Nitrogen gas may point to alien life on exoplanets

Observations of nitrogen in Earth's atmosphere by a NASA spacecraft 17 million miles away are giving astronomers fresh clues to how the gas may help in the search for life on exoplanets.
Finding and measuring nitrogen in the atmosphere of an exoplanet can be crucial to determining if that world might be habitable, since nitrogen can provide clues to surface pressure.
If nitrogen is found to be abundant in a planet's atmosphere, that world almost certainly has the right pressure to keep liquid water stable on its surface. Liquid water is one of the prerequisites for life.
Nitrogen is hard to spot from afar. It's often called an "invisible gas" because it has few light-altering features in visible or infrared light that would make it easy to detect.

The best way to detect nitrogen in a distant atmosphere is to measure nitrogen molecules colliding with each other. The resulting, instantaneously brief "collisional pairs" create a unique and discernable spectroscopic signature.
Researchers from University of Washington said that a future large telescope could detect this unusual signature in the atmospheres of terrestrial, or rocky planets, given the right instrumentation.

They used three-dimensional planet-modelling data from the UW-based Virtual Planetary Laboratory to simulate how the signature of nitrogen molecule collisions might appear in Earth's atmosphere, and compared this simulated data from the university's Virtual Planetary Laboratory to real observations of Earth by NASA's unmanned Deep Impact Flyby spacecraft.

The craft undertook a revised mission, called EPOXI, which included observation and characterisation of Earth as if it were an exoplanet.
By comparing the real data from the EPOXI mission and the simulated data, the researchers were able to confirm the signatures of nitrogen collisions in our own atmosphere, and that they would be visible to a distant observer.
"We were able to validate that nitrogen produces an impact on the spectrum of our own planet as seen by a distant spacecraft," said lead author Edward Schwieterman, a doctoral student at the university.
The researchers then used a suite of Virtual Planetary Laboratory models that simulated the appearance of planets beyond the solar system bearing varying amounts of nitrogen in their atmospheres.

The detection of nitrogen will help astronomers characterise the atmospheres of potentially habitable planets and determine the likelihood of oxygen production by nonliving processes, the researchers said.
"One of the interesting results from our study is that, basically, if there's enough nitrogen to detect at all, you've confirmed that the surface pressure is sufficient for liquid water, for a very wide range of surface temperatures," Schwieterman said.
The study was published in The Astrophysical Journal.