| The Magazine of the CALIFORNIA ACADEMY OF SCIENCES |
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Feature Message in a Meteor"Mars ain't the kind of place to raise your kids," sings Elton John in "Rocket Man." No argument there. The images beamed back from two Viking missions to Mars in the mid-1970s depicted a bleak and frigid rock-strewn desert. A trio of experiments failed to find any sign of extant life in the peroxide-laden surface sediment. But some scientists clung to the possibility that life existed on Mars long ago--a few billion years--when the Red Planet was most likely a much warmer and wetter place. Maybe it even still survived in hydro-thermal habitats deep underground. This hope received a startling boost in August when NASA announced to a slack-jawed world that its crack scientific team had uncovered what it believed to be evidence for ancient life on Mars. The announcement became instant front-page news, and more than a million Internet surfers perused the technical report in Science within the first two days of its posting. President Clinton was moved to remark, "If this discovery is confirmed, it will surely be one of the most stunning insights into our universe that science has ever uncovered. Its implications are as far-reaching and awe-inspiring as can be imagined." The chemical and purportedly biological clues come from small slices of a softball-sized meteorite from Antarctica that had been cleft from Mars by an impacting object, which sent the rock spinning into space on an Earthbound journey. Now scientists are clamoring to find more bits of Mars in their meteorite collections, turning to known Martian meteorites to see if they contain further clues to any life, and thinking anew about the role that meteorites and other astronomical visitors have played in the history of our planet and, perhaps, in the origin of life. An asteroid impact has been fingered as a favorite culprit in the mass extinction of dinosaurs 65 million years ago, for instance, but Earth's encounters with kamikaze crater-makers extend back as far as the planet's infancy. Our own Moon formed nearly 4.5 billion years ago when a Mars-sized planet or asteroid plowed into Earth, vaporized itself and the proto-Earth, and gas and rock reaggregated to make the Moon. Since that calamitous collision, from which Earth thankfully recovered, to what extent have meteorites, asteroids, and other celestial bodies served as creators, conveyors, and destroyers of life? Two days after Christmas 1984, geologist Roberta Score was snowmobiling across a vast Antarctic ice field with a handful of colleagues who journeyed there in search of meteorites. It was her day off, but Score couldn't resist investigating a green-looking rock dwarfed by mammoth formations of sculpted ice in the Allan Hills region. At the time, she thought there was something weird about it, and her instincts would prove dead right. As the first meteorite from the 1984 search to be processed at NASA Johnson Space Center, Score's discovery was labelled Allan Hills (ALH) 84001. Since 1969, scientists have known that Antarctica is a mother lode of meteorites. American teams have mounted annual National Science Foundation-funded expeditions there since 1976, scouring the ice for what has been called a "poor man's space probe" because each fragment of meteorite provides a glimpse of cosmic chemistry that could otherwise only come from costly, equipment-laden missions beyond Earth. It's not that more meteorites fall in Antarctica. Most disappear into oceans or seas; those that strike land soon erode away and blend in with surrounding rocks to an undiscerning eye. When a meteorite strikes Antarctic ice, it quickly becomes buried, reducing the chances for erosion and contamination. Due to the drop in elevation from the South Pole to the edge of the continent, Antarctic ice naturally shifts seaward, carrying its extraterrestrial payload. As the ice abuts prominent rock barriers such as the Transantarctic Mountains, it gets pushed upward and brings meteorites closer to the surface. Heavy winds sweep away snow and ablate the blue ice, eventually exposing the meteorites. Meteorite hunters can often easily spot their dark quarry contrasting with the bright ice, though they must endure 40-knot winds and freezing temperatures and avoid treacherous crevasses. They retrieve the rocks with sterilized tongs and place them in special collecting bags like those used for Moon rocks. The precious rocks are kept frozen for their journey to the Johnson Space Center (JSC) in Houston where they are thawed in a laboratory clean box containing nitrogen to remove any ice and water and prevent contamination. Curators knock chips off the old rocks and send them to scientists for study. About 2,500 meteorite specimens had been found worldwide before anyone looked in Antarctica, but intensive collecting there has supplied nearly half the world total of 16,357 specimens. The vast majority of meteorites are chunks of passing or incoming