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K K Khurana - One of the best experts on this subject based on the ideXlab platform.
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the origin of Ganymede s polar caps
Icarus, 2007Co-Authors: K K Khurana, R T Pappalardo, N W Murphy, T DenkAbstract:Since their discovery in Voyager images, the origin of the bright polar caps of Ganymede has intrigued investigators. Some models attributed the polar cap formation to thermal migration of water vapor to higher latitudes, while other models implicated plasma bombardment in brightening ice. Only with the arrival of Galileo at Jupiter was it apparent that Ganymede possesses a strong internal magnetic field, which blocks most of the plasma from bombarding the satellite's equatorial region while funneling plasma onto the polar regions. This discovery provides a plausible explanation for the polar caps as related to differences in plasma-induced brightening in the polar and the equatorial regions. In this context, we analyze global color and high resolution images of Ganymede obtained by Galileo, finding a very close correspondence between the observed polar cap boundary and the open/closed field lines boundary obtained from new modeling of the magnetic field environment. This establishes a clear link between plasma bombardment and polar cap brightening. High resolution images show that bright polar terrain is segregated into bright and dark patches, suggesting sputter-induced redistribution and subsequent cold trapping of water molecules. Minor differences between the location of the open/closed field lines boundary and the observed polar cap boundary may be due to interaction of Ganymede with Jupiter's magnetosphere, and our neglect of higher-order terms in modeling Ganymede's internal field. We postulate that leading-trailing brightness differences in Ganymede's low-latitude surface are due to enhanced plasma flux onto the leading hemisphere, rather than darkening of the trailing hemisphere. In contrast to Ganymede, the entire surface of Europa is bombarded by jovian plasma, suggesting that sputter-induced redistribution of water molecules is a viable means of brightening that satellite's surface.
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induced magnetic fields as evidence for subsurface oceans in europa and callisto
Nature, 1998Co-Authors: K K Khurana, David J Stevenson, M G Kivelson, C T Russell, G Schubert, R J Walker, C A PolanskeyAbstract:The Galileo spacecraft has been orbiting Jupiter since 7 December 1995, and encounters one of the four galilean satellites-Io, Europa, Ganymede and Callisto-on each orbit. Initial results from the spacecraft's magnetometer have indicated that neither Europa nor Callisto have an appreciable internal magnetic field, in contrast to Ganymede and possibly Io. Here we report perturbations of the external magnetic fields (associated with Jupiter's inner magnetosphere) in the vicinity of both Europa and Callisto. We interpret these perturbations as arising from induced magnetic fields, generated by the moons in response to the periodically varying plasma environment. Electromagnetic induction requires eddy currents to flow within the moons, and our calculations show that the most probable explanation is that there are layers of significant electrical conductivity just beneath the surfaces of both moons. We argue that these conducting layers may best be explained by the presence of salty liquid-water oceans, for which there is already indirect geological evidence in the case of Europa.
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absence of an internal magnetic field at callisto
Nature, 1997Co-Authors: K K Khurana, M G Kivelson, C T Russell, Raymond J Walker, D J SouthwoodAbstract:Little is known about the internal properties of Callisto—the outermost of Jupiter's four large galilean moons—other than the average density (about 1.8gem-3). The recent unexpected discovery1–4 that Ganymede, and perhaps Io, has an internally generated magnetic field, combined with gravity results5,6 suggesting that both Ganymede and Io are internally differentiated with metallic cores and rocky mantles, has heightened anticipation of the results obtained by the Galileo spacecraft in its recent fly-by of Callisto. Here we report that the spacecraft, passing the moon at a distance of only ∼1,100 km from the surface, detected only a small enhancement of the field strength (∼7nT), which maybe related to changes in the jovian plasma environment caused by Callisto7. Callisto does not have an internally generated magnetic field.
