astrobiology about methane on Mars comes not only from the discovery of the gas, but also from where on the planet and when in the Martian year it was found. So far, the methane (a carbon atom surrounded by four hydrogen atoms) has been detected at five locations, and all have at least one thing in common: They are areas where the remnants of early Mars, that is, when it was most Earthlike and most hospitable to life, are least disturbed. Specifically, that means a time when liquid water appears to have run freely around the planet, and when Mars had a magnetic field surrounding it that enabled a much thicker atmosphere to act as a shield against the ravages of the solar wind and the ultraviolet radiation that now desiccate the surface. This was some 3.5–4.5 billion years ago in what is called the Noachian age of Mars, yet remainders of those days have been detected in low levels of magnetism still present in some areas. Mars scientists have also found evidence of minerals that can only be formed and transformed in the presence of liquid water. In other words, the methane seems to be coming from areas most protected from the yet-unknown catastrophic events that turned Mars from a watery planet shielded from the harsh forces of the sun and cosmic radiation to the parched landscape of today, with its thin atmosphere unable to ward off those formidable assaults.