asteroids, from the asteroid belt between Mars and Jupiter. A tiny sample, however, of a dozen each can be traced to the Moon and Mars. The lunar specimens matched the mineralogy of rocks collected by Apollo astronauts on the Moon. Tracing meteorites back to Mars proves a bit trickier, as no rocks have been collected from its surface. But that planet has been inferred as the source for twelve specimens based on a distinctive signature of trapped gases in one meteorite, which is similar to Viking's measurement of the atmosphere of Mars, and the fact that most Martian specimens are much younger than asteroids. This select dozen are collectively called SNCs ("snicks"), an abbreviation for Shergotty, Nakhla, and Chassigny, locations in India, Egypt, and France, respectively, where three meteorites were discovered in the nineteenth and early twentieth centuries. Most SNCs are small fragments of larger objects that split apart in space or during impact. The largest one, the 18-kilogram Zagami meteorite, landed in 1962 with a puff of smoke and a shock wave just a few feet from a farmer tending his cornfield in Nigeria. All six SNCs retrieved since then have come from Antarctica. When ALH84001 was found, no one recognized it as Martian. That realization happened nine years later. Despite its unique mineral make-up compared to other SNCs, a characteristic ratio of oxygen isotopes indicated that this rock also arrived from Mars. A recent statistical analysis by Nadine Barlow of the University of Central Florida has pinpointed two craters as the most likely sources of the impact that launched ALH84001 on its epic journey. Barlow winnowed 42,283 impact craters down to 23 with the combined criteria of ancient geologic age and youthful crater appearance to best match the evidence from ALH84001. These she narrowed further to two candidates: both elliptical craters in the Southern Highlands of Mars with respective diameters of 23 and 11 kilometers. According to three dating techniques, the ALH84001 rock crystallized 4.5 billion years ago from molten lava beneath the Martian surface. Carbonates in the meteorite, on which centers the interpretation of early life, have been dated by the argon-argon method to 3.6 billion years ago, although other researchers using an alternative technique, rubidium-strontium dating, recently came up with an age of only 1.4 billion years, comparable to the age of the next oldest SNC meteorite. If the younger age turns out to be correct, it raises the question of where on Mars life could have survived, since the surface at that time was probably as inhospitable as it is today. That is, if the NASA team has in fact detected relics of once-living organisms. The evidence for biological traces in ALH84001 falls into three categories: the presence of calcium carbonate globules within the meteorite; distinctive mineral and chemical compounds and organic matter associated with these globules; and structures in the globules interpreted as fossilized bacteria. Individually, none of these lines of evidence offers convincing or perhaps even compelling testimony to early life on Mars. But when all are considered together, conclude David McKay and Everett K. Gibson, Jr., of NASA JSC and their seven co-authors, a biological explanation becomes the simplest answer. Sometime around four billion years ago, shocks from impacts on Mars cracked the ALH84001 rock. Calcium carbonate gradually filled these fissures, suggesting that the rock had been submerged under relatively cool water, possibly in a deep hydrothermal environment. Within the fissures are scattered tiny orange carbonate globules ringed by layered, black-and-white "oreo cookie" rims containing iron- and magnesium-rich minerals. The NASA team homed in on the globules with special high-resolution scanning electron and transmission electron microscopes, and after months of searching they observed some curious sights. Long particles of iron sulfide and teardrop- and cube-shaped crystals of magnetite occurred in the center and around the rims of the globules. The iron sulfide minerals bear resemblance to greigite, which on Earth is an organic by-product of sulfur-metabolizing bacteria. And the minute magnetite grains, a billion of which would fit on a pinhead, share similar size, shape, and composition with magnetite found in terrestrial bacteria. These could have been made inorganically, or they may have helped Martian bacteria get oriented in a planetary magnetic field. Even more provocative evidence concerns clusters of oval and skinny structures and numerous longer, apparently segmented bodies. Ranging from 20 to 700 nanometers long (or one thousandth to three hundredths the width of a human hair), these could be fossil bacteria, as McKay and his colleagues suggest, or they could just be eroded blobs of carbonate. The structures are a mere fraction of the size of unambiguous bacterial fossils from Earth; however, at least one contempory bacterium, Coxiella, can be as small as 200 by 400 