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discovery of Ganymede s magnetic field by the galileo spacecraft
Nature, 1996Co-Authors: M G Kivelson, K K Khurana, C T Russell, D J Southwood, R J Walker, C A Polanskey, F. V. Coroniti, J Warnecke, G SchubertAbstract:THE Galileo spacecraft has now passed close to Jupiter's largest moon—Ganymede—on two occasions, the first at an altitude of 838 km, and the second at an altitude of just 264 km. Here we report the discovery during these encounters of an internal magnetic field associated with Ganymede (the only other solid bodies in the Solar System known to have magnetic fields are Mercury, Earth and probably lo1). The data are consistent with a Ganymede-centred magnetic dipole tilted by ∼10° relative to the spin axis, and an equatorial surface-field strength of ∼750 nT. The magnetic field is strong enough to carve out a magnetosphere with clearly defined boundaries within Jupiter's magnetosphere. Although the observations require an internal field, they do not indicate its source. But the existence of an internal magnetic field should in itself help constrain models of Ganymede's interior.
G Schubert - One of the best experts on this subject based on the ideXlab platform.
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the tidal response of Ganymede and callisto with and without liquid water oceans
Icarus, 2003Co-Authors: W B Moore, G SchubertAbstract:Calculations of the tidal responses of Ganymede and Callisto reveal that tidal amplitudes on these bodies may be as large as a few meters if a liquid ocean exists to decouple the surface ice from the interior. Tides on Ganymede's surface can exceed 7 m peak-to-peak variation, while on Callisto the tidal amplitude can exceed 5 m in the presence of a liquid ocean. Without an ocean, tidal amplitudes are less than 0.5 m on Ganymede and less than 0.3 m on Callisto. An orbiting spacecraft using an altimeter for crossover analysis and Doppler tracking from Earth should be able to achieve sufficient accuracy to identify the tidal amplitude to within about a meter over the course of a few months (observing tens of tidal cycles).
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induced magnetic fields as evidence for subsurface oceans in europa and callisto
Nature, 1998Co-Authors: K K Khurana, David J Stevenson, M G Kivelson, C T Russell, G Schubert, R J Walker, C A PolanskeyAbstract:The Galileo spacecraft has been orbiting Jupiter since 7 December 1995, and encounters one of the four galilean satellites-Io, Europa, Ganymede and Callisto-on each orbit. Initial results from the spacecraft's magnetometer have indicated that neither Europa nor Callisto have an appreciable internal magnetic field, in contrast to Ganymede and possibly Io. Here we report perturbations of the external magnetic fields (associated with Jupiter's inner magnetosphere) in the vicinity of both Europa and Callisto. We interpret these perturbations as arising from induced magnetic fields, generated by the moons in response to the periodically varying plasma environment. Electromagnetic induction requires eddy currents to flow within the moons, and our calculations show that the most probable explanation is that there are layers of significant electrical conductivity just beneath the surfaces of both moons. We argue that these conducting layers may best be explained by the presence of salty liquid-water oceans, for which there is already indirect geological evidence in the case of Europa.
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gravitational evidence for an undifferentiated callisto
Nature, 1997Co-Authors: John D Anderson, G Schubert, Eunice L Lau, W L Sjogren, W B MooreAbstract:Before the arrival of the Galileo spacecraft at Jupiter, models for the interior structure of the four galilean satellites-lo, Europa, Ganymede and Callisto-ranged from uniform mixtures of rock and ice (that is, undifferentiated objects) or rocky cores surrounded by a mantle of water ice. Now it appears that lo has a large metallic core and that Ganymede is strongly differentiated, most probably into a three-layer structure consisting of a metallic core, a silicate mantle and a deep outer layer of ice. Direct information on the interior structure of Callisto determined from previous spacecraft fly-bys was essentially limited to an estimate of the mean density being intermediate between pure ice and pure rock. Here we report measurements of Callisto's gravitational field which reveal that, in contrast to lo and Ganymede, this galilean satellite is most probably a homogeneous object consisting of a solar mixture of 40% compressed ice and 60% rock (including iron and iron sulphide). Callisto's undifferentiated state is consistent with the apparent lack of an intrinsic magnetic field, and indicates that the outermost galilean satellite has not experienced a heating phase sufficiently high to separate its rock and metal components from the lighter ices.
