Publications (10)0 Total impact
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Bruno Christophe,
Linda J. Spilker,
John D. Anderson,
Nicolas André,
Sami W. Asmar,
Jonathan Aurnou, Don Banfield,
Antonella Barucci,
Orfeu Bertolami,
Robert Bingham, [......],
Joachim Saur,
Kunio M. Sayanagi,
Nicole Schmitz,
Hanns Selig,
Frank Sohl,
Thomas R. Spilker,
Ralf Srama,
Katrin Stephan,
Pierre Touboul,
Peter Wolf
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ABSTRACT: The present OSS mission continues a long and bright tradition by associating
the communities of fundamental physics and planetary sciences in a single
mission with ambitious goals in both domains. OSS is an M-class mission to
explore the Neptune system almost half a century after flyby of the Voyager 2
spacecraft. Several discoveries were made by Voyager 2, including the Great
Dark Spot (which has now disappeared) and Triton's geysers. Voyager 2 revealed
the dynamics of Neptune's atmosphere and found four rings and evidence of ring
arcs above Neptune. Benefiting from a greatly improved instrumentation, it will
result in a striking advance in the study of the farthest planet of the Solar
System. Furthermore, OSS will provide a unique opportunity to visit a selected
Kuiper Belt object subsequent to the passage of the Neptunian system. It will
consolidate the hypothesis of the origin of Triton as a KBO captured by
Neptune, and improve our knowledge on the formation of the Solar system. The
probe will embark instruments allowing precise tracking of the probe during
cruise. It allows to perform the best controlled experiment for testing, in
deep space, the General Relativity, on which is based all the models of Solar
system formation. OSS is proposed as an international cooperation between ESA
and NASA, giving the capability for ESA to launch an M-class mission towards
the farthest planet of the Solar system, and to a Kuiper Belt object. The
proposed mission profile would allow to deliver a 500 kg class spacecraft. The
design of the probe is mainly constrained by the deep space gravity test in
order to minimise the perturbation of the accelerometer measurement.
06/2011;
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ABSTRACT: A steady-state scheme for data assimilation in the context of a single, short period (relative to a day), sun-synchronous, polar-orbiting satellite is examined. If the satellite takes observations continuously, the gains, which are the weights for blending observations and predictions together, are steady in time. For a linear system forced by random noise, the optimal steady-state gains (Wiener gains) are equivalent to those of a Kalman filter. Computing the Kalman gains increases the computational cost of the model by a large factor, but computing the Wiener gains does not. The latter are computed by iteration using prior estimates of the gains to assimilate simulated observations of one run of the model, termed 'truth' into another run termed 'prediction'. At each stage, the prediction errors form the basis for the next estimate of the gains. Steady state is achieved after three or four iterations. Further simplification is achieved by making the gains depend on longitudinal distance from the observation point, not on absolute longitude. For a single-layer primitive equation model, the scheme works well even if only the mass field is observed but not the velocity field. Although the scheme was developed for Mars Observer, it should be applicable to data retrieved from Earth atmosphere satellites, for example, UARS.
04/1995;
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ABSTRACT: Gains are the spatial weighting of an observation in its neighborhood versus the local values of a model prediction. They are the key to data assimilation, as they are the direct measure of how the data are used to guide the model. As derived in the broad context of data assimilation by Kalman and in the context of meteorology, for example, by Rutherford, the optimal gains are functions of the prediction error covariances between the observation and analysis points. Kalman introduced a very powerful technique that allows one to calculate these optimal gains at the time of each observation. Unfortunately, this technique is both computationally expensive and often numerically unstable for dynamical systems of the magnitude of meteorological models, and thus is unsuited for use in PMIRR data assimilation. However, the optimal gains as calculated by a Kalman filter do reach a steady state for regular observing patterns like that of a satellite. In this steady state, the gains are constants in time, and thus could conceivably be computed off-line. These steady-state Kalman gains (i.e., Wiener gains) would yield optimal performance without the computational burden of true Kalman filtering. We proposed to use this type of constant-in-time Wiener gain for the assimilation of data from PMIRR and Mars Observer.
02/1993;
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ABSTRACT: The strong jet, with a speed between 500 and 600 m/s, is inferred in the equatorial region of Saturn by combining the nadir and limb observations of Composite Infrared Spectrometer (CIRS) aboard the Cassini spacecraft. A similar jet was discovered on Jupiter (F. M. Flasar et al., 2004a). These discoveries raise the possibility that intense jets are common in the equatorial stratospheres of giant planets. An equatorial wave with wavenumber ~9 is revealed in the stratosphere of Saturn by the CIRS high spatial-resolution thermal maps. Our discussion based on the phase velocity suggests that the equatorial wave is probably a Rossby-gravity wave. The discovery of an equatorial wave in the stratosphere suggests that Saturn's equatorial oscillations (T. Fouchet et al., 2008; G. S. Orton et al., 2008) may be driven by vertically propagating waves, the same mechanism that drives the quasi-biennial oscillation (QBO) on Earth.
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ABSTRACT: Photometrically calibrated grism spectra of Saturn in the H-(1.45–1.80 μm) and K-band (1.95–2.50 μm) are presented. The spectra were obtained with the 200-inch Hale telescope at Palomar mountain three days after the ring plane crossing of August 10, 1995. By inversion of the spectra, the vertical distribution of the scattering density is obtained as a function of latitude along the central meridian, for pressures ranging from about 10 to 600 mbar. At all latitudes, we find a vertical structure consisting of a stratospheric haze layer and a more dense upper tropospheric haze layer, with density minima both above and below the tropospheric layer. At northern midlatitudes, the upper tropospheric haze is located deeper into the atmosphere than at similar southern midlatitudes. This hemispherical asymmetry can be explained by seasonal influences. The largest scattering densities in the upper tropospheric haze layer are found at tropical latitudes, between about −10° and +15°.
