The Interstellar Amino Acid TestApr. 04, 2002
by Seth Shostak- Senior Astronomer
It's one of the great, unsolved mysteries: how did life get started?
Puzzling out how biology gained a foothold on Earth has obvious importance for gauging whether it's likely to be widespread in the universe. After all, if life required some really special circumstances to jump-start its existence on our planet, then you can safely figure that the bulk of the universe is as sterile as steam.
However, scientists have just weighed in with some amazing results that suggest the contrary.
The old-school view, first espoused by Chuck Darwin, is that terrestrial life may have begun in some "warm, little pond." More modern suggestions have implicated deep-sea vents, or maybe muddy clays as the birthplace of us all. But an idea that's been jazzing the research community for some time is the possibility that the seeds of life may have come drizzling down from space: not as full-blown microbes, mind you, but as those infamous building blocks of biology: amino acids.
Every reader who's slogged his way through high school biology knows that the proteins of life are constructed from 20 different amino acids; a set of rather simple organic molecules. If you made it to college biology, then you might also be aware that the amino acids found in bacteria, your dog, and you are all left-handed. What does that mean? Well, just like catchers' mitts, amino acids come in two styles: left-handed, and right-handed. Both styles function identically, and are simply mirror images of one another. If you cook up amino acids using chemical reagents, you'll end up with equal amounts of each style.
So why are life's amino acids left-handed? One possibility that's been bandied about for years is that they were made - not in a tepid pond - but in space; perhaps in giant, watery comets. In the dusty interstellar clouds that form solar systems, starlight tends to get polarized. In other words, the light in these clouds is itself "handed". So it's possible that somehow, some way, left-handed light has produced left-handed amino acids that eventually end up on Earth, and lead to left-handed life.
Amino acids, mostly left-handed, have been found in meteorites, suggesting that this idea might be right. After all, the meteors are just fragments of old comets, where presumably the amino acids were born.
There's a problem, however. For various chemical reasons, it seems that the amino acids found in the rocks from space were made at very, very low temperatures, hundreds of degrees below the freezing point of water. So the idea that the building blocks of life were all cooked up in a (relatively) warm little puddle of water inside a comet just doesn't seem to well, hold water.
But now Max Bernstein and Jason Dworkin of the SETI Institute, together with colleagues Scott Sandford, George Cooper, and Lou Allamandola of NASA's Ames Research Center, have made an astounding discovery. They did so by going into their lab and simulating the dusty, interstellar clouds that formed the planets and Sun. Icy dust grains were stuffed into a vacuum chamber at bitterly cold temperatures, and exposed to artificial starlight. After giving the grains some time, the researchers checked out the grains, and found four of the amino acids that make up earthly life - the building blocks of biology, constructed without the benefit of liquid water!
You might be thinking, what's the big deal? Acids made in warm water or in cold ice? What do I care? The reason to care is this: comets and asteroids that might occasionally harbor pools of liquid water could be rare. Maybe a lot of solar systems don't have an asteroid belt or a cloud of handy comets. If cooking up the ingredients of life requires liquid water in space, then life itself might be as rare as white rhinos.
But all planetary systems are made from cold dust clouds. They're truly universal. So forming amino acids on cold dust grains is more than an esoteric laboratory exercise. The implications of this remarkable experiment are that biology might be as common as planets themselves.
As Max Bernstein says, "It may not be that efficient, but a cloud 100 light-years across makes a pretty big beaker."