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Utilizing Helium Ion Chemistry to Derive Mixing Ratios of Heavier Neutral Species in Saturn's Equatorial Ionosphere
Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy. Uppsala University, Disciplinary Domain of Science and Technology, Physics, Swedish Institute of Space Physics, Uppsala Division.ORCID iD: 0000-0003-3038-3359
Uppsala University, Disciplinary Domain of Science and Technology, Physics, Swedish Institute of Space Physics, Uppsala Division.ORCID iD: 0000-0003-2647-8259
European Space Agcy, European Space Res & Technol Ctr, Noordwijk, Netherlands..
Waite Sci LLC, Pensacola, FL USA..
2023 (English)In: Journal of Geophysical Research - Space Physics, ISSN 2169-9380, E-ISSN 2169-9402, Vol. 128, no 6, article id e2023JA031488Article in journal (Refereed) Published
Abstract [en]

A surprisingly strong influx of organic-rich material into Saturn's upper atmosphere from its rings was observed during the proximal obits of the Grand Finale of the Cassini mission. Measurements by the Ion and Neutral Mass Spectrometer (INMS) gave insights into the composition of the material, but it remains to be resolved what fraction of the inferred heavy volatiles should be attributed as originating from the fragmentation of dust particles in the instrument versus natural ablation of grains in the atmosphere. In the present study, we utilize measured light ion and neutral densities to further constrain the abundances of heavy volatiles in Saturn's ionosphere through a steady-state model focusing on helium ion chemistry. We first show that the principal loss mechanism of He+ in Saturn's equatorial ionosphere is through reactions with species other than H-2. Based on the assumption of photochemical equilibrium at altitudes below 2,500 km, we then proceed by estimating the mixing ratio of heavier volatiles down to the closest approaches for Cassini's proximal orbits 288 and 292. Our derived mixing ratios for the inbound part of both orbits fall below those reported from direct measurements by the INMS, with values of similar to 2 x 10(-4) at closest approaches and order-of-magnitude variations in either direction over the orbits. This aligns with previous suggestions that a large fraction of the neutrals measured by the INMS stems from the fragmentation of infalling dust particles that do not significantly ablate in the considered part of Saturn's atmosphere and are thus unavailable for reactions. Plain Language Summary During the final orbits of the Cassini mission, the spacecraft flew between Saturn's rings and the planets upper atmosphere. The onboard plasma instruments detected a large amount of ring particles falling toward the planet, but direct measurements of the composition of these grains are complicated due to the high spacecraft speed and instrumental effects. In this study, we present an independent method to estimate the abundance of heavier neutral species entering the atmosphere from infalling ring material. This method relies on helium ion chemistry and the measured light ion and neutral densities. Our results generally fall below those inferred from direct measurements. Together with comparisons to other studies, this potentially suggests that a large fraction of the infalling neutral species do not significantly ablate in the considered part of Saturn's atmosphere (and remain bound to the dust grains instead) and are thus unavailable for reactions.

Place, publisher, year, edition, pages
American Geophysical Union (AGU), 2023. Vol. 128, no 6, article id e2023JA031488
Keywords [en]
Saturn, Cassini, ionosphere, space plasma, ion chemistry, planetary science
National Category
Fusion, Plasma and Space Physics Astronomy, Astrophysics and Cosmology
Identifiers
URN: urn:nbn:se:uu:diva-508434DOI: 10.1029/2023JA031488ISI: 001011943200001OAI: oai:DiVA.org:uu-508434DiVA, id: diva2:1785341
Funder
Swedish National Space Board, 143/18Available from: 2023-08-02 Created: 2023-08-02 Last updated: 2023-10-06Bibliographically approved
In thesis
1. Diving Deep into Saturn's Equatorial Ionosphere with Cassini: Insights from the Grand Finale
Open this publication in new window or tab >>Diving Deep into Saturn's Equatorial Ionosphere with Cassini: Insights from the Grand Finale
2023 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

In the summer of 2017, the Cassini mission concluded its nearly 13 years orbiting Saturn with a series of daring dives between the rings and the upper reaches of Saturn's atmosphere. This last phase of the mission, called the Grand Finale, revealed a highly variable equatorial ionosphere dominated by a large influx of ring material from Saturn's D ring. The papers included in this thesis utilize data gathered during these proximal orbits to gain insights into the nature and effects of the infalling ring material.

Initially, we derive upper limits for the effective recombination coefficient in Saturn's equatorial ionosphere at altitudes below 2500 km, where photochemical equilibrium can be assumed, to constrain the composition of the positive ion species. Our inceptive results indicate that ion species with low recombination coefficients are dominant.

We follow up on this by developing a photochemical model, incorporating grain charging, to investigate the effects of the ring influx on the plasma composition. The model results at an altitude of 1700 km yield vastly different abundances of two types of neutral species when compared to those derived from measurements, ultimately representing the difficulty of reconciling the observed H+ and H3+ densities with our and other model results.

Exploring the nature of narrow decreases in the ionospheric H2+ densities reveals a time shift in the ion data. After correcting for this, the decreases line up very well with calculated shadows for substructures in Saturn's C ring. We can further estimate the optical depths of these substructures and investigate at which altitudes photochemical equilibrium for H2+ is applicable.

The direct measurement of heavier neutral species during the proximal orbits is complicated by the high spacecraft speed. We devise a method to utilize helium ion chemistry to independently derive the mixing ratios of these heavier neutrals in Saturn's ionosphere. Our results show considerable variability, which may suggest temporal and/or spatial changes in the ring influx. A comparison with other studies indicates that potentially only the most volatile ring-sourced species significantly ablate to enter the gas phase in this region of Saturn's ionosphere.

Finally, we compare the fixed-bias Langmuir probe electron densities and the light ion densities. They exhibit a strong positive correlation for most parts of the proximal orbits even on short timescales. We find three distinct regions in the proximal orbits, which can provide further insight into the ionospheric composition, connection to the rings, and measurement uncertainties.

Place, publisher, year, edition, pages
Uppsala: Acta Universitatis Upsaliensis, 2023. p. 59
Series
Digital Comprehensive Summaries of Uppsala Dissertations from the Faculty of Science and Technology, ISSN 1651-6214 ; 2313
Keywords
Saturn, Cassini, Grand Finale, Ionosphere, Photochemistry, Planetary Rings, Planetary Science, Space Plasma, Space Physics
National Category
Fusion, Plasma and Space Physics
Research subject
Physics with specialization in Space and Plasma Physics
Identifiers
urn:nbn:se:uu:diva-512834 (URN)978-91-513-1910-0 (ISBN)
Public defence
2023-11-23, Sonja Lyttkens (101121), Ångströmlaboratoriet, Lägerhyddsvägen 1, Uppsala, 13:00 (English)
Opponent
Supervisors
Funder
Swedish National Space Board, 143/18
Available from: 2023-11-02 Created: 2023-10-03 Last updated: 2023-11-02

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