Saturn's dynamic D ring Hedman, Matthew M.; Burns, Joseph A.; Showalter, Mark R. ...
Icarus (New York, N.Y. 1962),
05/2007, Letnik:
188, Številka:
1
Journal Article
Recenzirano
The Cassini spacecraft has provided the first clear images of the D ring since the Voyager missions. These observations show that the structure of the D ring has undergone significant changes over ...the last 25 years. The brightest of the three ringlets seen in the Voyager images (named D72), has transformed from a narrow, <40-km wide ringlet to a much broader and more diffuse 250-km wide feature. In addition, its center of light has shifted inwards by over 200 km relative to other features in the D ring. Cassini also finds that the locations of other narrow features in the D ring and the structure of the diffuse material in the D ring differ from those measured by Voyager. Furthermore, Cassini has detected additional ringlets and structures in the D ring that were not observed by Voyager. These include a sheet of material just interior to the inner edge of the C ring that is only observable at phase angles below about 60°. New photometric and spectroscopic data from the ISS (Imaging Science Subsystem) and VIMS (Visual and Infrared Mapping Spectrometer) instruments onboard Cassini show the D ring contains a variety of different particle populations with typical particle sizes ranging from 1 to 100 microns. High-resolution images reveal fine-scale structures in the D ring that appear to be variable in time and/or longitude. Particularly interesting is a remarkably regular, periodic structure with a wavelength of
∼
30
km
extending between orbital radii of 73,200 and 74,000 km. A similar structure was previously observed in 1995 during the occultation of the star GSC5249-01240, at which time it had a wavelength of
∼
60
km
. We interpret this structure as a periodic vertical corrugation in the D ring produced by differential nodal regression of an initially inclined ring. We speculate that this structure may have formed in response to an impact with a comet or meteoroid in early 1984.
The New Horizons spacecraft will achieve a wide range of measurement objectives at the Pluto system, including color and panchromatic maps, 1.25–2.50 micron spectral images for studying surface ...compositions, and measurements of Pluto’s atmosphere (temperatures, composition, hazes, and the escape rate). Additional measurement objectives include topography, surface temperatures, and the solar wind interaction. The fulfillment of these measurement objectives will broaden our understanding of the Pluto system, such as the origin of the Pluto system, the processes operating on the surface, the volatile transport cycle, and the energetics and chemistry of the atmosphere. The mission, payload, and strawman observing sequences have been designed to achieve the NASA-specified measurement objectives and maximize the science return. The planned observations at the Pluto system will extend our knowledge of other objects formed by giant impact (such as the Earth–moon), other objects formed in the outer solar system (such as comets and other icy dwarf planets), other bodies with surfaces in vapor-pressure equilibrium (such as Triton and Mars), and other bodies with N
2
:CH
4
atmospheres (such as Titan, Triton, and the early Earth).
Images of Saturn's narrow and contorted F ring returned by the Cassini spacecraft have revealed phenomena not previously detected in any planetary ring system. The perturbing effect of the inner ...shepherding satellite, Prometheus, seems to introduce channels through the F ring and a 'streamer'-a line of particles that link the ring to the satellite. The detailed mechanism for the formation of these features has been lacking an explanation. Here we show that these phenomena can be understood in terms of a simple gravitational interaction as Prometheus approaches and recedes from the F ring every 14.7 hours. Our numerical models show that as Prometheus recedes from its closest approach to the F ring, it draws out ring material; one orbital period later, this affected region has undergone keplerian shear and is visible as a channel, in excellent agreement with structures seen in the Cassini images. Prometheus' periodic disruption of the F ring will become more pronounced as the two orbits approach their minimum separation in 2009. The model predicts that the appearance of streamers and the associated channels will vary in a regular fashion on a timescale of one orbital period.
Vertical wind shear on Jupiter from Cassini images Li, Liming; Ingersoll, Andrew P.; Vasavada, Ashwin R. ...
Journal of Geophysical Research - Planets,
April 2006, Letnik:
111, Številka:
E4
Journal Article
Recenzirano
Odprti dostop
Multifilter images of Jupiter acquired by the Cassini Imaging Science Subsystem (ISS) are used to derive zonal winds at altitudes above and below the visible cloud deck. Small features unique to the ...ultraviolet images of ISS are tracked to get the systematic high‐altitude zonal winds. Comparison between the zonal winds from ultraviolet images and the vertical profile of zonal winds from the Cassini Composite Infrared Spectrometer (CIRS) shows that the zonal winds from the ultraviolet images are from a pressure level that is ∼0.2 scale heights higher than the pressure level of the zonal winds from continuum‐band images. Deeper zonal winds at different latitudes of the equatorial region are measured by tracking cloud features observed within hot spots on continuum‐band images. The deeper zonal winds in this study extend the measurement of the Galileo probe to different latitudes of the equatorial region. Comparison between the Galileo probe and this study suggests that these fast‐moving clouds within hot spots are deeper than 3 bars and are therefore probably water clouds.
