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.