hot potato
During the last days of 1984, Roberta Score, the curator of the Antarctic Meteorite Laboratory at the Johnson Space Center in Houston, Texas, was hunting for meteorites in Antarctica. She had been hired by NASA in 1978, fresh out of UCLA where she had majored in geology, to help get this Meteorite Laboratory up and running. In the late 1960s and early 1970s, NASA had learned how to carefully curate and store 840 pounds of rocks and dirt collected by Apollo astronauts on the surface of the Moon. Now, in support of the National Science Foundation’s Antarctic Search for Meteorites (ANSMET) program, the lab was adding to the moon rock collection more extraterrestrial rocks: pristine meteorites collected on the blue ice of Antarctica that had fallen thousands or millions of years ago, then been buried by snowfall, preserved in ice, and finally unburied by the motion of glaciers and the scouring action of the wind.
Score and the other members of the 1984–1985 collection team, John Schutt, Carl Thompson, Scott Sandford, Bob Walker, and Catherine King-Frazier, lived in tents in the remote Antarctic cold; they rode snowmobiles across dangerous terrain, working diligently to avoid plunging into crevasses or acquiring frostbitten fingers and noses. Robbie Score learned, quickly, that scouring the Antarctic in search of meteorites is an extreme sport, not child’s play, but for the scientific community these meteorites were a scientific windfall, well worth the difficult work involved in obtaining them.
Figure 11.1. Catherine King-Frazier, a member of the 1984–1985 National Science Foundation–sponsored research team tasked to search for meteorites in Antarctica, walking across the Allan Hills Main Ice Field on a blustery day in early December 1984. The meteorite ALH 84001 was found by meteorite search team member Roberta Score later that month about 50–60 kilometers from this location, in the Allan Hills Far Western Ice Field. See Plate 4. Image courtesy of Roberta Score.
On the twenty-seventh of December, Score got lucky when she found an odd-looking, greenish-gray-colored, four-and-a-quarter-pound rock that she correctly surmised was a meteorite. When she lifted that potato-shaped rock off the Antarctic ice, Earth shifted, figuratively at least. She released a shock wave that grew in energy until it triggered an explosion of ideas a dozen years later. That later explosion sent ripples through the staid worlds of astronomy, biology, geology, geophysics, and planetary sciences that dramatically reshaped all of those disciplines.
On that bitter cold Antarctic morning, albeit a morning after a night during which the Sun never set, Robbie Score was careful not to contaminate the meteorite by touching it with her bare hands. With help from the rest of the collection team, she put this unusual looking, black-encrusted rock into a NASA-provided “clean bag” and sealed the bag with tape. Months later, back in Houston, she had the responsibility of assigning numbers to each meteorite she and her teammates had collected in Antarctica. Score chose to pull out her favorite meteorite first. It was her favorite because it was green, and meteorites are not typically green, though she was surprised to find that in the light of the laboratory environment it looked mostly gray. She tagged it and then numbered it 001. Score’s green rock was not the first one found in the 1984–1985 Antarctic season of meteorite hunting, but it fittingly became the first one tagged upon her return to her day job in Houston. In doing so, she gave a name to the meteorite that a decade hence would compete with the Rosetta Stone for the title of the most famous rock on Earth. Robbie Score’s rock, collected in the Allan Hills section of Antarctica in 1984, is now known as ALH 84001, and in her honor the section of the Meteorite Hills region of the Darwin Mountains where she found this rock that, thousands of years ago, fell from the sky is now named Score Ridge.1
Figure 11.2. Martian meteorite ALH 84001, photographed in the laboratory at Johnson Space Center. For a sense of scale, the small black cube (lower right) is 1 cm (four-tenths of an inch) on a side. Parts of the outside of the meteorite are covered with a black fusion crust, while the inside is greenish gray. See Plate 5. Image courtesy of NASA’s Johnson Space Center.
As was done with all meteorites that were entered into the NASA Antarctic meteorite collection at Johnson Space Center, a technician carefully sliced off a half-gram chip of ALH 84001 and shipped the rock fragment to the Smithsonian Museum of Natural History, in Washington, DC. At the Smithsonian, a meteorite specialist carried out a quick examination of this slice of ALH 84001 and then assigned the parent meteorite to a classification category within the known framework of meteorite families. This cursory examination is performed for every meteorite collected in Antarctica, and the subsequent publication of those write-ups in the Antarctic Meteorite Newsletter allows meteoriticists from around the world to decide whether any of the new arrivals are worthy of further study in their laboratories.
Because ALH 84001 was numbered 001 by Robbie Score and because its greenish color was so unusual, this rock was a high-priority meteorite when it arrived in Houston, and the small chip of ALH 84001 was a high-priority sample for Glenn MacPherson when it showed up in his lab at the Smithsonian. However, MacPherson, the curator who examined the small slice of ALH 84001, quickly decided it was not a particularly interesting object. He did, however, discover the first important clue that, one day, would lead to ALH 84001 becoming a worldwide phenomenon: he correctly identified the rock as igneous, or volcanic, in origin.
ALH 84001, like almost all small rocks, is a chip off an old block. The old block, the larger “parent” rock of which ALH 84001 is a fragment, must have formed in an environment in which the temperature was high enough that the precursor materials that formed the rock were molten. Magma (molten or semi-molten material that remains below the surface of a planet or moon or asteroid) and lava (magma that erupts onto the surface) can only exist on objects in the solar system that are as large or larger than the biggest asteroids. Only large objects can both generate enough heat from the decay of radioactive minerals, like certain isotopes of aluminum, potassium, thorium, and uranium, and then trap that heat within them for long enough for their internal temperatures to rise to a level that produces volcanic activity on their surfaces or magmatic activity deep within. Meteorites that formed as igneous rocks that cooled slowly, deep inside a large parent-body, are called diogenites. ALH 84001 was born as a diogenite in such an environment.