Saturn's emitted power Li, Liming; Conrath, Barney J.; Gierasch, Peter J. ...
Journal of Geophysical Research: Planets,
November 2010, Letnik:
115, Številka:
E11
Journal Article
Recenzirano
Odprti dostop
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.
We describe a model that accounts for the complex morphology of spiral density waves raised in Saturn's rings by the co-orbital satellites, Janus and Epimetheus. Our model may be corroborated by ...future Cassini observations of these time-variable wave patterns.
High-resolution images of Saturn's southern hemisphere acquired by the Cassini Imaging Science Subsystem between February and October 2004 are used to create maps of cloud morphology at several ...wavelengths, to derive zonal winds, and to characterize the distribution, frequency, size, morphology, color, behavior, and lifetime of vortices. Nonequatorial wind measurements display only minor differences from those collected since 1981 and reveal a strong, prograde flow near the pole. The region just southward of the velocity minimum at 40.7 deg S is especially active, containing numerous vortices, some generated in the proximity of convective storms. The two eastward jets nearest the pole display periodicity in their longitudinal structure, but no direct analogs to the northern hemisphere's polar hexagon or ribbon waves were observed. Characteristics of winds and vortices are compared with those of Saturn's northern hemisphere and Jupiter's atmosphere.
We have developed a semianalytic method of parameterizing N-body simulations of self-gravity wakes in Saturn's rings, describing their photometric properties by means of only six numbers: three ...optical depths and three weighting factors. These numbers are obtained by fitting a sum of three Gaussians to the results of a density-estimation procedure that finds the frequencies of various values of local density within a simulated ring patch. Application of our parameterization to a suite of N-body simulations implies that rings dominated by self-gravity wakes appear to be mostly empty space, with more than half of their surface area taken up by local optical depths around 0.01. Such regions will be photometrically inactive for all viewing geometries. While this result might be affected by our use of identically sized particles, we believe the general result that the distribution of local optical depths is trimodal, rather than bimodal as previous authors have assumed, is robust. The implications of this result for the analysis of occultation data are more conceptual than practical, as we find that occultations can only distinguish between bimodal and trimodal models at very low opening angles. Thus, the only adjustment needed in existing analyses of occultation data is that the model parameter Delta *tgap should be interpreted as representing the area-weighted average optical depth within the gaps (or inter-wake regions), keeping in mind the possibility that the optical depth within those inter-wake regions may vary significantly. The most significant consequence of our results applies to the question of why 'propeller' structures observed in the mid-A ring are seen as relative-bright features, even though the most prominent features of simulated propellers are regions of relatively low density. Our parameterization of self-gravity wakes lends preliminary quantitative support to the hypothesis that propellers would be bright if they involve a local and temporary disruption of self-gravity wakes. Even though the overall local density is lower within the propeller-shaped structure surrounding an embedded central moonlet, disruption of the wakes would flood these same regions with more 'photometrically active' material (i.e., material that can contribute to the rings' local optical depth), raising their apparent brightnesses in agreement with observations. We find for a wide range of input parameters that this mechanism indeed can plausibly make propellers brighter than the wake-dominated background, though it is also possible for propellers to blend in with the background or even to remain dark. We suggest that this mechanism be tested by future detailed numerical models.
High-resolution Cassini stereo images of Saturn's moon Phoebe have been used to derive a regional digital terrain model (DTM) and an orthoimage mosaic of the surface. For DTM-control a network of 130 ...points measured in 14 images (70–390
m/pixel resolution) was established which was simultaneously used to determine the orientation of the spin-axis. The J2000 spin-axis was found at Dec=78.0°±0.1° and RA=356.6°±0.3°, substantially different from the former Voyager solution. The control points yield a mean figure radius of 107.2
km with RMS residuals of 6.2
km demonstrating the irregular shape of this body. The DTM was computed from densely spaced conjugate image points determined by methods of digital image correlation. It has a horizontal resolution of 1–2
km and vertical accuracies in the range 50–100
m. It is limited in coverage, but higher in resolution than the previously derived global shape model of Phoebe Porco et al., 2005. Cassini imaging science: initial results on Phoebe and Iapetus. Science 307, 1237–1242 and allows us to study the morphology of the surface in more detail. There is evidence for unconsolidated material from a steep and smooth slope at the rim of a 100
km impact feature. There are several conically shaped craters on Phoebe, which may hint at highly porous and low compaction material on the surface.
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