The outer planets of our Solar System display a myriad of interesting cloud features, of different colors and sizes. The differences between the types of observed clouds suggest a complex interplay ...between the dynamics and chemistry at play on these atmospheres. Particularly, the stark difference between the banded structures of Jupiter and Saturn, vs the sporadic clouds on the ice giants highlights the varieties in dynamic, chemical and thermal processes that shape these atmospheres. Since the early explorations of these planets by spacecrafts, such as Voyager and Voyager 2, there are many outstanding questions about the long term stability of the observed features. One hypothesis is that the internal heat generated during the formation of these planets is transported to the upper atmosphere through latent heat release from convecting clouds (i.e., moist convection). In this review, we present evidences of moist convective activity on the gas giant atmospheres of our Solar System from remote sensing data, both from ground- and space-based observations. We detail the processes that drive moist convective activity, both in terms of the dynamics as well as the microphysical processes that shape the resulting clouds. Finally, we also discuss the effects of moist convection on shaping the large scale dynamics (such as jet structures on these planets).
The Ice Giants represent a unique and relatively poorly characterized class of planets that have been largely unexplored since the brief Voyager 2 flyby in the late 1980s. Uranus is particularly ...enigmatic, due to its extreme axial tilt, offset magnetic field, apparent low heat budget, mysteriously cool stratosphere and warm thermosphere, as well as a lack of well-defined, long-lived storm systems and distinct atmospheric features. All these characteristics make Uranus a scientifically intriguing target, particularly for missions able to complete
in situ
measurements. The 2023-2032 Decadal Strategy for Planetary Science and Astrobiology prioritized a flagship orbiter and probe to explore Uranus with the intent to “...transform our knowledge of Ice Giants in general and the Uranian system in particular” (National Academies of Sciences, Engineering, and Medicine in Origins, worlds, and life: a decadal strategy for planetary science and astrobiology 2023-2032, The National Academies Press, Washington,
2022
). In support of this recommendation, we present community-supported science questions, key measurements, and a suggested instrument suite that focuses on the exploration and characterization of the Uranian atmosphere by an
in situ
probe. The scope of these science questions encompasses the origin, evolution, and current processes that shape the Uranian atmosphere, and in turn the Uranian system overall. Addressing these questions will inform vital new insights about Uranus, Ice Giants and Gas Giants in general, the large population of Neptune-sized exoplanets, and the Solar System as a whole.
We present numerical simulations to systematically explore the differences in the formation and maintenance of polar vortices under Saturnian (“S-Regime”) and Ice Giant (“I-Regime”) dynamical ...conditions. The wide variation of polar vortices observed on the gas giants Jupiter and Saturn by the Juno and Cassini spacecraft, respectively, and on the ice giants Uranus and Neptune by ground- and space-based telescopes, was recently captured in simulations by Brueshaber et al. (2019) (hereafter, ‘B19’) using the EPIC shallow-water numerical model. B19, expanding on a prior finding by O’Neill et al. (2015), reconfirmed this prior discovery that the dynamical regimes of giant planet polar vortices are controlled primarily by the planetary Burger number, Bu=(Ld0∕a)2, where Ld0 is the first-baroclinic deformation length at the pole, and a is the planetary radius. Small Bu, matching estimates for Jupiter produce a Jupiter-like regime of multiple circumpolar cyclones (“J-Regime”). Larger Bu, matching estimates of Bu for Saturn and the Ice Giants, both produce a single cyclone over each pole; the resulting polar vortex has a larger diameter in the I-Regime than in the S-Regime. However, B19 found that the effect of Bu alone was not sufficient to explain the differences in the polar vortices in the S- and I-Regimes. B19 speculated that the turbulent forcing scale and intensity had an impact on the size of the resulting polar vortices, which motivate the current study.
Here, like in B19, we employ a shallow-water model forced by mass pulses that represent thunderstorms in which positive/negative mass pulses geostrophically adjust to form anticyclones/cyclones. We develop a new four-parameter experimental design to systematically test the role of (1) storm size, and (2) storm wind speed. In addition, we (3) investigate the role of the storm polarity fraction (the fraction of small-scale anticyclones to cyclones), using (4) Bu values that sample the S- and I-Regimes.
