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  • 1
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    Saturn's Deep Atmospheric Flows Revealed by the Cassini Grand Finale Gravity Measurements (2019)
    Wiley
    Details
    Publication Date: 2019
    Description: Abstract How deep do Saturn's zonal winds penetrate below the cloud level has been a decades‐long question, with important implications not only for the atmospheric dynamics but also for the interior density structure, composition, magnetic field, and core mass. The Cassini Grand Finale gravity experiment enables answering this question for the first time, with the premise that the planet's gravity harmonics are affected not only by the rigid body density structure but also by its flow field. Using a wide range of rigid body interior models and an adjoint based optimization for the flow field using thermal wind balance, we calculate the flow structure below the cloud level and its depth. We find that with a wind profile, largely consistent with the observed winds, when extended to a depth of around 8,800 km, all the gravity harmonics measured by Cassini are explained. This solution is in agreement with considerations of angular momentum conservation and is consistent with magnetohydrodynamics constraints.
    Print ISSN: 0094-8276
    Electronic ISSN: 1944-8007
    Topics: Geosciences , Physics
    Published by Wiley on behalf of American Geophysical Union (AGU).
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  • 2
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    Titan's rotation reveals an internal ocean and changing zonal winds (2008)
    American Association for the Advancement of Science (AAAS)
    Details
    Publication Date: 2008-03-22
    Description: Cassini radar observations of Saturn's moon Titan over several years show that its rotational period is changing and is different from its orbital period. The present-day rotation period difference from synchronous spin leads to a shift of approximately 0.36 degrees per year in apparent longitude and is consistent with seasonal exchange of angular momentum between the surface and Titan's dense superrotating atmosphere, but only if Titan's crust is decoupled from the core by an internal water ocean like that on Europa.〈br /〉〈span class="detail_caption"〉Notes: 〈/span〉Lorenz, Ralph D -- Stiles, Bryan W -- Kirk, Randolph L -- Allison, Michael D -- Del Marmo, Paolo Persi -- Iess, Luciano -- Lunine, Jonathan I -- Ostro, Steven J -- Hensley, Scott -- Intramural NIH HHS/ -- New York, N.Y. -- Science. 2008 Mar 21;319(5870):1649-51. doi: 10.1126/science.1151639.〈br /〉〈span class="detail_caption"〉Author address: 〈/span〉Space Department, Johns Hopkins University Applied Physics Laboratory, Laurel, MD 20723, USA. ralph.lorenz@jhuapl.edu〈br /〉〈span class="detail_caption"〉Record origin:〈/span〉 〈a href="http://www.ncbi.nlm.nih.gov/pubmed/18356521" target="_blank"〉PubMed〈/a〉
    Keywords: Atmosphere ; Extraterrestrial Environment ; Ice ; *Saturn ; Spacecraft ; *Water ; Wind
    Print ISSN: 0036-8075
    Electronic ISSN: 1095-9203
    Topics: Biology , Chemistry and Pharmacology , Computer Science , Medicine , Natural Sciences in General , Physics
    Published by American Association for the Advancement of Science (AAAS)
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  • 3
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    Gravity field, shape, and moment of inertia of Titan (2010)
    American Association for the Advancement of Science (AAAS)
    Details
    Publication Date: 2010-03-13
    Description: Precise radio tracking of the spacecraft Cassini has provided a determination of Titan's mass and gravity harmonics to degree 3. The quadrupole field is consistent with a hydrostatically relaxed body shaped by tidal and rotational effects. The inferred moment of inertia factor is about 0.34, implying incomplete differentiation, either in the sense of imperfect separation of rock from ice or a core in which a large amount of water remains chemically bound in silicates. The equilibrium figure is a triaxial ellipsoid whose semi-axes a, b, and c differ by 410 meters (a-c) and 103 meters (b-c). The nonhydrostatic geoid height variations (up to 19 meters) are small compared to the observed topographic anomalies of hundreds of meters, suggesting a high degree of compensation appropriate to a body that has warm ice at depth.