Engineering topics

Nicholson, Philip D.

Publications and source records attributed to Nicholson, Philip D..

At least 19 records

NASA NTRS2016

Observing Planetary Rings and Small Satellites with the James Webb Space Telescope: Science Justification and Observation Requirements

The James Webb Space Telescope (JWST) will provide unprecedented opportunities to observe the rings and small satellites in our Solar System, accomplishing three primary objectives: (1) discovering new rings and moons, (2) unprecedented spectroscopy, and (3) time-domain observations. We give details on these science objectives and describe requirements that JWST must fulfill in order to accomplish the science objectives.

observational - planets and satellites↗

NASA NTRS2011

Thermal Structure and Dynamics of Saturn's Northern Springtime Disturbance

This article combined several infrared datasets to study the vertical properties of Saturn's northern springtime storm. Spectroscopic observations of Saturn's northern hemisphere at 0.5 and 2.5 / cm spectral resolution were provided by the Cassini Composite Infrared Spectrometer (CIRS, 17). These were supplemented with narrow-band filtered imaging from the ESO Very Large Telescope VISIR instrument (16) to provide a global spatial context for the Cassini spectroscopy. Finally, nightside imaging from the Cassini Visual and Infrared Mapping Spectrometer (VIMS, 22) provided a glimpse of the undulating cloud activity in the eastern branch of the disturbance. Each of these datasets, and the methods used to reduce and analyse them, will be described in detail below. Spatial maps of atmospheric temperatures, aerosol opacity and gaseous distributions are derived from infrared spectroscopy using a suite of radiative transfer and optimal estimation retrieval tools developed at the University of Oxford, known collectively as Nemesis (23). Synthetic spectra created from a reference atmospheric model for Saturn and appropriate sources of spectroscopic line data (6, 24) are convolved with the instrument function for each dataset. Atmospheric properties are then iteratively adjusted until the measurements are accurately reproduced with physically-realistic temperatures, compositions and cloud opacities.

Fletcher, Leigh N.↗

NASA NTRS2006

On the Discovery of CO Nighttime Emissions on Titan by Cassini/VIMS: Derived Stratospheric Abundances and Geological Implications

We present a quantitative analysis of CO thermal emissions discovered on the nightside of Titan by Baines et al. [2005. The atmospheres of Saturn and Titan in the near-infrared: First results of Cassini/VIMS. Earth, Moon, and Planets, 96, 119-147] in Cassini/VIMS spectral imagery. We identify these emission features as the P and R branches of the 1-0 vibrational band of carbon monoxide (CO) near 4.65 microns. For CH3D, the prominent Q branch of the nu(2) fundamental band of CH3D near 4.55 microns is apparent. CO2 emissions from the strong nu(3) vibrational band are virtually absent, indicating a CO2 abundance several orders of magnitude less than CO, in agreement with previous investigations. Analysis of CO emission spectra obtained over a variety of altitudes on Titan's nightside limb indicates that the stratospheric abundance of CO is 32 +/- 15 ppm, and together with other recent determinations, suggests a vertical distribution of CO nearly constant at this value from the surface throughout the troposphere to at least the stratopause near 300 km altitude. The corresponding total atmospheric content of CO in Titan is similar to 2.9 +/- 1.5 x 10(exp 14) kg. Given the long lifetime of CO in the oxygen-poor Titan atmosphere (similar to 0.5-1.0 Gyr), we find a mean CO atmospheric production rate of 6 +/- 3 x 10(exp 5) kg yr(exp -1). Given the lack of primordial heavy noble gases observed by Huygens [Niemann et al., 2005. The abundances of constituents of Titan's atmosphere from the GCMS on the Huygens probe. Nature, 438, 779-784], the primary source of atmospheric CO is likely surface emissions. The implied CO/CH4 mixing ratio of near-surface material is 1.8 +/- 0.9 x 10(exp -4), based on an average methane surface emission rate over the past 0.5 Gyr of 1.3 x 10(exp -13) gm cm(exp -2) s(exp -1) as required to balance hydrocarbon haze production via methane photolysis [Wilson and Atreya, 2004. Current state of modeling the photochemistry of Titan's mutually dependent atmosphere and ionosphere. J. Geophys. Res. 109, E06002 Doi: 10.1029/2003JE002181]. This low CO/CH4 ratio is much lower than expected for the sub-nebular formation region of Titan and supports the hypothesis [e.g., Atreya et al., 2005. Methane on Titan: photochemical-meteorological-hydrogeochemical cycle. Bull. Am. Astron. Soc. 37, 735] that the conversion of primordial CO and other carbon-bearing materials into CH4-enriched clathrate-hydrates occurs within the deep interior of Titan via the release of hydrogen through the serpentinization process followed by Fischer-Tropsch catalysis. The time-averaged predicted emission rate of methane-rich surface materials is approximately 0.02 km(exp 3) yr (exp -1), a value significantly lower than the rate of silicate lava production for the Earth and Venus, but nonetheless indicative of significant geological processes reshaping the surface of Titan.

carbon compounds↗

NASA NTRS1998

Analysis of Stellar Occultation and Imaging Data for the Rings of Saturn and Uranus

The principal goals of this research were as follows: 1. To carry out photometric modelling of Saturn's B Ring; 2. To re-examine dynamical perturbations of the Uranian c ring, 'While delivering relevant occultation data to the PDS Rings Node; and 3. Prepare data from the Saturn ring-plane crossing observations for delivery to the Rings Node.

