Magnetic field observations obtained by the Cassini spacecraft as it traversed regions inside of Saturn's D ring packed a genuine surprise. The azimuthal component of the magnetic field recorded a ...consistent positive perturbation with a strength of 15–25 nT near closest approach. The closest approaches were near the equatorial plane of Saturn and were distributed narrowly around local noon and brought the spacecraft to within 2,550 km of Saturn's cloud tops. Modeling of this perturbation shows that it is not of internal origin but is produced by external currents that couple the low‐latitude northern ionosphere to the low‐latitude southern ionosphere. The azimuthal perturbations diminish at higher latitudes on field lines that connect to Saturn's icy rings. The sense of the current system suggests that the southern feet of the field lines in the ionosphere leads their northern counterparts. We show that the observed field perturbations are consistent with a field‐aligned current whose strength is ~1 MA/radian, that is, comparable in strength to the planetary‐period‐oscillation‐related current systems observed in the auroral zone. We show that the Lorentz force in the ionosphere extracts momentum from the faster moving low‐latitude zonal belt and delivers it to the northern ionosphere. We further show that the electric current is generated when the two ends of a field line are embedded in zonal flows with differing wind speeds in the low‐latitude thermosphere. The wind‐generated currents dissipate 2 × 1011W of thermal power, similar to the input from the solar extreme ultraviolet flux in this region.
Plain Language Summary
The Cassini spacecraft observed strong electric currents aligned along Saturn's magnetic field as it traversed regions inside Saturn's D ring. Modeling and analysis of these currents show that their origin lies in the neutral winds observed in the upper atmosphere of Saturn. The wind‐generated currents dissipate a modest amount of energy, which is roughly equal to that coming from solar extreme ultraviolet rays.
Key Points
During Cassini's Grand Finale orbits, the spacecraft identified strong but variable field‐aligned currents inside Saturn's D ring
The current is generated when the two ends of a field line are embedded in zonal atmospheric flows that have differing wind speeds
The wind‐generated currents dissipate 2 × 1011TW of thermal power in the low‐latitude thermosphere, similar to the input from the solar EUV flux in this region
•Stable stratification near the core of Saturn allows for gravity modes with frequencies near fundamental mode frequencies.•F-modes and g-modes mix due to the Coriolis force, the centrifugal force, ...and the ellipticity of the planet.•Mode mixing between g-modes and f-modes can explain some features of Saturn’s mode spectrum inferred from ring observations.•Additional effects, such as differential rotation, may be required to robustly explain ring observations.
Seismology allows for direct observational constraints on the interior structures of stars and planets. Recent observations of Saturn’s ring system have revealed the presence of density waves within the rings excited by oscillation modes within Saturn, allowing for precise measurements of a limited set of the planet’s mode frequencies. We construct interior structure models of Saturn, compute the corresponding mode frequencies, and compare them with the observed mode frequencies. The fundamental mode frequencies of our models match the observed frequencies (of the largest amplitude waves) to an accuracy of ∼1%, confirming that these waves are indeed excited by Saturn’s f-modes. The presence of the lower amplitude waves (finely split in frequency from the f-modes) can only be reproduced in models containing gravity modes that propagate in a stably stratified region of the planet. The stable stratification must exist deep within the planet near the large density gradients between the core and envelope. Our models cannot easily reproduce the observed fine splitting of the m=-3 modes, suggesting that additional effects (e.g., significant latitudinal differential rotation) may be important.
