Astrobiology Isn't a Dirty Word Anymore

THE CONTROVERSIAL STONE

Astrobiology is also more than a marketing slogan, thanks to recent discoveries that do indeed give astrobiologists a subject matter to study. By far the most important, the Allan Hills meteorite, a 1.9 kilogram hunk of Martian rock, was discovered in Antarctica in 1984. In 1996, David McKay and Everett Gibson of NASA's Johnson Space Center in Houston published a paper¹ claiming that the meteorite contained molecular fossils that were byproducts of organic processes. "That, more than anything else, was the birth of astrobiology as a serious subject," says Carol Cleland, a member of the philosophy department at the University of Colorado, who has studied the emerging field and its implications for society. "The claims are still as controversial as when they first came out, but it made people realize how easy it could be for life to be happening elsewhere." With that realization came a new-found respect for the concept of astrobiology.

Indeed, the concept that organic molecules can form autonomously has been known since the 1950s, when Stanley Miller and Harold Urey ran an electric current through a vacuum containing organic compounds and ammonia gas, and 2% of the carbon in the bell jar formed into amino acids. Even though amino acids have long been documented in meteorites and even in samples of interstellar space dust, no direct evidence confirms that they resulted randomly from the heat and pressure of a hurtling meteorite or whether they comprise the remains of a living organism.

The Allan Hills meteorite and the Miller-Urey experiment were not alone in lending credence to the astrobiology movement. Equally important was the discovery of extremophilic organisms in deep-sea vents. The organisms live off the thermal energy of the vents, which provide the same wet and hot conditions that are thought to have prevailed on Earth during the initial formation of prokaryotes. Similar conditions existed on other planets, most notably on Mars and Venus, during other periods of their existence.

Much astrobiology research aims to find conditions that mimic early Earth. That explains why David Kring, a geologist at the University of Arizona, is preparing to drill a hole 1.5 kilometers deep in Mexico this year. While other astrobiologists study Martian meteorites and steam vents, Kring specializes in impact craters. "It can take up to a million years for the heat of an impact crater to dissipate," Kring says. "During that time, that heat can provide all of the energy you need to start the chemistry of life."

So, funded with a large grant from the National Science Foundation, Kring will drill his massive hole in the Chicxulub crater in the Yucatan Peninsula to determine, through geological findings and temperature readings, exactly when the enormous crater was formed. "Around the time that Earth was being pelted by these enormous meteorites [an estimated 1.9 billion years ago], there was an explosion in evolutionary activity. If that activity continued at that rate today, we would have 10²⁷ families of species." Instead, Kring says, only about 10³ families exist today. Clearly, there's a correlation between the asteroid impacts and the development of early life.

The irony of a geologist looking for evidence of biological activity doesn't escape Kring, who revels in his adopted field of astrobiology. "When I got my PhD in 1988, I thought of myself as a geologist. I wanted to know what other planets were made of. Now I'm an astrobiologist. I don't care what I'm called as long as I get to keep doing this kind of work." Runnegar, as head of the NAI, was even more amazed by the course of his own career: "My degree was in paleontology. I never expected I would one day be overseeing biology projects."

Courtesy of NASA

The Spirit rover is the size of a small car. It contains two cameras and six instruments, including a rock abrasion tool, Mossbauer and alpha particle x-ray spectrometers and a microscopic imager.

BEEN THERE, DONE THAT

Some scientists express no surprise that they now call themselves astrobiologists. Christopher Chyba, who has been awarded a MacArthur Fellowship for his work at Stanford University and at the SETI (Search for Extraterrestrial Intelligence) Institute in Mountain View, Calif., never doubted that he would be exploring planetary biology. He earned his PhD at Cornell University ten years ago and worked in Carl Sagan's lab. "A lot of what we do today can be traced directly back to Carl's vision," Chyba says, who also lauds the astrobiology shift at NASA and thinks it was inevitable. "At some point they had to address the central questions of why we are going into space. We are doing it to find out if there is life elsewhere. That's why we were doing it all along, but we were just too timid to say it." As proof, Chyba points to the Viking landers.

