Predictably, a major motivation for multiple atmospheric probe measurements at Uranus is the understanding of dynamic processes that create and maintain spatial variation in thermal structure, ...composition, and horizontal winds. But origin questions---regarding the planet's formation and evolution, and conditions in the protoplanetary disk---are also major science drivers for multiprobe exploration. Spatial variation in thermal structure reveals how the atmosphere transports heat from the interior, and measuring compositional variability in the atmosphere is key to ultimately gaining an understanding of the bulk abundances of several heavy elements. We review the current knowledge of spatial variability in Uranus' atmosphere, and we outline how multiple probe exploration would advance our understanding of this variability. The other giant planets are discussed, both to connect multiprobe exploration of those atmospheres to open questions at Uranus, and to demonstrate how multiprobe exploration of Uranus itself is motivated lessons learned about the spatial variation at Jupiter, Saturn, and Neptune. We outline the measurements of highest value from miniature secondary probes (which would complement more detailed investigation by a larger flagship probe), and present the path toward overcoming current challenges and uncertainties in areas including mission design, cost, trajectory, instrument maturity, power, and timeline.
► We analyzed Saturn’s Great Storm of 2010–2011 as seen by Cassini Orbiter. ► The new storm erupted from the String of Pearls feature. ► The new storm was the longest-lasting storm on Saturn in ...record. ► The storm spawned the largest tropospheric vortex ever seen on Saturn. ► We captured the convective storm’s beginning and end using two instruments.
Saturn’s quasi-periodic planet-encircling storms are the largest convecting cumulus outbursts in the Solar System. The last eruption was in 1990 (Sánchez-Lavega, A. 1994. Chaos 4, 341–353). A new eruption started in December 2010 and presented the first-ever opportunity to observe such episodic storms from a spacecraft in orbit around Saturn (Fischer, G. et al. 2011. Nature 475, 75–77; Sánchez-Lavega, A. et al. 2011. Nature 475, 71–74; Fletcher, L.N. et al. 2011. Science 332, 1413). Here, we analyze images acquired with the Cassini Imaging Science Subsystem (ISS), which captured the storm’s birth, evolution, and demise. In studying the end of the convective activity, we also analyze the Saturn Electrostatic Discharge (SED) signals detected by the Radio and Plasma Wave Science (RPWS) instrument. The storm’s initial position coincided with that of a previously known feature called the String of Pearls (SoPs) at 33°N planetocentric latitude. Intense cumulus convection at the westernmost point of the storm formed a particularly bright “head” that drifted at −26.9±0.8ms−1 (negative denotes westward motion). On January 11, 2011, the size of the head was 9200km and up to 34,000km in the north–south and east–west dimensions, respectively. RPWS measurements show that the longitudinal extent of the lightning source expanded with the storm’s growth. The storm spawned the largest tropospheric vortex ever seen on Saturn. On January 11, 2011, the anticyclone was sized 11,000kmby12,000km in the north–south and east–west directions, respectively. Between January and September 2011, the vortex drifted at an average speed of −8.4ms−1. We detect anticyclonic circulation in the new vortex. The vortex’s size gradually decreased after its formation, and its central latitude shifted to the north. The storm’s head moved westward and encountered the new anticyclone from the east in June 2011. After the head–vortex collision, the RPWS instrument detected that the SED activities became intermittent and declined over ∼40days until the signals became undetectable in early August. In late August, the SED radio signals resurged for 9days. The storm left a vast dark area between 32°N and 38°N latitudes, surrounded by a highly disturbed region that resembles the mid-latitudes of Jupiter. Using ISS images, we also made cloud-tracking wind measurements that reveal differences in the cloud-level zonal wind profiles before and after the storm.
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.
•In contrast to expectations, extreme storms were observed on Uranus in August 2014.•One storm reflected 30% as much light as the rest of the planet at 2.2μm.•Another cloud, deeper in the atmosphere, ...was later seen by amateur astronomers.•We report the first detection of a long-awaited haze over the north polar region.
In spite of an expected decline in convective activity following the 2007 equinox of Uranus, eight sizable storms were detected on the planet with the near-infrared camera NIRC2, coupled to the adaptive optics system, on the 10-m W.M. Keck telescope on UT 5 and 6 August 2014. All storms were on Uranus’ northern hemisphere, including the brightest storm ever seen in this planet at 2.2μm, reflecting 30% as much light as the rest of the planet at this wavelength. The storm was at a planetocentric latitude of ∼15°N and reached altitudes of ∼330mbar, well above the regular uppermost cloud layer (methane-ice) in the atmosphere. A cloud feature at a latitude of 32°N, that was deeper in the atmosphere (near ∼2bar), was later seen by amateur astronomers. We also present images returned from our HST ToO program, that shows both of these cloud features. We further report the first detection of a long-awaited haze over the north polar region.
