A new study provides strong evidence that the nitrogen in the atmosphere of Titan – the largest of the Saturn’s known 62 moons – originated as ammonia ice formed in the protosolar nebula under conditions similar to that of cometary formation. The study rules out the possibility that Titan’s building blocks formed within the warm disk of material thought to have surrounded Saturn during its formation.
This image is a composite of several images taken during two separate Titan flybys in 2006. Image credit: NASA/JPL/University of Arizona.
Nitrogen is the main ingredient in the atmosphere of Earth, as well as on Titan. The planet-sized moon of Saturn is frequently compared to an early version of Earth, locked in a deep freeze.
The new study, published in the Astrophysical Journal Letters, suggests that information about Titan’s original building blocks is still present in the icy moon’s atmosphere, allowing researchers to test different ideas about how the moon might have formed.
Dr Kathleen Mandt of Southwest Research Institute in San Antonio, who is the lead author on the study, and her 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 one isotope, or form, of nitrogen, called nitrogen-14, to another isotope, called nitrogen-15.
The scientists find that the Solar System is not old enough for this nitrogen isotope ratio to have changed significantly. This is contrary to what astronomers commonly have 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 4 billion years of Solar System history,” Dr Mandt said.
The small amount of change in this isotope ratio over long time periods makes it possible for researchers to compare Titan’s original building blocks to other Solar System objects in search of connections between them.
The study also has implications for Earth. It supports the emerging view that ammonia ice from comets is not likely to be the primary source of Earth’s nitrogen.
“Some have suggested that meteorites brought nitrogen to Earth, or that nitrogen was captured directly from the disk of gas that formed the Sun. This is an interesting puzzle for future investigations,” Dr Mandt said.
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Kathleen E. Mandt et al. 2014. Protosolar Ammonia as the Unique Source of Titan’s nitrogen. ApJ 788, L24; doi: 10.1088/2041-8205/788/2/L24
