Cassini’s swan song reveals surprising Saturn chemistry

The National Aeronautics & Space Administration’s Cassini probe ended its 20-year mission to Saturn with a series of passes between the planet and its rings before taking a final plunge into the atmosphere. This grand finale revealed something unexpected: The rings contain methane, carbon dioxide, molecular nitrogen, and more that rain down into the atmosphere (Science 2018, DOI: 10.1126/science.aat2382).

Data from ground observations and earlier spacecraft suggested Saturn’s rings were mostly water ice and that hydrogen and helium dominated its upper atmosphere. J. Hunter Waite Jr. of the Southwest Research Institute and colleagues found something more complex in data from Cassini’s mass spectrometer. The planet’s innermost D ring also contains CH₄, CO, _2,, and likely CO₂, NH₃, and larger organic species. Those molecules don’t stay in orbit, either; Cassini observed them falling into the planet at an estimated rate of 10,000 kg/s. The ring molecules react with protons and ions when they hit the planet’s upper atmosphere.

“This not only illustrates a hitherto unknown connection between the rings and upper atmosphere, but it also shows us that Saturn’s upper atmosphere is really quite complex chemically, which isn’t what anyone had expected,” says Ingo Mueller-Wodarg, a planetary scientist at Imperial College London. Reactions of CH₄, CO, and other carbon-containing molecules in the upper atmosphere could change scientists’ interpretation of models of how much carbon is on Saturn, though Waite cautions that it’s not certain that what Cassini detected in the upper atmosphere near Saturn’s equator extends to the rest of the planet.

Waite says the D ring could maintain the measured rate of material loss for only between 7,000 and 66,000 years before disappearing. The researchers think the material from the D ring originates in its neighbor, the larger C ring, but even that ring has only enough material to last an estimated 700,000 to 7 million years. Those calculations support the idea that comets trapped by Saturn’s gravity resupply the rings’ materials. Astronomers think that this has happened regularly throughout the planet’s life.

Cassini’s data also suggest that the ratio of helium to hydrogen in Saturn’s upper atmosphere is between 0.11 and 0.16, in line with recent models based on indirect observations. These measurements will help scientists refine models of Saturn’s interior and its formation and evolution, says Tommi Koskinen, a planetary scientist at the University of Arizona.

In related research based on data from Cassini’s passes and final plunge, scientists found silicate and water ice grains among the rain from Saturn’s rings ( Science 2018, DOI: 10.1126/science.aat3185) and detected a ring of high-energy protons between the rings and the planet (Science 2018, DOI: 10.1126/science.aat1962).