Our results provide new key insights into the dynamics of solitary polar cyclones that emerge on giant planets as a result of moist-convective forcing. We find that the wind speed of the polar cyclones within S- and I-Regimes is substantially influenced by storm size, storm wind speed, and storm polarity fraction. The radius of the polar cyclone is also influenced by the storm polarity fraction, but, is not influenced by the storm size or storm wind speed. Our new results clarify the role of storm forcing on the intensity and size of S- and I-Regime polar cyclones. Lastly, we resolve the conundrum found in B19 regarding polar cyclone circulations, providing a more dynamically consistent set of results of classifying polar cyclones into dynamical regimes based on Bu.
•Six key findings are presented for dynamic regimes that feature a solitary pole-centered cyclone on the giant planets.•The effects of turbulent forcing size and intensity on the resulting polar cyclones are investigated.•The polar cyclone’s winds are substantially influenced by storm size, storm wind speed, and storm polarity fraction.•The polar cyclone’s size is also influenced by the storm polarity fraction but not by the storm size or storm intensity.
The paper presents velocity fields with ~3‐km spatial resolution of Saturn's north polar vortex (NPV) retrieved using the optical flow method from a sequence of polar‐projected cloud images captured ...by the Imaging Science Subsystem camera on board NASA's Cassini spacecraft. The fields of the velocity magnitude, velocity variation, relative vorticity, divergence, and second invariant are determined to characterize the flow structures of the inner core of the NPV. The mean zonal and mean meridional velocity profiles of the NPV are compared with previous measurements. We also describe the relevant details of application of the optical flow method to planetary cloud‐tracking wind measurements. The mean zonal velocity profile is consistent with the previous measurements using correlation image velocimetry methods. The small but significant meridional velocity corresponds to outwardly spiraling streams observed in the region near the north pole (NP). The concentrated vorticity and second invariant within 1° planetographic latitude of the NP indicate strong rotational motion of the fluid. An analysis is presented to explore a possible physical origin of the observed spiraling node at the NP.
Plain Language Summary
A swirling flow pattern with wind speeds peaking at about 100 m/s was revealed in Saturn's north polar vortex in high‐resolution images captured by the Imaging Science Subsystem camera on board NASA's Cassini spacecraft in November 2012. Using sequences of images that show clouds in the north polar vortex, the motions of these clouds were analyzed to measure the wind speeds in the north polar region. The high‐precision wind measurements presented in the current report are enabled by the optical flow cloud‐tracking method. The time‐averaged wind field shows a well‐defined counterclockwise (cyclonic) vortex at the pole. In particular, the observed flow structures and wind shear near the pole indicate strong rotational motion of the north polar atmosphere with upwelling at the center.
Key Points
The high‐resolution velocity fields of Saturn's north polar vortex (NPV) are extracted from cloud images by using the optical flow method
The vorticity, divergence, and second invariant are obtained to characterize the flow structures of the NPV
The strong rotational motion of the fluid with upwelling is found near the pole
We present a numerical model that reveals a mechanism governing the polar atmospheric dynamics of Jupiter, Saturn, Uranus and Neptune. Exploration of the polar regions of the gas giants has produced ...surprisingly diverse results, with Cassini finding a single, intense, compact polar cyclone precisely centered on each pole of Saturn, and Voyager data and ground-based observations suggesting Uranus and Neptune have dominant, single polar cyclones as well. The Juno spacecraft at Jupiter finds several tightly packed cyclones surrounding a central cyclone offset from the poles. These discoveries raise questions about the mechanism that differentiates these polar atmospheric dynamics regimes. To help determine what physical mechanisms control these differences, we use the Explicit Planetary Isentropic Coordinate (EPIC) model to carry out forced-turbulence shallow-water simulations in a gamma-plane configuration, i.e. a Cartesian grid with a pole placed at the center. The model is forced by small-scale stochastic mass pulses that parametrically represent cumulus storms. The effects of three parameters, the planetary Burger number, Bu = (Ld / a)2 (Ld is the Rossby deformation radius, a is the planetary radius), input storm strength, s, and proportion of cyclonic and anticyclonic storms injected into the domain, α, are systematically investigated. Bu emerges to be the most important, able to distinguish between four distinct dynamical regimes, matching those of the giant planets, which from large to small Bu, are: i) a large cyclonic polar vortex (i.e., Uranus/Neptune-like), ii) a compact intense cyclonic polar vortex (Saturn-like), iii) two large vortices or one vortex offset from the pole (transitional), and iv) meandering jets with no centrally dominant vortex, or with multiple circumpolar cyclones (Jupiter-like). The boundaries of these regimes are found to be only slightly modulated by the values of s and α. By applying this correlation with respect to Bu in reverse, an observation of a particular polar regime could in principle be used to constrain Ld.
