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.