Finding comets’ hidden nitrogen

For decades, comets have been at the center of a nitrogen mystery. The ratio of carbon-to-nitrogen within comets should be similar to that of the sun, because the sun and the rest of the solar system share the same fundamental building blocks. But comets appeared to be nitrogen-deficient by comparison. Now, the case of the missing nitrogen may finally be solved. A new study suggests it was there all along, hiding in ammonium salts (Science 2020, DOI: 10.1126/science.aaw7462).

The crucial clue came in 2014, when the European Space Agency’s Rosetta spacecraft was orbiting a comet called 67P/Churyumov-Gerasimenko (67P for short). The craft’s imaging spectrometer captured a mystifyingly broad peak in spectra of the comet’s surface. Water ice likely contributes to this band, but it doesn’t explain the entire feature. Some scientists attributed the band to carboxylic acids on the comet’s surface, while others thought nitrogen-rich ammonium salts were the source. Based on previous lab experiments, it seemed plausible that ammonium salts existed on comets, but researchers had never matched lab data with observations from space.

To test these theories, a team from several European institutions created artificial comets, made of water ice, dust, and either carboxylic acids or ammonium salts. The synthetic comets had a grainy and porous nature similar to 67P, and the researchers placed them in space-like conditions using a simulation chamber. Based on spectroscopic analysis of the artificial comets, the researchers determined that the ammonium-containing ones best matched the spectra from 67P.

Although ammonium salts reproduced the general shape of the mysterious peak, the researchers acknowledge there are likely other materials on the comet’s surface, including organic molecules, contributing to the spectral feature.

“We’ve found a new nitrogen carrier, which could shed light on its evolution within the solar system,” says coauthor Olivier Poch, a postdoc at the Institute of Planetology and Astrophysics of Grenoble.

There are several possible explanations for how these ammonium salts formed. Poch and colleagues think ammonia reacted with acid molecules in the comet’s ice as it warmed and transitioned from a solid to a gas.

Prior studies indicated that nitrogen was present on 67P in organic matter and some simple molecules, such as NH₃, HCN, and N₂, but the ammonium salts could constitute the comet’s largest nitrogen reservoir. The concentration of ammonium salts on the surface is difficult to calculate due to the comet’s porosity and graininess. But preliminary estimates from Poch and his team surmise that the concentration is between 5 and 40% of the cometary dust—rendering the relative abundance of nitrogen compared to the other elements in this comet higher than previously thought, and closer to that of the sun. This adds credence to the theory that comets share a common origin with the sun. The researchers suggest that some asteroids, too, may contain ammonium salts.

Bernard Marty of the University of Lorraine, who worked on Rosetta’s mass spectrometer data, says more information about which nitrogen isotopes comprise the ammonium salts would help to verify a common origin. Analyses have shown that comets are nitrogen-15-enriched compared with the sun, so understanding the isotopic composition of 67P’s ammonium salts could confirm or deny comets’ relationship to the sun and early solar system. “Do comets contain solar system material, or do they contain components coming from outside the solar system?” Marty says. “It’s still an open question.”

Michael Mumma, founding director of the National Aeronautics and Space Administration’s Goddard Center for Astrobiology, says ammonium salts and their hidden nitrogen reservoirs could have allowed comets to deliver nitrogen to the barren Earth, along with water and organic acids, providing the basis for life and atmosphere formation. Thanks to this study, he says, “the whole question of missing nitrogen has been given new life.”