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Li L.,University of Houston | Jiang X.,University of Houston | Trammell H.J.,University of Houston | Pan Y.,University of Houston | And 10 more authors.
Geophysical Research Letters | Year: 2015

We analyze the relationship between Saturn's radiant energies and the 2010 giant storm with the Cassini observations. The storm increased the emitted power in a wide latitudinal band (20-55N) with a maximum change of 9.2 ± 0.1% around 45N from 2010 to 2011. Such a regional change caused the global-average emitted power to increase by ∼2.0 ± 0.2%. Saturn's giant storm occurs quasiperiodically (i.e., period approximately one Saturnian year), so it is possible that giant storms continuously modify the emitted power if the storm modification has a lifetime close to one Saturnian year. The hemispheric-average emitted power in the southern hemisphere, which was mainly affected by the seasonal change, decreased by 8.5 ± 0.3% from 2004 to 2013. Our estimates also imply that the 2010 giant storm significantly modified the absorbed solar power of Saturn. The significant temporal variations of radiant powers should be considered in reexamining the value of Saturn's internal heat flux. ©2015. American Geophysical Union. All Rights Reserved.

Roatsch Th.,German Aerospace Center | Kersten E.,German Aerospace Center | Wahlisch M.,German Aerospace Center | Hoffmeister A.,German Aerospace Center | And 6 more authors.
Planetary and Space Science | Year: 2012

The Cassini Imaging Science Subsystem (ISS) acquired 370 high-resolution images (<500 m/pixel) of Rhea during two close flybys and 9 non-targeted flybys between 2004 and 2010. We combined these images with lower-resolution Cassini images and others taken by the Voyager cameras to produce a high-resolution semi-controlled mosaic of Rhea. This global mosaic is the baseline for a high-resolution Rhea atlas. The nomenclature used in this atlas was proposed by the Cassini imaging team and approved by the International Astronomical Union (IAU). The atlas is available to the public through the Imaging Teams website 〈〉 and the Planetary Data System 〈〉. This atlas completes the series of the atlases of the Saturnian medium-sized satellites Mimas, Enceladus, Tethys, Dione, Rhea, and Iapetus. © 2011 Elsevier Ltd. All rights reserved.

Li L.,University of Houston | Li L.,Cornell University | Conrath B.J.,Cornell University | Gierasch P.J.,Cornell University | And 18 more authors.
Journal of Geophysical Research E: Planets | Year: 2010

Long-term (2004-2009) on-orbit observations by Cassini Composite Infrared Spectrometer are analyzed to precisely measure Saturn's emitted power and its meridional distribution. Our evaluations suggest that the average global emitted power is 4.952 ± 0.035 W m-2 during the period of 2004-2009. The corresponding effective temperature is 96.67 ± 0.17 K. The emitted power is 16.6% higher in the Southern Hemisphere than in the Northern Hemisphere. From 2005 to 2009, the global mean emitted power and effective temperature decreased by ∼2% and ∼0.5%, respectively. Our study further reveals the interannual variability of emitted power and effective temperature between the epoch of Voyager (∼1 Saturn year ago) and the current epoch of Cassini, suggesting changes in the cloud opacity from year to year on Saturn. The seasonal and interannual variability of emitted power implies that the energy balance and internal heat are also varying. Copyright © 2010 by the American Geophysical Union.

Helfenstein P.,Cornell University | Porco C.C.,University of California at Berkeley | Porco C.C.,CICLOPS Space Science Institute
Astronomical Journal | Year: 2015

