471 research outputs found
The Christiansen Effect in Saturn's narrow dusty rings and the spectral identification of clumps in the F ring
Stellar occultations by Saturn's rings observed with the Visual and Infrared
Mapping Spectrometer (VIMS) onboard the Cassini spacecraft reveal that dusty
features such as the F ring and the ringlets in the Encke and the Laplace Gaps
have distinctive infrared transmission spectra. These spectra show a narrow
optical depth minimum at wavelengths around 2.87 microns. This minimum is
likely due to the Christiansen Effect, a reduction in the extinction of small
particles when their (complex) refractive index is close to that of the
surrounding medium. Simple Mie-scattering models demonstrate that the strength
of this opacity dip is sensitive to the size distribution of particles between
1 and 100 microns across. Furthermore, the spatial resolution of the
occultation data is sufficient to reveal variations in the transmission spectra
within and among these rings. For example, in both the Encke Gap ringlets and F
ring, the opacity dip weakens with increasing local optical depth, which is
consistent with the larger particles being concentrated near the cores of these
rings. The strength of the opacity dip varies most dramatically within the F
ring; certain compact regions of enhanced optical depth lack an opacity dip and
therefore appear to have a greatly reduced fraction of grains in the few-micron
size range.Such spectrally-identifiable structures probably represent a subset
of the compact optically-thick clumps observed by other Cassini instruments.
These variations in the ring's particle size distribution can provide new
insights into the processes of grain aggregation, disruption and transport
within dusty rings. For example, the unusual spectral properties of the F-ring
clumps could perhaps be ascribed to small grains adhering onto the surface of
larger particles in regions of anomalously low velocity dispersion.Comment: 42 pages, 15 figures, accepted for publication in Icarus. A few small
typographical errors fixed to match correction in proof
The weather report from IRC+10216: Evolving irregular clouds envelop carbon star
High angular resolution images of IRC+10216 are presented in several near-infrared wavelengths spanning more than 8 years. These maps have been reconstructed from interferometric observations obtained at both Keck and the VLT, and also from stellar occultations by the rings of Saturn observed with the Cassini spacecraft. The dynamic inner regions of the circumstellar environment are monitored over eight epochs ranging between 2000 January and 2008 July. The system is shown to experience substantial evolution within this period including the fading of many previously reported persistent features, some of which had been identified as the stellar photosphere. These changes are discussed in the context of existing models for the nature of the underlying star and the circumstellar environment. With access to these new images, we are able to report that none of the previously identified bright spots in fact contains the star, which is buried in its own dust and not directly visible in the near-infrared
Cassini VIMS and Altimeter Joint Study of Titan Surface
The joint NASA-ESA-ASI Cassini- Huygens mission reached the saturnian system on July 1st 2004. It started the observations of Saturn's environment including its atmosphere, rings, and satellites (Phoebe, Iapetus and Titan). Titan, one of the primary scientific interests of the mission, is veiled by an ubiquitous thick haze [1]. Its surface is unreachable to ultraviolet and visible wavelengths, but can be seen in some infrared atmospheric windows and for greater wavelengths, in the case of an unclouded low atmosphere [2,3]. Onboard the Cassini spacecraft, the VIMS (Visual and Infrared Mapping Spectrometer) instrument has already proved to be able to successfully pierce the veil of the hazy moon and image its surface in the infrared wavelengths, taking hyperspectral images in the range 0.4 to 5.2 ?m. Since July 2004, VIMS acquired image cubes with spatial resolution ranging from a few tens of kilometers down to less than one kilometer per pixel, demonstrating its capability for mapping more than 70% of Titan's surface and studying its composition and geology [4,5,6,7,8,9,10,11]. Also in the Cassini orbiter payload is the Ku-band RADAR experiment that can operate in altimeter mode. Exclusively dedicated to Titan's observations, this second active mode has been designed primarily to retrieve Titan's surface elevation and study its topography. We present here the comparative analysis of the altimeter track recorded during the first Titan flyby (26 October 2004, tagged TA) and VIMS images over the same regions acquired during the 13th flyby (30 April 2006). In particular, we present here the first nontopographic analysis of Cassini altimeter data along with a tentative correlation with VIMS observations
Cassini VIMS and Altimeter Joint Study of Titan Surface
