Saturn’s Moon Titan And Oort Cloud Comets Share An Ancient Link

Saturn’s moon Titan has a thick atmosphere of nitrogen and hydrocarbons, which form a dense pink haze that hides the surface. The smaller moon Tethys is visible in the background in this photo taken by the Cassini spacecraft. Credit: NASA/JPL

Let’s start with what Titan and Earth share in common. The atmospheres of both contain lots of nitrogen. 90% for Titan and 78% for Earth. Astronomers have recently discovered that nitrogen in Titan’s atmosphere may have originated far from Saturn’s womb in the dimly-lit and bitter cold Oort Cloud, ancient birthplace of comets.

Named for Dutch astronomer Jan Oort, the roughly spherical cloud contains billions of icy comets orbiting the sun at up to 4.5 trillion miles away or about three-quarters of a light year. The temperature in this frontier of solar influence climbs no higher than a few degrees above absolute zero.

Southwest Research Institute scientists are studying the atmosphere of Titan, Saturn’s largest moon and the only known extraterrestrial body with a hydrologic cycle similar to the Earth’s, where methane rains down on Titan surface much like water falls to the Earth. This artist’s rendition shows a methane rainstorm. (by Michael Carroll, based upon radar mapping data from Cassini)

Kathleen Mandt, a planetary scientist at the Southwest Research Institute in San Antonio and an international team of researchers studied the ratio of two different forms of nitrogen in Titan’s atmosphere called isotopes. Isotopes are two or more forms of the same element that have the same number of protons in their atomic nuclei but a different number of neutrons. For instance, 99.7% of the oxygen on Earth is in the form of oxygen-16, which has 16 protons and 16 neutrons. But nature makes tiny amounts of two other stable oxygen ‘isotopes’ – oxygen 17 and 18.

Most scientists had assumed that the ratio of isotopes nitrogen 15 and 16 in Titan’s atmosphere as well as the solar system at large had changed over the their 4.6 billion year history. Mandt and team, studying Titan’s nitrogen, discovered otherwise.

The building blocks of comets, and apparently Saturn’s largest moon, Titan, formed under similar conditions in the disk of gas and dust that formed the sun rather than from the material surrounding Saturn. Credit: NASA

“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.

But here’s the rub. The ratio isn’t the same on Titan compared to Earth’s nitrogen but matches that of comets dropping by from the remote Oort Cloud. That rules out the formation of Titan’s elemental building blocks in the warm disk of material around Saturn that eventually coalesced into its moons.

The study also has implications for Earth. Ammonia ice from comets, which contains nitrogen, appears not to be the primary source of Earth’s nitrogen.

“In the past, researchers assumed a connection between comets, Titan and Earth, and supposed the nitrogen isotope ratio in Titan’s original atmosphere was the same as that ratio is on Earth today. Measurements of the nitrogen isotope ratio at Titan by several instruments of the Cassini mission showed that this is not the case, while measurements of the ratio in comets have borne out their connection to Titan,” according to a recent press release.

This means the sources of Earth’s and Titan’s nitrogen must have been different. This is interesting because it’s believed that comets delivered much of the water that still runs through our taps early in Earth’s history during the days of heavy meteorite/comet bombardment. Presumably they would also bring the same mix of nitrogen isotopes as found on Titan … but no. So where did our nitrogen – and water – come from? And how did the material Titan-stuff migrate from the frigid outer solar system inward to Saturn? Questions, questions.