We apply histogram analysis, photogeological methods, and tidal stress modeling to Porco et al.'s survey of 101 Enceladus South Polar Basin geysers and their three-dimensional orientations to test if the jet azimuths are influenced by their placement relative to surface morphology and tectonic structures. Geysers emplaced along the three most active tiger stripe fractures (Damascus Sulcus, Baghdad Sulcus, and Cairo Sulcus) occur in local groupings with relatively uniform nearest-neighbor separation distances (∼5 km). Their placement may be controlled by uniformly spaced en echelon Riedel-type shear cracks originating from left-lateral strike-slip fault motion inferred to occur along tiger stripes. The spacing would imply a lithosphere thickness of ∼5 km in the vicinity of the tiger stripes. The orientations of tilted geyser jets are not randomly distributed; rather their azimuths correlate with the directions either of tiger stripes, cross-cutting fractures, or else fine-scale local tectonic fabrics. Diurnal tidal stress modeling suggests that periodic changes of plume activity are significantly affected by cross-cutting fractures that open and close at different times than the tiger stripes that they intersect. We find evidence of sub-kilometer scale morphological modification of surface geological features surrounding geysers from sublimation-aided erosion, and ablation, and scouring. We propose that the simultaneous crushing and shearing action of periodic transpressional tidal stress on ice condensing on the inside walls of geyser conduits is the mechanism that extrudes the peculiar, paired narrow ridges known as "shark fins" that flank the medial tiger stripe fissures. We present a gallery of high-resolution image mosaics showing the placement of all the jets in their source region and consequently their geological context. © 2015. The American Astronomical Society. All rights reserved..

Li L.,University of Houston | Jiang X.,University of Houston | Ingersoll A.P.,California Institute of Technology | Del Genio A.D.,NASA | And 12 more authors.
Nature Geoscience | Year: 2011

The zonal jets on the giant planets have been thought to be stable in time1-3. A decline in the velocity of Saturn's equatorial jet has been identified, on the basis of a comparison of cloud-tracking data across two decades4, but the differences in cloud speeds have since been suggested to stem from changes in cloud altitude in combination with vertical wind shear, rather than from temporal changes in wind strength at a given height5. Here, we combine observations of cloud tracks and of atmospheric temperatures taken by two instruments on the Cassini spacecraft to reveal a significant temporal variation in the strength of the high-altitude equatorial jet on Saturn. Specifically, we find that wind speeds at atmospheric pressure levels of 60 mbar, corresponding to Saturn's tropopause, increased by about 20 m s-1 between 2004 and 2008, whereas the wind speed has been essentially constant over time in the southern equatorial troposphere. The observations further reveal that the equatorial jet intensified by about 60 m s-1 between 2005 and 2008 in the stratosphere, that is, at pressure levels of 1-5 mbar. Because the wind acceleration is weaker near the tropopause than higher up, in the stratosphere, we conclude that the semi-annual equatorial oscillation of Saturn's middle atmosphere6,7 is also damped as it propagates downwards. © 2011 Macmillan Publishers Limited. All rights reserved.

West R.,Jet Propulsion Laboratory | Knowles B.,CICLOPS Space Science Institute | Birath E.,CICLOPS Space Science Institute | Charnoz S.,University Paris Diderot | And 9 more authors.
Planetary and Space Science | Year: 2010

We describe in-flight calibration of the Cassini Imaging Science Sub-system narrow- and wide-angle cameras using data from 2004 to 2009. We report on the photometric performance of the cameras including the use of polarization filters, point spread functions over a dynamic range greater than 10 7, gain and loss of hot pixels, changes in flat fields, and an analysis of charge transfer efficiency. Hot pixel behavior is more complicated than can be understood by a process of activation by cosmic ray damage and deactivation by annealing. Point spread function (PSF) analysis revealed a ghost feature associated with the narrow-angle camera Green filter. More generally, the observed PSFs do not fall off with distance as rapidly as expected if diffraction were the primary contributor. Stray light produces significant signal far from the center of the PSF. Our photometric analysis made use of calibrated spectra from eighteen stars and the spectral shape of the satellite Enceladus. The analysis revealed a shutter offset that differed from pre-launch calibration. It affects the shortest exposures. Star photometry results are reproducible to a few percent in most filters. No degradation in charge transfer efficiency has been detected although uncertainties are large. The results of this work have been digitally archived and incorporated into our calibration software CISSCAL available online. © 2010 Elsevier B.V.. All rights reserved.

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