Space Weathering Provides a Lower Limit on the Age of Saturn’s Rings

Cassini observations of the micrometeoroid bombardment flux, ring mass and fractional pollution constrain the origin and history of Saturn’s rings. In the simplest model, the age of the rings can be estimated by assuming the rings are a closed system with constant bombardment at the current rate. However, the rings are not a closed system and Cassini spectroscopy is consistent with space weathering of the cosmic dust polluting the rings. The remote sensing of the rings shows a red slope, with higher pollution at the shortest wavelengths, consistent with reddening due to space weathering of atmosphereless bodies. If processes at the time of the micrometeorite impacts or subsequent chemical and physical weathering can degrade the original pollutants, this means that laboratory spectra are not appropriate to determine the total extrinsic material that has struck the rings over their lifetime. Laboratory results for irradiation of icy outer solar system analogues indicate oxidation of organics and other pollutants over time. It is now generally agreed that the radiolysis of ice by energetic ions, electrons and solar UV photons produces the oxygen, ozone and peroxide seen at many icy satellites. The porosity of ice provides sufficient space for chemical reactions and mobility. The ring particle surfaces are in addition continually gardened by particle collisions and meteoritic impacts. Because of the loss processes, the current fractional pollution provides only a lower limit on the total integrated pollution flux, and thus a lower limit for the ring age. Rosetta data on the dust composition and surface reflectivity of Comet 67P provide our starting point for the composition of the bombarding material: Two independent analyses of Cassini UVIS spectra of Saturn’s rings give fractional pollution in the outer B ring of 1.4 -3%. This provides a lower limit of 400 to 1600 million years for the most opaque parts of Saturn’s B ring, depending on whether we use the maximum or minimum values for the bombardment rate reported by Cassini CDA. This result shows Saturn’s rings may be as old as the planet Saturn itself, consistent with the estimates for the orbital evolution of ring moons Atlas and Epimetheus. The A and C rings may have formed more recently.