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, was published this week in the Astrophysical Journal Letters. Co-authors on the study include colleagues from France’s National Center for Scientific Research (CNRS) and Observatoire de Paris.
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.
NASA’s Cassini spacecraft looks toward the night side of Saturn’s largest moon and sees sunlight scattering through the periphery of Titan’s atmosphere and forming a ring of color. Image credit: NASA/JPL-Caltech/SSI.