Parts of Saturn's Moon Titan Are Older Than Saturn Itself

We know that planets form around their parent star (or stars) when an orbiting cloud of gas and dust cools and coalesces. The same model has been extended to explain how some moons might have formed around parent planets, but a new discovery from NASA and the European Space Agency (ESA) is throwing a wrench in that idea.

Scientists found that the nitrogen in Titan’s atmosphere originated in the same cold conditions as ancient comets of the Oort cloud, which is home to icy planetesimals, meaning the moon could not be made exclusively from the material that once surrounded a young Saturn.

Different isotopes of nitrogen exist in nature, and it’s these isotopes (of nitrogen or any other gas) that help scientists date and find similarities between bodies. The amount of elemental isotopes in an atmospheres or surface material provides comparison points that can reveal the conditions in which the atmosphere or material was formed.

In the case of Titan’s atmospheric nitrogen, scientists noticed something interesting. The ratio between two isotopes—nitrogen-14 and nitrogen-15—isn’t consistent with what scientists expected to find. Our solar system isn’t old enough for the observed nitrogen isotope ratio to have changed significantly, so the ratio seen today must come from an older source. It might have originated in the cold, distant Oort cloud region, and not from the dust that surrounded a young Saturn.

“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,” said Kathleen Mandt of Southwest Research Institute in San Antonio. "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.”

This finding isn’t just interesting where Titan is concerned. There are implications for Earth wrapped up in this study, too.

Titan is of perennial interest because of its similarities with the Earth. This largest moon of Saturn is the only body in the Solar System that we know has the right kind and amount of organic substances on its surface to help explain how life might have started on Earth. Some scientists have even called Titan a natural laboratory that replicates and the environment of the early Earth.

There’s another interesting link between the Earth and Titan: nitrogen is the dominant gas in both bodies’ atmospheres.

Finding that Titan’s nitrogen formed in the same time and place as the Oort cloud supports the emerging idea that ammonia ice from comets isn’t how the Earth got its nitrogen. Scientists have found that the nitrogen isotope ratios on Earth and Titan are different, but that Titan’s ratio is similar to that of comets. That means the Earth likely gained its nitrogen from some other source, not a comet like scientists previously thought.

"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," Mandt said.

Mandt and colleagues will get a chance to add data to their latest findings when ESA's Rosetta mission visits and studies comet 67P/Churyumov-Gerasimenko later this year. If Rosetta finds that two isotopes of hydrogen in methane ice is lower than the nitrogen isotope ratio on Titan (the isotope ratio is what’s important here more than the specific gas), it will support their theory that Titan’s nitrogen originated in the Oort cloud. 67P/ Churyumov-Gerasimenko is a Kuiper Belt comet, so it’s isotope ratio will be different than Oort cloud objects.

The more we learn about Titan the more reasons it seems we have to explore the distant moon. Maybe one day the futuristic missions NASA is investigating will come to pass and we’ll finally get to explore the intriguing little body.