nanometers. Another key clue to the interpretation of early life emerged from the Stanford University chemistry lab of Academy Fellow Richard Zare. For more than two decades Zare and numerous graduate students have developed a sensitive technique called microprobe two-step laser mass spectrometry that can analyze minute amounts of molecules. It relies on two lasers that straddle a time-of-flight mass spectrometer, a six-foot metal tube taking center stage in Zare's lab beneath a dangling array of electrical cables. One laser emits an infrared beam that briefly blasts a sample with intense heat to lift the molecules into the path of a perpendicular beam from an ultraviolet laser that vaporizes and ionizes the molecules. The charged ions then hurtle from a vacuum chamber to a collector at the far end of the mass spectrometer, which detects the presence and amounts of various molecules from the mass of the careening ions as measured by how long they take to reach the collector. What Zare's team found on thin slices cleaved from chunks of ALH84001 was plenty of polycyclic aromatic hydrocarbons (PAHs). PAHs are a group of organic molecules that can be formed by both organic and inorganic processses. Organic ones occur on Earth, for instance, in chimney soot, diesel exhaust fumes, or the char on a barbequed burger, and they accumulate as dead organisms decay. Inorganic PAHs result from other processes, such as methane or acetylene gas contacting hot metal or rock, and are common inorganic components of interplanetary dust. So, as Zare says, the identification of PAHs alone does not prove that they came from something once alive. The PAHs on the meteorite increased in frequency the further inside the meteorite the scientists probed, and they correlate closely with the location of carbonate globules. This suggests that the PAHs are intrinsic features rather than contaminants from outside. Zare says that the limited variety and simple chemical skeletons of the PAHs analyzed in ALH84001 mirror the pattern in old coal, that is, something that has long been fossilized or decayed. "This sample we have could have formed at high temperatures or long ago," he says, "and I can't tell the difference between the two." Therein lies a conundrum. Although scientists are now scrutinizing each piece of evidence for the presence of life in ALH84001, perhaps the biggest challenge comes from a paper published in Nature a month before the NASA team's startling announcement. Planetary scientists Ralph Harvey, of Case Western Reserve University, and Harry McSween, of the University of Tennessee, concluded that the carbonates in ALH84001 formed not by immersion in cool water, as a previous model proposes, but by the sudden reaction between rock and boiling, carbon-dioxide-rich water during an impact. The collision created fracture patterns in the rock along which traveled CO2 from melted polar ice or surface frost. McSween contends that the meteorite's carbonates do not resemble ones on Earth that form at low temperatures, such as below 80 ¼C. Since the rock is predominantly made of pyroxene, there should be ample clays evident to support the conclusion that the rock came in contact with water. The large concentration of calcium present in ALH84001 matches the level in terrestrial carbonates formed at temperatures exceeding 650 ¼C. Such intense heat would wither the chemical bonds of any carbon-based life. And while he admires the Stanford team's technical triumph in making "the first unambiguous measurement of organic matter from Mars," McSween says that the PAHs conceivably could have condensed from the same hot vapor as the carbonates. In other words, they may not be from microbes. "They don't look like what forms from the decay of organisms on Earth. They look much simpler, and one way to make them simpler is to heat them." NASA's Gibson counters that several observations point to a different origin for the carbonates. The ratio of two oxygen isotopes within the sample is consistent with formation at low temperature. Extreme temperatures would have homogenized the minerals and decomposed the organic molecules. The iron sulfide compounds that have been interpreted as greigite would degrade at temperatures above 250 ¼C. He also says that their sample does contain evidence of clay. A more recent theoretical study by Kevin Hutchins and Bruce Jakosky, of the University of Colarado at Boulder, raises more uncertainty that the carbonates formed at low temperature and considers their formation in the context of the dramatic changes Mars has experienced. Mars once had a thick atmosphere and warm temperatures, but now just the opposite exists. Their analysis of isotopes of oxygen, carbon, and argon concluded that lighter gas isotopes were lost into space after volcanoes spewed the gases into the atmosphere. The resulting enrichment of heavier isotopes suggests to Hutchins and Jakosky that the carbonates in ALH84001 most likely formed in a hydrothermal setting--as volcanic rock seeped into the