Twenty high‐inclination ring‐grazing orbits occurred in the final period of the Cassini mission. These orbits intercepted a region of intense Z‐mode and narrowband (NB) emission (Ye et al., 2010, ...https://doi.org/10.1029/2009JA015167) along with isolated, intense upper hybrid resonance (UHR) emissions that are often associated with NB source regions. We have singled out such UHR emission seen on earlier Cassini orbits that also lie near the region crossed by the ring‐grazing orbits. These previous orbits are important because Cassini electron phase‐space distributions are available and dispersion analysis can be performed to better understand the free energy source and instability of the UHR emission. We present an example of UHR emission on a previous orbit that is similar to that observed during the ring‐grazing orbits. Analysis of the observed plasma distribution of the previous orbit leads us to conclude that episodes of UHR emission and NB radiation observed during the ring‐grazing orbits are likely due to plasma distributions containing loss cones, temperature anisotropies, and strong density gradients near the ring plane. Z‐mode emissions associated with UHR and NB emission can be in Landau resonance with electrons to produce scattering or acceleration (Woodfield et al., 2018, https://doi.org/10.1038/s41467‐018‐07549‐4).
Key Points
Upper hybrid resonances (UHR) occur at Saturn near the magnetic equator on high‐inclination inner magnetospheric orbits
These regions can be sources of Z‐mode and narrowband (NB) emission
Observed electron plasma distribution contains a weak loss cone unstable to Z‐ and O‐mode wave growth.
Cassini/ISS imagery and Cassini/VIMS spectral imaging observations from 0.35 to 5.12 μm show that Saturn’s north polar region (70°–90° N) evolved significantly between 2012 and 2017, with the region ...poleward of the hexagon changing from dark blue/green to a moderately brighter gold color, except for the inner eye region (88.2°–90° N), which remained relatively unchanged. These and even more dramatic near-IR changes can be reproduced by an aerosol model of four compact layers consisting of a stratospheric haze at an effective pressure near 50 mbar, a deeper haze of putative diphosphine particles typically near 300 mbar, an ammonia cloud layer with a base pressure between 0.4 bar and 1.3 bar, and a deeper cloud of a possible mix of NH4SH and water ice particles within the 2.7 to 4.5 bar region. Between the eye and the hexagon boundary near 75° N were many small discrete bright cloud features that VIMS spectra indicate have increased opacity in the ammonia cloud layer. Our analysis of the background clouds between the discrete features shows that between 2013 and 2016 the effective pressures of most layers changed very little, except for the ammonia ice layer, which decreased from about 1 bar to 0.4 bar near the edge of the eye, but increased to 1 bar inside the eye. Inside the hexagon there were large increases in optical depth, by up to a factor of 10 near the eye for the putative diphosphine layer and by a factor of four over most of the hexagon interior. Inside the eye, aerosol optical depths were very low, suggesting downwelling motions. The high contrast between eye and surroundings in 2016 was due to substantial increases in optical depths outside the eye. The color change from blue/green to gold inside most of the hexagon region can be explained in our model almost entirely by changes in the stratospheric haze, which increased between 2013 and 2016 by a factor of four in optical depth and by almost a factor of three in the short-wavelength peak of its wavelength-dependent imaginary index. A plausible mechanism for increasing aerosol opacity with time is the action of photochemistry as the north polar region became increasingly exposed to solar UV radiation. For 2013 we found an ammonia mixing ratio of about 50×10−6 in the depleted region between 4 bars and the NH3 condensation level (∼ 1 bar), but the NH3 results for 2016 are unclear due to very high retrieval uncertainties associated with increased aerosol opacity. We retrieved a deep abundance of about 5×10−6 for PH3 and a pressure breakpoint (where the PH3 abundance begins to decline with altitude) that coincided with the main cloud top near 300 mbar, except when that cloud opacity was very low, at which point the PH3 breakpoint pressure generally increased substantially, consistent with prior suggestions that the cloud layers shield PH3 from destruction by UV radiation above the clouds. We found an average AsH3 mixing ratio of 2×10−9 with some evidence for a decline with altitude above the main cloud layer.
•From 2012 to 2017 Saturn’s north polar region changed color from blue/green to gold.•We analyzed Cassini/VIMS spectral imaging at wavelengths from 0.35 to 5.15 microns.•We used a compact 4-layer model with distinct compositions to model the evolution.•Stratospheric haze increases in opacity(X4) and absorption(X3) fit the color change.•Larger near-IR changes arise from 10X increases in opacity of a putative P2H4 layer.
