The Case for Mars (52 page)

Usually when investigating a sample, Mittlefehldt would take a general, overall look at a thin section to get the lay of the land, as it were. His approach to the Elephant Moraine sample was a bit different in this instance, though. He wanted a specific bit of information on the sulfides—compounds of iron and sulfur—within the sample, so he placed the sample under the gaze of an electron microprobe, which would allow him to spot sulfides easi
ly. He started his analysis, but, as he would later recall, “the results were coming out screwy.” In diogenites, sulfides are composed of one atom of sulfur for each atom of iron. In the sample, though, he was finding two atoms of sulfur for each atom of iron, iron disulfide, a compound commonly found in Mars rocks, but not in diogenites.

Mittlefehldt “stepped back” to take an overall loothe section. He noted the disulfides. He noted the zoned carbonates that would play a leading role in the stone’s notoriety years later. He did a double take; such objects had no business being in an ordinary meteorite. There was little doubt in his mind: The sample was not Elephant Moraine, but Allan Hills. And Allan Hills was a piece of Mars.

Mittlefehldt went to Marilyn Lindstrom, curator of meteorites, to tell her of his discovery. A sample of ALH84001 was sent to the University of Chicago for oxygen isotope analysis. Though Mittlefehldt was certain the stone was Martian, the oxygen isotope analysis would nail the identification. It did.

In December 1993, the Meteorite Working Group released a special edition of the
Antarctic Meteorite Newsletter
announcing the reclassification and availability of a new SNC meteorite. As might be expected, the discovery of a hitherto unrecognized Martian rock caused a bit of a stir in the planetary science community. Before long, samples of ALH84001 were winding their way to curious researchers, including Everett Gibson, a planetary scientist at the Johnson Space Center, and Chris Romanek, a National Research Council postdoctoral fellow working in Gibson’s lab.

Gibson and Romanek received their samples of the stone early in 1994 and started an investigation of the carbonates Mittlefehldt had identified within ALH84001. The carbonates appeared as tiny, orange-brown blobs outlined by a thin black-white-black band, or what the investigators called “Oreo cookie rims.” Just 250 microns in diameter, or about five times the diameter of a human hair, the globules nonetheless proved intriguing.

Within a few months, the pair had measured the carbon and oxygen isotopes within the carbonates. Previously, Mittlefehldt had suggested that the carbonates formed deep within Mars’ crust at temperatures approaching 700°C. Gibson and Romanek, on
the basis of the carbonates’ isotopic composition, came to an entirely different conclusion. Their investigations suggested that the carbonates had been deposited by a fluid, most likely water, percolating through the Martian crust at temperatures of 0° to 80°C. In addition to investigating the carbonates, Gibson and Romanek scrutinized the various minerals associated with the carbonates. Again, ALH84001 proved full of surprises, as the two researchers noted some unusual relationships in the mineral assemblages, relationships that, if found in a terrestrial rock, would be attributed to “biogenic” processes—biology, not chemistry, formed the minerals.

Gibson and Romanek were not alone in their investigations. Also poking into the nature of the stone was David McKay, a planetary scientist at JSC who had cut his teeth studying lunar materials returned by the Apollo crews. ALH84001 intrigued McKay as well. He had been exploring the innards of the rock with a scanning electron microscope, an instrument that magnified the surfaces he gazed at by a factor of around 30,000. What he found were extremely small, tubular structures, sometimes packs of them. These tubular structures could, of course, simply be odd bits of geology, flecks of clay. Or they could be microfossils. Gibson and Romanek had also spotted these alluring, odd forms, and they wanted to study them further, but both realized they were not tremendously experienced microscopists. McKay was, and over the course of the summer Gibson and McKay decided to collaborate on their investigations. Working with the scanning electron microscope, the pair continued to uncover what they interpreted as evidence for the biogenic origin of the minerals they saw. However, by summer’s end, they realized that they would need state-of-the-art equipment to fullail down their interpretations. They looked to Kathie Thomas-Keprta, a planetary scientist working for Lockheed-Martin who was skilled with a transmission electron microscope, a probe that had a resolving power five to ten times that of the equipment McKay and Gibson had used.

