When Comets Are Fertilizer

By John Upton

One of the main ingredients needed for life might have arrived here after an ancient meteor grabbed a cold one for Mother Earth out of an equally ancient chemical ice chest.

On Earth, nitrogen is wildly abundant, making up the vast majority of the air that we breathe. Conveniently, nitrogen is also critical for life as we know it. Every strand of DNA and every piece of protein that its genetic code describes contains nitrogen. When you drive a car, you’re spraying fertilizer out of your tailpipe, often helping weeds outgrow native species. That’s because fossil fuels harbor the nitrogen that was in the algae and other life forms that fossilized and provided you with your fuel.

Weirdly, though, the  types of nitrogen isotopes found here on Earth are different than those found through much of the solar system. That suggests that the nitrogen we breathe today was not floating freely in the clouds of space dust that clumped together to form the planets. At least some of it arrived here, as German researchers wrote in a letter published Monday in Nature Geoscience, from a “primordial cosmochemical source.”

The part of a meteor that survives its incendiary passage through Earth’s atmosphere is called a meteorite,  and some meteorites contain the mineral carlsbergite. After chemically analyzing carlsbergite and putting it under a powerful microscope, the researchers concluded that its nitrogen isotopes more closely resembled the nitrogen in Earth’s atmosphere than that found in regular cometary ice. The finding adds to speculation that these carlsbergite-wielding meteorites fertilized our planet with its nitrogen, making life as we know it possible here.

Illustrated by Perry Shirley.
Illustrated by Perry Shirley.

“Some nitrogen might have come from other sources, but we don’t know what they are,” one of the researchers, Dennis Harries, of the Friedrich-Schiller-Universität, told us. “We don’t know how much ammonia came directly to Earth, but it is quite certain that ammonia reacted in the meteorite’s parent body to form nitrogen-bearing organic molecules like amino acids. These molecules might have brought nitrogen to Earth and might have helped life to emerge.”

So where did these planet fertilizing comets come from?

Perhaps, the scientists say, vast regions of icy ammonia that pocked the early solar system were broken up by powerful shock waves that sent their chunks spinning away as meteors. Such shock waves might have been produced back when Jupiter’s sunward trajectory was bumped into a new direction by the effects of Saturn’s gravity.

“Jupiter being involved is just an idea — our research is primarily about the existence of the ammonia in this ice,” Harries said. “The icy bodies could have been brought to Earth by Jupiter’s change in tack.”