n Aug. 16th, 1996, NASA scientists David S. McKay and Everett K. Gibson, along with a team of government, corporate and university scientists, dropped a bombshell on the astrobiology community by announcing evidence for microbial life on the planet Mars. The source? A meteorite called ALH84001, found in Antarctica and analyzed at the Johnson Space Center in Houston. Their historic paper in SCIENCE, and the subsequent storm of press conferences, outlined the nature of this evidence and the reasoning behind their stunning hypothesis.
That was five years ago, almost to the day. How well are their arguments holding up? In science, of course, a hypothesis can't be proven—only disproven or supported by new evidence and new reasoning. And throughout the world, a great many researchers have tested five key preconditions for the meteorite samples to be considered genuine Martian microfossils:
1) That the rock is a meteorite.
2) That the meteorite is from Mars.
3) That the rock was exposed to liquid water while on Mars, creating conditions where life could potentially survive.
4) That the structures found in the rock, which resemble the remains and excreta of bacteria, are in fact of biological rather than mineral origin.
5) That the bacterial remains originated not on Earth following the meteorite's landing, but on Mars prior to its ejection.
Martian life has arrived—sort of
This first question is easy enough to answer: ALH84001 was found sitting on top of the thick ice sheet in the Alan Hills region of Antarctica. (ALH stands for Alan Hills, and the numeric code indicates that it was the first rock sample collected there in 1984.) Meteorites are no more or less common here than anywhere else on Earth, but since there is no way for rocks to get on top of the ice sheet except by falling, they are easier to distinguish from ordinary, terrestrial stones. Basically, if you find a rock in the Alan Hills, it's either a meteorite or the ejecta from a large volcano somewhere on Earth. But an earthly rock of ALH84001's extreme age should be buried under the ice, not sitting on top of it.
The second question is only slightly more difficult; thanks to the Viking landers, which touched down on the Martian surface in 1976, we know the exact mix of isotopes (heavier and lighter forms of the same element) in the Martian soil and atmosphere, and can compare these against the Alan Hills meteorite. The match—a unique planetary fingerprint—is very close indeed. There is very little doubt that ALH84001 is from Mars.
In fact, using the isotope fingerprint, cosmic ray tracks (essentially microscopic bullet holes through the material which accumulate at a known rate), wear patterns and other structural features, we can accurately deduce the rock's entire history. (This process, known as "geology," has an excellent track record here on Earth. Ask Exxon.) The rock was formed volcanically on Mars some 4.5 billion years ago, and then apparently ejected by an asteroid strike some 16 million years ago, and landed on Earth a mere 13,000 years ago. So it's been in space a long time. And yeah, it does appear to have been hydrated (i.e., soaked in water) prior to its rude departure. Most Earth rocks are hydrated, and they look completely different from the dry ones we find on the moon, although in the case of ALH84001 the contrast is not nearly so definitive.
Number four is the trickiest question of the bunch, and in their original paper, McKay and Everett advanced four separate findings to support their hypothesis: a) globules of a carbonate material inside the rock, b) microscopic grains of an inorganic iron oxide called magnetite, along with grains of two iron sulfides, found inside the carbonate granules, c) chemicals called polycyclic aromatic hydrocarbons, or PAHs, found in the same areas, and d) "elliptical, rope-like and tubular" mineral structures which might be the fossils of small bacteria. Earth bacteria can form microfossils as well, although these are usually (though not always) much larger. Some Earth bacteria also create magnetite, for use as a kind of internal compass, and excrete iron sulfides as a waste product.
This all struck me as pretty fishy back in 1996, and I remain politely skeptical. But it's harder today to feel sure about it. As Everett himself points out, "Five years later, all four lines of evidence still stand. Not one has been disproven." For example: Some characteristics of the carbonate globules could indicate they were formed at high temperature—650 degrees Celsius, well above the tolerance of even the hardiest life forms. But bacterial action could cause these same features, so in fact the evidence is not conclusive either way. Similarly, while PAHs are common products of bacterial decay, they are also common in inorganic minerals, including airless asteroids.
The size of the purported microfossils—between 20 and 100 nanometers, or billionths of a meter—was probably the single greatest point of skepticism; earthly bacteria are normally a micron or more in size (one millionth of a meter), and the smallest confirmed fossils are hundreds of nanometers across. However, subsequent research has shown that "nanobacteria" as small as 50 nanometers exist on Earth in certain extreme environments, such as rocks buried kilometers deep in the planetary crust, and the Foresight Institute's Robert Freitas reports theoretical minimum size for a viable bacterium at a mere 40 nanometers. Of course, looking at rocks is like looking at clouds; it's easy to find the shapes of bacteria, bunny rabbits, Richard Nixon or whatever. Still, these particular shapes were found only inside the carbonate globules, and nowhere else in the rock. Findings are similar in another Martian meteorite under study, called EETA79001.