The joint NASA-ESA-ASI Cassini- Huygens mission reached the saturnian system on July 1st 2004. It started the observations of Saturn's environment including its atmosphere, rings, and satellites (Phoebe, Iapetus and Titan). Titan, one of the primary scientific interests of the mission, is veiled by an ubiquitous thick haze [1]. Its surface is unreachable to ultraviolet and visible wavelengths, but can be seen in some infrared atmospheric windows and for greater wavelengths, in the case of an unclouded low atmosphere [2,3]. Onboard the Cassini spacecraft, the VIMS (Visual and Infrared Mapping Spectrometer) instrument has already proved to be able to successfully pierce the veil of the hazy moon and image its surface in the infrared wavelengths, taking hyperspectral images in the range 0.4 to 5.2 ?m. Since July 2004, VIMS acquired image cubes with spatial resolution ranging from a few tens of kilometers down to less than one kilometer per pixel, demonstrating its capability for mapping more than 70% of Titan's surface and studying its composition and geology [4,5,6,7,8,9,10,11]. Also in the Cassini orbiter payload is the Ku-band RADAR experiment that can operate in altimeter mode. Exclusively dedicated to Titan's observations, this second active mode has been designed primarily to retrieve Titan's surface elevation and study its topography. We present here the comparative analysis of the altimeter track recorded during the first Titan flyby (26 October 2004, tagged TA) and VIMS images over the same regions acquired during the 13th flyby (30 April 2006). In particular, we present here the first nontopographic analysis of Cassini altimeter data along with a tentative correlation with VIMS observations
Geomorphological significance of Ontario Lacus on Titan: Integrated interpretation of Cassini VIMS, ISS and RADAR data and comparison with the Etosha Pan (Namibia)
International audienceOntario Lacus is the largest lake of the whole southern hemisphere of Titan, Saturn's major moon. It has been imaged twice by each of the Cassini imaging systems (Imaging Science Subsystem (ISS) in 2004 and 2005, Visual and Infrared Mapping Spectrometer (VIMS) in 2007 and 2009 and Radar in 2009 and 2010). In this study, we take advantage of each imaging dataset to establish a global survey of Ontario Lacus' environment from 2005 to 2010. We perform a geomorphological mapping and interpretation of Ontario Lacus, mainly based on a joint analysis of VIMS and Radar SAR datasets, along with the T49 altimetric profile acquired in December 2008. The morphologies observed on Ontario Lacus are compared to landforms of a semi-arid terrestrial analog, which closely resembles Titan's lakes: the pans of the Etosha Basin, located in Namibia. From this comparison, we infer that Ontario Lacus is an extremely flat depression where liquids, only located in the darkest areas in the Radar data, cover topographic lows where the "alkanofer" would raise above the depression floor. The rest of the depression appears rather as a muddy flat surface likely composed of a thick coating of photon-absorbing materials, explaining its still rather dark appearance in the infrared and radar data. We also determined whether surface changes occurred during the 5 years time interval between 2005 and 2010. We found that the depression contour is constant at the resolution of ISS and VIMS data, both being consistent with the depression contour derived from the Radar data. Our interpretation, in which the liquids are located only in some parts of Ontario Lacus, agrees with the lack of significant change of the depression contour between 2007 (and 2005 with more uncertainties) and 2010
Saturn's icy satellites and rings investigated by Cassini - VIMS. III. Radial compositional variability
In the last few years Cassini-VIMS, the Visible and Infared Mapping
Spectrometer, returned to us a comprehensive view of the Saturn's icy
satellites and rings. After having analyzed the satellites' spectral properties
(Filacchione et al. (2007a)) and their distribution across the satellites'
hemispheres (Filacchione et al. (2010)), we proceed in this paper to
investigate the radial variability of icy satellites (principal and minor) and
main rings average spectral properties. This analysis is done by using 2,264
disk-integrated observations of the satellites and a 12x700 pixels-wide rings
radial mosaic acquired with a spatial resolution of about 125 km/pixel. The
comparative analysis of these data allows us to retrieve the amount of both
water ice and red contaminant materials distributed across Saturn's system and
the typical surface regolith grain sizes. These measurements highlight very
striking differences in the population here analyzed, which vary from the
almost uncontaminated and water ice-rich surfaces of Enceladus and Calypso to
the metal/organic-rich and red surfaces of Iapetus' leading hemisphere and
Phoebe. Rings spectra appear more red than the icy satellites in the visible
range but show more intense 1.5-2.0 micron band depths. The correlations among
spectral slopes, band depths, visual albedo and phase permit us to cluster the
saturnian population in different spectral classes which are detected not only
among the principal satellites and rings but among co-orbital minor moons as
well. Finally, we have applied Hapke's theory to retrieve the best spectral
fits to Saturn's inner regular satellites using the same methodology applied
