► 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 Cassini Visual and Infrared Mapping Spectrometer (VIMS) 5‐μm images are used to derive Saturn's global zonal winds around the 2,000‐hPa level. The comparison of zonal winds between 2,000 and ...300–500 hPa shows a general consistency of wind structure between the two pressure levels on a global scale. However at some latitudes, the magnitude of the zonal winds differs between these levels. The equatorial zonal winds are stronger downward, while the zonal winds in the middle and high latitudes are generally weaker downward. These new wind measurements also imply that barotropic and baroclinic instabilities probably exist through the relatively deep atmosphere at some latitudes. Finally, our analysis reveals that the VIMS winds in the two polar regions are basically constant with time except for a westerly jet centered at ~88°N, which decreased from 135 ± 7 m/s in 2008 to 91 ± 12 m/s in 2017.
Plain Language Summary
Images of giant planets at the visible wavelengths are widely used to track visible clouds and hence estimate the atmospheric winds at the pressure levels of the visible clouds. On the other hand, images at the infrared wavelengths (e.g., 5 μm), which are sensitive to the pressure levels below the visible clouds, can be used to measure the relatively deep winds. Here we use the infrared images recorded by the Cassini spacecraft to measure Saturn's zonal winds (i.e., atmospheric wind in the longitudinal direction) at the relatively deep pressure levels around 2,000 mbar. We provide the global profile of the zonal winds around 2,000 mbar for the first time. The comparison of the global profile of zonal winds between 2,000 mbar and 300–500 mbar reveals interesting characteristics of the vertical shear of zonal winds and the related stabilities on Saturn. In addition, the comparison of the 2,000‐mbar zonal winds among different years suggests important temporal characteristics of zonal winds in the polar region of Saturn. This observational study will not only provide key information about the large‐scale atmospheric dynamics but also help us develop the theories and models of the general circulation on the giant planets.
Key Points
The global profile of zonal winds around 2,000 mbar is generated for the first time; the new profile suggests vertical wind shear
The vertical shear of zonal winds helps better understand the atmospheric dynamics (e.g., stability)
Measurements of zonal winds in multiple years suggest the temporal variations of zonal winds in the polar region of Saturn
•Cassini spacecraft images detected Saturn’s visible-light aurora.•The altitudes of aurora are up to 1500km above the horizon.•The color of the aurora changes with height from pink to purple.•The ...aurora corotates with Saturn at changing period of 10.65–10.8±0.15h.•Auroral oval brightens in an interval of about 1h.
The first observations of Saturn’s visible-wavelength aurora were made by the Cassini camera. The aurora was observed between 2006 and 2013 in the northern and southern hemispheres. The color of the aurora changes from pink at a few hundred km above the horizon to purple at 1000–1500km above the horizon. The spectrum observed in 9 filters spanning wavelengths from 250nm to 1000nm has a prominent H-alpha line and roughly agrees with laboratory simulated auroras. Auroras in both hemispheres vary dramatically with longitude. Auroras form bright arcs between 70° and 80° latitude north and between 65° and 80° latitude south, which sometimes spiral around the pole, and sometimes form double arcs. A large 10,000-km-scale longitudinal brightness structure persists for more than 100h. This structure rotates approximately together with Saturn. On top of the large steady structure, the auroras brighten suddenly on the timescales of a few minutes. These brightenings repeat with a period of ∼1h. Smaller, 1000-km-scale structures may move faster or lag behind Saturn’s rotation on timescales of tens of minutes. The persistence of nearly-corotating large bright longitudinal structure in the auroral oval seen in two movies spanning 8 and 11 rotations gives an estimate on the period of 10.65±0.15h for 2009 in the northern oval and 10.8±0.1h for 2012 in the southern oval. The 2009 north aurora period is close to the north branch of Saturn Kilometric Radiation (SKR) detected at that time.
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•Cassini spacecraft imaged Saturn’s lightning, including first day-side images.•We characterize lightning storms with lightning and cloud images, and radio data.•Lightning occurs in ...clearly observable cloud structures.•Storm in 2011 produced powers comparable with total Saturn’s radiation to space.
