14,127 research outputs found
The B-ring's surface mass density from hidden density waves: Less than meets the eye?
Saturn's B ring is the most opaque ring in our solar system, but many of its
fundamental parameters, including its total mass, are not well constrained.
Spiral density waves generated by mean-motion resonances with Saturn's moons
provide some of the best constraints on the rings' mass density, but detecting
and quantifying such waves in the B ring has been challenging because of this
ring's high opacity and abundant fine-scale structure. Using a wavelet-based
analyses of 17 occultations of the star gamma Crucis observed by the Visual and
Infrared Mapping Spectrometer (VIMS) onboard the Cassini spacecraft, we are
able to examine five density waves in the B ring. Two of these waves are
generated by the Janus 2:1 and Mimas 5:2 Inner Lindblad Resonances at 96,427 km
and 101,311 km from Saturn's center, respectively. Both of these waves can be
detected in individual occultation profiles, but the multi-profile wavelet
analysis reveals unexpected variations in the pattern speed of the Janus 2:1
wave that might arise from the periodic changes in Janus' orbit. The other
three wave signatures are associated with the Janus 3:2, Enceladus 3:1 and
Pandora 3:2 Inner Lindblad Resonances at 115,959 km, 115,207 km and 108,546 km.
These waves are not visible in individual profiles, but structures with the
correct pattern speeds can be detected in appropriately phase-corrected average
wavelets. Estimates of the ring's surface mass density derived from these five
waves fall between 40 and 140 g/cm^2, even though the ring's optical depth in
these regions ranges from 1.5 to almost 5. This suggests that the total mass of
the B ring is most likely between one-third and two-thirds the mass of Saturn's
moon Mimas.Comment: 40 Pages, 21 Figures, Accepted for publication in Icarus, a few typos
fixe
Low complexity method for large-scale self-consistent ab initio electronic-structure calculations without localization
A novel low complexity method to perform self-consistent electronic-structure
calculations using the Kohn-Sham formalism of density functional theory is
presented. Localization constraints are neither imposed nor required thereby
allowing direct comparison with conventional cubically scaling algorithms. The
method has, to date, the lowest complexity of any algorithm for an exact
calculation. A simple one-dimensional model system is used to thoroughly test
the numerical stability of the algorithm and results for a real physical system
are also given
Empirical studies of upper atmospheric species
The first month of spin-scan ozone imaging (SOI) data (October 1981) was processed and compared with total ozone mapping spectrometer and ground based data. Short term variations in the ozone field have been revealed using animated sequences of SOI data. High correlations were observed between SOI ozone and upper tropospheric meteorological data. The relationship between ozone and temperature in the stratosphere was investigated by examining Nimbus 4 backscattered ultraviolet ozone and selective chopper radiometer temperature measurements as well as solar Nimbus 7 solar backscattered ultraviolet ozone and stratospheric and mesospheric sounder temperature measurements. Results from these ozone temperature studies were compared with calculations from theoretical 2-D models. All of the lims infrared monitor of the stratosphere data was processed at 10 pressure levels and correlations between various species have been performed. The Venus Atmospheric Drag model was modified taking into account recent investigations on the Venus thermosphere and the resulting model was considered for incorporation into the Venus International Reference Atmosphere
The Smallest Particles in Saturn's A and C Rings
Radio occultations of Saturn's main rings by spacecraft suggest a power law
particle size-distribution down to sizes of the order of 1 cm (Marouf et al.,
1983), (Zebker et al., 1985). The lack of optical depth variations between
ultraviolet and near-IR wavelengths indicate a lack of micron-sized particles.
Between these two regimes, the particle-size distribution is largely unknown. A
cutoff where the particle-size distribution turns over must exist, but the
position and shape of it is not clear from existing studies.
Using a series of solar occultations performed by the VIMS instrument
on-board Cassini in the near-infrared, we are able to measure light forward
scattered by particles in the A and C rings. With a model of diffraction by
ring particles, and the previous radio work as a constraint on the slope of the
particle size distribution, we estimate the minimum particle size using a
truncated power-law size distribution. The C Ring shows a minimum particle size
of mm, with an assumed power law index of q=3.1 and a
maximum particle size of 10 m.
The A Ring signal shows a similar level of scattered flux, but modeling is
complicated by the presence of self-gravity wakes and higher optical depths. If
q<3, our A Ring model requires a minimum particle size below one millimeter (<
0.34 mm for an assumed q=2.75, or mm for a steeper
q=2.9) to be consistent with VIMS observations. These results might seem to
contradict previous optical(Dones et al., 1993) and infrared (French and
Nicholson, 2000) work, which implied that there were few particles in the A
Ring smaller than 1 cm. But, because of the shallow power law, relatively
little optical depth (between 0.03 and 0.16 in extinction, or 0.015 - 0.08 in
absorption) is provided by these particles.Comment: 47 pages, 16 figures, 3 Table
- …