A combined NASA and European Space Agency (ESA)-funded study has found firm evidence that nitrogen in the atmosphere of Saturn's moon Titan originated in conditions similar to the cold birthplace of the most ancient comets from the Oort cloud. The finding rules out the possibility that Titan's building blocks formed within the warm disk of material thought to have surrounded the infant planet Saturn during its formation.
The main implication of this new research is that Titan's building blocks formed early in the solar system's history, in the cold disk 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.
The research, led by Kathleen Mandt of Southwest Research Institute in San Antonio, and including an international team of researchers, was published this week in the Astrophysical Journal Letters.
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 paper 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. 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 one isotope, or form, of nitrogen, called nitrogen-14, to another isotope, called nitrogen-15.
The team finds that our solar system is not old enough for this nitrogen isotope ratio to have changed significantly. This is contrary to what scientists 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 four billion years of solar system history," 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.
As planetary scientists investigate the mystery of how the solar system formed, isotope ratios are one of the most valuable types of clues they are able to collect. In planetary atmospheres and surface materials, the specific amount of one form of an element, like nitrogen, relative to another form of that same element can be a powerful diagnostic tool because it is closely tied to the conditions under which materials form.
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