Those two crafts, which landed on the surface of Mars in the summer of 1976, conducted the first real astrobiology experiments. Each carried crude instruments to search for microbial life and to perform four experiments. The first experiment analyzed the composition of Martian soil in a gas chromatograph. Scientists had hoped to find evidence of organic molecules. They didn't.

In the second experiment, Viking shot a short sticky string at the ground and collected the dust that clung to it. That dust was deposited into a test tube containing radioactively labeled nutrients, and then the brew was slightly heated. Any microbes present in the dust would grow in the test tube, feed off the nutrients, and belch off radioactive gas byproducts, which would in turn be detected by a Geiger counter. The last two experiments were variations of this method. Two experiments showed no activity, but one caused the Geiger counter to start clicking.²

The designer and coordinator of the Viking experiments, Gilbert Levin, now says that the radioactivity suggested the presence of microbial life on Mars. Most other astrobiologists, however, discount the positive results as signifying only the effects of oxidative minerals in the Martian soil.

In early fall the Cassini spacecraft will reach Saturn and will drop the Huygens probe onto its moon Titan. It should answer the question of the existence of methane oceans on Titan once and for all.

Nevertheless, the lessons for the Viking mission scientists go far beyond the crude data gathered. "They sent a lander to Mars the moment technology allowed it, which is commendable in terms of their bravery," Chyba says. "But the Viking missions also set astrobiology back because of its 'grab-for-the-brass-ring' approach." Primitive instrumentation and a poorly designed set of experiments (approved by engineers, not biologists) deflated interest in finding traces of life on Mars.

FOLLOW THE WATER

The NASA teams behind the landers that hit Mars this month hope to avoid such mistakes and further boost the credibility of astrobiology. Rather than going for broke with a life-proving experiment, they simply hope to explore the terrain, delve a few millimeters into the regolith crust, and concentrate on straight geology rather than what will probably be a vain search for microbial life. "If Mars ever had microbial life, it probably existed deep under its crust," Chyba says. "The radiation on the surface is an enormous hurdle to overcome."

The two Martian rovers, dubbed Spirit and Opportunity, are designed to wander about 100 meters each day from their landing zones in the Gusev Crater and the Terra Meridiani Plains. The driving idea behind the landers, says Chyba, is to follow the water. "We're doing it much more smartly this time," he says. "With Viking, the main concern was safety. Now we've designed much more robust landers and they're dropping in areas where water was most likely to have existed." Occasionally, they can stop to scoop up rocks with their robotic arms and then analyze them with the onboard cameras, spectrometers, and microscopes. Each lander also can use its rock abrasion tool to drill five millimeters into the surface of the regolith to obtain material unaffected by Martian weather.

Meanwhile, the European Space Agency's Beagle II lander, which was designed to be a stationary probe akin to Viking, failed during the landing process and is believed to be lost for good. Even before the failure of the Beagle, Europe was far behind the United States in adopting the astrobiology banner. "I proposed an astrobiology workshop at the European Molecular Biology Organization meeting and I was met with blank stares," says Víctor de Lorenzo Prieto, a biochemist at Spain's Centro de Astrobiologia just outside of Madrid. Funding is also more difficult to obtain for European astrobiologists, because the field isn't included in the centralized Sixth Framework Research Programme for the European Union. "We don't get the respect and we don't get the money that our colleagues enjoy in the United States."

That respect is paying off in an explosion of upcoming astrobiology-based NASA missions. Next up is the Mars Science Laboratory, due to launch in 2009, which will have much more advanced biology and geology experiments, including possibly a deep drill. Other probes to Mars will be launched every couple of years after that, in addition to probes to the moons of Saturn and Jupiter, and the Kepler mission, a deep-space telescope in search of other planetary systems. And all the while the lab notebooks of astrobiologists will continue to fill up with subject matter to study.

Sam Jaffe can be contacted at sjaffe@the-scientist.com.