Paul. R. Estrada | Searcharxiv

arXiv2023

Large mass inflow rates in Saturn's rings due to ballistic transport and mass loading

The Cassini mission provided key measurements needed to determine the absolute age of Saturn's rings, including the extrinsic micrometeoroid flux at Saturn, the volume fraction of non-icy pollutants in the rings, and the total ring mass. These three factors constrain the ring age to be no more than a few 100 Myr (Kempf et al., 2023). Observations during the Cassini Grand Finale also showed that the rings are losing mass to the planet at a prodigious rate. Some of the mass flux falls as "ring rain" at high latitudes. However, the influx in ring rain is considerably less than the total measured mass influx of 4800 to 45000 kg/s at lower latitudes (Waite et al., 2018). In addition to polluting the rings, micrometeoroid impacts lead to ballistic transport, the mass and angular momentum transport due to net exchanges of meteoroid impact ejecta. Because the ejecta are predominantly prograde, they carry net angular momentum outward. As a result, ring material drifts inward toward the planet. Here, for the first time, we use a simple model to quantify this radial mass inflow rate for dense rings and find that, for plausible choices of parameters, ballistic transport and mass loading by meteoroids can produce a total inward flux of material in the inner B ring and in the C ring that is on the order of a few x 10^3 to a few x 10^4 kg/s, in agreement with measurements during the Cassini Grand Finale. From these mass inflow rates, we estimate that the remaining ring lifetime is ~15 to 400 Myr. Combining this with a revised pollution age of ~120 Myr, we conclude that Saturn's rings are not only young but ephemeral and probably started their evolution on a similar timescale to their pollution age with an initial mass of one to a few Mimas masses.

astro-ph.EP↗

arXiv2020

Streaming Instability in Turbulent Protoplanetary Disks

The streaming instability for solid particles in protoplanetary disks is re-examined assuming the familiar alpha ($α$) model for isotropic turbulence. Turbulence always reduces the growth rates of the streaming instability relative to values calculated for globally laminar disks. While for small values of the turbulence parameter, $α< 10^{-5}$, the wavelengths of the fastest-growing disturbances are small fractions of the local gas vertical scale height $H$, we find that for moderate values of the turbulence parameter, i.e., $α\sim 10^{-5}-10^{-3}$, the lengthscales of maximally growing disturbances shift toward larger scales, approaching $H$. At these moderate turbulent intensities and for local particle to gas mass density ratios $ε< 0.5$, the vertical scales of the most unstable modes begin to exceed the corresponding radial scales so that the instability appears in the form of vertically oriented sheets extending well beyond the particle scale height. We find that for hydrodynamical turbulent disk models reported in the literature, with $α= 4\times 10^{-5} - 5\times 10^{-4}$, together with state of the art global evolution models of particle growth, the streaming instability is predicted to be viable within a narrow triangular patch of $α$--$τ_s$ parameter space centered on Stokes numbers, $τ_s \sim 0.01$ and $α\sim 4\times 10^{-5}$ and, further, exhibits growth rates on the order of several hundred to thousands of orbit times for disks with 1 percent ($Z= 0.01$) cosmic solids abundance or metallicity. Our results are consistent with, and place in context, published numerical studies of streaming instabilities.

astro-ph.EP↗