Icarus.
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ABSTRACT: Photometrically calibrated grism spectra of Jupiter in the H (1.45–1.8 μm) and K (1.95–2.5 μm) bands with a resolution of about 100 were taken with the 5-m Hale telescope at Palomar in August of 1995. The spectra were obtained as meridional cuts at six different longitudes on the planet, with one cut crossing the Great Red Spot.The technique outlined in D. Banfieldet al.(1996,Icarus121, 389–410) for the retrieval of scatterer density with altitude from near-infrared spectra is used and refined. It is expected that this retrieval technique will find use in the interpretation of many atmospheric near-infrared reflection spectra, especially those from Galileo NIMS and Cassini VIMS. For the wavelengths and spectral resolution used in this study, the sensitivity of the inversions extends from pressure levels near 400 mbar up to about 20 mbar. Employing this inversion technique on the spectra yields well-resolved jovian cloud densities for τ ≲ 0.1, as a function of latitude and altitude.The density of scatterers is minimum at a height where the pressure is about 100 mbar and increases both upward and downward from this level. The minimum near 100 mbar can be explained by coagulation of settling particles, leading to an increase in fall speed. The results indicate that stratospheric haze particles are generated at heights wherep≲ 20 mbar.
Icarus.
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ABSTRACT: A principal components analysis was performed on Hubble Space Telescope WFPC2 images of Jupiter from October 1995 and October 1996. Global maps in the F410M, F555W, F673N, and F953N filters were analyzed. These are continuum wavelengths in Jupiter's spectrum, sensitive to reflection from the visible cloud deck. The primary principal components correspond approximately to gray spectral brightness variations, accounting for ∼91% of the variance in the images, and a component with a red spectral slope, which accounts for ∼8% of the variance. This color component probably corresponds to a constituent in the ammonia cloud deck. Another color component, which is blue/green, may correspond to upper tropospheric clouds or haze and accounts for ∼1% of the image variance. Although small, this component may explain differences seen in the color of the Great Red Spot and other features.
Icarus.
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Don Banfield
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ABSTRACT: We examine a scenario involving the capture origin of Triton, and infer the dynamical history of the Neptune satellite system. Triton's post-capture orbit forced chaotic perturbations on the original inner satellites of Neptune, leading to their mutual collisions and self-destruction. Neptune's current inner satellite system re-formed equatorially after Triton's orbital circularization. The 4.7° inclination of 1989N6 is probably due to a temporary inclination resonance. The 2:1 secondary resonance of the 1989N6-1989N3 12:10 resonance would eject 1989N6 at 4.7°, matching the observations. We have established limits for Neptune's Q: 12,000 < QN < 330,000. We examine a steady-state scheme for data assimilation in the context of a single, sun-synchronous, polar-orbiting satellite. The optimal (Wiener) gains are steady in time, and equivalent to those of a Kalman filter. The gains are computed by iteration using prior estimates to assimilate simulated observations of one model run ('Truth') into another run. The resulting prediction errors then form the next estimate of the gains. In model tests, the scheme works well even if only the mass field is observed. Although the scheme was developed for Mars Observer, it should be applicable to data retrieved from Earth atmosphere satellites, e.g., UARS. Spring and fall Viking IRTM T15 observations are used to estimate the Martian weather correlation length scale in the range 0.5-1 mbar. The results are important in providing a benchmark for validating Martian GCMs, determining the optimal placement of a network of landers, and guiding data assimilation efforts. Atmospheric temperature observations are used to compute an atmospheric mean state, which is subtracted from the observations to yield weather temperature residuals. These residuals are correlated with each other to determine the weather temperature correlation length scale (~ 1500km) and the weather temperature variance (~ 4-11K2). This work suggests that ~110 landers are needed to globally observe Mars' weather.
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Liming Li,
Barney J. Conrath,
Peter J. Gierasch,
Richard K. Achterberg,
Conor A Nixon,
Amy A. Simon-Miller,
F. Michael Flasar, Don Banfield,
Kevin H. Baines,
Robert A West, [......],
Ashwin R. Vasavada,
Anthony D Del Genio,
Carolyn C. Porco,
Andrei A. Mamoutkine,
Marcia E. Segura,
Gordon L. Bjoraker,
Glenn S. Orton,
Leigh N. Fletcher,
Patrick G. J. Irwin,
Peter L. Read
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ABSTRACT: 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.
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ABSTRACT: 1] Analysis of temperature retrievals from Mars Global Surveyor Thermal Emission Spectrometer data has revealed the presence of regular, eastward propagating waves in the Northern Hemisphere. A large amplitude, zonal wave 1 with a long ($20 sol) period is particularly prominent during early winter (L s = 220– 270°). After L s = 270°, a weaker and more rapidly propagating (6.5 sol period) zonal wave 1 is dominant. These waves have a deep vertical structure (>40 km) correlated with the axis of the winter hemisphere westerly jet. Simulations with a Mars general circulation model suggest that the fast wave is associated with baroclinic instability due to the strong meridional temperature gradient at the surface and is consistent with surface pressure oscillations seen in Viking Lander data. By contrast, the slow wave has the appearance of a large-amplitude Rossby wave that is coupled with an inertially unstable region in the subtropics.
