Saturn's moon Titan may be older than the ringed planet itself, say scientists who've studied gases in the moon's atmosphere that suggest it may have formed somewhere other than around its parent.
Researchers in a study conducted by NASA and the European Space Agency say nitrogen detected in Titan's atmosphere is thought to have originated in circumstances that existed long before Saturn was formed.
Nitrogen on Titan more closely resembles that found in ancient comets that formed long ago in the distant Oort cloud, a vast region of icy bodies that orbit in the far reaches of the solar system, they say.
That rules out a likelihood that the building blocks of Titan all came from the disk of gas and dust that would have surrounded Saturn through its early formation, study leader Kathleen Mandt of Southwest Research Institute in San Antonio, Texas, says.
The researchers came to their conclusion by comparing different istotopes of nitrogen that exist in nature that allow for the dating of bodies in the solar system.
On Titan the ratio between two isotopes -- nitrogen-14 and nitrogen-15 -- isn't consistent with what scientists would expect to find if Titan was formed exclusively from material around Saturn.
The solar system hasn't existed long enough for significant changes to have occurred in the ratio, so the ratio observed on Titan today, which displays considerable change, must come from an older source, Mandt says.
"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," she says.
The finding suggests Titan's building blocks formed much earlier in the solar system's history, in the original cold disk of gas and dust that formed the sun.
Comets also formed in that disk and today retain their primitive and for the most part unchanged composition, the researchers note.
Measurements made of nitrogen isotope ratios in comets have confirmed their connection to Titan, they say.
Mandt and her research colleagues say they are eagerly awaiting the results from a European Space Agency mission that will see the Rosetta spacecraft study a comet, 67P/ Churyumov-Gerasimenko, later this year.
They expect the comet will display a lower ratio of the two isotopes than is seen on Titan because it formed in the Kuiper Belt, beginning around the orbit of Neptune, rather than in the more distant and older Oort cloud.
That would be more evidence Titan is similar to Oort cloud comets, they said.