Research Article

Chemical interactions between Saturn’s atmosphere and its rings

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Science  05 Oct 2018:
Vol. 362, Issue 6410, eaat2382
DOI: 10.1126/science.aat2382

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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

Past remote observations of Saturn by Pioneer 11, Voyager 1 and 2, Earth-based observatories, and the Cassini prime and solstice missions suggested an inflow of water from the rings to the atmosphere. This would modify the chemistry of Saturn’s upper atmosphere and ionosphere. In situ observations during the Cassini Grand Finale provided an opportunity to study this chemical interaction.

RATIONALE

The Cassini Grand Finale consisted of 22 orbital revolutions (revs), with the closest approach to Saturn between the inner D ring and the equatorial atmosphere. The Cassini Ion Neutral Mass Spectrometer (INMS) measured the composition of Saturn’s upper atmosphere and its chemical interactions with material originating in the rings.

RESULTS

Molecular hydrogen was the most abundant constituent at all altitudes sampled. Analysis of the atmospheric structure of H2 indicates a scale height with a temperature of 340 ± 20 K below 4000 km, at the altitudes and near-equatorial latitudes sampled by INMS.

Water infall from the rings was observed, along with substantial amounts of methane, ammonia, molecular nitrogen, carbon monoxide, carbon dioxide, and impact fragments of organic nanoparticles. The infalling mass flux was calculated to be between 4800 and 45,000 kg s−1 in a latitude band of 8° near the equator.

The interpretation of this spectrum is complicated by the Cassini spacecraft’s high velocity of 31 km s−1 relative to Saturn’s atmosphere. At this speed, molecules and particles have 5 eV per nucleon of energy and could have fragmented upon impact within the INMS antechamber of the closed ion source. As a result, the many organic compounds detected by INMS are very likely fragments of larger nanoparticles.

Evidence from INMS indicates the presence of molecular volatiles and organic fragments in the infalling material. Methane, carbon monoxide, and nitrogen make up the volatile inflow, whereas ammonia, water, carbon dioxide, and organic compound fragments are attributed to fragmentation inside the instrument’s antechamber of icy, organic-rich grains. The observations also show evidence for orbit-to-orbit variations in the mixing ratios of infalling material; this suggests that the source region of the material is temporally and/or longitudinally variable, possibly corresponding to localized source regions in the D ring.

CONCLUSION

The large mass of infalling material has implications for ring evolution, likely requiring transfer of material from the C ring to the D ring in a repeatable manner. The infalling material can affect the atmospheric chemistry and the carbon content of Saturn’s ionosphere and atmosphere.

INMS mass spectra from the Grand Finale.

The graphic depicts the Cassini spacecraft as it passes from north to south between Saturn and its rings. The inset spectrum shows the mass deconvolution of compounds measured by INMS on rev 290. The x axis is in units of mass per charge (u) and extends over the full mass range of INMS (1 to 99 u). The y axis is in counts per measurement cycle integrated over the closest-approach data. The mass influx rate for rev 290, derived from mass deconvolution of the rev-integrated spectrum, is shown as embedded text in the spectrum. The side panel gives the average of the mass deconvolution of revs 290, 291, and 292 in mass density units (g cm–3). The composition of the ring-derived compounds in terms of percentage mass density is also shown.

IMAGE COURTESY OF NASA/JPL-CALTECH/SWRI

Abstract

The Pioneer and Voyager spacecraft made close-up measurements of Saturn’s ionosphere and upper atmosphere in the 1970s and 1980s that suggested a chemical interaction between the rings and atmosphere. Exploring this interaction provides information on ring composition and the influence on Saturn’s atmosphere from infalling material. The Cassini Ion Neutral Mass Spectrometer sampled in situ the region between the D ring and Saturn during the spacecraft’s Grand Finale phase. We used these measurements to characterize the atmospheric structure and material influx from the rings. The atmospheric He/H2 ratio is 10 to 16%. Volatile compounds from the rings (methane; carbon monoxide and/or molecular nitrogen), as well as larger organic-bearing grains, are flowing inward at a rate of 4800 to 45,000 kilograms per second.

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