•The plumes decreased by a factor of ∼2 from 2005 to 2015.•Decadal trend roughly matches a decrease in eccentricity.•Interannual stochasic variability of plumes is likely.•Launch velocity is less at ...apocenter, greater at pericenter.•A secondary maximum occurs at 90 deg mean anomaly.
The brightness of the Enceladus plumes varies with position in the satellite's eccentric orbit, with altitude above the surface, and with time from one year to the next. Hedman et al. (2013, hereinafter H13) were the first to report these variations. They used data from Cassini's Visible and Infrared Mapping Spectrometer (VIMS). Here we present brightness observations from Cassini's Imaging Science Subsystem (ISS), which has 40 times higher spatial resolution than VIMS. Our unit of measure is slab density, the total mass of particles in a horizontal slab per unit thickness of the slab. Using slab density is one approach to correcting for the variation of brightness with wavelength and scattering angle. Approaches differ mainly by a multiplicative scaling factor that depends on particle density, which is uncertain. All approaches lead to the same qualitative conclusions and agree with the conclusions from VIMS. We summarize our conclusions as follows: At all altitudes between 50 and 200 km, the corrected brightness is 4–5 times greater when Enceladus is farther from Saturn (near apocenter) than when it is closer (near pericenter). A secondary maximum occurs after pericenter and before apocenter. Corrected brightness vs. altitude is best described as a power law whose negative exponent is greatest in magnitude at apocenter, indicating a slower launch speed for the particles at apocenter than at other points in the orbit. Corrected brightness decreased by roughly a factor of two during much of the period 2005–2015. The last is our principal result, and we offer three hypotheses to explain it. One is a long-period tide—the decreasing phase of an 11-year cycle in orbital eccentricity; another is buildup of ice at the throats of the vents; and the third is seasonal change—the end of summer at the south pole.
► 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.
► Total mass of ice in plumes is (1.45±0.50)×105kg. ► Large ice/vapor ratio (0.35–0.70) implies a liquid water source for plumes. ► Particulate mass leaves the vents at (51±18)kgs−1. ► Fraction that ...escape Enceladus ∼9%. ► Average lifetime of particles in E ring ∼8years.
The eclipse mosaic (PIA08329) of the Saturn system, taken on September 15, 2006 when Cassini was in Saturn’s shadow, contains numerous color images of the Enceladus plume and the E ring at phase angles ranging from 173° to 179°. These forward-scattering observations sample the diffraction peak for particle radii in the 1–5μm range. The phase angle dependence and total brightness are sensitive indicators of the total mass of solid material in the plume. We fit the data with a variety of particle shapes and size distributions, and find that the median radius of the equivalent-volume sphere is 3.1μm, with an uncertainty of ±0.5μm. The total mass of particles in the plume is (1.45±0.5)×105kg. We have not considered variations with altitude in the particle size and shape distribution, and we leave that for another paper. We find that the brightness of the E ring varies with position in the orbit, not only because of the viewing geometry, e.g., variations in phase angle, but also because of some unknown intrinsic variability. The total mass of solid material in the E ring is (12±5.5)×108kg. For the plume, the production rate of particles – the mass per unit time leaving the vents is 51±18kgs−1. We estimate that 9% of these particles are escaping from Enceladus, implying lifetimes of ∼8years for the E ring particles. Based on three comparisons with vapor amounts from ultraviolet spectroscopy, the ice/vapor ratio is in the range 0.35–0.70. This high ratio poses a problem for theories in which particles form by condensation from the gas phase, and could indicate that particles are formed as spray from a liquid reservoir.
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
•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.
•We mapped the spatial distribution of ice-block features on Enceladus.•Ice blocks are heavily concentrated in the geologically active south-polar region.•Ice blocks exhibit no clear pattern in ...relation to fracture margins or jet sources.•We propose possible mechanisms for ice-block origin, emplacement and evolution.
We have mapped the locations of over 100,000 ice blocks across the south polar region of Saturn’s moon Enceladus, thus generating the first quantitative estimates of ice-block number density distribution in relation to major geological features. Ice blocks were manually identified and mapped from twenty of the highest resolution (4–25m per pixel) Cassini Imaging Science Subsystem (ISS) narrow-angle images using ArcGIS software. The 10–100m-diameter positive-relief features are marginally visible at the resolution of the images, making ice-block identifications difficult but not impossible. Our preliminary results reveal that ice blocks in the southern hemisphere are systematically most concentrated within the geologically active South Polar Terrain (SPT) and exhibit peak concentrations within 20km of the tiger-stripe fractures as well as close to the south pole. We find that ice blocks are concentrated just as heavily between tiger-stripe fractures as on the directly adjacent margins; although significant local fluctuations in ice-block number density do occur, we observe no clear pattern with respect to the tiger stripes or jet sources. We examine possible roles of several mechanisms for ice-block origin, emplacement, and evolution: impact cratering, ejection from fissures during cryovolcanic eruptions, tectonic disruption of lithospheric ice, mass wasting, seismic disturbance, and vapor condensation around icy fumeroles. We conclude that impact cratering as well as mass wasting, perhaps triggered by seismic events, cannot account for a majority of ice-block features within the inner SPT. The pervasiveness of fracturing at many size scales, the ubiquity of ice blocks in the inner SPT, as well as the occurrence of linear block arrangements that parallel through-cutting crack networks along the flanks of tiger stripes indicate that tectonic deformation is an important source of blocky-ice features in the SPT. Ejection during catastrophic cryovolcanic eruptions and condensation around surface vents, however, cannot be ruled out. Further, sublimation processes likely erode and disaggregate ice blocks from solid exposures of ice, especially near the warm tiger-stripe fractures. The relative paucity of blocks beyond the bounds of the SPT, particularly on stratigraphically old cratered terrains, may be explained in part by mantling of the surface by fine particulate ice grains that accumulate over time.
Display omitted
•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
A global dust storm occurred on Mars between June and October 2001. The storm began near Hellas just before southern spring equinox (∼Ls = 177°). Local storms, likely forced by a combination of ...slope‐flow and ice cap thermal contrasts, were observed to propagate along the northwestern rim of Hellas, apparently triggering the global storm. Cap‐edge storm activity for much of late southern winter was similar in 2001 to one Mars year earlier; however, a very large storm propagated into the basin just after Ls = 177°. Subsequently, the total area of storm activity in 2001 was roughly double that of the previous year. For about 10 days, dust lifting was limited to the Hellas region. As additional storms propagated into Hellas, activity built and extended northward into Syrtis and eastward into Hesperia. It is not clear whether transport or spreading of lifting were of greatest importance for expansion. At Ls = 185° the storm began to spread rapidly to the east, along a line from the southern pole to the northern tropics. Essentially no storm propagation to the west occurred, yielding strong zonal asymmetry of expansion. As the dust storm reached the western edge of Tharsis, secondary dust lifting centers developed in Daedalia and Solis (southeastern Tharsis). Subsequently, the storm rapidly encompassed the planet (by Ls = 193°). Once fully global, the Syria/Solis/Daedalia lifting center appeared to dominate (on the basis of cloud top morphology), with Hellas quiescent. By Ls = 212°, lifting could no longer be discerned. Thereafter, dust haze appeared uniform and diffuse, and decay appeared to have set in.
PDF