•Differences between the polar vortex configurations of Jupiter, Saturn, Uranus, and Neptune explained by our simulations.•Key controlling parameter is the planetary Burger number (square of the Rossby deformation radius to planetary radius).•The polar configurations are only weakly affected by the strength and sign of small storms used to force the turbulence.
The temperature structure of a giant planet was traditionally thought to be an adiabat assuming convective mixing homogenizes entropy. The only in-situ measurement made by the Galileo Probe detected ...a near-adiabatic temperature structure within one of Jupiter's 5μm hot spots with small but definite local departures from adiabaticity. We analyze Juno's microwave observations near Jupiter's equator (0– 5 oN) and find that the equatorial temperature structure is best characterized by a stable super-adiabatic temperature profile rather than an adiabatic one. Water is the only substance with sufficient abundance to alter the atmosphere's mean molecular weight and prevent dynamic instability if a super-adiabatic temperature gradient exists. Thus, from the super-adiabaticity, our results indicate a water concentration (or the oxygen to hydrogen ratio) of about 4.9 times solar with a possible range of 1.5– 8.3 times solar in Jupiter's equatorial region.
•The Juno/MWR finds a super-adiabatic temperature gradient across the water condensation layer at Jupiter's equatorial zone.•The deep atmosphere has a higher potential temperature than the shallow atmosphere at the equatorial zone.•The deep O/H ratio on Jupiter is between 1.4 - 8.3 times solar with the optimal estimate at about 4.9 solar.
Cloud‐tracked wind observations document the role of eddies in putting momentum into the zonal jets. Chemical tracers, lightning, clouds, and temperature anomalies document the rising and sinking in ...the belts and zones, but questions remain about what drives the flow between the belts and zones. We suggest an additional role for the eddies, which is to generate waves that propagate both up and down from the cloud layer. When the waves break they deposit momentum and thereby replace the friction forces at solid boundaries that enable overturning circulations on terrestrial planets. By depositing momentum of one sign within the cloud layer and momentum of the opposite sign above and below the clouds, the eddies maintain all components of the circulation, including the stacked, oppositely rotating cells between each belt‐zone pair, and the zonal jets themselves.
Plain Language Summary
The dark belts and bright zones that circle the planet at constant latitude, along with the jet streams on the belt‐zone boundaries, are the iconic dynamical features of Jupiter's atmosphere. But the circulation cells with rising, sinking, and cross‐latitude motion are just as important because they maintain the storms and turbulent eddies. Voyager and Cassini have shown that the turbulent eddies put energy into the jet streams. We argue that the eddies also put energy into the circulation cells. They do this by generating waves that break as they propagate above and below the clouds. The breaking waves provide the essential forces that replace those that occur on planets with solid boundaries.
Key Points
This study proposes a dynamical mechanism that maintains the circulation cells connecting neighboring belts and zones of Jupiter
Waves that propagate down from the cloud layer are key; when they break they produce a drag force that mimics the effect of a solid boundary
Eddies within the clouds drive the zonal jets and probably drive the waves, thereby driving all aspects of the zonal mean circulation
On May 31, 2020 a short-lived convective storm appeared in one of the small cyclones of Jupiter's South Temperate Belt (STB) at planetographic latitude 30.8°S. The outbreak was captured by amateur ...astronomer Clyde Foster in methane-band images, became widely known as Clyde's Spot, and was imaged at very high resolution by the Junocam instrument on board the Juno mission 2.5 days later. Junocam images showed a white two-lobed cyclonic system with high clouds observed in the methane-band at 890 nm. The storm evolved over a few days to become a dark feature that showed turbulence for months, presented oscillations in its drift rate, and slowly expanded, first into a Folded Filamentary Region (FFR), and later into a turbulent segment of the STB over a timescale of one year. On August 7, 2021, a new storm strikingly similar to Clyde's Spot erupted in a cyclone of the STB. The new storm exhibited first a similar transformation into a turbulent dark feature, and later transformed into a dark cyclone fully formed by January 2022. We compare the evolution into a FFR of Clyde's Spot with the formation of a FFR observed by Voyager 2 in 1979 in the South South Temperate Belt (SSTB) after a convective outburst in a cyclone that also developed a two-lobed shape. We also discuss the contemporaneous evolution of an additional cyclone of the STB, which was similar to the one were Clyde's Spot developed. This cyclone did not exhibit visible internal convective activity, and transformed from pale white in 2019, with low contrast with the environment, to dark red in 2020, and thus, was very similar to the outcome of the second storm. This cyclone became bright again in 2021 after interacting with Oval BA. We present observations of these phenomena obtained by amateur astronomers, ground-based telescopes, Hubble Space Telescope and Junocam. This study reveals that short-lived small storms that are active for only a few days can produce complex long-term changes that extend over much larger areas than those initially covered by the storms. In a second paper Iñurrigarro et al., 2022 we use the EPIC numerical model to simulate these storms and study moist convection in closed cyclones.