〈br /〉〈span class="detail_caption"〉Notes: 〈/span〉Iess, Luciano -- Rappaport, Nicole J -- Jacobson, Robert A -- Racioppa, Paolo -- Stevenson, David J -- Tortora, Paolo -- Armstrong, John W -- Asmar, Sami W -- New York, N.Y. -- Science. 2010 Mar 12;327(5971):1367-9. doi: 10.1126/science.1182583.〈br /〉〈span class="detail_caption"〉Author address: 〈/span〉Dipartimento di Ingegneria Aerospaziale ed Astronautica, Universita La Sapienza, via Eudossiana 18, 00184 Rome, Italy. luciano.iess@uniroma1.it〈br /〉〈span class="detail_caption"〉Record origin:〈/span〉 〈a href="http://www.ncbi.nlm.nih.gov/pubmed/20223984" target="_blank"〉PubMed〈/a〉
    Keywords: Extraterrestrial Environment ; Gravitation ; Ice ; *Saturn ; Spacecraft ; Water
    Print ISSN: 0036-8075
    Electronic ISSN: 1095-9203
    Topics: Biology , Chemistry and Pharmacology , Computer Science , Medicine , Natural Sciences in General , Physics
    Published by American Association for the Advancement of Science (AAAS)
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  • 4
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    A rigid and weathered ice shell on Titan (2013)
    Nature Publishing Group (NPG)
    Details
    Publication Date: 2013-08-30
    Description: Several lines of evidence suggest that Saturn's largest moon, Titan, has a global subsurface ocean beneath an outer ice shell 50 to 200 kilometres thick. If convection is occurring, the rigid portion of the shell is expected to be thin; similarly, a weak, isostatically compensated shell has been proposed to explain the observed topography. Here we report a strong inverse correlation between gravity and topography at long wavelengths that are not dominated by tides and rotation. We argue that negative gravity anomalies (mass deficits) produced by crustal thickening at the base of the ice shell overwhelm positive gravity anomalies (mass excesses) produced by the small surface topography, giving rise to this inverse correlation. We show that this situation requires a substantially rigid ice shell with an elastic thickness exceeding 40 kilometres, and hundreds of metres of surface erosion and deposition, consistent with recent estimates from local features. Our results are therefore not compatible with a geologically active, low-rigidity ice shell. After extrapolating to wavelengths that are controlled by tides and rotation, we suggest that Titan's moment of inertia may be even higher (that is, Titan may be even less centrally condensed) than is currently thought.〈br /〉〈span class="detail_caption"〉Notes: 〈/span〉Hemingway, D -- Nimmo, F -- Zebker, H -- Iess, L -- England -- Nature. 2013 Aug 29;500(7464):550-2. doi: 10.1038/nature12400.〈br /〉〈span class="detail_caption"〉Author address: 〈/span〉Department of Earth and Planetary Sciences, University of California Santa Cruz, 1156 High Street, Santa Cruz, California 95064, USA. djheming@ucsc.edu〈br /〉〈span class="detail_caption"〉Record origin:〈/span〉 〈a href="http://www.ncbi.nlm.nih.gov/pubmed/23985871" target="_blank"〉PubMed〈/a〉
    Print ISSN: 0028-0836
    Electronic ISSN: 1476-4687
    Topics: Biology , Chemistry and Pharmacology , Medicine , Natural Sciences in General , Physics
    Published by Nature Publishing Group (NPG)
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  • 5
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    The tides of Titan (2012)
    American Association for the Advancement of Science (AAAS)
    Details
    Publication Date: 2012-06-30
    Description: We have detected in Cassini spacecraft data the signature of the periodic tidal stresses within Titan, driven by the eccentricity (e = 0.028) of its 16-day orbit around Saturn. Precise measurements of the acceleration of Cassini during six close flybys between 2006 and 2011 have revealed that Titan responds to the variable tidal field exerted by Saturn with periodic changes of its quadrupole gravity, at about 4% of the static value. Two independent determinations of the corresponding degree-2 Love number yield k(2) = 0.589 +/- 0.150 and k(2) = 0.637 +/- 0.224 (2sigma). Such a large response to the tidal field requires that Titan's interior be deformable over time scales of the orbital period, in a way that is consistent with a global ocean at depth.