In a laboratory at the University of Chicago, Jennifer Blank places a steel capsule in the firing path of a 15-meter-long gun that shoots soda-can-size bullets. Sensibly, she then leaves the room. From next door, she and her colleagues trigger the gun to fire. They hear only a muffled pop, but they know they’ve just caused violence of almost unworldly proportions.
This oversize gun play is how a cadre of researchers including Blank, a geochemist from the University of California, Berkeley, simulate comet and meteorite impacts. They’re out to answer an awe-inspiring question: Could life’s building blocks have stowed away on such space debris and then survived an impact with Earth?
For nearly a hundred years, researchers have wondered whether the newborn Earth possessed a primordial stew with all the ingredients necessary for creating life. In a famously successful 1953 chemistry experiment at the University of Chicago, Stanley Miller and Harold Urey demonstrated that a spark of electricity coursing through a brew of hydrogen, water, ammonia, and methane could ignite a reaction that makes amino acids–the building blocks of proteins and, therefore, of all Earth’s life.
But when Miller and Urey performed their experiment, they assumed that the early Earth’s atmosphere was very different from today’s. In the 1950s, scientists thought that the atmosphere 4 billion years ago was rich in gases that contain hydrogen and poor in those bearing oxygen. Chemists call such an atmosphere “reducing.” But many researchers now hold that the ancient Earth’s atmosphere, compared with the earlier view, had more oxygen and less hydrogen–as the atmosphere does today. Amino acids don’t form as readily under that condition as they did in the 1953 experiment, and when they do form, they tend to break apart.
There’s no shortage of theories for explaining how life could have arisen on Earth, even in the primitive atmosphere now considered likely. For example, some researchers suggest that life got its first toehold in one of Earth’s more exotic locales, such as deep-sea hydrothermal vents or on the surface of certain rocks (SN: 5/5/01, p. 276).
Others, including Blank, have looked outward. These researchers speculate that precursors to life might have arrived on an asteroid, meteorite, comet, or even interplanetary dust (SN: 2/3/01, p. 68).
If you make a shopping list of all the chemicals you’d need to create life, they’re all found in space, says astrobiologist Max Bernstein of NASA’s Ames Research Center and the SETI (Search for Extraterrestrial Intelligence) Institute, both in Mountain View, Calif. “It seems unlikely that that’s just a coincidence,” he adds.
The next question is, Could those chemicals have traveled from their out-of this-world venues to Earth’s surface?
No one knows if the delicate chemicals could have survived the intense heat and pressure of an arrival via comet or meteorite. Nor does anyone know how an asteroid, meteorite, or comet impact might have altered Earth’s atmosphere locally, perhaps making it more friendly to life.
Short of witnessing a collision firsthand–not a desirable scenario in the case of a comet or an asteroid hitting Earth–researchers need to attack these questions with simulations, such as mathematical calculations and computer models. Still another way is Blank’s way. Right in the lab, she and her colleagues recreate the behavior of comets and meteorites striking Earth.
Large comets
According to Christopher Chyba of SETI and Stanford University, large comets probably supplied vast amounts of extraterrestrial material to the early Earth. Since potentially life-spawning chemicals almost certainly were within that heap of space stuff, Chyba and Elisabetta Pierazzo of the University of Arizona in Tucson mathematically modeled comets and asteroids slamming into Earth.
They concluded in 1999 that some amino acids on kilometer-wide comets would indeed survive a direct hit to Earth’s surface. Even more of the molecules could endure a low-angle encounter. The scientists also found that some types of amino acids would be more rugged than others in such impacts.
Intriguing as such theoretical calculations are, their value is limited unless they can be tested. These tests call for big guns.
Blank suspects that comets, in particular, could have given Earth everything required for life. Spectroscopy studies have shown that comets contain organic material, including the components of amino acids. Comets also carry water, and an impact’s energy could have driven some of the initial reactions that led to life, Blank adds.
There’s a problem with this idea, however. In a beaker, these molecules break down well below the high temperatures present during an impact. So, how could they survive an actual impact?
For one thing, says Blank, a comet isn’t a beaker. And small, tabletop experiments ignore one of an impact’s crucial factors–pressure. “We have very poor intuition for the effects of pressure,” she says.
To figure out whether amino acids can withstand pressures of up to 200,000 times that of Earth’s atmosphere and temperatures of 500 to 600C, Blank and her colleagues created some of the best physical simulations so far. In each trial, they encased a solution of amino acids in a stainless steel, nickel-size disk and put the disk in a large tank. They then shot it with a metal-tipped projectile that traveled at nearly 2 km per second, or 4,500 miles per hour. Blank says that the first apparatus they used looked like a 3-ton cannon. The Chicago fire marshals were not pleased, she recalls.
After retrieving the steel container, Blank and her colleagues found that some portion of each of the five amino acids she tested survived the shock of the impact. In fact, the violent reactions even stitched some amino acids into peptides, the chainlike molecules that make up proteins, says Blank. She reported her team’s work in San Diego in April at a national meeting of the American Chemical Society.
Using a similar megagun at Los Alamos National Laboratory in New Mexico, Blank’s team first froze the steel container to better simulate the conditions of a comet. Compared with the Chicago experiments, an even larger proportion of the amino acids in the disks survived. The researchers also found that different peptides formed when they varied the pressure and duration of the impacts by changing the speed of the projectile and the thickness of its tip.
