Noble gases introduced by comets

Published: April 7, 2016

Staff writer

The volatile inventory of the inner planets’ atmospheres, including water, nitrogen, carbon and the noble gases, are the presumed results of asteroid collisions with planets. Asteroids, however, may not have been the sole origin of all the volatile elements, according to a study released March 9 in “Earth and Planetary Science Letters.” The article reveals new data from the recent spacecraft mission Rosetta, which suggests that comets may have significantly contributed to the presence of noble gases in the terrestrial atmosphere. Because noble gases are highly stable, they repel other chemical reactions. Therefore, their abundances have not varied drastically since the beginning of the solar system.

Comets are celestial bodies orbiting the sun that remain from the formation of the solar system. The composition of comets includes ice, dirt, rocky materials and other volatiles, including noble gases. When the sun begins to melt the ice of a comet, some of its other components are also released, resulting in the appearance of the comet’s coma and tail. The analysis of a comet’s coma and its distinguishing noble gases can reveal the manner of the comet’s origin and possible contributions to the inner planets.

The International Rosetta Mission launched in March of 2004 was designed to rendezvous with the Jupiter family Comet 67P/Churyumov-Gerasimenko (67P) in order to study the nucleus and environment of the comet for two years as well as to land a probe on its surface for in-situ analysis. It was the first spacecraft to land on and orbit a comet. Analysis was made using the Rosetta Orbiter Spectrometer for Ion and Neutral Analysis (ROSINA) aboard the Rosetta spacecraft. Measurements of the volatile composition of the 67P’s coma revealed a significantly high deuterium-hydrogen ratio within the comet’s ice in addition to the presence of carbon monoxide, carbon dioxide, nitrogen gas and, importantly, argon.

Using the analysis from the Rosetta mission and other data from literature, the study in “Earth and Planetary Science Letters” established a model of cometary composition and applied the calculations to estimate the percentage of volatiles within Earth’s atmosphere resulting from cometary impact. The mass balance calculations of the study revealed a weak contribution of water, carbon and nitrogen species to the Earth’s volatile inventory, however, contribution of noble gases, such as argon, proved significant.

The rough calculations regarding the presence of argon within comets allows for inferences regarding the degree of prebiotic materials on Earth originating from comets. Specifically, the amount of amino acids delivered by comets can be estimated by using similar calculations and cometary samples.

Though the study was a major step for determining the degree to which space projectiles served as sources for volatiles of the terrestrial atmosphere, there is still much to be discovered regarding comets’ roles within the development of the solar system and its planets. This study in particular focused on the amount of argon present in 67P, but the composition of comets is variable and different isotopes of different noble gases may be present within a comet.

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