Life is a lot like Supreme Court Justice Potter Stewart’s famous definition of pornography — hard to define, but you know it when you see it. At least, that’s what scientists hope when they search the cosmos for life, or the remnants of life. But how do scientists know what to look for on planets where life could have evolved under drastically different circumstances than it did here on Earth? Would they really know it when they saw it?
There would be chemical clues, says Judith Herzfeld, professor of biophysical chemistry. Even if an organism itself didn’t look like any we’ve seen before, its appearance and its impact on its environment would be distinct from that of lifeless material, she says.
There’s a famous experiment that shows how life can change the surrounding environment. About 60 years ago, University of Chicago chemists Stanley Miller and Harold Urey built an apparatus that recreated pre-life conditions on the Earth’s surface. The experiment combined water and the gases in Earth’s primitive atmosphere — hydrogen, water vapor, methane and ammonia — with an electric spark to simulate lightning. In the experiment the simple molecules were electrically stimulated to produce more complex molecules, like amino acids and sugars that we closely identify with life.
“Life has a remarkable tendency to throw things out of whack,” says Herzfeld, who won a NASA grant a few years ago to study a curious aspect of the Urey-Miller experiment.
In the original experiment, hydrogen cyanide and some amorphous gunk that could be precursors to biological polymers, chains of molecules, were also produced. Chemists suspected a relationship between the two, as hydrogen cyanide — a deadly gas, formerly used in gas chambers — is known to spontaneously form polymers. Herzfeld and graduate student Irena Mamajanov studied the structure of these polymers and discovered three forms that were nothing like scientists had imagined.
But what does this have to do with finding life on other planets?
First, it means that life can emerge from unlikely places. Second, how life changed Earth’s environment can provide clues for finding life on other planets. Before life, our atmosphere (well, it wasn’t really ours then) was composed primarily of methane, ammonia, hydrogen and water vapor. Once life took hold, the new organisms consumed methane and caused oxygen and carbon dioxide to accumulate.
“What would really point to life is if the balance of the chemistry somewhere doesn’t make sense with all the ordinary factors on the planet, like overall composition, temperature or radiation,” Herzfeld says. “We are looking for something distinctive.”
Of course, whatever life on other planets looks like, it will likely need water, Herzfeld says.
“It’s hard to imagine life without water because it’s just such an unusual molecule,” she says. “Liquid water buffers temperature really well, it has a way of helping proteins fold and cell membranes form, it readily dissolves ions. Each water molecule can participate in hydrogen bonds with four neighbors, so it can do pretty amazing stuff.”
So, what are the chances of finding a planet with liquid water and chemistry that doesn’t make a lot of sense?
“The universe is a big place,” Herzfeld says. “There’s a good chance there is something really interesting out there. If we find it, we’ll probably argue about what it means for a while. But, then again, we argue about what is life on Earth, too.”
To learn how Brandeis alumna Valerie Scott is developing devices to chemically analyze soil and rock samples on celestial bodies, perhaps to find life, check out our profile on BrandeisNow.