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This has produced a sort of "We'll know it when we see it" attitude in the field, and even that is iffy. Selective pressures on other planets could have pushed evolution in directions we can't even imagine, and scientists have to struggle against their Earthcentric biases. For example, Mathies says, you wouldn't want to go to Mars and sample for DNA, because what if Mars life doesn't have DNA? "One of the hard things about structuring an experiment that looks for life is that if you make your hypothesis too specific you may not find it," he says. "And if you make it too general, you may not learn anything."
Yet scientists do believe there are some themes the cosmic creation process is likely to play repeatedly. Earth life is based on the most common chemical elements in the universe so other life is likely to use them too. Liquid water seems to be, as one scientist puts it, the "the cosmic cocktail mixer" that allows biochemistry to occur, and water also is commonly available. Amino acids are the building blocks of Earth life, and they form readily in nature, even through nonbiological processes. They have been found in meteorites, showing that they don't have to originate on Earth.
Inspired by science Mathies developed while working on the Human Genome Project, his chip exploits an amino acid quirk particular to Earth life. In nature, amino acids have two forms that are mirror images of each other: They can be either "right-handed" or "left-handed." Nonbiological processes produce an equal mix of these two forms. By contrast, Earth life uses only left-handed amino acids. As McKay puts it, "Biology selects; chemistry doesn't."
So if an extraterrestrial specimen has an equal mix of the two forms, it's not biological. But if its amino acids are uniformly left- or right-handed, it's nearly a sure sign of life. If they all happen to be right-handed, well, that's even more exciting it indicates not only life, but life different from that of Earth, compelling evidence of an independent genesis.
This is the sort of scenario that makes McKay light up with glee. He's already got plans for what we should do if we discover we have neighbors: unfreeze them by creating global warming on the Red Planet, a sci-fi notion if ever there was one. "If I found alien life on Mars, a second genesis, my vote would be to bring it back to life and restore the planet to habitable conditions habitable for that life form," he says. "My slogan would be 'Mars for Martians.'"
Even if that meant a planet covered in bacteria?
"Yep," he says. "As long as they're alien bacteria."
But wouldn't that set in motion an evolutionary chain whose endpoint we can't possibly predict?
"Exactly," McKay responds. "What if three billion years ago somebody came to Earth and said, 'Oh, nothing here but microbes, hit the erase button, alt-shift-delete'? We'd be unhappy with that."
Fine, but imagine the equally likely scenario in which the amino acids are left-handed, like ours. "Then the simplest conclusion is that they're our cousins, and that Earth and Mars exchanged material and share a common origin, and the planets Earth and Mars are not any more isolated from each other biologically than the continents on Earth than Australia is from North America," McKay says.
After all, we know that meteors can criss-cross between the two planets some theories even speculate that Earth life originated on Mars, then flourished here, meaning that we have been the Martians all along. More tests would be needed to determine whether and how we map onto one another's trees of life. To McKay, that would be disappointing. "I would really much prefer we found a second genesis and we just smashed the Rare Earth hypothesis to smithereens," he says.
Another plot twist to consider if Mars life looks like ours: that we've contaminated the planet by repeatedly landing on it. Although great pains are taken to sterilize spacecraft, and Mars' harsh UV radiation is likely to have killed off any Earth organisms that could have hitched a ride, it's still a possibility. That's why some people think that if you want conclusive proof of a second genesis in our own solar system, and a chance of finding life totally unrelated to ours, you have to look further out than Mars. Someplace colder, darker, and altogether weirder.
The Case for Europa
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