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discovery of Ganymede s magnetic field by the galileo spacecraft
Nature, 1996Co-Authors: M G Kivelson, K K Khurana, C T Russell, D J Southwood, R J Walker, C A Polanskey, F. V. Coroniti, J Warnecke, G SchubertAbstract:THE Galileo spacecraft has now passed close to Jupiter's largest moon—Ganymede—on two occasions, the first at an altitude of 838 km, and the second at an altitude of just 264 km. Here we report the discovery during these encounters of an internal magnetic field associated with Ganymede (the only other solid bodies in the Solar System known to have magnetic fields are Mercury, Earth and probably lo1). The data are consistent with a Ganymede-centred magnetic dipole tilted by ∼10° relative to the spin axis, and an equatorial surface-field strength of ∼750 nT. The magnetic field is strong enough to carve out a magnetosphere with clearly defined boundaries within Jupiter's magnetosphere. Although the observations require an internal field, they do not indicate its source. But the existence of an internal magnetic field should in itself help constrain models of Ganymede's interior.
D J Southwood - One of the best experts on this subject based on the ideXlab platform.
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the magnetic fields of the galilean moons of jupiter the galileo spacecraft magnetometer results
The Three Galileos the Man the Spacecraft the Telescope, 1998Co-Authors: D J Southwood, M G KivelsonAbstract:We review magnetic field measurements from flybys of the Galilean moons. Large or at least detectable magnetic perturbations during flybys were expected. Intrinsic fields were not. Ganymede has an intrinsic field, as probably do Io and Europa, constituting a major discovery in planetary science.
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absence of an internal magnetic field at callisto
Nature, 1997Co-Authors: K K Khurana, M G Kivelson, C T Russell, Raymond J Walker, D J SouthwoodAbstract:Little is known about the internal properties of Callisto—the outermost of Jupiter's four large galilean moons—other than the average density (about 1.8gem-3). The recent unexpected discovery1–4 that Ganymede, and perhaps Io, has an internally generated magnetic field, combined with gravity results5,6 suggesting that both Ganymede and Io are internally differentiated with metallic cores and rocky mantles, has heightened anticipation of the results obtained by the Galileo spacecraft in its recent fly-by of Callisto. Here we report that the spacecraft, passing the moon at a distance of only ∼1,100 km from the surface, detected only a small enhancement of the field strength (∼7nT), which maybe related to changes in the jovian plasma environment caused by Callisto7. Callisto does not have an internally generated magnetic field.
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discovery of Ganymede s magnetic field by the galileo spacecraft
Nature, 1996Co-Authors: M G Kivelson, K K Khurana, C T Russell, D J Southwood, R J Walker, C A Polanskey, F. V. Coroniti, J Warnecke, G SchubertAbstract:THE Galileo spacecraft has now passed close to Jupiter's largest moon—Ganymede—on two occasions, the first at an altitude of 838 km, and the second at an altitude of just 264 km. Here we report the discovery during these encounters of an internal magnetic field associated with Ganymede (the only other solid bodies in the Solar System known to have magnetic fields are Mercury, Earth and probably lo1). The data are consistent with a Ganymede-centred magnetic dipole tilted by ∼10° relative to the spin axis, and an equatorial surface-field strength of ∼750 nT. The magnetic field is strong enough to carve out a magnetosphere with clearly defined boundaries within Jupiter's magnetosphere. Although the observations require an internal field, they do not indicate its source. But the existence of an internal magnetic field should in itself help constrain models of Ganymede's interior.
John R. Spencer - One of the best experts on this subject based on the ideXlab platform.