Most recently, a detailed analysis of the magnetite grains by the Lockheed Martin Corporation reveals them to be "indistinguishable" from grains produced by a strain of magnetobacteria here on Earth known as MV-1. No known mineralogical process can duplicate the exact size, shape and purity of these grains, which are orders of magnitude smaller than the magnetic particles on today's magnetic disks and tapes. The grains are also found in chains, like ultratiny strings of pearls, which again is consistent with a biological rather than a mineral origin. However, a recent NASA experiment did show that carbonate globules, when heated, could produce magnetite grains, though not necessarily identical to those found in ALH84001.
As for question 5, it's difficult to completely rule out Earthly contamination as a possible source for some features. Still, the oxygen and carbon in the carbonate globules carry, once again, the isotopic fingerprints of Mars. Also, the deposits themselves are about 3.9 billion years old, dating back to the planet's warm, wet period, when its atmosphere was thicker. Certainly they're much older than the impact event which lifted the rock off Mars' surface. And the PAHs, if they'd soaked in from some Antarctic source, should be found in greater quantity near the rock's surface, with concentrations decreasing toward the center. This is not the case with ALH84001. The PAHs themselves are also quite different from those normally found in Earth's atmosphere, or in ordinary carbonaceous meteorites. As for the "bacterial" shapes, these don't resemble any species known to infiltrate rocks in Antarctica, and none of the other meteorites found in the Alan Hills were found to contain bacteria. Of course, the other meteorites didn't contain carbonates either, so McKay and Everett acknowledge that if an unknown carbonate-loving species were resident in the area, it could well colonize 84001 while leaving the others alone.
Earth can still make Mars
So much of the evidence is subject to multiple interpretations that skeptics like myself have a hard time simply accepting it for what it appears to be. But in essence there is a fifth leg in the pro-microfossil argument: that all of these features are found together, in tiny pockets of the meteorite. Where carbonates are found, so are PAHs and magnetites and nanobacteria-like imprints in the rock, even though these all require wildly different chemical processes to create. And where the carbonates are not found, neither are these other features. This is exactly what you'd expect from ancient biological traces, and would be very unusual indeed if the origin were purely mineral. But then, the discovery of Martian life would be unusual, too. Didn't the Viking probes rule it out? Well, actually, this evidence is also being re-examined, and in some cases reinterpreted.
Martian life has certainly not been proven beyond the shadow of a doubt, or even a reasonable doubt, but there certainly is a growing heap of circumstantial evidence. If ALH84001 were on trial, the jury might have a tough time finding it innocent. Fortunately, these things aren't decided in an afternoon, but over decades of study and debate. Continental drift took 40 years to be widely accepted, and the asteroid impact theory of the dinosaur-killing K-T extinction event took almost 20. And really, one of the largest obstacles to acceptance is simple bias on the part of researchers and journalists. Case in point: I personally don't want there to be native bacteria on Mars, because I'm terribly afraid that would make the colonization of the planet politically impossible. Endangered species habitat: hands off.
Still, it does seem clear that Earth and Mars—and possibly other planets as well—are capable of exchanging material through meteorite impacts, and in principle this could well include biological material. Bacterial spores can survive hardships similar to or worse than those encountered in outer space, and there is evidence that they can remain viable for thousands or even millions of years. In 1976, when it was first proposed, "panspermia" sounded like a damned kooky idea; life on Earth began with a drifting space seed? Get real. But now we know there is nothing impossible about it, or even all that implausible.
This summer found an astronomer in Wales claiming to have found spaceborne bacteria in Earth's upper atmosphere, and another astronomer in Italy claiming to have revived actual organisms from a (non-Martian) meteorite. In 1995, these men would have been dismissed as lunatics, but today the press and the scientific community will at least perk their ears up and listen. And here my own prejudices show through as well, because if Mars is indeed contaminated, then I do at least hope that everyplace else is, too. If life is everywhere, and everywhere it's all the same, then Mars is no more or less sacred than any other bit of rock, and we can, with clean conscience and dusty red hands, move in and make the place our own.
Wil McCarthy is a rocket guidance engineer, robot designer, science fiction author and occasional aquanaut. He has contributed to three interplanetary spacecraft, five communication and weather satellites, a line of landmine-clearing robots, and some other "really cool stuff" he can't tell us about. His short fiction has graced the pages of Analog, Asimov's, Science Fiction Age and other major publications, and his novel-length works include Aggressor Six, the New York Times notable Bloom, and The Collapsium.
