Title: First results from Cassini’s Grand Finale published — Short Story: In September 2017, the Cassini orbiter plunged into Saturn after completing a highly successful 13 year orbital tour, that started with the delivery of a Titan entry probe – a concept and design first developed at Ames. Saturn entry culminated a six month “Grand Finale” when the orbiter slipped repeatedly between the rings and the planet, taking high-resolution remote sensing data and making in-situ measurements of ring material and plasma properties not even contemplated when Cassini was launched. The journal Science is publishing five articles this week as the first of two installments reporting the results of the Grand Finale phase.
Three of the articles deal with the properties of Saturn’s inner rings, and their interaction with the planet’s upper atmosphere. The three papers taken together make a compelling story about inflow of ring material into the planet as particles so small (1-30nm) that they have never even been seen before. Moreover, the results have important implications about the composition of Saturn’s majestic rings – in particular, why are they so red compared to the water ice they are primarily composed of?
Jeff Cuzzi (SST) is involved with two of these papers, in his capacity as Interdisciplinary Scientist for Rings and Dust. The three studies come to rather different conclusions about the amount of mass falling into the planet, suggesting it might be strongly variable in time. This means they can’t cast much light on the key question of how old the rings are. Indeed, other Cassini studies suggest the rings are much younger than the planet, and the ramifications of this possibility are under active study. But the published work is critical in one way. The Cassini Ion-Neutral Mass Spectrometer sees unmistakable evidence for complex organic molecules – larger than a single carbon ring- that are surely only fragments of even larger molecules. This material is thought to derive most immediately from Saturn’s very faint, innermost D Ring, which is heavily irradiated and processed by a newly-discovered inner radiation belt as well. The D Ring (and indeed all the faint inner rings) are probably supplied in large part from the main rings, so the composition of this material does constrain the main ring composition. Prior to this measurement, there has been a vigorous debate as to whether the redness of the rings was due to organic material or good old fashioned rust (and related tiny metal grains), which would suggest rather different ring parents and origin scenarios. The new likelihood that the rings’ redness is due to organics (only a small fraction of a percent by mass is needed) supports a growing sense that rock, ice, and complex organic materials were the building blocks of the outer solar system.