crust, heating and circulating the groundwater above--at temperatures between 50 and 350 ¼C. "As the temperature for the formation of carbonates goes up," says Hutchins, "the possibility for the evolution of life goes down." Although McSween considers himself a "hopeful skeptic" about microbial life on Mars and thinks we need to keep looking regardless of how science ultimately evaluates the evidence from ALH84001, he says, "It seems highly improbable that one of the twelve rocks that we've received from Mars should have fossils in it. We ought to have to look harder than that." But Gibson points out that this particular rock has probably been around almost as long as Mars itself and ought to record major episodes in the planet's history. "There is an abundance of data in this rock that is astounding," says Gibson. "All I can say is stay tuned." In late October, a team of British planetary geochemists reported confirming evidence for the presence of life in ALH84001. Colin Pillinger and Ian Wright, of the Open University, and Monica Grady, of the Natural History Museum, detected in the meteorite an abundance of organic carbon with an isotopic ratio matching that produced on Earth by methane-producing microbes. More intriguingly, a similar chemical signature turned up in a second meteorite analyzed by the team, EETA79001, which formed only 175 million years ago and was launched from Mars a mere 600,000 years ago. Gas trapped within this same specimen had previously provided conclusive evidence linking the SNCs to Mars, and the new result marks the best evidence yet that life may reside there today. Much more study of these and other SNC meteorites can be expected soon. As for the reported bacterial bodies, UCLA paleontologist William Schopf, who has discovered the oldest bacterial fossils on Earth, says, "We've got to look inside these things--see if they have cell walls, see if they are compartmentalized...see if they are composed of organic material." That's exactly what the JSC scientists have in mind: plucking some of the miniscule objects from the meteorite, encasing them in epoxy, slicing them open, and peeking inside. They've already begun honing their skills with tiny bacterial fossils from the Columbia River basalts before tackling the meteorite. Meanwhile at Stanford, Zare's team is busy retooling equipment to search for amino acids, the building blocks of proteins, within their samples of ALH84001. Other researchers will try to detect the remnants of lipids or fatty acids; a team in Japan will search for organic material using a technique called microfluorescence; British researchers will employ an atomic force microscope, able to analyze the most minute bacteria in detail. But all this international scrutiny may not be enough to affirm that past life resides in this rock. As Michael Meyer, NASA's exobiology program manager, told a reporter for Science, "That's a lot to ask from something the size of a potato." And that's the whole specimen; all the evidence reported to date comes from less than three grams of rock. Confirmation may have to wait until at least early in the next century when NASA plans to mount a mission to Mars designed to bring back specimens, particularly sedimentary rocks, from carefully chosen targets. Scientists aren't awaiting further evidence, though, to ponder the implications arising from the two-kilogram meteorite. If life once existed on Mars, did it get there from here, or vice versa? Could life have evolved independently on both planets? Alternatively, did life originate in some third location and get transmitted later to both Earth and Mars? Science slides quickly into speculation in attempting to explore any of these questions, but more data are being brought to bear. A study in the March 8 issue of Science used sophisticated computer simulations to model the orbital meanderings of hypothetical meteorites from various planets as well as the Moon. Relatively frequent impacts on the Moon have released many smaller rocks that can reach Earth quickly, many within 50,000 years. Martian meteorites, however, average a third more mass than lunar ones, and most take their time reaching Earth. The space treks of known Martian meteorites, judging from the amount of accumulated radioisotopes from bombarding cosmic rays, lasted from 700,000 years for the EETA79001 specimen to 16 million years for ALH84001. The shortest simulated meteorite trip from Mars to Earth lasted 16,000 years, implying that relatively rapid transfer could have occurred in the past. Perhaps 40 percent of material launched from the Moon will make it to Earth, but for Mars the figure drops to four percent. Meteorite expert Ralph Harvey has written, "The Earth is not some cosmic fishbowl sheltering us from the outside universe, but a giant wrecking ball crashing through the swarm that envelops us, sampling whatever happens into our path." It samples a lot. An estimated 40,000 tons of interstellar dust lands on Earth each year, about two tons of that coming from