〈br /〉〈span class="detail_caption"〉Notes: 〈/span〉Iess, Luciano -- Jacobson, Robert A -- Ducci, Marco -- Stevenson, David J -- Lunine, Jonathan I -- Armstrong, John W -- Asmar, Sami W -- Racioppa, Paolo -- Rappaport, Nicole J -- Tortora, Paolo -- New York, N.Y. -- Science. 2012 Jul 27;337(6093):457-9. doi: 10.1126/science.1219631. Epub 2012 Jun 28.〈br /〉〈span class="detail_caption"〉Author address: 〈/span〉Dipartimento di Ingegneria Meccanica e Aerospaziale, Universita La Sapienza, via Eudossiana 18, 00184 Rome, Italy. luciano.iess@uniroma1.it〈br /〉〈span class="detail_caption"〉Record origin:〈/span〉 〈a href="http://www.ncbi.nlm.nih.gov/pubmed/22745254" target="_blank"〉PubMed〈/a〉
    Keywords: Extraterrestrial Environment ; Gravitation ; Ice ; *Saturn ; Spacecraft ; *Water
    Print ISSN: 0036-8075
    Electronic ISSN: 1095-9203
    Topics: Biology , Chemistry and Pharmacology , Computer Science , Medicine , Natural Sciences in General , Physics
    Published by American Association for the Advancement of Science (AAAS)
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  • 6
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    The gravity field and interior structure of Enceladus (2014)
    American Association for the Advancement of Science (AAAS)
    Details
    Publication Date: 2014-04-05
    Description: The small and active Saturnian moon Enceladus is one of the primary targets of the Cassini mission. We determined the quadrupole gravity field of Enceladus and its hemispherical asymmetry using Doppler data from three spacecraft flybys. Our results indicate the presence of a negative mass anomaly in the south-polar region, largely compensated by a positive subsurface anomaly compatible with the presence of a regional subsurface sea at depths of 30 to 40 kilometers and extending up to south latitudes of about 50 degrees . The estimated values for the largest quadrupole harmonic coefficients (10(6)J2 = 5435.2 +/- 34.9, 10(6)C22 = 1549.8 +/- 15.6, 1sigma) and their ratio (J2/C22 = 3.51 +/- 0.05) indicate that the body deviates mildly from hydrostatic equilibrium. The moment of inertia is around 0.335MR(2), where M is the mass and R is the radius, suggesting a differentiated body with a low-density core.〈br /〉〈span class="detail_caption"〉Notes: 〈/span〉Iess, L -- Stevenson, D J -- Parisi, M -- Hemingway, D -- Jacobson, R A -- Lunine, J I -- Nimmo, F -- Armstrong, J W -- Asmar, S W -- Ducci, M -- Tortora, P -- New York, N.Y. -- Science. 2014 Apr 4;344(6179):78-80. doi: 10.1126/science.1250551.〈br /〉〈span class="detail_caption"〉Author address: 〈/span〉Dipartimento di Ingegneria Meccanica e Aerospaziale, Sapienza Universita di Roma, via Eudossiana 18, 00184 Rome, Italy.〈br /〉〈span class="detail_caption"〉Record origin:〈/span〉 〈a href="http://www.ncbi.nlm.nih.gov/pubmed/24700854" target="_blank"〉PubMed〈/a〉
    Keywords: Extraterrestrial Environment ; *Gravitation ; Ice ; *Saturn ; Spacecraft ; *Water
    Print ISSN: 0036-8075
    Electronic ISSN: 1095-9203
    Topics: Biology , Chemistry and Pharmacology , Computer Science , Medicine , Natural Sciences in General , Physics
    Published by American Association for the Advancement of Science (AAAS)
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  • 7
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    Titan's topography and shape at the end of the Cassini mission (2017)
    Wiley
    Details
    Publication Date: 2017-11-09
    Description: With the conclusion of the Cassini mission, we present an updated topographic map of Titan, including all the available altimetry, SARtopo, and stereo-photogrammetry topographic datasets available from the mission. We use radial basis functions to interpolate the sparse dataset, which covers only ∼9% of Titan's global area. The most notable updates to the topography include higher coverage of the poles of Titan, improved fits to the global shape, and a finer resolution of the global interpolation. We also present a statistical analysis of the error in the derived products and perform a global minimization on a profile-by-profile basis to account for observed biases in the input dataset. We find a greater flattening of Titan than measured, additional topographic rises in Titan's southern hemisphere, and better constrain the possible locations of past and present liquids on Titan's surface.