Since these results suggest that impact wouldn’t destroy amino acids on an incoming comet, “we couldn’t rule out this extraterrestrial origin” for primordial organic molecules, says Blank. The experiment “is support for the possibility” that comets brought amino acids and other biologically important material to Earth, she says.
Blank and her team achieve realistic pressures and temperatures, even though the 2-km-per-second speed of the projectile is much lower than the minimum speed at which a comet would hit Earth. However, the duration of the shock the researchers generate is much shorter than what they would expect for a large impact. Blank notes that, so far, her team’s results suggest that lengthening the duration of the shock creates more peptides.
For the moment, these laboratory results are helping provide reality checks on theoretical models of high-speed impacts, says Pierazzo, who’s now working with Chyba to apply impact calculations to the possible survival of comet- or asteroid-delivered amino acids on Mars and on Jupiter’s moon Europa. What’s more, Chyba adds, the velocities in experiments like Blank’s will probably increase as new technology becomes available.
Impact simulations
Simulating comets is only one goal for aficionados of impact simulations. Consider Seiji Sugita of the University of Tokyo. Like Blank, he relies on large guns, but he has a different goal in mind. Using an instrument at NASA’s Ames Research Center in Mountain View, Calif., Sugita has studied how meteorites interact with the local atmosphere during impacts.
In his experiments, Sugita has found that iron-rich meteorites could have temporarily altered the atmosphere around their impact site. Most notably, they could decrease oxygen concentrations, creating the type of environment around them that would encourage amino acids to form efficiently.
With this type of atmospheric change, organic material deposited by meteorites might have participated in reactions that formed life’s first biologically important molecules, says Sugita, who also presented his results at the April American Chemical Society meeting.
To reach this conclusion, Sugita and Peter Schultz of Brown University in Providence, R.I., used Ames’ 10-meter-long vertical gun–a device originally designed to study how Apollo spacecraft would land on the moon. This instrument can launch objects to velocities of 7 km per second. In their experiments, Sugita and Schultz shot a small plastic projectile at a piece of copper and carefully analyzed the light emitted during the ensuing impact. The projectile represented a meteorite, while the copper provided a hard surface for breaking the projectile apart.
The researchers observed in detail how the projectiles, during the intense impacts, shattered at temperatures of several thousand degrees C. At millionth-of-a-second intervals throughout the experiment, spectroscopy data revealed temperature and chemical changes, says Sugita. With a plastic projectile moving at 6 km per second, he adds, the experiment simulates an iron meteorite impact of about 20 km per second, which is close to the speed calculated for some incoming meteorites.
Sugita and Schultz found that the plastic projectile fragmented into tiny particles, which reacted extensively with molecules of the experiment’s earthly, nitrogen-rich atmosphere. Extrapolating this result with a carbon-rich plastic projectile to an iron-rich meteorite, the researchers concluded that the latter would react with carbon dioxide on the young Earth, temporarily creating a reducing atmosphere around the impact site.
Sugita’s experiments suggest how even an early Earth atmosphere that was hostile to the formation of biological molecules could have been locally, temporarily transformed into a friendlier status, comments Bernstein. This could have helped precursor molecules already on the planet or delivered from space take the next steps toward life.
Small asteroids
Many researchers say large impacts by asteroids, comets, and meteorites, such as those modeled by Blank and Sugita, would have been the primary source of organic molecules for the early Earth. Still, some scientists are examining the opposite end of the extraterrestrial spectrum–small, millimeter-size particles of asteroids and comets. Perhaps these grains, too, could have ferried biology’s precursor molecules to Earth’s surface.
“Maybe it’s the small stuff,” says Duncan Steel of the University of Salford in England.
Daniel P. Glavin and Jeffrey L. Bada of the University of California, San Diego study whether amino acids could survive as such small objects enter Earth’s atmosphere. Even if only one amino acid, glycine, withstood the intense heat that incoming space particles encounter, it could spawn life, they say.
Bada says that, at first glance, glycine alone might seem too limited a chemical palette. Scientists used to think that life needed “every amino acid under the sun,” he says, but now, some scientists suspect everything may not be required.
Glycine could have made the backbone of a molecule called peptide nucleic acid that’s been suggested to have served as a forerunner to DNA and RNA, he and Glavin suggest.
Taking another angle on space dust, Steel and Christopher McKay of NASA’s Ames Research Center looked at a population of small meteoroids that vaporize higher in the atmosphere than the typical ones, which are made of rock and metal.
The researchers suspect the reason for this quick-burn property may be the presence of massive organic molecules. They say such organics wouldn’t be volatile enough to escape from these so-called tarry meteoroids as they travel near the sun, and they’ve calculated that heavy organic molecules could theoretically survive thousands of years in space. The constituents of these small tarry meteoroids wouldn’t disintegrate completely in the atmosphere and could shower down on Earth, claims Steel.
“There appears to be an ongoing rain coming down on our head,” he says.
It may be that the best clues to life’s first molecules remain out in space. Researchers can theorize with computers about impacts, simulate them in the laboratory, and test meteorites that have fallen to Earth. But they’ve yet to get their hands on untainted, extraterrestrial samples of space stuff.
The Stardust mission is on its way to collect much anticipated samples from the comet Wild 2. The mission is scheduled to return to Earth in 2006. Various international attempts to retrieve other comet and asteroid samples could follow over the next several years.