•We present a detailed study of Clyde's Spot, a convective storm in Jupiter's STB.•Clyde's Spot and a similar storm developed inside small cyclones in the STB.•Clyde's Spot transformed into a large turbulent segment of the STB after 1 year.•The second storm, initially similar to Clyde's Spot, transformed in a dark cyclone.•Small convective storms in Jupiter can produce long-lasting effects in large areas.
My research investigates the polar atmospheric dynamics of the giant planets: Jupiter and Saturn (gas giants), and Uranus and Neptune (ice giants). I conduct my research modifying and applying the ...Explicit Planetary Isentropic Coordinate global circulation code to model the polar regions of the four giant planets.The motivation behind my research is to uncover the reason why giant planet polar atmospheric dynamics differ. Jupiter features multiple circumpolar cyclones arranged in geometrical configurations, whereas Saturn features a single pole-centered cyclone. Uranus and Neptune also appear to have single pole-centered cyclones, albeit, larger than those on Saturn. It is widely accepted that moist-convective processes such as thunderstorms, are a leading candidate in generating small-scale turbulence, which self-organizes into larger structures, via a process called the inverse-cascade. In the polar regions, cyclonic vortices are the naturally preferred outcome of this self-organization. I model small-scale turbulence by continually adding or removing mass into the domain throughout the simulation at scales matching the size of thunderstorms. The continual injection of turbulence is known as a “forced-turbulence” model. The storms geostrophically balance into small cyclones (anticyclones) if mass is removed (added). Cyclones (anticyclones) drift poleward (equatorward) via the beta-drift mechanism, which leads to an accumulation of cyclonic vorticity at the pole. The resulting configurations, dynamics, and morphologies of polar cyclones are the subject of my numerical simulations.In Chapter 4, I show that the Burger Number, Bu—the ratio of the Rossby deformation radius to the planet radius squared—controls the morphology and number of polar cyclones. If Bu is sufficiently small, as expected for Jupiter, multiple circumpolar cyclones emerge from the forced-turbulent simulations. If Bu is sufficiently large, as expected for Saturn and the ice giants, a single pole-centered cyclone emerges instead. Four dynamical regimes are found from my experiments, three of which match the Bu and configurations observed on the giant planets: the Jupiter (J)-Regime, the Saturn (S)-Regime, and the Ice Giant (I)-Regime. This first set of numerical simulations also tests the effect of the mass injection rate (a proxy for storm intensity), and tests the effect of the storm polarity fraction, i.e., fraction of cyclonic to anticyclonic storms, on the dynamics of polar cyclones.Chapter 5, focuses on the effect of turbulent forcing for the S- and I-Regimes. Here, I test the effect of turbulent scale and turbulent intensity by varying the storm size and wind speed, which are not a part of my first set of experiments. I find that the intensity of the resulting single polar cyclone is affected by the storm size, storm wind speed, and storm polarity fraction. However, the radius of the polar cyclone is not affected by the size of the storms.The results of my numerical experiments advance the state of knowledge of giant planet polar atmospheric dynamics by revealing a fundamental mechanism behind the differing configurations and morphologies of polar cyclones. Furthermore, my results provide insight crucial in developing spacecraft exploration missions to the ice giant planets, which will likely include atmosphere science objectives.
PDF
