A false-color cross-section of the Tissint meteorite.
If you were a passenger aboard the meteorite from Mars bearing down on the town of Tata, Morocco in July 2011, you would be in a decidedly unenviable position. For one thing you’d be a bacterium — a nifty Martian bacterium, to be sure, but still. For another thing you’d be either freeze-dried or in a state of suspended animation — the better to survive the thousands or even millions of years you might have spent in space. And the odds are that even if you were alive and well when you stowed away on the rock while it was on Mars, the life would have been snuffed out of you the moment the asteroid hit that blasted you into space in the first place.
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On the other hand, maybe you’d survive — maybe the shock would not have been so great or you’d be tucked away deep enough in the meteorite that you were spared its full power. And if you did survive and arrived on Earth a few billion years ago, maybe you’d have have adapted well to your new home and grown and thrived and served as the seed for every organism that ever populated your new planet including, eons later, the human species itself. In which case, modern humans never have to contemplate meeting Martians again because we are the Martians.
That, anyway, is the thinking behind panspermia — the theory that life on Earth may have come from beyond, if not bacteria from Mars, then organic raw materials from comets or asteroids that rained down on the planet back in the heavy bombardment days of the early solar system. It’s an idea that’s spawned a lot of faculty-lounge theorizing for more than a century, but in recent years, it’s attained a new credibility. Observations from both space-based and Earthbound telescopes as well as interplanetary probes have shown that organic chemistry is common everywhere in the cosmos. Space is rich in hydrocarbons, water and even the amino acids essential to life. Nucleobases, amino acids and sugars have been found in meteors that have crashed to Earth. Our planet’s water itself is thought by many scientists to have been imported to us aboard ice-heavy comets long ago, preparing the planet for the life it would one day support.
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Panspermia is quietly becoming a go-to area of study for astronomers and astrobiologists, and a new paper published in the current edition of Science is a very good example of what the field is doing. A team of investigators got their hands on a few bits of that Moroccan meteorite — known as the Tissint meteorite for its mineral composition — and subjected it to both mineralogical and chemical analyses, and while no traces of bacteria were found (and none were expected to be found) the scientists did learn more about what it would take for such a planetary tissue exchange to occur.
Martian meteorites look like any other meteorite, but decades of samplings of the Mars’s air and soil by NASA probes have made it easy for scientists to match the chemistry of a space rock with the chemistry of the Red Planet — and the Tissint meteorite is a perfect fit. The debris the researchers studied has what’s known as a fusion crust, a black outer skin that was the result of the super-heating that occurred when the body entered Earth’s atmosphere. The interior is shot through with black glass veins that are rich in traces of Martian air. It’s the glass that makes the meteorite so improbably lovely — to geologists, at least — and also reveals the most about its past.
“You’ve got a rock sitting at the surface of Mars, the impact takes place and the shock wave passes through and kicks it up into space,” says Chris Herd, a meteorite expert at the University of Alberta and one of the co-authors of the paper. “But the shock wave can also cause very local melting where there are open fractures or cavities in the rock. That space collapses and it melts.”
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That would be bad news for any Martian bacteria that happened to be aboard. “Where would you expect microorganisms to be?” asks Herd. “You’d expect them to be in the fractures. Maybe water percolates there. Maybe they set up shop there like organisms that live in rocks on Earth do. Those guys would be vaporized when the melt pockets form.”
That doesn’t stop scientists like Herd from looking for such signs of extraterrestrial life in meteorites both from Mars and elsewhere in space — nor should it. For one thing, just because some bacteria would be flash-cooked when the fragment takes off doesn’t mean all of them would; and some rocks, Herd points out, undergo less peak shock than others. The shortest transit time from Mars to Earth for a randomly drifting meteor would be about 10,000 years — survivable for bacteria that can enter a state of self-protective suspended animation, which some bacteria on Earth do in harsh desert or polar climates. And while such a long time in interplanetary space exposes any organism to lethal levels of cosmic radiation, at least one species of Earthly bacteria, known as Deinococcus radiodurans, has been found to shrug off even the most high-energy radiative roasting. “A lot has to go right,” for bacteria to make a planet-to-planet trip, Herd concedes, but it’s by no means an impossible scenario.
The idea that organics — or even organisms — drift from world to world like spores on the wind is an unfamiliar one for most people. But unfamiliar ideas have a way of becoming familiar, and even commonplace, quickly. The great, thriving terrarium that is Earth may be unique in our solar system. But if the new findings show anything, it’s that our planet may not be at all unique — or even terribly special — in the cosmos as a whole.
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