Mars. Meteorites may bring about one hundred pounds of Martian rock to Earth's surface in the same period, and over a million-year period, Earth may receive up to 100 million Martian meteorites. Early in the history of the solar system, when orbiting rocks were rampant as the planets and satellites took shape, these numbers added up to billions. What are the odds that some of these travelers between Earth and Mars carried organic matter or even living organisms? "There's a huge chance to transport life between the two planets," contends Stanford geophysicist Norman Sleep. He envisions a scenario with life emerging in one spot and then colonizing nearby planets as incoming comets or asteroids eject microbe-carrying chunks that get transported to a new home, where intact survivors might become established. Or, Sleep muses, jettisoned rocks might provide refugia for microbes after an impact renders their home planet suddenly inhospitable. As long as the cosmic journey is long enough for conditions back home to improve but short enough to avoid lethal exposure to radiation or other hazards, the microbes might return to Earth during a subsequent impact and be available to restore the devastated populations--giving new meaning to the phrase circle of life. Such scenarios, while undeniably speculative, are more scientifically grounded updates of "panspermia," the idea that our solar system was innoculated with living organisms from outside the galaxy, presumably by some hypothetical intelligent lifeforms. First proposed in 1908 by Swedish chemist Svante Arrhenius, and more recently championed by Fred Hoyle and Francis Crick, panspermia begs the question of where and how life originated. As Sleep points out, geological evidence on Earth suggests that life evolved pretty quickly, and if it arose independently on Mars through convergent evolution then life must arise easily when given half a chance, eliminating the need to invoke an extragalactic source. If there was life on Mars, could it have originated there and then come to Earth? At a NASA press conference announcing August's Science paper, Zare remarked, "Who is to say that we are not all Martians, that Mars was the place where life first started?" Sleep and other scientists argue that Mars may have offered a more hospitable home for life during the early days of the solar system. First of all, Mars probably had a solid surface--if ALH84001 is accurately dated to 4.5 billion years ago--before there was an Earth around to put life upon. Mars is a smaller target than Earth for potential impactors and it may have been spared collisions with objects big enough to vaporize oceans and sterilize sediments on Earth. The deep subsurface of Mars may have always been safe enough from impacts and their consequences to support life. And with less gravity than Earth, Mars provides a more efficient launch pad for any life-encasing projectiles. But what about Earth? Earth's earliest rocks have already been recycled into the crust or eroded to dust, so if the planet solidified about 4.5 billion years ago, there is nearly a billion years with practically no record of early geology and, presumably, the first glimmerings of life. The oldest rocks known to contain fossilized organisms come from the Apex Chert of Australia. They indicate that by 3.5 billion years ago at least eleven distinct kinds of bacteria existed. Some closely resemble modern cyanobacteria, and if these first fossils were also creatures capable of photosynthesis, then something less complicated must have preceded them. In Nature this past November 7, an international scientific team announced the discovery of chemical fossils--a distinctive carbon signature for life--within 3.8 billion-year-old rocks from Greenland's Akilia Island. When our planet was taking shape, the solar system was a rough neighborhood. Today a major impact may occur every 100 million years, but back then Earth collided with large extraterrestrial objects every million or few million years. During the 500 million years after the giant Moon-making impact obliterated Earth, the planet was pelted by perhaps 17 more objects larger than 200 kilometers in diameter, plus hundreds of smaller objects. Subsurface life could conceivably have survived an impact by anything up to 300 kilometers in diameter, but calculations by Sleep and colleague Kevin Zahnle show that larger objects--a few of which likely struck Earth--would create a global catastrophe for any life. Intense heat released from a 400-kilometer-diameter asteroid vaporized the ocean 2.5 kilometers deep and turned the sky into a sterilizing steam bath of rock vapor. As the atmosphere slowly cooled, hot rock condensed and rained down for two months, coating the globe 300 meters thick. Rain refilled the ocean basins but only after 3,000 years. Nothing on or near the surface could have endured the frying temperatures. For comparison, the Cretaceous asteroid widely credited with snuffing out dinosaurs was only 20 kilometers in diameter, judging