Sticks and stones: the Martian Meteorite debate rages on
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Billions of years ago, Mars was a very different place, with a thick atmosphere and liquid water either on, or very near, the surface. Liquid water seems to have changed the chemistry of the rock, dissolving parts away and leaving globs of carbon-rich minerals. The globs were also rich in organic compounds called PAH (polycyclic aromatic hydrocarbons). It's actually not unheard of to find complex organic molecules in a meteorite. Many meteorites contain them, and some scientists think that may be how organic chemistry came to Earth in the first place. Still, the rock proved to be intriguing.Skip to next paragraph
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When the scientists turned an electron microscope on these carbon globs, they got the shock of their lives. Inside, clustered tightly together, were hundreds of tiny, wormy shapes. Only about 100 billionths of a meter across, the wormy things looked alarmingly similar to the fossilized remains of ancient Earth bacteria. They certainly didn't look like anything that had been seen inside a meteorite before.
And it wasn't only their shapes that were surprising; all around the "worm" were pure strings of iron crystals, called magnetite. Similar magnetite deposits are left behind when Earth bacteria die and decay. And at that point, scientists knew of no natural process that could produce pure magnetite crystals in the shapes and sizes observed in the meteorite. In fact, up until then, similar magnetite deposits had been used as a tracer to find bacteria in rocks. Did that still hold true? Had they, in fact, found the first example of life outside Earth?
At this point there was a bit of a media circus, and a lot of facts got distorted. In truth, no scientist had ever claimed that the meteorite definitely contained life; there were just a lot of tantalizing loose ends, and no good way of explaining them. Nature (and, it seems, the publicity machine) abhor a vacuum, so in the absence of any conclusions, many people got the idea that we had, in fact, discovered ancient Martian bacteria. But, as is often the problem with front-page news, any subsequent detractions seem to get buried somewhere on the back page.
In the last few months, scientists have done a bit of back-pedaling. What happened, in the best of scientific tradition, is that people went back to their labs and got to work. Was it possible, they wondered, to re-create everything in the mysterious meteorite by natural geologic processes? The wormy shapes were the first to go. There are plenty of ways to create similar shapes from minerals embedded in the meteorite, no life needed. And as stated before, PAH's, although highly-complex organic molecules, exist in abundance in space. There is still some arguing back and forth as to exactly what flavors of PAH's are commonly found in meteorites as opposed to those in ALH 84001, but that particular debate has reached no closure.
In March of 2002, a team of scientists announced a discovery that may turn out to be the last nail in the coffin for the Martian Meteorite. The team had, in their laboratory, created very similar magnetite crystals to the ones in ALH 84001, using nothing but repeated heating and shocking. From what little we know of the meteorite's history, it seems to have undergone plenty of both. Other scientists countered that the artificially created magnetite didn't have the exact same structure as bacteria-produced crystals, which may prove to be true. The crystals are so tiny that much of the discussion has centered on inventing better ways to probe the chemical structure of the crystals.
In the end, no one has proved beyond a shadow of a doubt that the shapes and chemicals in ALH 84001 are due to the presence of fossilized bacteria. But no one has disproved it either, and the whole debate brings up a fundamental issue that NASA will have to face as it begins to search for life outside the Earth: how do you recognize ancient life when you see it?
Billions of years ago, when ALH 84001 was forming inside some long-extinct Martian volcano, life in our Solar System had just barely taken hold. We're not just talking about Mars, either. Even on Earth, only the very first bacteria were emerging. So much of the chemistry of primitive life is indistinguishable from natural changes in rocks and minerals. That's no coincidence; that's what early life had to work with. What was the subtle change in chemistry, somewhere deep inside a rock or miles underneath the oceans, that allowed life to begin? We just don't know. So, it seems that before we can pass judgment on life elsewhere, we may need to get to know ourselves, and our origins, a whole lot better.