In 1984, the object considered the principal source of all known diogenites was Vesta. Vesta is a very large asteroid (about 525 kilometers, or 326 miles, in diameter), second in size only to Ceres among objects in the asteroid belt. Because of Vesta’s large size, planetary scientists are certain that soon after it formed, early in the history of the solar system, radioactive materials buried deep inside of it, probably mostly the unstable isotope of aluminum known as aluminum-26, decayed, released heat, and melted the inside of Vesta. As a result, reduced iron, which is the element iron that has been chemically isolated from oxygen, drizzled downward through the magma until it reached Vesta’s center and formed an iron-rich, oxygen-poor core. In addition, as gravity pulled the iron downward through the softened innards of Vesta, the iron dragged with it other iron-loving (siderophile) elements. At the same time, lighter, rocky (lithophilic) elements bubbled up toward Vesta’s surface. The basaltic lavas that rose up and then solidified to form the mantle and outer crust of Vesta are presumed to be the source for igneous meteorites like ALH 84001.
Of course, objects in the solar system that are larger than Vesta, for instance Earth’s Moon, or even the Earthlike planets, Mercury, Venus, and Mars, are also places where internal melting took place, convection brought lighter magmas upward, and surface volcanic activity occurred. Therefore, they also could be sources of basaltic meteorites, although some were recognized as less likely potential sources of meteorites than others.
In order for a meteorite to come from a large solar system body, several things must happen. First, an asteroid of significant size must hit the surface of that moon or planet and some of the debris from that impact must be lofted off the surface intact and at high speed. The impact debris kicked off the surface then must drill a hole through the planet’s atmosphere, if the planet has one, and emerge above the atmosphere with a high enough velocity (known as “escape velocity”) to escape the gravitational clutches of the planet.
Venus has a massive atmosphere and an escape velocity (6.34 miles per second, or 10.4 kilometers per second) almost as large as that of Earth (6.96 miles per second, or 11.2 kilometers per second). The likelihood of any meteorites being splattered off the surface of Venus, pushing through Venus’s dense atmosphere, emerging with a velocity greater than escape velocity, and getting to Earth is thought to be nearly zero. At this time, we know of no meteorites from Venus in our collections.
Mercury has no atmosphere and an escape velocity (2.7 miles per second, or 4.3 kilometers per second) that is fairly small compared to that of Venus; but even though Mercury lacks a substantive atmosphere, to get from Mercury to Earth a meteorite also would have to battle against the Sun’s gravitational pull to move outward away from the Sun from Mercury’s orbit to Earth’s orbit. Rising 57 million miles upward from a place near the Sun in order to reach Earth is a serious energy challenge. As with Venus, we know of no meteorites from Mercury in our collections. (One 4.5-billion-year-old meteorite, NWA 7325, which was found in southern Morocco in 2012, might be from Mercury, thought that claim has been challenged.2) As a result, the planetary science community, in 1984, considered neither Mercury nor Venus seriously as a likely candidate source for igneous meteorites.
On the other hand, both the Moon (escape velocity only 1.5 miles per second, or 2.4 kilometers per second) and Mars (escape velocity of 3.1 miles per second, or 5.0 kilometers per second) were considered plausible meteorite sources because of the combination of their lower escape velocities, their absent (Moon) or thin (Mars) atmospheres, and their relative proximity to Earth; however, in the early 1980s the absence of any actual meteorites in meteorite collections from either object suggested otherwise.
A meteoritic breakthrough occurred on January 18, 1982, when John Schutt, of Spokane, Washington, the leader of the 1981–1982 U.S. search party looking for meteorites in Antarctica (and the leader of Robbie Score’s team three years later), found an unusual-looking, 31-gram (about 1 ounce) rock now known as ALH 81005, which showed similarities to lunar rocks. By 1983, several teams of meteoriticists, working independently, had confirmed that this specimen was, without any doubt, a lunar meteorite.
Finally, the planetary science community had confirmed that meteorites could escape the gravitational grip of the Moon and survive a trip to Earth. Nevertheless, since ALH 84001 did not bear any resemblance to Moon rocks, as did ALH 81005, and since the evidence on the ground in the form of actual meteorites seemed to demonstrate that meteorites either cannot or do not travel from Mars to Earth, in 1984 Vesta remained the most likely source for all diogenites.
Vesta had another very significant plus on its side of the ledger. The spectrum of sunlight reflected from the surface of Vesta and observed by astronomers appears very similar to the results of laboratory experiments in which meteoriticists study the spectra of light reflected from meteorites known to have volcanic origins. In contrast, the reflected light spectra from other kinds of meteorites do not mimic the spectrum of Vesta. In addition, the reflected light spectra of other asteroids do not match the spectra of the diogenites or of Vesta. Vesta and these meteorites were a unique match: no other meteorites are a good match to Vesta, and no other asteroids are a good match to these meteorites. All of these experimental results strongly suggested that Vesta was the likely source of all meteorites with igneous origins. This result was not controversial; rather, it was part of the broad consensus as to how the planetary science community understood the rocks in our meteorite collections.
When the tiny rock fragment chipped off ALH 84001 arrived at the Smithsonian, it did not present a particularly difficult challenge for MacPherson. With ease, he immediately classified ALH 84001 as a diogenite. As a diogenite, ALH 84001 is nothing special, just one more meteorite among the nearly 1,000 such objects thought to be from Vesta.a
ALH 84001 remained unbothered and unstudied until 1988, at which time David Mittlefehldt, a geochemist from Lockheed Engineering working for NASA in Houston, began a study of meteorites presumed to be from Vesta. Mittlefehldt used a device called an electron microprobe, which bombards a microscopic sample under study with a beam of electrons. This is a nondestructive technique that causes the elements in the sample to emit X-rays. The energies of the emitted X-rays are diagnostic of the elements present in the material under study.
