Research Article

Dust grains fall from Saturn’s D-ring into its equatorial upper atmosphere

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Science  05 Oct 2018:
Vol. 362, Issue 6410, eaat2236
DOI: 10.1126/science.aat2236

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Cassini's final phase of exploration

The Cassini spacecraft spent 13 years orbiting Saturn; as it ran low on fuel, the trajectory was changed to sample regions it had not yet visited. A series of orbits close to the rings was followed by a Grand Finale orbit, which took the spacecraft through the gap between Saturn and its rings before the spacecraft was destroyed when it entered the planet's upper atmosphere. Six papers in this issue report results from these final phases of the Cassini mission. Dougherty et al. measured the magnetic field close to Saturn, which implies a complex multilayer dynamo process inside the planet. Roussos et al. detected an additional radiation belt trapped within the rings, sustained by the radioactive decay of free neutrons. Lamy et al. present plasma measurements taken as Cassini flew through regions emitting kilometric radiation, connected to the planet's aurorae. Hsu et al. determined the composition of large, solid dust particles falling from the rings into the planet, whereas Mitchell et al. investigated the smaller dust nanograins and show how they interact with the planet's upper atmosphere. Finally, Waite et al. identified molecules in the infalling material and directly measured the composition of Saturn's atmosphere.

Science, this issue p. eaat5434, p. eaat1962, p. eaat2027, p. eaat3185, p. eaat2236, p. eaat2382

Structured Abstract

INTRODUCTION

Ring material has long been thought to enter Saturn’s atmosphere, modifying its atmospheric and ionospheric chemistry. This phenomenon, dubbed “ring rain,” involves the transport of charged dust particles from the main rings along the planetary magnetic field.

RATIONALE

At the end of the Cassini mission, measurements by onboard instruments tested this hypothesis as well as whether ring material falls directly into the equatorial atmosphere. The final 22 orbits of the Cassini mission sent the spacecraft through the gap between the atmosphere and the innermost of the broad ring system, the D-ring.

RESULTS

The Magnetospheric Imaging Instrument—designed to measure energetic neutral atoms, ions, and electrons—recorded very small dust grains [8000 to 40,000 unified atomic mass units (u), or roughly 1- to 3-nm radius] in two sensors. At 3000-km altitude, a peak rate of ~300,000 counts s–1 was detected by one sensor as Cassini crossed the equatorial plane. At lower altitude (1700 to 2000 km), a second sensor recorded positively charged dust in the upper atmosphere and ionosphere over a size range of ~8000 to 40,000 u (~1 to 2 nm, assuming the density of ice).

Consistent with this observation, larger dust in the 0.1- to 1-µm range was detected by the Cassini Dust Analyzer and the Radio and Plasma Wave Science instrument.

CONCLUSION

We modeled the interaction of dust with the H and H2 exospheric populations known to populate the gap. Collisions between small dust grains and H atoms provide sufficient drag to de-orbit the dust, causing it to plunge into the atmosphere over ~4 hours. The analysis indicates that at least ~5 kg s−1 of dust is continuously precipitating into the atmosphere. At 3000-km altitude, the dust is distributed symmetrically about the equator, mostly between ±2° latitude with a peak density of ~0.1 cm−3. On the wings of the distribution, consistent with ring rain transport along the magnetic field, almost all of the dust was observed to be charged. At the 2000 to 1700 km altitude, the dust has reached a diffusive terminal velocity and, although showing some bias toward the equator, is ordered mostly by a scale height of ~180 km in altitude. The most probable source for this dust population is the innermost bright ringlet of the D-ring, known as the D68 ringlet.

We predict that this kinetic process generates a highly anisotropic neutral hydrogen population, concentrated near the equatorial plane with periapses between ~4000 and 7000 km, and apoapses ranging to as high as 10 Saturn radii, with a small fraction on escape trajectories.

Ring dust.

(Top left) Data and model fits for the equatorial dust population near 3000-km altitude for three orbits through the D-ring gap. HV, plate detector high voltage. Red line uses left scale (percent). (Bottom left) Dust counts (blue) from ~2000 to 1700 km (modulated by sensor energy/charge steps, green), ordered by altitude and latitude, consistent with diffusive transport. (Right) Model trajectory of a dust particle in three frames of reference, as collisions with exospheric hydrogen degrade its velocity. Saturn has been shrunk to expand the gap for clarity.

Abstract

The sizes of Saturn’s ring particles range from meters (boulders) to nanometers (dust). Determination of the rings’ ages depends on loss processes, including the transport of dust into Saturn’s atmosphere. During the Grand Finale orbits of the Cassini spacecraft, its instruments measured tiny dust grains that compose the innermost D-ring of Saturn. The nanometer-sized dust experiences collisions with exospheric (upper atmosphere) hydrogen and molecular hydrogen, which forces it to fall from the ring into the ionosphere and lower atmosphere. We used the Magnetospheric Imaging Instrument to detect and characterize this dust transport and also found that diffusion dominates above and near the altitude of peak ionospheric density. This mechanism results in a mass deposition into the equatorial atmosphere of ~5 kilograms per second, constraining the age of the D-ring.

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