Saturn's Atmosphere at 1–10 Kilometer Resolution Ingersoll, Andrew P.; Ewald, Shawn P.; Sayanagi, Kunio M. ...
Geophysical research letters,
16 August 2018, Letnik:
45, Številka:
15
Journal Article
Recenzirano
Odprti dostop
We present images of Saturn from the final phases of the Cassini mission, including images with 0.5 km per pixel resolution, as high as any Saturn images ever taken. Notable features are puffy clouds ...resembling terrestrial cumulus, shadows indicating cloud height, dome and bowl shaped cloud structures indicating upwelling and downwelling in anticyclones and cyclones respectively, and filaments, which are thread‐like clouds that remain coherent over distances of 20,000 km. From the coherence of the filaments, we give upper bounds on the diffusivity and kinetic energy dissipation. A radiative transfer analysis by Sanz‐Requena et al. (2018) indicates that methane‐band imagery is most useful in determining cloud and haze properties in the 60–250 mbar pressure range. Our methane‐band imagery finds haze in this pressure range covering 64°‐74°planetocentric latitude. Filaments lie within the haze, and cumulus clouds lie below it, but pressure levels are uncertain below the 250 mbar level.
Plain Language Summary
During its final half‐year, the Cassini spacecraft took close‐up images from 3000–4000 km above Saturn's cloud tops. The spatial resolution was as high as any images ever taken of the planet. They revealed isolated puffy clouds like terrestrial cumulus and also more exotic, threadlike clouds ‐ filaments ‐ that would stretch halfway around the Earth if they were on our planet. The filamentary clouds indicate that the level of turbulence in Saturn's atmosphere is very low. The images showed dome‐shaped clouds indicative of upwelling at the centers of anticyclones and bowl‐shaped clouds indicative of downwelling at the centers of cyclones.
Key Points
Close‐up imaging by the Cassini spacecraft reveals long filamentary clouds and puffy cumulus clouds at 0.5 km resolution
A dome‐shaped cloud structure in anticyclones suggests upwelling, and a bowl‐shaped cloud structure in cyclones suggests downwelling
Thread‐like filamentary clouds 20,000 km long suggest a laminar flow with extremely low values of diffusivity and dissipation
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 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).
Using images from the Cassini spacecraft, we analyzed three ribbon waves in Saturn's 42°N eastward jet at 45°N, 42°N, and 39°N planetocentric latitudes. In this report, we demonstrate that the ...morphology, wavelength, and propagation of the ribbon waves are consistent with barotropic Rossby waves with a smaller baroclinic component. We report on the appearance and disappearance of these waves during Cassini's mission. We suggest that the temporal evolution of these waves are related to the great Saturn storm of 2010–2011.
Plain Language Summary
During their 1980 and 1981 flybys of Saturn, the Voyager spacecraft imaged a dark, sinuous line encircling the planet. This feature, dubbed the ribbon wave after its visual appearance, was embedded in an atmospheric jet stream at 42N latitude. The Cassini spacecraft also discovered waves in the 42N jet during its 2004–2017 Saturn mission. Using images taken by Cassini, we have identified the ribbon waves as Rossby waves, that is, planet‐scale waves that are common in atmospheres, including that of the Earth. Unlike Earth's atmospheric Rossby waves, which are only visible as undulations on weather maps, Saturn's ribbons are visually striking and may be some of the most prominent examples of Rossby waves in the Solar System. The ribbons are composed of a number of wavelengths, each of which is affected differently by the atmosphere and move at different speeds. By measuring the differing speed of these wavelength components, we compared the behavior of the ribbons to theoretical predictions for Rossby waves and estimated basic properties of the atmosphere. Because the ribbons likely extend deep into the atmosphere, they may help shed light on the how the atmosphere behaves at depths that Cassini was not able to observe directly.
Key Points
Cassini observed three wave‐like ribbon features in Saturn's 42N atmospheric jet from 2005 to 2014
The ribbons' morphology, mean wavelengths, and propagation are consistent with Rossby waves
Their propagation places constraints on atmospheric conditions within the jet
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.
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