previously for Rhea data discussed in Ciarniello et al. (2011).Comment: 44 pages, 27 figures, 7 tables. Submitted to Icaru
Occultation observations of Saturn's rings with Cassini VIMS
We describe the prediction, design, execution and calibration of stellar and solar occultation observations of Saturn's rings by the Visual and Infrared Mapping Spectrometer (VIMS) instrument on the Cassini spacecraft. Particular attention is paid to the technique developed for onboard acquisition of the stellar target and to the geometric and photometric calibration of the data. Examples of both stellar and solar occultation data are presented, highlighting several aspects of the data as well as the different occultation geometries encountered during Cassini's 13-year orbital tour. Complete catalogs of ring stellar and solar occultations observed by Cassini-VIMS are presented, as a guide to the standard data sets which have been delivered to the Planetary Data System's Ring Moon Systems Node (Hedman and Nicholson, 2019b)
BINGO: A code for the efficient computation of the scalar bi-spectrum
We present a new and accurate Fortran code, the BI-spectra and
Non-Gaussianity Operator (BINGO), for the efficient numerical computation of
the scalar bi-spectrum and the non-Gaussianity parameter f_{NL} in single field
inflationary models involving the canonical scalar field. The code can
calculate all the different contributions to the bi-spectrum and the parameter
f_{NL} for an arbitrary triangular configuration of the wavevectors. Focusing
firstly on the equilateral limit, we illustrate the accuracy of BINGO by
comparing the results from the code with the spectral dependence of the
bi-spectrum expected in power law inflation. Then, considering an arbitrary
triangular configuration, we contrast the numerical results with the analytical
expression available in the slow roll limit, for, say, the case of the
conventional quadratic potential. Considering a non-trivial scenario involving
deviations from slow roll, we compare the results from the code with the
analytical results that have recently been obtained in the case of the
Starobinsky model in the equilateral limit. As an immediate application, we
utilize BINGO to examine of the power of the non-Gaussianity parameter f_{NL}
to discriminate between various inflationary models that admit departures from
slow roll and lead to similar features in the scalar power spectrum. We close
with a summary and discussion on the implications of the results we obtain.Comment: v1: 5 pages, 5 figures; v2: 35 pages, 11 figures, title changed,
extensively revised; v3: 36 pages, 11 figures, to appear in JCAP. The BINGO
code is available online at
http://www.physics.iitm.ac.in/~sriram/bingo/bingo.htm
Origin and Evolution of Saturn's Ring System
The origin and long-term evolution of Saturn's rings is still an unsolved
problem in modern planetary science. In this chapter we review the current
state of our knowledge on this long-standing question for the main rings (A,
Cassini Division, B, C), the F Ring, and the diffuse rings (E and G). During
the Voyager era, models of evolutionary processes affecting the rings on long
time scales (erosion, viscous spreading, accretion, ballistic transport, etc.)
had suggested that Saturn's rings are not older than 100 My. In addition,
Saturn's large system of diffuse rings has been thought to be the result of
material loss from one or more of Saturn's satellites. In the Cassini era, high
spatial and spectral resolution data have allowed progress to be made on some
of these questions. Discoveries such as the ''propellers'' in the A ring, the
shape of ring-embedded moonlets, the clumps in the F Ring, and Enceladus' plume
provide new constraints on evolutionary processes in Saturn's rings. At the
same time, advances in numerical simulations over the last 20 years have opened
the way to realistic models of the rings's fine scale structure, and progress
in our understanding of the formation of the Solar System provides a
better-defined historical context in which to understand ring formation. All
these elements have important implications for the origin and long-term
evolution of Saturn's rings. They strengthen the idea that Saturn's rings are
very dynamical and rapidly evolving, while new arguments suggest that the rings
could be older than previously believed, provided that they are regularly
renewed. Key evolutionary processes, timescales and possible scenarios for the
rings's origin are reviewed in the light of tComment: Chapter 17 of the book ''Saturn After Cassini-Huygens'' Saturn from
Cassini-Huygens, Dougherty, M.K.; Esposito, L.W.; Krimigis, S.M. (Ed.) (2009)
537-57
Measurement of the B0-anti-B0-Oscillation Frequency with Inclusive Dilepton Events
The - oscillation frequency has been measured with a sample of
23 million \B\bar B pairs collected with the BABAR detector at the PEP-II
asymmetric B Factory at SLAC. In this sample, we select events in which both B
mesons decay semileptonically and use the charge of the leptons to identify the
flavor of each B meson. A simultaneous fit to the decay time difference
distributions for opposite- and same-sign dilepton events gives ps.Comment: 7 pages, 1 figure, submitted to Physical Review Letter
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