By 1990, Mittlefehldt had determined with a high degree of confidence that the X-rays produced by ALH 84001, when subjected to his probing, did not match the X-ray signature produced by other meteorite fragments presumed also to be from Vesta. Clearly, in ways that were obvious but that he did not understand at that time, ALH 84001 was different in some fundamental way from all other known diogenites. After three more years of struggling with ideas and searching for a plausible explanation of this discrepancy, he figured out part of the answer. He had to challenge and then toss the assumption that ALH 84001 was a fragment of Vesta. Yes, the other diogenites are all from Vesta, but ALH 84001 is not. Of course, knowing that ALH 84001 is not from Vesta simply created a new mystery: which other solar system body, one large enough to generate magma and slowly cool that material deep inside, was the birthplace of ALH 84001?
The X-ray spectrum of ALH 84001, which told him the atomic contents of the rock, is equivalent to a set of fingerprints. In this case, Mittlefehldt relied on the published work of other meteoriticists who had previously measured the X-ray fingerprints of some other unusual meteorites. He found that the fingerprints of ALH 84001 did match the known atomic fingerprints of a very small handful of other meteorites, and those other meteorites were not diogenites from Vesta. The remarkable thing about the match was that all of the other meteorites whose X-ray signatures matched that of ALH 84001 were rocks from Mars. Mystery solved: ALH 84001 did not come from Vesta. It came from Mars3; however, it was different from the other known Martian meteorites.
In 1993, Robert N. Clayton, a highly respected meteoriticist at the University of Chicago, confirmed the Martian origin of this meteorite by analyzing the isotopes of oxygen in the rock.4 Clayton, in 1973, had discovered that the ratios of the three stable isotopes of oxygen—oxygen-16, oxygen-17, and oxygen-18—were of enormous significance for cosmochemistry. That is, they could be used to identify the location in the solar system from which a rock sample originated and could even be used to identify the distinct events that created the oxygen isotopes before the Sun and solar system formed. Clayton was elected to the Royal Society of Canada in 1980, the Royal Society of London in 1981, and the National Academy of Sciences in 1996. Confirmation by Robert Clayton of the Martian origin of ALH 84001, based on the oxygen isotope ratios in the meteorite, was akin to being blessed by the pope for a member of the Roman Catholic faith.
What are these isotopes of oxygen? The oxygen atom always has eight positively charged protons in the atomic nucleus and usually also commonly has eight uncharged neutrons in the nucleus. Such an atom is known as oxygen-16 (16O). But stable atoms of oxygen can also have nine neutrons (17O) or 10 neutrons (18O) sharing the nucleus with the eight protons. The relative percentages of 16O atoms (99.76 percent) to 17O atoms (0.039 percent) and 18O atoms (0.201 percent) on Earth (the so-called Standard Mean Ocean Water, or SMOW, abundances) are well known. In addition, the fact that these relative abundances can change, or fractionate, as a result of mass-dependent processes like evaporation (what remains in liquid form is enriched in the heavier isotopes because of the effect of gravity pulling downward more strongly on the heavier isotopes) or chemical bonding (heavier isotopes are bound more strongly than lighter isotopes) is well understood. Any sample of water from any environment on Earth will yield deviations from the SMOW abundances that are directly related to the original SMOW values, which were determined by the isotopic abundances that were present in the primordial reservoir of materials out of which Earth formed. When these deviations are plotted on a graph that depicts the deviations in the 18O values versus the deviations in the 17O values, every drop of terrestrial water will plot on a single line known as the terrestrial fractionation line. This terrestrial fractionation line is identical for both Earth and the Moon, which tells us that they formed from the same original reservoir of material. Other solar system bodies, however, whether Mars or Jupiter or different types of meteorites, have their own, unique oxygen isotope fractionation lines. These isotopic differences tell us that each planet formed from materials with unique isotopic ratios. Once known and understood, these isotopic ratios become a signature of origin for meteorites and for planets. They are, effectively, fingerprints that identify the home planet (or moon or asteroid) of a rock.
In 1985, a year after ALH 84001 descended into obscurity as just another diogenite, and at a time when no planetary scientist had yet identified even a single meteorite from another planet, Robert Pepin, of the University of Minnesota, published a remarkable research result that revealed a one-to-one correlation between the relative amounts of rare gases in the atmosphere of Mars, as had been originally measured by the Viking lander in 1976,5 and the relative amounts of those same gases trapped in air bubbles in the meteorite known as EETA 79001.6 EETA 79001, like ALH 84001, was an Antarctic meteorite, collected in 1979 in the Elephant Moraine region. Pepin found that the relative amounts of xenon-132, krypton-84, argon-36, argon-40, neon-20, molecular nitrogen (N2), and carbon dioxide in the air bubbles in EETA 79001 were indistinguishable from, in fact a perfect match to, the gases found by Viking in the atmosphere of Mars. Without any doubt, EETA 79001 was a piece of Mars.
How did it get knocked off of Mars, travel through the solar system, and finally land on Earth, in Antarctica, where it sat for thousands of years, quietly waiting to be collected and tagged by an Antarctic meteorite collection team? Those are questions worth asking, but for ALH 84001, the critical piece of information is not how EETA 79001 left Mars and traveled to Earth. No, the critical data point is that when EETA 79001 was knocked off of Mars, it trapped some Martian atmospheric gases in air pockets within the meteorite, protected those air bubbles for millions of years, and successfully navigated the Mars-to-Earth trip. As one last contribution to our understanding of other meteorites, EETA 79001 provides the instruction set for testing whether other meteorites are from Mars: test the gases trapped in air bubbles in a particular meteorite and see how they compare to the Martian atmosphere.
