Saturn's atmosphere bears a latent image of its icy rings, implying that electrically charged bits of water ice are being transported along magnetic-field lines of force from sources in the ring plane to the upper atmosphere. See Letter p.193
The origin and evolution of Saturn's rings has been a particularly vexing problem over the past several decades of planetary exploration, and is one that has obvious implications for the formation of the Solar System. And although much progress has been made in understanding the dynamics of the ring system, both observationally and theoretically, the big questions remain unanswered. How and when did Saturn's rings form? Can they be the last surviving remnants of a much more massive ring system that formed 4.5 billion years ago1, coeval with the primordial gaseous disk that condensed to form Saturn? Or are the rings relatively young, say about 100 million years old, as implied by a suite of mechanisms that would disperse2, darken3 and erode4,5,6 them? Observations obtained by O'Donoghue and colleagues7 with the Keck telescope, and reported on page 193 of this issue, point ominously to an electromagnetic erosion mechanism that siphons away ring mass and deposits it in Saturn's upper atmosphere — a process that may explain some of the gross structure of the rings4,5,6.
Saturn's rings are comprised of nearly pure water-ice objects of all sizes, from submicrometre-sized grains to embedded moonlets kilometres across. However, the vast majority of ring mass, equivalent to that of a 500-km icy sphere, is contained in objects ranging in size from centimetres to a few metres8. Such objects reside in Keplerian orbit about Saturn, and their motion lies well within the domain of Newtonian physics (classical mechanics). Indeed, many interesting features in the rings may be explained by the dynamics of a collisional, self-gravitating ensemble of particles confined to a thin disk in orbit about a central body, behaving like a dense gas, characterized by viscosity, temperature and pressure9. It may require coupled hydrodynamic and gravitational models, and a fast computer, to describe the collective motion of so many particles, but it remains a problem of classical mechanics.
By contrast, the motion of very small (submicrometre-sized) ice particles will be quite different, if they acquire sufficient electrical charge, for example by photoionization or exposure to dense plasma evolving from a micrometeorite impact. Particles with a high charge-to-mass ratio (one electron charge per 1,000 water molecules is sufficient) gyrate about magnetic lines of force in response to the magnetic Lorentz force, which acts in a direction perpendicular to the magnetic field. The motion of such a particle can be described as the combination of a circular motion about the magnetic line of force and the motion of this 'guiding centre' along the magnetic field. In essence, the particle is constrained to move along the magnetic field like a bead on a wire (Fig. 1). These particles will slide along the magnetic field in response to the components of the gravitational and centrifugal forces that are parallel to the magnetic field, and in response to a third force, the 'magnetic mirror' force, which is parallel to the magnetic field and points in the direction of weaker magnetic-field strength (towards the 'magnetic equator'). The latter force is a simple function of the particles' velocity with respect to the magnetic field.
In the case of any other magnetized planet, such forces would quickly disperse small ring particles that acquire a charge. But Saturn is unique among all the magnetized planets of the Solar System in that its magnetic field is symmetric about its rotation axis10,11; there is a unique pair of conjugate latitudes, north and south, that map to a specific radial distance in the ring plane. Mass excavated from the rings in the form of particles of high charge-to-mass ratio, if not returned to the rings and reabsorbed, must therefore be deposited at specific latitudes in Saturn's atmosphere12 (ring-plane conjugates). The current rate of mass erosion as a function of radial distance in the ring plane could be read13 from the variation with latitude of water influx at the top of Saturn's atmosphere — if only one could measure it.
O'Donoghue and colleagues did not measure water influx, but they did observe a good proxy for it: emissions of the H3+ ion. Water introduced into the upper atmosphere facilitates the rapid chemical recombination of the major ionospheric (upper atmosphere) ions by charge exchange14, so a greater depletion of H3+-ion density will be observed at latitudes that receive more water. The authors' measurements clearly show that water is being supplied, along magnetic field lines, to the ionosphere, from sources throughout the ring plane — a ring rain, as it is called. Gaps in the rings are evidently weak sources, not surprisingly, as there is little ring material therein to be eroded. Their measurements also demonstrate that the current ring-erosion rate (as a function of radial distance) differs from that thought to have shaped the C-ring/B-ring boundary4 and the inner B-ring transparency5,6 over tens of millions of years of evolution. Whether the water is transported in the form of ions or, more efficiently, in the form of charged submicrometre grains is not yet clear. And much work remains to be done before the mass-erosion rate can be worked out, quantitatively, from H3+ emission intensities, because it has been difficult to precisely match variations in the observed ionospheric electron density15,16 with models that use an exogenous water influx17,18.
The potential of this observational technique for improving our understanding of the electromagnetic erosion of Saturn's rings is particularly exciting. The near-infrared region of the electromagnetic spectrum is replete with many discrete H3+ emission lines that span a rather broad methane absorption band (Fig. 2). By judicious choice of emission lines, it should be possible to image ionospheric H3+ with high signal-to-noise against a planetary disk darkened by methane absorption deeper in the atmosphere. All that is needed is a telescope with a large effective aperture and time to integrate.
Saturn's rings as observed today probably bear little resemblance to the rings that originally formed. They are highly evolved, much like our Solar System, albeit over a shorter span of time2,4,14. To understand when and how they formed, one needs to understand the processes that shaped the rings we see today. One of those processes — electromagnetic erosion — has projected an image of the rings upon the disk of Saturn7 and perhaps left clues in the rings as well4,5,6. If so, these brilliant rings have yet a tale to tell, in the language of both Newton and Lorentz.
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Geophysical Research Letters (2017)