    Print ISSN: 0094-8276
    Electronic ISSN: 1944-8007
    Topics: Geosciences , Physics
    Published by Wiley on behalf of American Geophysical Union (AGU).
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  • 8
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    Jupiter gravity field estimated from the first two Juno orbits (2017)
    Wiley
    Details
    Publication Date: 2017-05-26
    Description: The combination of the Doppler data from the first two Juno science orbits provides an improved estimate of the gravity field of Jupiter, crucial for interior modeling of giant planets. The low-degree spherical harmonic coefficients, especially J 4 and J 6 , are determined with accuracies better than previously published by a factor of 5 or more. In addition, the independent estimates of the Jovian gravity field, obtained by the orbits separately, agree within uncertainties, pointing to a good stability of the solution. The degree 2 sectoral and tesseral coefficients, C 2,1 , S 2,1 , C 2,2 , and S 2,2 , were determined to be statistically zero as expected for a fluid planet in equilibrium.
    Print ISSN: 0094-8276
    Electronic ISSN: 1944-8007
    Topics: Geosciences , Physics
    Published by Wiley on behalf of American Geophysical Union (AGU).
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  • 9
    Electronic Resource
    Electronic Resource
    The interaction of a hollow cathode with the ionosphere (1989)
    New York, NY : American Institute of Physics (AIP)
    Physics of Fluids 1 (1989), S. 1880-1889 
    Details
    ISSN: 1089-7666
    Source: AIP Digital Archive
    Topics: Physics
    Notes: The expansion of the plasma emitted by a hollow cathode source and its interaction with an ambient ionospheric plasma are described in a spherically symmetric, stationary, fluid model, which includes anomalous collisions due to ion acoustic or Buneman instability. Currents and potential profiles are self-consistently computed for different densities and polarizations of the hollow cathode plasma. The model indicates that the region where the potential has suprathermal values is quite large and that hollow cathodes strongly enhance the charge collection of a polarized body from the ambient plasma. These results are significant to the problem of power generation via tethered systems in space.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.858921
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  • 10
    Electronic Resource
    Electronic Resource
    Hyperion’s sponge-like appearance (2007)
    [s.l.] : Nature Publishing Group
    Nature 448 (2007), S. 50-56 
    Details
    ISSN: 1476-4687
    Source: Nature Archives 1869 - 2009
    Topics: Biology , Chemistry and Pharmacology , Medicine , Natural Sciences in General , Physics
    Notes: [Auszug] Hyperion is Saturn’s largest known irregularly shaped satellite and the only moon observed to undergo chaotic rotation. Previous work has identified Hyperion’s surface as distinct from other small icy objects but left the causes unsettled. Here we report high-resolution images that ...
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1038/nature05779
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