Thomas-Keprta joined the collaboration as, quite literally, a doubting Thomas. They had told her what they were looking for—microfossils, evidence of past life. She was more than a bit leery of the entire concept of life’s signature buried in this chunk of stone. If anything, Thomas-Keprta was set to prove her collea
gues wrong as she started her investigations of the rock and the mineralogy of the “Oreo” rims. Two years later, though, she would find herself explaining to a gaggle of reporters that, “even though there could be very complicated inorganic explanations for the presence of these mineral grains, the simplest explanation is that these are products from microorganisms that were produced on Mars.”

About the same time Thomas-Keprta joined the collaboration, Gibson and McKay thought it would be worthwhile to investigate the rock for traces of organic compounds. After all, if what they were seeing resulted from biological processes, it would follow that organics might be present, however minutely, within ALH84001. The JSC researchers sent samples of their Martian stone to Richard Zare, a chemistry professor at Stanford whose lab is home to one of the most sensitive instruments in the world for detecting minuscule amounts of organic compounds. The samples were sent “blind”—Zare didn’t know he was getting a piece of Mars that might, just might host evidence of biologic activity in the planet’s distant past. By late fall, Zare’s team was reporting the presence of organic molecules dubbed PAHs—polycyclic aromatic hydrocarbons. Not only were organics present in the samples Zare had received, they were associated with the carbonates that had originally intrigued Gibson and colleagues.

By this time it was clear that the contents of the rock could make it the find of the century. But the possibility of terrestrial contamination of ALH84001—in the field, in the meteorite laboratory, and in the respective researchers’ labs—had to be ruled out. To this end, the curator’s office was informed of the data the team was taking and asked to put in place strict protocols against contamination while the researchers themselves began the tortuous task of investigating possible avenues of contamination and eliminating them one by one. Gibson would later note that the researchers had “the story” within the first year of working with the meteorite, but would work another year and a half simply to prove that “what we had was real and not a contaminant.”

Meantime, work continued on the mineral chemistry and composition of the stone. Gibson, McKay, and Thomas-Keprta continued both scanning and transmission electron microscopic work through the summer and fall of 1995. During one summer evening, Gibson and McKa
y stumbled across an object they would later dryly describe as a “tubular, segmented structure” some 0.4 microns long. The words don’t quite describe the object, which looks for all the world (Earth or Mars) suspiciously wormy. You can look at the image they captured and imagine a front end to the “structure” and a back end and a middle. You can imagine it wriggling along. Chemical clues of life were one thing, but this infinitesimal, wormy-looking thing shook both researchers to the core. It was hard sleeping that night, for both.

Other discoveries were made. During the fall of 1995 the team, with the help of a new mer, Hojatollah Vali, positively identified minerals within the carbonates that, at least on Earth, are often created by bacteria. The size, shape, and chemistry of the mineral grains were identical to compounds produced by anaerobic bacteria and other earthly microscopic organisms. Additional high resolution images of “unusual features” and “interesting morphologies” were obtained. Thomas-Keprta’s doubts fell away as she positively identified minute grains of the mineral greigite, which, in the size range she discovered, is more often than not produced by bacteria, at least here on Earth. Finally, in February 1996, Gibson wrote the first draft of a paper that would be submitted to
Science
, one of the most prestigious journals in the scientific community. The paper was officially submitted to
Science
in April with David McKay as first author, and eight other co-authors, including Gibson, Thomas-Keprta, Romanek, Vali, Zare, and two of Zare’s graduate students, Simon Clemett and Claude Maechling, along with Stanford postdoc Xavier Chillier.

It took nearly four months and several iterations for the paper to wend its way through the peer-review and editorial process, but on July 16th,
Science
accepted the paper, entitled “Search for Past Life on Mars: Possible Relic Biogenic Activity in Martian Meteorite ALH84001.”

News of the paper’s acceptance quickly traveled up the NASA chain of command. NASA administrator Dan Goldin was undoubtedly pleased to find himself fretting over whether to tell the president that researchers within his organization had possibly discovered evidence for life on ancient Mars. After a night of pondering the question, Goldin called the White House for a meeting with the president’s chief of staff, Leon Panetta. The next day Goldin
found himself briefing not only Panetta, but President Clinton and Vice President Gore, and, unknowingly, setting the stage for one the more bizarre news leaks in recent memory.