Nicholson, Philip D.↗

NASA NTRS1995

Saturn's central flash from the 3 July 1989 occultation of 28 Sgr

We present observations of Saturn's central flash obtained from Palomar and McDonald Observatories during the 3 July 1989 occultation of 28 Sgr. As the star passed close to the geometric center of Saturn's shadow, the focusing of the incident starlight by the planet's atmosphere formed multiple stellar images along the limb which were detected in infrared images obtained at wavelengths of 3.9 (Palomar) and 2.1 microns (McDonald). These are the first reported observations of a central flash due to Saturn, and the first of any planet in which the signal flash from each stellar image could be determined separately, permitting a comparison of both intensity and position for each image with model predictions. Four separate flashes were observed from each observatory, corresponding to points on the limb where the starlight passed through the Cassini Division and the relatively transparent C Ring, with maximum brightness reaching 1-2% of the unocculted stellar intensity. The timing of the flashes is quite sensitive to the shape of Saturn's limb, which depends in turn on the planet's zonal gravity harmonics and on the zonal wind profile in the lower stratosphere, near the 2.5-mbar pressure level. The locations of the images along the limb, as well as the timing, shapes, and amplitudes of the individual flash light curves, are well matched by a smoothed model based on a tropospheric zonal wind profile obtained from tracking cloud features in Voyager images and the Saturn ring optical depth profile obtained from the Voyager Photopolarimeter experiment. The smoothing required to give the best match to the data exceeds that attributable to the finite angular extent of the occulted star and may be due to refractive scattering by turbulence or wave structure in Saturn's atmosphere. There is no evidence of significant atmospheric absorption at the observed wavelengths, which correspond to spectral regions of weak methane absorption.

Nicholson, Philip D.↗

NASA NTRS1994

The Rings Node for the Planetary Data System

The Planetary Data System's Rings Node is devoted to the archiving and distributing of scientific data sets relevant to planetary ring systems. The two major classes of ring data are images and occultation profiles, although a variety of additional data types (e.g. spectra, particle absorption signatures, etc.) are also of interest. A large fraction of our data sets are from the Voyager missions to the outer planets, but Earth-based and Hubble Space Telescope data sets are also represented. Archiving work often includes re-formatting the data into standardized formats and reconstructing some of the data processing steps. The Rings Node also performs a variety of services to support research into these data sets. These services include developing on-line catalogs and information systems, filling orders for data, developing software tools, and coordinating special observing campaigns.

Showalter, Mark R.↗

NASA NTRS1993

Geometry of the Saturn system from the 3 July 1989 occultation of 28 Sgr and Voyager observations

The pole direction and radius scale for the ring system of Saturn are presently determined by combining the July 3, 1989 observations of 28 Sgr's occultation by Saturn with Voyager 1 and 2 photopolarimeter occultation measurements. The computation of ring-orbit models is separately conducted via a sky-plane method and a solar system barycentric vector approach; the results obtained by both methods are in excellent agreement. If Voyager trajectory uncertainties are assumed to be negligible, these observations can be used to determine the precession rate of the pole of Saturn about the solar system's invariable pole.

French, Richard G.↗

NASA NTRS1993

Orbital evolution of the PSR1257+12 planetary system

The detection of orbital perturbation effects in the PSR1257+12 timing data would provide irrefutable confirmation that planets are indeed orbiting the pulsar. Here we give an overview of how perturbation effects are expected to affect the orbital elements of the two planets over the next few years. In particular, we give a simple calculation of resonant perturbations, including the nonlinear effects which could be important if sin i less than about 0.1. We also present a new analysis of the effects of close encounters between the two planets and we discuss their detectability.

Rasio, Frederic A.↗

NASA NTRS1992

New observations of Saturn's coorbital satellites

The strong planetary methane and hydrogen absorption at 2.0-2.4 microns are exploited in the present observations of the Saturnian coorbital satellites Janus and Epimetheus as they passed over the north pole of Saturn at superior conjunction. These observations confirm the orbital model results of Yoder at al. (1989), especially in the question as to the low density of both satellites; in addition, a much stronger solution is furnished which is essentially independent of 1966 data for Epimethius. The low density results are interpreted as indicative that the objects are composed of relatively pure water ice, but with porosities of the order of 30 percent.

