The Earth as seen by the Polychromatic Imaging Camera aboard NASA’s
Deep Space Climate Observatory satellite, July 2015.NASA
(September 6, 2015) Observations of nitrogen in Earth’s atmosphere by a NASA spacecraft 17 million miles away are giving astronomers fresh clues to how that gas might reveal itself on faraway planets, thus aiding in the search for life.
Finding and measuring nitrogen in the atmosphere of an exoplanet — one outside our solar system — can be crucial to determining if that world might be habitable. That’s because 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.
Should life truly exist on an exoplanet, detecting nitrogen as well as oxygen could help astronomers verify the oxygen’s biological origin by ruling out certain ways oxygen can be produced abiotically, or through means other than life.
The trouble is, 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.