In the case of ALH 84001, Robbie Score’s rock did have tiny bubbles of gas trapped within it, and the isotopic signatures of the atoms in those bubbles of trapped gas were identical to those of the gases in the Martian atmosphere. Bingo. ALH 84001 was now known to be similar in this way to a handful of meteorites already known to be from Mars, because they also had tiny bubbles of Martian gas trapped within them. Thus, the Martian origin of ALH 84001 was firmly established and remains uncontroversial.
Even among the small family of Martian meteorites, however, ALH 84001 stood apart. The small number of known Martian meteorites include the Shergottites, the Nakhlites, and the Chassignites (collectively known as SNCs). The Shergottites are named for the town of Shergotty (now Sherghati), in northeastern India, where the Shergotty meteorites fell in 1865. More than one hundred Shergottites have been identified, collected from Antarctica, California, Libya, Algeria, Tunisia, Mali, Mauritania, Nigeria, and Oman. The largest is just over 8.5 kilograms (about 19 pounds), the smallest only 4.2 grams (0.15 ounces). The Nakhlites are named for the village of Nakhla, a tobacco-farming village about 25 miles (40 kilometers) southeast of Alexandria, Egypt, where these meteorites fell on June 28, 1911. The Nakhlites, composed of forty individual fragments, were originally found after a farmer reported seeing smoke trails and explosions in the sky and claimed that he witnessed a dog that was killed by a falling rock fragment. Although the Egyptian Survey Department, after later interviews, concluded that the dog story was likely imagined, the Nakhla dog remains one of only a very few living things (possibly) struck and killed by meteorites.b A total of eighteen Nakhlites are known, including the Lafayette meteorite, found in Indiana in 1931; the Governador Valadares meteorite, found in Brazil in 1958; and a handful of others found in the last two decades in Antarctica, Morocco, and Mauritania. The largest Nakhlite is 13.71 kilograms (30.2 pounds); the smallest is only 7.2 grams (0.25 ounces). And the Chassignites are named for the hamlet in northeastern France called Chassigny, where these meteorites fell in 1815. Only two other Chassignites have been identified, both small rocks found in northwest Africa.
All of the SNCs formed from a molten reservoir on the surface of Mars about 1.3 billion years ago. In stark contrast, ALH 84001 formed inside Mars 4.1 billion years ago7 and is nearly as old as Mars. ALH 84001 formed almost as soon as Mars was cool enough to allow any rocks to form on that planet or on any planet in the solar system. Suddenly, as the oldest known rock in the solar system from a planet—a few meteorites known as carbonaceous chondrites, which are not from planets, are just a bit older—plus as a rock from Mars, ALH 84001 became a hot potato, with meteorite specialists from all over the world asking for a piece of it to study.
fossils from mars
On August 7, 1996, ALH 84001 became the most interesting and controversial rock in the world. On that day, David McKay, Everett Gibson, Jr., and Kathie Thomas-Keprta, all highly respected career scientists based at the Johnson Space Center in Houston, and professor of chemistry and of physics Richard Zare, a distinguished laser chemist at Stanford University and a member of the National Academy of Sciences, represented their team of nine co-authors at a NASA-sponsored press conference in Washington, DC. They announced that they were publishing a paper in Science, the contents of which NASA thought worthy of having members of this team present to the world at a press conference hosted by NASA. In that paper, they were putting forth the claim that they had discovered evidence inside ALH 84001 of fossils that strongly suggested that life had existed on Mars in the ancient past.8 “Although inorganic formation is possible,” they wrote, “formation of the globules by biogenic processes could explain many of the observed features, including the PAHs.c The PAHs, the carbonate globules, and their associated secondary mineral phases and textures could thus be fossil remains of a past Martian biota.”
Unlike the observational evidence for canals, chlorophyll, lichens, or the Sinton bands, which, whether right or wrong, were only indirect evidence of life on Mars, at the press conference and in their published research paper McKay’s team was showing pictures—pictures!—of objects they claimed were fossils of ancient Martian life-forms. No hyperbole is needed: if this discovery is correct, it is absolutely phenomenal, the discovery of the century.
Assuming the evidence reported by McKay, Gibson, Thomas-Keprta, and Zare is proven correct, how did life arise on Mars? Life may have emerged independently on both Earth and Mars. Or, life may have arisen first on Mars and then been transferred to Earth via an asteroid collision with Mars. That collision may have launched a life-bearing meteorite into orbit around the Sun, which millions of years later fell to Earth!
The discovery reported by McKay and his team was important enough to warrant special remarks by President Bill Clinton, carefully orchestrated and coordinated by NASA. Speaking from the South Lawn of the White House, just before the NASA press conference began, Clinton told the world:
It is well worth contemplating how we reached this moment of discovery. More than 4 billion years ago this piece of rock was formed as a part of the original crust of Mars. After billions of years it broke from the surface and began a 16-million-year journey through space that would end here on Earth. It arrived in a meteor shower 13,000 years ago. And in 1984 an American scientist on an annual U.S. government mission to search for meteors on Antarctica picked it up and took it to be studied. Appropriately, it was the first rock to be picked up that year—rock ALH 84001. Today, ALH 84001 speaks to us across all those billions of years and millions of miles. It speaks of the possibility of life. 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.9
President Clinton’s remarks included a few mistakes: American scientists in Antarctica were looking for meteorites, not meteors. And ALH 84001 was the first rock from the 1984 collection mission to be catalogued, but not the first to be collected. But he conveyed the awe and wonder and importance of this discovery extremely well to a national audience.