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condensed o2 on europa and callisto
The Astronomical Journal, 2002Co-Authors: John R. Spencer, Wendy M CalvinAbstract:High signal-to-noise ratio spectra of Europa and Callisto's trailing side show a 0.3% deep 5771 A absorption band due to condensed O2 at the same wavelength as a stronger band previously identified on Ganymede. Excellent longitudinal coverage for Europa shows that unlike Ganymede, where the band is much stronger on the trailing side, Europa shows no significant longitudinal variation in the O2 band strength.
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mass movement and landform degradation on the icy galilean satellites results of the galileo nominal mission
Icarus, 1999Co-Authors: Jeffrey M Moore, John R. Spencer, Erik Asphaug, David Morrison, C R Chapman, B Bierhaus, R Sullivan, Frank C Chuang, J E Klemaszewski, R GreeleyAbstract:The Galileo mission has revealed remarkable evidence of mass movement and landform degradation on the icy Galilean satellites of Jupiter. Weakening of surface materials coupled with mass movement reduces the topographic relief of landforms by moving surface materials down-slope. Throughout the Galileo orbiter nominal mission we have studied all known forms of mass movement and landform degradation of the icy galilean satellites, of which Callisto, by far, displays the most degraded surface. Callisto exhibits discrete mass movements that are larger and apparently more common than seen elsewhere. Most degradation on Ganymede appears consistent with sliding or slumping, impact erosion, and regolith evolution. Sliding or slumping is also observed at very small (100 m) scale on Europa. Sputter ablation, while probably playing some role in the evolution of Ganymede's and Callisto's debris layers, appears to be less important than other processes. Sputter ablation might play a significant role on Europa only if that satellite's surface is significantly older than 108 years, far older than crater statistics indicate. Impact erosion and regolith formation on Europa are probably minimal, as implied by the low density of small craters there. Impact erosion and regolith formation may be important on the dark terrains of Ganymede, though some surfaces on this satellite may be modified by sublimation–degradation. While impact erosion and regolith formation are expected to operate with the same vigor on Callisto as on Ganymede, most of the areas examined at high resolution on Callisto have an appearance that implies that some additional process is at work, most likely sublimation-driven landform modification and mass wasting. The extent of surface degradation ascribed to sublimation on the outer two Galilean satellites implies that an ice more volatile than H2O is probably involved.
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Galileo Photopolarimeter-Radiometer Observations of Jupiter and the Galilean Satellites
Science, 1996Co-Authors: Glenn S. Orton, John R. Spencer, L. Travis, Terry Z. Martin, Leslie K. TamppariAbstract:Photopolarimeter-Radiometer (PPR) maps of daytime temperatures on Ganymede at a resolution of 220 kilometers show the expected anticorrelation with albedo, but morning temperatures were about 10 kelvin warmer than expected. Europa had a subsolar temperature of 128 kelvin and a lower effective thermal inertia than either Ganymede or Callisto, and Io9s night side was cooler than predicted by recent models, perhaps requiring revision of heat-flow estimates. The lowest 250-millibar temperatures in the Great Red Spot (GRS) generally corresponded to the visually darkest regions. Temperatures remained cold north of the GRS, but they rose by as much as 6 kelvin to the south over the 2800-kilometer PPR resolution. A visually bright region northwest of the GRS was also relatively cold. It is likely that NH3 clouds affected the determination of the 500-millibar temperature field, which appears qualitatively different.
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observing the icy jovian satellites with the galileo photopolarimeter radiometer instrument
Journal of Geophysical Research, 1995Co-Authors: Leslie K. Tamppari, John R. Spencer, T Z MartinAbstract:The photopolarimeter/radiometer (PPR) instrument aboard the Galileo spacecraft will go into orbit around Jupiter in December 1995. The 23-month tour offers PPR four Ganymede encounters, three Callisto encounters, and three Europa encounters with maximum PPR resolution varying from 0.5 km to 8 km and typical resolution of 200 km. In addition, there will be one Ganymede, one Callisto, and two Europa “nontargeted” encounters, giving maximum PPR resolution from 58 km to 200 km, with a typical resolution of 300 km. There is a single Io encounter before Jupiter orbit insertion that will provide resolutions ranging from 400 km to 2.5 km, and numerous subsequent opportunities to observe Io with resolution as good as 600 km. The PPR will be used to study the polarization of reflected sunlight from each satellite over a wide range of phase angles. It will also map daytime and nighttime surface temperatures to look for spatial variations in thermophysical properties, study volcanic activity on Io, and look for possible endogenic thermal activity on Europa. These observation plans are presented.