Visible lightning on Saturn was first detected by the Cassini camera in 2009 at ∼35° South latitude. We report more lightning observations at ∼35° South later in 2009, and lightning in the 2010–2011 giant lightning storm at ∼35° North. The 2009 lightning is detected on the night side of Saturn in a broadband clear filter. The 2011 lightning is detected on the day side in blue wavelengths only. In other wavelengths the 2011 images lacked sensitivity to detect lightning, which leaves the lightning spectrum unknown.
The prominent clouds at the west edge, or the “head” of the 2010–2011 storm periodically spawn large anticyclones, which drift off to the east with a longitude spacing of 10–15° (∼10,000km). The wavy boundary of the storm’s envelope drifts with the anticyclones. The relative vorticity of the anticyclones ranges up to −f/3, where f is the planetary vorticity. The lightning occurs in the diagonal gaps between the large anticyclones. The vorticity of the gaps is cyclonic, and the atmosphere there is clear down to level of the deep clouds. In these respects, the diagonal gaps resemble the jovian belts, which are the principal sites of jovian lightning.
The size of the flash-illuminated cloud tops is similar to previous detections, with diameter ∼200km. This suggests that all lightning on Saturn is generated at similar depths, ∼125–250km below the cloud tops, probably in the water clouds. Optical energies of individual flashes for both southern storms and the giant storm range up to 8×109J, which is larger than the previous 2009 equinox estimate of 1.7×109J. Cassini radio measurements at 1–16MHz suggest that, assuming lightning radio emissions range up to 10GHz, lightning radio energies are of the same order of magnitude as the optical energies.
Southern storms flash at a rate ∼1–2per minute. The 2011 storm flashes hundreds of times more often, ∼5times per second, and produces ∼1010W of optical power. Based on this power, the storm’s total convective power is of the order 1017W, which is uncertain by at least an order of magnitude, and probably is underestimated. This power is similar to Saturn’s global internal power radiated to space. It suggests that storms like the 2010–2011 giant storm are important players in Saturn’s cooling and thermal evolution.
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 analyzed Saturn’s String of Pearls as seen by Cassini Orbiter.•The String of Pearls feature’s cloud morphology is thoroughly studied.•The pearls’ vorticity has been determined to be cyclonic.•The ...pearls are not dispersive, and do not appeared to be generated by an obstacle in the flow.
We present the dynamics of the String of Pearls (SoPs) feature observed by the Cassini spacecraft’s Imaging Science Subsystem (ISS) camera between 2007 and 2010. The SoPs was originally discovered in the 5μm images captured by Cassini VIMS instrument, where it appeared as a chain of infrared-bright spots (Momary, T.W., et al. 2006. The Zoology of Saturn: The Bizarre Features Unveiled by the 5 Micron Eyes of Cassini/VIMS. AAS/Division for Planetary Sciences Meeting Abstracts 38, 499). Using ISS images of Saturn, we found a chain of 23–26 dark spots at 33.2°N planetocentric latitude with characteristics that are consistent with those of SoPs. Our measurements imply that the feature propagated at −2.26±0.02°day−1 in longitude (−22.27±0.2ms−1, negative values denote westward) during the observed period that spans three Earth years. Our measurements imply that the SoPs is a chain of cyclones, which we infer from the motion of clouds on the periphery of the individual pearls. We tracked the motion of 26 pearls for 6months in 2008 and noted a few pearls appearing and disappearing, all near the east–west termini of the SoPs feature. During this period, a few of the pearls, varying between 6 and 10, harbored a small circular cloud at the center, which we call the central peaks. In general, a group of vortices with the same sign of vorticity tend to merge; however, our measurements did not detect merger of pearls. The interest in the feature was heightened when the latest planet-encircling storm erupted from the SoPs on December 5, 2010 (Sayanagi, K.M., Dyudina, U.A., Ewald, S.P., Fischer, G., Ingersoll, A.P., Kurth, W.S., Muro, G.D., Porco, C.C., West, R.A. 2013. Icarus 223, 460–478). The storm severely disrupted the region; the SoPs was last seen on December 24, 2010 in the turbulent wake of the storm, and has not reappeared as of August 2013.