McKay’s discovery birthed NASA’s Astrobiology Institute, which has grown over the decades since the McKay press conference; it inspired aggressive searches for extreme life-forms, now referred to as extremophiles, on Earth; and it initiated a vigorous public and scientific debate about what life is. This discovery also invigorated NASA with the support of the public and the U.S. Congress that has continued for several decades. As a result, NASA has sent a mission to Mars, and sometimes multiple missions, almost every other year for more than two decades. These missions have included two rovers that have finished their missions (Mars Pathfinder Rover 1996–1997; Mars Exploration Rover Spirit 2003–2011), two rovers that continue to work (Mars Exploration Rover Opportunity 2003–present; Mars Science Laboratory Rover Curiosity 2011–present), and another rover scheduled for launch in 2020 (the Mars 2020 Rover). NASA has also sent two landers to Mars: the Mars Polar Lander (1999) failed while the Mars Phoenix Lander (2007–2008) was a great success. A third, the InSight (Interior Exploration using Seismic Investigations, Geodesy and Heat Transport) Mars Lander, is scheduled for launch in spring 2018 and due to land on Mars that November. Finally, since 1990, NASA has put five spacecraft in orbit around Mars and sent a sixth orbiter (Mars Climate Orbiter, 1998–1999) that was lost upon arrival at Mars. The five successes include two completed missions—Mars Observer (1992–1993) and Mars Global Surveyor (1996–2006)—and three missions that remain active—Mars Odyssey (2001–present), Mars Reconnaissance Orbiter (2005–present), and Mars Atmospheric and Volatile Evolution (MAVEN, 2013–present).d In addition, teams of scientists and engineers are planning sample-return missions and for human exploration of the red planet in the not-too-distant future.
To their credit, McKay, Gibson, Thomas-Keprta, Zare, and their colleagues and co-authors Hojatolla Vali, Christopher Romanek, Simon Clemett, Xavier Chillier, and Claude Maechling followed established protocols when they published their scientific results. They submitted their research paper to Science before they announced their results to the press. The editors of Science subject every submitted manuscript to a refereeing process by other experts, known as peer review. The bar for publication in Science is very high, as high as in any journal—only about 10 percent of manuscripts submitted to Science survive this process and are published. The McKay et al. paper rose above that bar. While publication in Science does not guarantee that the published results are correct, it does guarantee that a handful of impartial experts examined the scientific claims rigorously, and it does give the published results a great deal of credibility (and visibility).
In practice, Science likes to publish papers that will generate a great deal of interest beyond the narrow community of scientific experts in the niche field associated with the research project. High visibility usually means cutting-edge science, and quite often science done at the cutting edge turns out to be wrong. Papers published in Science naturally lead to press releases, newspaper headlines, television reports, and other forms of publicity. Of course, fossil evidence of possible ancient life on Mars, being almost unimaginably big news, spawned a press conference and quickly led to worldwide fame and notoriety for the authors.
Virtually all scientists who studied ALH 84001 over the decade that followed the McKay-team press conference would do so with open minds as to whether the McKay team’s claims for ancient biogenic activity on Mars, as seen in the evidence contained in a Martian meteorite, might be correct. Within the broader scientific community, the McKay team’s claims were immediately met with a strong and healthy skepticism. Battle lines were quickly drawn and scientists chose sides.
The meteorite community had been down the extraterrestrial-life-in-a-meteorite road before, three decades earlier, with the Orgueil meteorite. This previous episode actually began in the nineteenth century, when a meteorite shower occurred over Orgueil, France, on May 14, 1864. Only seventeen days later, a claim was put forward that an analysis of that meteorite revealed it to contain some kind of chemical residue that resembled humic acid, which is derived from the decay of organic matter. The author of this claim suggested that the presence of humic acid in the Orgueil meteorite implied that the parent body of this meteorite must have had living things in or on it.
According to Edward Anders, then of the Enrico Fermi Institute at the University of Chicago, who retells this story, the Orgueil meteorite fell just a month after Louis Pasteur’s famous lecture of April 7, 1864, entitled “On Spontaneous Generation,” which Pasteur presented at the Sorbonne in Paris. Pasteur thoroughly debunked the idea that spontaneous generation occurs. “No,” Pasteur thundered, “there is not a single known circumstance in which microscopic beings may be asserted to have entered the world without germs, without parents resembling them. Those who think otherwise have been deluded by their poorly conducted experiments, full of errors they neither knew how to perceive, nor how to avoid.”10 Anders suggested that Pasteur’s report “may conceivably have inspired a person of the proper disposition into playing a little practical joke on the scientists.” Perhaps the practical joker was also less than thrilled with Charles Darwin’s evolutionary arguments, presented only a few years earlier in Darwin’s Origin of Species, and was attempting to stir up anti-Darwinian sentiments. “Somehow,” Anders concluded, “the plot failed, and the contaminated stone went unrecognized for 98 years.”