M G Kivelson - One of the best experts on this subject based on the ideXlab platform.
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induced magnetic fields as evidence for subsurface oceans in europa and callisto
Nature, 1998Co-Authors: K K Khurana, David J Stevenson, M G Kivelson, C T Russell, G Schubert, R J Walker, C A PolanskeyAbstract:The Galileo spacecraft has been orbiting Jupiter since 7 December 1995, and encounters one of the four galilean satellites-Io, Europa, Ganymede and Callisto-on each orbit. Initial results from the spacecraft's magnetometer have indicated that neither Europa nor Callisto have an appreciable internal magnetic field, in contrast to Ganymede and possibly Io. Here we report perturbations of the external magnetic fields (associated with Jupiter's inner magnetosphere) in the vicinity of both Europa and Callisto. We interpret these perturbations as arising from induced magnetic fields, generated by the moons in response to the periodically varying plasma environment. Electromagnetic induction requires eddy currents to flow within the moons, and our calculations show that the most probable explanation is that there are layers of significant electrical conductivity just beneath the surfaces of both moons. We argue that these conducting layers may best be explained by the presence of salty liquid-water oceans, for which there is already indirect geological evidence in the case of Europa.
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the magnetic fields of the galilean moons of jupiter the galileo spacecraft magnetometer results
The Three Galileos the Man the Spacecraft the Telescope, 1998Co-Authors: D J Southwood, M G KivelsonAbstract:We review magnetic field measurements from flybys of the Galilean moons. Large or at least detectable magnetic perturbations during flybys were expected. Intrinsic fields were not. Ganymede has an intrinsic field, as probably do Io and Europa, constituting a major discovery in planetary science.
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absence of an internal magnetic field at callisto
Nature, 1997Co-Authors: K K Khurana, M G Kivelson, C T Russell, Raymond J Walker, D J SouthwoodAbstract:Little is known about the internal properties of Callisto—the outermost of Jupiter's four large galilean moons—other than the average density (about 1.8gem-3). The recent unexpected discovery1–4 that Ganymede, and perhaps Io, has an internally generated magnetic field, combined with gravity results5,6 suggesting that both Ganymede and Io are internally differentiated with metallic cores and rocky mantles, has heightened anticipation of the results obtained by the Galileo spacecraft in its recent fly-by of Callisto. Here we report that the spacecraft, passing the moon at a distance of only ∼1,100 km from the surface, detected only a small enhancement of the field strength (∼7nT), which maybe related to changes in the jovian plasma environment caused by Callisto7. Callisto does not have an internally generated magnetic field.
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discovery of Ganymede s magnetic field by the galileo spacecraft
Nature, 1996Co-Authors: M G Kivelson, K K Khurana, C T Russell, D J Southwood, R J Walker, C A Polanskey, F. V. Coroniti, J Warnecke, G SchubertAbstract:THE Galileo spacecraft has now passed close to Jupiter's largest moon—Ganymede—on two occasions, the first at an altitude of 838 km, and the second at an altitude of just 264 km. Here we report the discovery during these encounters of an internal magnetic field associated with Ganymede (the only other solid bodies in the Solar System known to have magnetic fields are Mercury, Earth and probably lo1). The data are consistent with a Ganymede-centred magnetic dipole tilted by ∼10° relative to the spin axis, and an equatorial surface-field strength of ∼750 nT. The magnetic field is strong enough to carve out a magnetosphere with clearly defined boundaries within Jupiter's magnetosphere. Although the observations require an internal field, they do not indicate its source. But the existence of an internal magnetic field should in itself help constrain models of Ganymede's interior.
