If you were on a distant planet, and if you had instruments that could tell you the composition of Earth's atmosphere, how would you know there was life on this planet?
Water in the atmosphere would suggest there could be water on the surface, and as we all know water is considered crucial to life. But water would only suggest that life is possible. It wouldn't prove it's there.
Carbon? That basic component of "life as we know it?" Not necessarily. A diamond is pure carbon, and it may be pretty, but it isn't alive.
What really sets Earth apart is nitrogen, which makes up 80 percent of the planet's atmosphere. And it's there only because there is abundant life on Earth, say scientists at the University of Southern California, who have come up with a provocative approach to searching for life on other planets.
"If there wasn't anything biological on Earth, there wouldn't be very much nitrogen in the atmosphere," says Douglas Capone, professor of environmental biology at USC and lead author of a report in a recent issue of the journal Science. The report grew out of a class discussion two years ago in a course taught by Capone and Kenneth Nealson, professor of earth sciences.
Students were asked to come up with different ideas about searching for extraterrestrial life. What is a distinct "signature," as Capone puts it, that would show there is life on another planet?
That's a question that has been kicked around in many quarters in recent decades, especially since all efforts to find some form of life, no matter how primitive, either on Mars or in the distant reaches of space, have failed. At least so far.
The current effort to search for some evidence of life, even fossilized life, on Mars focuses primarily on the search for water, because it has long been believed that water, or at least some fluid, is necessary for the chemical processes that lead life to take place. But that's probably the wrong approach, the USC group argues.
"It's hard to imagine life without water, but it's easy to imagine water without life," says Nealson, who was on the Mars team at NASA's Jet Propulsion Laboratory before moving to USC.
But nitrogen would be a much clearer signature of life.
"If you found nitrogen in abundance on Mars, you would get extremely excited because it shouldn't be there [if there is no biological activity]," Nealson adds.
Here's the real downside to this story for folks who are still hoping to find life on Mars.
Only about 2 percent to 3 percent of the Martian atmosphere is nitrogen. That's just a trace, and it probably means there is no life on Mars today, and if there was in the past, it probably ended many, many years ago.
But, the USC team adds quickly, that doesn't mean there's no life anywhere else in the universe. They don't know where, of course, but they may have found a way to narrow down the search. Look first for nitrogen, then look for biological activity that should be there.
Capone, who says he has received no negative feedback from other scientists since proposing a shift in the focus of the search for extraterrestrial life, says nitrogen functions very differently from carbon, that other crucial building block of life.
Carbon turns up everywhere, even in rocks and other minerals. But nitrogen has a different lifestyle. It is vital to the lives of plants and animals, and so it is found only in rocks that were formed in sedimentary basins. There are no nitrogen-based mountains, or deep reservoirs, other than the atmosphere.
"The nitrogen cycle on Earth is dominated by a lot of microbiological reactions," says Capone. "Life takes it up through a process called nitrogen fixation. And that converts nitrogen gas to forms that the rest of the biosphere can use, like ammonium. There's a biological pathway for that."
The nitrogen used by plants and animals eventually returns to the soil, as all things must, and then washes down to the sea, where it evaporates into the atmosphere to begin the process all over again.
"So we have this very dynamic cycle that is really dictated by these specific groups of microorganisms on Earth," Capone says. "The cycle is dictated by life on Earth."
So if life exists elsewhere, and is similar to life as we know it, there should be nitrogen, and that's what we should be looking for first, the researchers say.
For now, they would be satisfied with a close look at a few sedimentary rocks from Mars. If they don't find nitrogen, Capone says, "that will probably bring us to the conclusion that there likely never was life on Mars."
But how about elsewhere? Could this technique be used to search for life in other solar systems?
It might be possible to detect a nitrogen-rich atmosphere around a planet orbiting another star, but not yet. Current instruments aren't that sensitive.
If they ever are, the search for life might be narrowed down to the most promising prospects, chiefly because of the presence of nitrogen. And won't that be fun.