News of organic matter in the Orgueil meteorite became known widely enough to have inspired Swedish playwright August Strindberg, who while living in France in 1887 wrote The Father. The father, a retired captain and active scientist, claims to have “submitted meteoric stones to spectrum analysis, with the result that I have found carbon, that is to say, a clear trace of organic life.” His wife uses the captain’s apparently crazy idea as part of a scheme to have a doctor declare him insane. After the doctor arranges for the captain to be placed in a straitjacket in preparation for having him removed to an asylum, the captain suffers a stroke and dies.11
After almost a century, the practical joke reemerged in 1962 when Bart Nagy, a chemist at Fordham University, and his collaborators began their study of the Orgueil meteorite. They found evidence of extraterrestrial life, or so they thought, in one fragment of Orgueil, in the form of “fossilized, organic, organized structures, that are not likely to be minerals, organic artefacts or terrestrial, microbiological contaminations.” They went further, writing in their paper in Nature, “At present, we are of the opinion that the organized elements are microfossils apparently indigenous to the meteorite parent body.”12 For two years, a scientific battle raged until Anders and his colleagues ended the war of ideas with a tour de force paper in Science whose title says everything one needs to know: “Contaminated Meteorite.”13
“There can be little question,” Anders wrote, “that stone No. 9419 has been contaminated. Most probably, the contamination occurred in 1864, shortly before or after the meteorite was put in the museum.” Somebody added coal fragments and some local plant material (identified by Anders as identical to the perennial reed plant juncus conglomeratus, which grows abundantly throughout Europe and is commonly known as Compact Rush) to the meteorite, moistened it, and glued the contaminants to the specimen. Since coal was not used for household heating in France in the 1860s and likely must have been obtained from a blacksmith’s forge, the conclusion that the contamination was intentional is highly plausible. Anders strongly implied that the contamination was done as an intentional hoax, perpetrated in 1864 as a practical joke on the French scientific community.
The Orgueil hoax failed miserably in the sense that it did not trap its intended audience of mid-nineteenth-century French intellectuals and went undiscovered for a century. This hoax also succeeded remarkably well in demonstrating the robustness of the scientific process. Scientific results must be reproducible by impartial members of the scientific community. An especially important and remarkable discovery will be tested, including by some whose scientific agendas are simply to prove that the original discoverers are wrong. By challenging an idea that looms large in both the international and public domains, challengers can establish their own international reputations by debunking claims made by others.
If ALH 84001 contains evidence for ancient biogenic activity on Mars, that evidence must survive extraordinary scientific scrutiny, as such an extraordinary claim should have to do. Beginning as early as the first press conference on ALH 84001 in 1996, the evidence presented by McKay and his colleagues in their paper in Science was challenged. Sometimes those challenges were offered respectfully and through the scientific process via papers and debates at conferences, but other times publicly via mudslinging and name-calling. The stakes were high; after all, the opportunity to be the first humans to discover fossil evidence of life on another planet occurs only once.
What, then, is the evidence for ancient life on Mars, in addition to the indisputable fact that the meteorite itself came from Mars? McKay and his colleagues identified four distinct and independent lines of evidence as proof that ancient Martian life-forms had affected the contents of ALH 84001.
First, they saw small (20–40 nanometerse wide), rod-shaped structures composed of carbon-bearing molecules that resemble rod-shaped bacteria. Without any doubt, ALH 84001 contains tubular, rope-like structures that look like certain kinds of terrestrial bacteria. But are they fossil bacteria? They are smaller than any bacteria that, in 1996, were known to exist on Earth. Could bacteria of such small sizes exist? The shapes alone do not make the objects fossil bacteria, but the small sizes alone do not mean they cannot be.
Second, they found orange-colored, pancake-shaped, carbonate globules; that is, the globules are rich in minerals that contain the carbonate ion CO3−2. Intimately associated with these globules, they identified microscopic mineral grains that they believed appeared to be of bacterial origin. That is, on Earth these mineral grains are all commonly manufactured as products of biological activity. Contamination by terrestrial carbonate sources is not considered an issue by any critics, in large part because the carbonate globules, which comprise about 1 percent of the mass of this meteorite, are physically associated with the tubular structures that might be fossil bacteria. Another particularly fascinating aspect of the carbonate blobs is that they almost certainly formed in a liquid water environment, so they formed on a location on Mars that was warmer and wetter than any part of the Martian surface is today.
Third, they found organic (carbon-bearing) compounds known as polycyclic aromatic hydrocarbons (PAHs), which they asserted were formed from the “diagenesis of microorganisms,” which is the process by which organic matter turns into sediment. A PAH is a ring-shaped molecule that contains both hydrogen and carbon atoms. More than one hundred different PAHs have been identified on Earth, and they can be of either biological or nonbiological origin. Some are manufactured; others form from the incomplete combustion of organic matter (e.g., cooking meat or burning tobacco, coal, or oil) or from the slow but natural decomposition of dead organisms. Notably, PAHs “are not produced by living organisms and do not possess any specific role in living processes.”14 But at the August 7 press conference, Richard Zare, who led the laboratory team that found and studied the PAHs, stated definitively, “It [these PAHs] very much resembles what you’d expect when you have simple organic matter decay.”15
Finally, they found magnetite crystals coexisting with iron sulfide grains in the carbonate globules. According to the McKay team, these magnetite particles “are similar (chemically, structurally, and morphologically) to terrestrial magnetite particles known as magnetofossils. . . . Some of the magnetite crystals in the ALH 84001 carbonates resemble extracellular precipitated superparamagnetic particles produced by the growth of anaerobic bacterium strain GS-15.”16 That is, they look like crystals that are made by a terrestrial species of bacteria. Of great importance to the case for the biological origin of the magnetite particles is the McKay team’s assertion that the magnetite crystals and the iron sulfide grains should not occur together naturally; they can, however, be found together in some living things on Earth, where biogenic processes force the creation and preservation of both.
How have these four lines of evidence of life in a Martian meteorite stood up under the test of time and under the withering scrutiny of skeptics? Not well.