NASA had kept a lid on the research for months, in part out of respect for the journal
Science’s
strict policy regarding prepublication publicity or announcements of research papers. In a word, it permits none. NASA planned to hold a press conference in mid-August, immediately following publication. News of the discovery, though, leaked out. The first mention came in a small piece by Leonard David in
Space News
. The basic details were there in the aerospace weekly’s report: NASA researchers had discovered what they believed to be evidence for past Martian life in their investigations of a Martian meteorite. Later, though, word abounded that galley copies of the
Science
article were available soon after Goldin’s White House meeting, and from an unusual source—the mistress of Richard Morris, a close political advisor to President Clinton. Whether the work of a scribe or a strumpet, the word was out and the rest, as they say, was history.

Carl Sagan is credited with having once noted that “extraordinary claims require extraordinary evidence.” It was an oft-repeated quote in the days following the announcement, and one that should be kept in mind as researchers line up to take shots at McKay, Gibson, and their colleagues’ work. The evidence the team presented will have to withstand what will undoubtedly be numerous challenges. It’s sometimes easier to think in terms of disproof rather than proof in scientific arguments. Absolute proof is hard to come by, especially in the case of the claim of fossilized Martian life. Disproof on the other hand can require no more than one small crack in the argument, and can be far easier to establish. So, will the claim for Martian life hold up? Let’s take a closer look at the SA team’s arguments, their strengths as well as their potential weaknesses.

McKay, Gibson, and their colleagues base their claims on five lines of evidence. There are, according to the team, alternative explanations for the phenomena present in each separate line of evidence. However, when viewed collectively, the researchers hold that life on Mars is the simplest explanation for the occurrence of all five lines of evidence. The evidence presented by the researchers thus far
is as follows.

ALH84001 Is an Ancient Piece of Mars

There’s little doubt that ALH84001 originally came from Mars. During the 1970s, researchers began to suspect that a small class of meteorites dubbed “SNCs” (after three sample types that were discovered in, respectively, Shergotty, India; Nakhla, Egypt; and Chassigny, France) may have come from Mars. All were igneous rocks—that is, formed from the crystallization of lava—and all were relatively young (about 1.3 billion years old) compared with the known ages of other igneous meteorites. Mars was one of the few planets in the solar system believed to have had active volcanoes that recently (in solar system terms), and the inclusion of water-bearing minerals in the SNCs helped point to Mars as the parent body for these strange rocks. Then, in 1983, researchers discovered Martian atmospheric gas in an SNC and, since then, in nearly all SNCs, including ALH84001. While far older then other SNCs, few would argue ALH84001’s Martian origin.

Formed more than four billion years ago, ALH84001 was blasted off the Martian surface some sixteen million years ago and plummeted to Earth sometime around 11,000
B.C
. The age of the stone has been determined via a technique known as mother-daughter isotope measurement. Researchers measure the relative amounts of specific radioactive chemical isotopes and what they have decayed to. Two isotope suites have been measured for ALH84001; samarium-neodymium and rubidium-strontium. Both have given ages of approximately 4.5 billion years for the time since the stone crystallized. Some time after the rock formed, it was apparently crushed and heated once again, most probably by a meteorite slamming into the Martian surface. This event fractured the rock and, according to McKay and colleagues, provided the fracture lines later filled by carbonates. Researchers have placed the age of this “shock event” (the “shock age”) anywhere from 4 billion to 1.39 billion years ago, again based on mother-daughter isotope measurements, though the NASA team points to a 3.6 billion year age for the shock event and subsequent formation of carbonate globules. A later impact on Mars blasted the stone into space, where it journeyed for millions of years before its plunge to Earth. During that time, high energy elementary particles—cosmic rays—wallope
d the stone, transforming chemical elements into new isotopes. Measurements of isotopes of helium, neon, and argon indicate that ALH84001 drifted through space for sixteen to seventeen million years. Finally, the “terrestrial age” age of the meteorite, how long ago it fell to Earth, has been determined via carbon-14 dating, a technique well known to archaeologists.