Saturn's rings age I.: Reconsideration of the exposure age

Abstract

At the end of the Cassini mission, Saturn's rings have been claimed to be spectacularly young compared to the age of the Solar System: their unusual ice-rich composition corresponds to initially pure ice rings polluted by interplanetary dust particles for 100 to 400 Myr. Since then, this exposure age has been commonly accepted as the real age of the rings. In this paper, we review the processes that are involved in determining the exposure age. We aim to see how the exposure age depends on various parameters and how relevant it is to define the real rings age. First, a new expression for the gravitational focusing onto planar rings, important parameter but crudely defined in the literature, is derived. Then, an analytical formula describing how the dust fraction varies with time in static or viscously evolving rings is provided, including possible vaporisation at impact. Finally, we introduce a cleaning process from space weathering to possibly alter dust and reduce its amount to make rings look younger than they are. We first found that the gravitational focusing is 5 times less important than previously thought, which automatically increases the exposure age from 0.5 to 2 Gyr. Moreover, depending on the impact properties (vaporisation rate, space weathering efficiency), several billion years can easily be reached. Finally, we find that the dust fraction in the rings converges towards a finite value which, in particular with an efficient space weathering mechanism, can be close to the observed one in the current rings. In this case, neither the age nor the initial composition of the rings can be derived, and the measure of the dust fraction and bombardment rate only constrains the physical parameters of the impacts and the efficiency of the space weathering. As long as the latter parameters are not known, the exposure age argument in favour of young rings is completely undercut.