Are the tubular structures that look like bacteria actually bacteria? They are small, very small, almost certainly too small, according to the very strong consensus opinion of experts. In 1998, the National Research Council convened a National Academy of Sciences panel to determine, to the best understanding of modern science, the limiting size of very small microorganisms.17 “Free-living organisms,” the panel concluded, “require a minimum of 250 to 450 proteins along with the genes and ribosomes necessary for their synthesis. A sphere capable of holding this minimal molecular complement would be 250 to 300 nm in diameter, including its bounding membrane.” As for the smallest bacteria yet observed, “bacteria with a diameter of 300 to 500 nm are common . . . smaller cells are not.” The National Academy panel, however, did not completely preclude the possibility that primitive microbes might have once been smaller, perhaps as small as 50 nanometers in diameter. Even that size, however, is larger than the diameters of the worm-shaped structures in ALH 84001.
Figure 11.3. High-resolution image, obtained with a scanning electron microscope, of the inside of the meteorite ALH 84001. At the center of this image is a tube-like structure whose diameter is less than 1/100th that of a human hair and that is located in a carbonate globule inside the meteorite. The scientific team that performed the original investigation of this sample argued that the tube-like structure was a fossil of a bacteria-like life-form. Image courtesy of NASA’s Johnson Space Center/Stanford University.
The consensus of the experts appointed by the National Academy was challenged by a handful of scientists who thought they had a better and different answer. Their challenge was rooted in a discovery made in the hot springs at Viterbo, Italy, in 1989 by Robert L. Folk, a geologist at the University of Texas at Austin, who found therein what he claimed were nanobacteria. The supposed Viterbo hot springs nanobacteria are exceedingly tiny, ranging in size from 10 to 200 nanometers. With McKay’s announcement of his discovery of possible 20–40-nanometer fossils in a Martian meteorite, Folk’s nanobacteria suddenly became tangible proof of the reality of such small living things on Earth. Alas, the overwhelming abundance of evidence that had emerged by 2010 reveals that the materials uncovered by Folk have been “conclusively shown to be nonliving nanoparticles crystallized from common minerals and other materials in their surroundings.”18
Additional arguments have been put forward regarding the speciousness of identifying a structure as biological simply because of its shape. Many studies have shown that “morphology alone is a poor and ambiguous indicator of biogenicity.”19 Many common minerals can resemble biological structures, like those structures seen in ALH 84001. Some of these are even created as artifacts in the process of preparing materials for examination under certain microscopes, as the materials being prepared for study must be painted with special coatings that allow them to properly respond to the examination technique.
What about the PAHs? PAHs are not special or unique. They exist throughout the length and breadth of the universe. Astronomers have identified them in interstellar clouds, in the atmospheres of red giant stars, and in the expanding shells of dying stars (planetary nebulae). Closer to home, meteoriticists and astronomers have found them in meteorites known as carbonaceous chondrites, on the surfaces of asteroids, and in the atmosphere of Saturn’s largest moon, Titan. In fact, not finding PAHs might be more difficult than finding them.
The PAHs in ALH 84001 have been studied intensively, and those studies have yielded a variety of explanations and challenges. One fundamental challenge is whether the PAHs are Martian in origin or are contaminants, either terrestrial or extraterrestrial. According to the McKay team, the PAHs in ALH 84001 were deposited when a low-temperature fluid penetrated the cracks in the rock. But not everyone agrees. Jan Martel, of the Laboratory of Nanomaterials at Chang Gung University in Taiwan, wrote in a major summary review published in the journal Annual Review of Earth and Planetary Science, in 2012, that we know some other meteorites “harbor PAHs similar to the ones found in ALH 84001,” and some studies “have concluded that the vast majority of the organic molecules found in this meteorite do, in fact, represent terrestrial contamination.” Whether any of the organic material in ALH 84001 originated on Mars, he wrote, “is still up for debate.”20
As for the carbonate globules that are associated with the PAHs, they are problematic as well. If the carbonates are of Martian origin, as McKay and his colleagues proposed, then a fluid must have gently washed organic material into the rock, whence the globules subsequently precipitated out of solution into the cracks within ALH 84001. In this case, both the temperature of the fluid that penetrated the rock and the environment in which this event occurred matter. One early challenge argued that the carbonates formed in a CO2-rich fluid environment at very high temperatures (>650°C, or >1200°F), as a consequence of an asteroidal impact with the Martian surface.21 In a 1998 study, Laurie Leshin, of UCLA, offered two other high-temperature (above the boiling point of water) formation alternatives—formation at 125°C (257°F) in a water-rich environment or formation at a temperature above 500°C (900°F) in a CO2-rich fluid environment. “Neither,” she concludes, “is consistent with biological activity.”22 Another study, conducted in 2005 by Edward Scott and his colleagues, of the Hawai’i Institute of Geophysics and Planetology, claimed to demonstrate “that the carbonates in ALH 84001 could not have formed at low temperatures, but instead crystallized from shock-melted material. This conclusion,” these authors wrote, “weakens significantly the arguments that these carbonates could host the fossilized remnants of biogenic activity.”23 Most recently, however, Itay Halevy, of Caltech, determined that the carbonate minerals “precipitated at a temperature of approximately 18°C (64°F) . . . pointing to deposition from a gradually evaporating, subsurface water body.” Halevy also concluded that “Though the mild temperatures point to an environment that might be considered habitable, the presence of water was also ephemeral, suggesting a time frame probably too short for life to have evolved de novo [edit: i.e., starting from non-living material].” While the arguments continue to go back and forth as to the temperature of the environment in which the organic material that eventually became the carbonate globules was deposited, none of the models support the possibility that life was involved, and all of them are consistent with explanations for their formation involving “nonbiological precipitation of minerals from supersaturated aqueous solutions.”24