from the size of the Chicxulub crater in Mexico, but it released a burst of energy equal to three months of sunshine--enough to set fires around the planet and to boil off the top meter of the oceans. A cooling cloud of dark dust and rock particles shrouded Earth for months, and plants could no longer collect sunlight to sustain themselves. Early life on Earth may have had to restart its evolutionary journey numerous times after being repeatedly extinguished by impacts almost before it could gain a cellhold. Given all the fire and brimstone occurring on the surface ("a good place to get hit on the head," says Stanford's Zare), the safest spots were deep underwater or underground. Subsurface sediments on Earth, of which we know virtually nothing as places for life, contain a quarter of the water in the world's oceans, or much more than in all freshwater habitats combined. These porous sediments offered a huge area where life could have retrenched to ride out hard times. Sleep suggests that extreme thermophiles in the Archaea (see "Horizons," Pacific Discovery Fall 1994), which appear to form the root of today's tree of life, may represent the common ancestor that survived a global sterilizing impact. Bombardment kept life off the surface for long periods until at least 3.8 billion years ago, but within 300 million years, life had become firmly established and was already displaying a semblance of diversity. Somewhere in that window, the 7,000 enzymes that comprise a typical cyanobacterium had been assembled from whatever chemicals were at hand. Long-time origin-of-life theorist Stanley Miller of UCLA argues that once conditions were right, life arose quickly, taking maybe just ten million years for thermophilic and photosynthetic bacteria to evolve from the very first beings. Just how that happened is a topic guaranteed to rile the scientists who make the origin of life their business, but for a while the debate has focused on whether life arose in a small pond--the proverbial primordial soup--or in a big sea, perhaps on the flanks of seafloor hydrothermal vents. Regardless of the locale, the intriguing possibility remains that the same astronomic artillery which would have impeded early life on Earth could have carried at least some of the prebiotic building blocks necessary for life to appear. One of the latest origin-of-life ideas posits that the surface of early Earth, both land and sea, was frozen solid because the sun put out nearly a third less light than it does today. Planetary scientists previously got around this sunlight problem by proposing an early atmosphere rich in carbon dioxide, a greenhouse gas that cast a warming blanket around the planet. But such an atmosphere spells "the kiss of death for organic chemistry," according to Jeffrey Bada of the Scripps Institution of Oceanography, and would not be conducive to creating life. In 1994, Bada, Scripps colleague Charles Bigham, and Miller proposed instead that impacts from asteroids and meteorites about 100 kilometers in diameter periodically thawed the ocean. A hole punched through the oceanic ice would have freed trapped greenhouse gases, spewed from deep-sea hydrothermal vents, into the atmosphere, possibly in great enough quantity to prevent the ocean from refreezing. The impacting object may have deposited its own organic matter that mixed with other compounds in the depths, where a cascade of chemical reactions could have paved the way for life to arise and survive even when the ocean surface refroze. Despite his scenario, Bada has had reason to doubt the importance of organic matter from space in the emergence of life on Earth. After two years of searching two dozen ice samples from Greenland and Antarctica he identified trace amounts of a space-borne amino acid, alpha-amino-isobutyric acid, embedded a half-mile deep in a 4,500-year-old ice core from Greenland. Yet the concentration of organic material is so small that, when placed in a vast ocean it would not go far to nourish potential life in a primordial soup. Recently, however, Bada was part of a team that reported the discovery of extraterrestrial helium trapped inside buckyballs, or fullerenes (soccer-ball-shaped molecules comprising 60 carbon atoms and named for inventor-philosopher Buckminster Fuller), that apparently came to Earth aboard a Mount Everest-sized object that crashed near Sudbury, Canada nearly two billion years ago and left a crater covering 1,800 square kilometers. These buckyballs came through the incredibly explosive impact intact, and Bada has since softened his view that space may have been but a sporadic source for the stuff of life. Whether life on Mars will play out to be a cosmic flash in the pan or the most stunning revelation since Copernicus remains to be seen. At the very least, though, the accumulating evidence should give us pause about overemphasizing our place in space.
Blake Edgar is Associate Editor of California Wild. |
Winter 1997
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