Earth, unlike Venus and Mars, exhaled nitrogen

One element that forms a unique part of the life-friendly chemical environment on Earth is the gas nitrogen. Its cyclic movement through the soil and the atmosphere via plants is a crucial part of how they produce energy.

Earth’s atmosphere is 78% nitrogen and 21% oxygen; the remainder includes carbon dioxide, methane and noble gases like argon and neon. In stark contrast, the ratio of nitrogen to argon by volume is almost ten times lower in the atmospheres of Venus and Mars.

There are competing explanations for why Earth’s atmosphere is rich in nitrogen, including physical similarities between the three rocky bodies, their distances from the Sun, etc. Now, a new study conducted by geologists from two American labs gives one of those explanations an upper hand. Unfortunately, it could also make the search for alien life harder.

According to them, Earth’s tectonic activity has allowed the planet to steadily exhale nitrogen from its interior into the atmosphere. Their findings were published in the journal Nature Geoscience on October 19.

Such activity has “added about 85% more nitrogen to Earth’s atmosphere over the course of geological time,” said Sami Mikhail, a geophysicist at the Carnegie Institute of Washington and lead author of the published paper.

The uppermost layer of Earth, on which we live, is not a continuous surface but a jigsaw of slowly moving plates called tectonic plates. They often grind into, slide over or under each other, forming mountains and deep trenches as the case may be. When one plate rises and another dives beneath it, the region where they meet is called a subduction zone. Often, a section of Earth’s mantle gets wedged between the two plates (see image).

How subduction zones could promote nitrogen degassing.
How subduction zones could promote nitrogen degassing. Image: Sami Mikhail

Mikhail and Dimitri Sverjensky, of Johns Hopkins University, Maryland, together developed a model to understand how the mantle could pump out nitrogen into the atmosphere in a continuous process. They found that if ammonium sediments brought downward by the diving plate entered the mantle, they would react with oxygen to form nitrogen. While ammonium can get trapped in minerals, nitrogen can’t and escapes through vents in the tectonic plates and volcanoes into the atmosphere.

“Because subduction only happens on Earth, this has not happened on Mars and Venus. So the atmospheric composition of the three planets diverged once plate tectonics got going on Earth,” Mikhail said.

The duo also thinks the oxidation of ammonium in the mantle wedge would also have lead to the formation of more water, deposited on Earth’s surface.

The search for alien life – whether within the Solar System or on faraway exoplanets – has taken many forms. Astronomers aren’t looking for a fixed set of conditions but some minimum requirements for life. On Earth, these have been the presence of liquid water, periodically changing seasons and the chemical environments necessary for the formation of macromolecules like DNA, among others.

A nitrogen-rich atmosphere is an important part of such an environment. If Mikhail’s conclusions are true, the search for alien life becomes trickier because Earth is the only known planet with subduction zones.

“Maybe life would have survived for billions of years without subduction zones, but without subduction zones the atmosphere would be drastically different and therefore so would life,” Mikhail speculated.

In September 2014, another study published in Nature Geoscience found that Jupiter’s moon Europa also harbored tectonic activity. The announcement raised scientists’ hopes of finding life because Europa also has a subsurface ocean of liquid water. However, Europa’s surface is nothing like Earth’s, and it’s hard to say if subduction can do for Europa what it did for Earth.