The Cassini-Huygens mission to Saturn provided a close-up study of the gas giant planet, as well as its rings, moons, and magnetosphere. The Cassini spacecraft arrived at Saturn in 2004, dropped the ...Huygens probe to study the atmosphere and surface of Saturn's planet-sized moon Titan, and orbited Saturn for the next 13 years. In 2017, when it was running low on fuel, Cassini was intentionally vaporized in Saturn's atmosphere to protect the ocean moons, Enceladus and Titan, where it had discovered habitats potentially suitable for life. Mission findings include Enceladus' south polar geysers, the source of Saturn's E ring; Titan's methane cycle, including rain that creates hydrocarbon lakes; dynamic rings containing ice, silicates, and organics; and Saturn's differential rotation. This Review discusses highlights of Cassini's investigations, including the mission's final year.
Abstract In April 2011 Saturn's midlatitude ionospheric H3(+) emissions were detected, exhibiting anomalous (nonsolar) H3(+) latitudinal variations consistent with the transport of water from ...specific locations in Saturn's rings, known as ''ring rain'' . These products, transported to the planet along the magnetic field, may help to explain the unusual pattern of peaks and troughs in electron densities discovered in Saturn's ionosphere by spacecraft flybys. In the present study, we analyzed H3(+) emissions recorded on 23 April 2013, showing for the first time since the original detection that Saturn's midlatitude H3(+) emissions are indeed heavily modified. Although the 2013 emissions are dimmer by almost a factor of 3.7, the latitudinal contrast is greater and uncertainties are lower. Increased H3(+) intensities were found near planetocentric latitudes of 43 deg, 51 deg, and 63 deg, previously identified with sources at the inner edge of the B ring, A ring, and the orbit of Enceladus and associated E ring.
Hot gas giant exoplanets can lose part of their atmosphere due to strong stellar irradiation, affecting their physical and chemical evolution. Studies of atmospheric escape from exoplanets have ...mostly relied on space-based observations of the hydrogen Lyman-α line in the far ultraviolet which is strongly affected by interstellar absorption. Using ground-based high-resolution spectroscopy we detect excess absorption in the helium triplet at 1083 nm during the transit of the Saturn-mass exoplanet WASP-69b, at a signal-to-noise ratio of 18. We measure line blue shifts of several km s
and post transit absorption, which we interpret as the escape of part of the atmosphere trailing behind the planet in comet-like form.
We present a study of Saturn's
H3+ northern auroral emission using data from 19 May 2013 from the Very Large Telescope's long‐slit spectrometer Cryogenic Infrared Echelle Spectrograph (VLT‐CRIRES). ...Adaptive optics, combined with the spectral resolution of VLT‐CRIRES (
λΔλ∼100,000), offers unprecedented spectrally resolved views of Saturn's infrared aurora. Discrete
H3+ emission lines—used to derive dawn‐to‐dusk profiles of auroral intensity, ion line‐of‐sight velocity, and thermospheric temperature—reveal a dawn‐enhanced aurora with an average temperature of 361 (±48) K and a localized dark region in the emission co‐located with a noon‐to‐midnight (and vice versa) flow in the ion velocity on the scale of ∼1 km/s, resembling an ionospheric polar vortex. A temperature hotspot of 379 (±66) K may be driving an emission region, corresponding to a location where
H3+ is failing to cool the thermosphere. Results presented here have implications for current understanding on the complex nature of Saturn's thermosphere‐ionosphere‐magnetosphere interaction.
Key Points
High spectral resolution profiles of
H3+ parameters from Saturn's northern aurora are derived using spectroscopic data from VLT‐CRIRES
The velocity flow of
H3+ ions is consistent with the behavior of a relatively small ionospheric polar vortex
The
H3+ temperature profile reveals a subtle and previously undetected gradient, which increases across the polar cap going from dawn to dusk
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