In the end, we are left with only the magnetite crystals as evidence that might support the life on Mars hypothesis. Perhaps these crystals provide compelling evidence for Martian life. On Earth, some bacteria build within themselves chains of magnetic crystals that they then use to orient themselves with respect to Earth’s magnetic field. Numerous studies by members of McKay’s research team yielded reports that the magnetic crystals in ALH 84001 were similar to those found in magnetotactic bacteria on Earth. Many of the arguments they have used to assert the biological origin of these magnetic crystals—that the crystals are extremely similar in their sizes and shapes and crystallography to those found for magnetic crystals in magnetotactic bacteria on Earth—have not, however, withstood the test of multiple scientific teams examining the same crystals. As Martel noted in the summary study in 2012, “Other researchers have shown that there exists considerable structural, morphological, and crystallographic variability in the magnetite crystals found in various species of magnetotactic bacteria, suggesting that it is difficult to confirm a biological origin for magnetite particles simply by comparing them with the magnetite crystals observed in terrestrial bacteria.”25 Other studies have questioned whether the magnetic crystals in ALH 84001 form chains. “Taken together,” Martel concluded, “these results call into question the hypothesis that the magnetite crystals found in ALH 84001 are of biogenic origins. . . . it can be safely said that the verdict on the presence of exobiological life on Mars cannot be reached on the basis of this thin line of evidence alone.”26
In 2003, Allan Treiman, of the Lunar and Planetary Institute, in Houston, Texas, wrote cautiously in a NASA report, “The hypothesis of McKay et al. has not been validated. . . . nearly all the data on ALH 84001 and on Earthlife developed since 1996 is [sic] not consistent with the claims, arguments, and hypothesis of McKay et al.”27 A decade later, Martel wrote similarly, “The main arguments that have been used to support the case for past life in the ALH 84001 meteorite can be best explained alternatively by nonliving chemical processes.”28
The larger scientific community has reached an equilibrium, if not a consensus, on how to interpret the evidence found in ALH 84001. Those few who believe that the mineralogical evidence in ALH 84001 demonstrates that life once existed on Mars continue to believe they are right and continue to do research that they believe generates additional support for their position. After all, while they have not provided incontrovertible proof for evidence of past biogenic activity in a Martian meteorite, they argue that their scientific opponents have not provided absolute proof that they are wrong. Meanwhile, every new measurement they make that suggests the magnetite crystals could be of biogenic origin inspires others to dig deeper, to make sure the modeling equations are correct (or to correct them), to fine-tune our understanding of how the geochemical reaction steps proceed (or to correct our understanding of these processes), and to make better measurements with their electron microscopes. With virtually every new positive report on the biogenic origin of the magnetite grains, the naysayers generate their own new scientific information that allows them to volley the conclusion of “yes, the minerals are evidence of life” back across the net with a firm “no, they are not.” As this cycle of healthy scientific debate continues, the science gets better and our knowledge of Mars, of Martian geochemistry and of ancient paleontology improves.
The work surrounding ALH 84001 is a case study of highly skilled scientists doing science extremely well, not an example of incompetent scientists doing science badly. While the interpretation by McKay and his team of their data was immediately controversial, neither the data published by the McKay team nor the remarkably high quality of those data were ever in question. Neither are the data that they and others continue to produce.
This case is also an example of how the many influences—politics, media, funding, fame—that scientists would like to think are external to the pursuit of truth that drives scientists can shape how science is reported and even done. Scientists often have no choice but to follow the money that funds their research; meanwhile, NASA directs money to research activities that the public and Congress support. The result is that nonscientific reasons often motivate what science is funded. In this instance, the big publicity splash resulting from a discovery made in a single meteorite found in Antarctica has had an enormous impact on science: big money from NASA has been channeled into meteoritics, into searches for extremophiles, and into the race to Mars, all because of the enormous potential effects of the discovery of life in one meteorite.
In this particular case, over a period of more than a decade the scientific method—test, retest, and then test again—has brought forward a consensus. Most of the original evidence for ancient life on Mars as found within this meteorite has not survived the rigors of scientific challenges. The full weight of the argument now appears to rest on whether the magnetite grains could have been produced by inorganic means or only by biological processes.
We do not yet have the extraordinary evidence we should demand that would lead us to agree that ALH 84001 presents evidence of ancient biogenic activity on Mars. The search goes on.
a The Meteoritical Society identifies 383 meteorites as diogenites (basaltic meteorites composed mostly of the mineral orthopyroxene). In addition, 279 meteorites known as eucrites (basaltic meteorites composed mostly of the mineral pigeonite) and 329 meteorites known as howardites (meteorites containing a mixture of diogenite and eucrite material) are also thought to be from Vesta.
b Reported deaths (not all with supporting evidence) from meteorites include a man in India in 1825, cattle in Brazil in 1836, a horse in Ohio in 1860, a whole family in China in 1907 (no evidence), the Nakhla dog in Egypt in 1911, a man in a wedding party in Yugoslavia in 1929, and hundreds of reindeer, among other creatures, by the Tunguska blast in Siberia in 1908 (see www.icq.eps.harvard.edu/meteorites-1.html).
c PAHs are defined and discussed later in this chapter.
d Three other spacecraft are active in Mars orbit. The Mars Express Orbiter (2003–present) was launched and is managed solely by ESA. In addition, the ExoMars Trace Gas Orbiter (TGO), a joint mission of ESA and the Russian space agency Roscosmos, was inserted into orbit around Mars in 2016; and the Mars Orbiter Mission (MOM, also called Mangalyaan), launched by the Indian Space Research Organisation, arrived at Mars in 2014. ESA also sent the British-built Beagle 2 Lander to Mars as part of the Mars Express mission, though it failed before reaching the surface. The ExoMars Lander, Schiaparelli, also failed to land safely in 2016.
e 1 nanometer is a billionth of a meter (or 4 hundred-millionths of an inch). A DNA molecule is 2–12 nanometers wide. A human hair is about 50,000–100,000 nanometers in diameter.