We report on Cassini Imaging Science Subsystem (ISS) data correlated with Radio and Plasma Wave Science (RPWS) observations, which indicate lightning on Saturn. A rare bright cloud erupt at ∼35° ...South planetocentric latitude when radio emissions (Saturn Electrostatic Discharges, or SEDs) occur. The cloud consisting of few consecutive eruptions typically lasts for several weeks, and then both the cloud and the SEDs disappear. They may reappear again after several months or may stay inactive for a year. Possibly, all the clouds are produced by the same atmospheric disturbance which drifts West at 0.45 °/day. As of March 2007, four such correlated visible and radio storms have been observed since Cassini Saturn Orbit Insertion (July 2004). In all four cases the SEDs are periodic with roughly Saturn's rotation rate (
10
h
39
m
), and show correlated phase relative to the times when the clouds are seen on the spacecraft-facing side of the planet, as had been shown for the 2004 storms in Porco, C.C., and 34 colleagues, 2005. Science 307, 1243–1247. The 2000-km-scale storm clouds erupt to unusually high altitudes and then slowly fade at high altitudes and spread at low altitudes. The onset time of individual eruptions is less than a day during which time the SEDs reach their maximum rates. This suggests vigorous atmospheric updrafts accompanied by strong precipitation and lightning. Unlike lightning on Earth and Jupiter, where considerable lightning activity is known to exist, only one latitude on Saturn has produced lightning strong enough to be detected during the two and a half years of Cassini observations. This may partly be a detection issue.
Dynamics of Saturn's South Polar Vortex Dyudina, Ulyana A; Ingersoll, Andrew P; Ewald, Shawn P ...
Science (American Association for the Advancement of Science),
03/2008, Letnik:
319, Številka:
5871
Journal Article
Recenzirano
Odprti dostop
The camera onboard the Cassini spacecraft has allowed us to observe many of Saturn's cloud features. We present observations of Saturn's south polar vortex (SPV) showing that it shares some ...properties with terrestrial hurricanes: cyclonic circulation, warm central region (the eye) surrounded by a ring of high clouds (the eye wall), and convective clouds outside the eye. The polar location and the absence of an ocean are major differences. It also shares properties with the polar vortices on Venus, such as polar location, cyclonic circulation, warm center, and long lifetime, but the Venus vortices have cold collars and are not associated with convective clouds. The SPV's combination of properties is unique among vortices in the solar system
We predict how a remote observer would see the brightness variations of giant planets similar to those in our solar system as they orbit their central stars. Our models are the first to use measured ...anisotropic scattering properties of solar system giants and the first to consider the effects of eccentric orbits. We model the geometry of Jupiter, Saturn, and Saturn's rings for varying orbital and viewing parameters, using scattering properties for the (forward scattering) planets and (backward scattering) rings as measured by the Pioneer and Voyager spacecraft at 0.6-0.7 mu m. Images of the planet with and without rings are simulated and used to calculate the disk-averaged luminosity varying along the orbit; that is, a light curve is generated. We find that the different scattering properties of Jupiter and Saturn (without rings) make a substantial difference in the shape of their light curves. Saturn-sized rings increase the apparent luminosity of a planet by a factor of 2-3 for a wide range of geometries, an effect that could be confused with a larger planet size. Rings produce asymmetric light curves that are distinct from the light curve that the planet would have without rings, which could resolve this confusion. If radial velocity data are available for the planet, the effect of the ring on the light curve can be distinguished from effects due to orbital eccentricity. Nonringed planets on eccentric orbits produce light curves with maxima shifted relative to the position of the maximum phase of the planet. Given radial velocity data, the amount of the shift restricts the planet's unknown orbital inclination and therefore its mass. A combination of radial velocity data and a light curve for a nonringed planet on an eccentric orbit can also be used to constrain the surface scattering properties of the planet and thus describe the clouds covering the planet. We summarize our results for the detectability of exoplanets in reflected light in a chart of light-curve amplitudes of nonringed planets for different eccentricities, inclinations, and azimuthal viewing angles of the observer.
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
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