Mars has always been a provocateur. The planet has a long history of making us uneasy, from the portents of violence our ancestors associated with its red glow, to our science-fiction nightmares of malicious, technologically superior alien invaders.
And Mars is still stirring things up in the scientific community. For several years now, there has been an on-going debate as to whether a meteorite from Mars contains the fossilized remnants of microbial life. Some scientists think we no longer have to wonder about whether there is other life in the universe; we have the remains of tiny Martian cousins in our laboratories at this very moment. Others remain skeptical, claiming that every structure and chemical in the meteorite could have been formed by natural processes that have nothing to do with life, like chemical weathering and heating. Despite the controversy, the Martian Meteorite debate has already taught us a lot about what kind of questions to ask the next time we get our hands on a sample of Martian soil, as well as shown us how little we understand about the threshold of life itself.
Backing up a little, how in the world did a piece of Mars find its way to Earth? Would you recognize a Martian rock if it were sitting in your backyard? The idea isn't as outlandish as it seems. We know of 24 meteorites that were originally part of Mars.
The first was recovered after it boomed down into a field outside of Chassigny, France, in 1815 (although the people of the time had no way of knowing that it came from Mars). The famous Martian Meteorite (designated ALH 84001) that has spurred all the debate was found more recently, lying on the Antarctic ice in 1984. Antarctica turns out to be a rich ground for meteorite hunters, as all of the indigenous rocks are buried beneath thousands of feet of glacial ice. If you find a rock sitting on top of the ice, there's a good chance it landed there from somewhere else. One of the meteorite-hunting teams even has a mascot of a penguin standing with a baseball glove aimed at the sky. Deserts like the Sahara and the Mojave are also a good bet, as the meteorites stand out from the sandy, eroded rocks around them.
So now that you've found a rock from space, how do you know it came from Mars? We've never brought a rock back from Mars, and our robotic landers have only been able to do crude chemical analysis of the rocks and soil there. Interestingly, that issue is not part of the debate. Scientists are almost certain that these meteorites are bits of the planet Mars due to careful chemical analysis of bits of the Martian atmosphere trapped in the rocks. We know the chemical composition of Mar's atmosphere very well (from spectroscopic measurements), and the rocks match it exactly. Really, there's no where else they could have come from.
Which leaves the next big question: How did they get here?
That might be the most amazing part of the whole story. The only way a rock could get to here from there is to be blasted off the surface of Mars at 11,000 miles per hour (that's the speed needed to escape Mars' gravity). There's no physical process on any planet that we know of that can achieve such speeds. Even rocks hurled out of giant volcanic explosions don't go anywhere near that fast. So, in fact, the only thing that can create a Martian meteorite is another meteorite. Probably a very big one, as big as a mile across.
That's right, scientists think these meteorites were chunks of Mars that got blasted into space after a violent (think dinosaurs) impact from a meteor or a comet. Judging from our maps of the Martian surface, this hasn't happened for a while. We have no way of knowing exactly when this impact took place, but we do have some idea how long ALH 84001 stayed drifting around between Earth and Mars. High energy particles called cosmic rays irradiate anything in space, leaving radioactive traces. ALH 84001 seems to have been exposed to these particles for about sixteen million years, although if it was the inner part of a larger meteor that broke up, it could have been in space much longer. And we didn't find it the moment it fell to Earth either, not by a long shot. Judging by other radioactive decay processes, the rock had been cooling its heels in Antarctica for about 13,000 years.
Right away, planetary scientists knew they'd found something interesting, as the rock showed signs of having been flooded with liquid water at least a billion years ago, perhaps as much as three billion. This piqued everyone's imagination, as the meteorite seemed to come from a lost age on Mars, when life might have taken hold.
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.
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.