Titanic isotopes: Mooning around nitrogen

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  • Published: Jul 1, 2014
  • Author: David Bradley
  • Channels: UV/Vis Spectroscopy
thumbnail image: Titanic isotopes: Mooning around nitrogen

Original nitrogen

The building blocks of comets, and perhaps Saturn's moon, Titan, formed under similar conditions Credit: NASA/JPL-Caltech

New evidence for the origins of nitrogen in the atmosphere of Saturn's moon has been unearthed by ESA and NASA funded scientists and suggests that the element comes from protosolar ammonia found in the cold birthplace of the most ancient comets of the Oort cloud. The discovery precludes the hypothesis that Titan's building blocks formed within the warm disk of material thought to have surrounded the fledgling planet Saturn itself during its formation.

Kathleen Mandt of Southwest Research Institute in San Antonio and colleagues at Cornell and at France's National Center for Scientific Research (CNRS) and Observatoire de Paris, have published details in Astrophysical Journal Letters in which they report the main implication of their research is that Titan's building blocks formed early in the solar system's history, in the cold disc of gas and dust that formed the sun. This was also the birthplace of many comets, which retain a primitive, or largely unchanged, composition today.

Titanic orb

Nitrogen is the main gas in Earth's atmosphere as well that of Titan, Saturn's largest moon and a planet-sized orb. Planetary scientists are fascinated by Titan because they suspect that conditions there are similar to those that were found on the Earth. The nitrogen question allows scientists to test different ideas about how Titan's moon may have formed. Mandt and colleagues demonstrate that a particular chemical hint as to the origin of Titan's nitrogen should be essentially the same today as when this moon formed, up to 4.6 billion years ago. That hint is the ratio of nitrogen-14 to nitrogen-15 isotopes.

Paradoxically, they have demonstrated that our solar system is simply not old enough for the nitrogen isotope ratio to have changed significantly, which is contrary to what scientists had generally assumed. "When we looked closely at how this ratio could evolve with time, we found that it was impossible for it to change significantly. Titan's atmosphere contains so much nitrogen that no process can significantly modify this tracer even given more than four billion years of solar system history," Mandt explains.

Oort and about

Other researchers had shown earlier that Earth's nitrogen isotope ratio likely has not changed significantly since our planet formed. Researchers assumed a connection between comets, Titan and Earth, and reasoned that the nitrogen isotope ratio in Titan's original atmosphere was the same as that ratio is on Earth today. Determination of the nitrogen isotope ratio on Titan based on measurements taken by several instruments of the NASA and ESA Cassini-Huygens mission, currently celebrating its tenth anniversary, showed the ratios to be different. The cometary nitrogen ratio seems to match that on Titan. This implies that Earth's nitrogen and Titan's nitrogen have different sources.

"Some have suggested that meteorites brought nitrogen to Earth, or that nitrogen was captured directly from the disc of gas and dust that formed the sun. This is an interesting puzzle for future investigations," Mandt adds.

For now, the team is keen to see whether their findings, suggesting cometary-like ammonia as Titan's source of nitrogen are supported by data from ESA's Rosetta mission, when it reaches comet 67P/ Churyumov-Gerasimenko later this year. If the analysis is correct, this comet will have a lower ratio of isotopic hydrogen in methane ice than observed on Titan suggesting that Titan's nitrogen came from an Oort cloud comet rather than closer Kuiper Belt comets of which 67P/ Churyumov-Gerasimenko is one.

"Our next step is to work with the Rosetta team to evaluate the implications of isotopic ratios that they will measure in ammonia and methane.  The target comet for Rosetta is likely to have originated from the Kuiper belt.  We expect the nitrogen isotopic ratio in ammonia to be similar to Titan's, but the deuterium to hydrogen isotopic ratio in methane to be different from what has been measured in Titan's methane," Mandt.

Related Links

Astrophys J Lett, 2014, 788 L24: "Protosolar Ammonia as the Unique Source of Titan's Nitrogen"

Article by David Bradley

The views represented in this article are solely those of the author and do not necessarily represent those of John Wiley and Sons, Ltd.

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