Nicholson, Philip D.↗

NASA NTRS1992

Stellar occultation experiment with the CASSINI VIMS instrument

The VIMS instrument (Visible and Infrared Mapping Spectrometer) will be flown in the late 1990s on the CASSINI mission to Saturn and its moons. VIMS is designed to generate two-dimensional multispectral images of planetary surfaces and their features in the visible and infrared spectra. Compared to earth-based instruments, the stellar occultation experiment will provide unprecedented thickness resolution and chemical composition of planetary atmospheres. It will also gather high SNR optical depth profiles and particle size distribution of the Saturnian rings. The stellar occultation mode ideally requires continuous data acquisition for periods of up to several hours as the instrument stares at a star. This presents a sizeable challenge to the instrument's operational mode: processes that normally occur during mirror retrace must be shifted to data acquisition cycles, thereby creating timing difficulties in the sequencing of these cycles. The presentation focuses on the analysis of the stellar occultation mode and presents solutions to the challenge of an uninterrupted data stream. Several options will be presented that minimize any possible degradation of the experiment's science content.

Walch, Marc↗

NASA NTRS1991

Near-infrared observations of the Jovian ring and small satellites

The Jovian ring has been observed at 2.2 microns at a near-maximum opening angle of 3.2 deg and a phase angle of 2.2 deg. The 126,100 +/- 500 km ring radius at maximum brightness obtained is compatible with Voyager data at high phase angles. Circular orbital elements have also been obtained for the two small satellites (Metis and Adrastea) associated with the ring. A combination of these orbits with the Voyager imaging-based measurements of Synnott yields a substantial improvement in mean motion estimates.

Nicholson, Philip D.↗

NASA NTRS1991

Planetary rings

This review discusses each planetary ring system and describes recent theoretical developments and models of ring evolution. The distribution of the inner rings and satellites of all four giant planets reduced to a uniform scale of 'planetary units', normalized by the equatorial radius of each planet, is summarized. Only major ring subunits are indicated, with the density of cross-hatching representing in a crude way the relative optical depths of the different components of each system. One result of the spacecraft observations has been the discovery at each planet of several small (10-100 km in radius) satellites that appear to be intimately related to the rings, dynamically and possibly genetically as well.

Nicholson, Philip D.↗

NASA NTRS1990

An absolute radius scale for Saturn's rings

Radio and stellar occultation observations of Saturn's rings made by the Voyager spacecraft are discussed. The data reveal systematic discrepancies of almost 10 km in some parts of the rings, limiting some of the investigations. A revised solution for Saturn's rotation pole has been proposed which removes the discrepancies between the stellar and radio occultation profiles. Corrections to previously published radii vary from -2 to -10 km for the radio occultation, and +5 to -6 km for the stellar occultation. An examination of spiral density waves in the outer A Ring supports that the revised absolute radii are in error by no more than 2 km.

Nicholson, Philip D.↗

NASA NTRS1990

Saturn's rings through a microscope - Particle size constraints from the Voyager PPS scan

The Voyager-2 photopolarimeter PPS experiment obtained the highest resolution of any ring observation of Saturn, profiling the variation of optical depth in radial steps of about 100 meters. A detailed treatment of the PPS statistics is presented here, and it is shown how these statistics can be related to the particle size distribution. An expression for the excess noise in the scan due to large particles is obtained, and the observed noise is used to constrain the upper end of the size distribution through the rings. It is shown that the Cassini Division and the C Ring have the smallest proportion of large particles, while the A ring has the largest proportion.

Showalter, Mark R.↗

NASA NTRS1990

Five stellar occultations by Neptune - Further observations of ring arcs

Diffraction model fits to the light curves obtained during April 18, 1984 and August 20, 1985 stellar occultations that revealed incomplete rings, or 'arcs' around Neptune, are found to indicate arc radial widths of 8-26 km, and normal optical depths of 0.07-0.14. On the basis of the Neptune pole direction ascertained from Voyager navigation analyses, it is confirmed that the arc observed on August 20, 1985, together with the Hubbard et al. (1986) and Covault et al. (1986) arcs, seem to match the radius of the outermost ring found in Voyager images analyzed by Smith et al. (1989).

Nicholson, Philip D.↗

NASA NTRS1990

Models of Neptune's arc rings

Models proposed to explain the dynamical confinement of such structures as incomplete arc rings in orbit about the planet Neptune are reviewed and tested against the currently available observational data. It is shown that satellites can confine arc rings to very limited radial and azimuthal extents via a combination of corotation and Lindblad resonances. The resonances can be produced by the same moon or by different moons.

Lissauer, Jack J.↗

NASA NTRS1989

On the obliquity and tidal heating of Triton

Although tidal heating is generally associated with spin-locked satellites on eccentric orbits, a satellite with a large obliquity can undergo substantial heating due to obliquity tides, even on a circular orbit. The near-100-deg obliquity of the Neptune moon, Triton, could generate significant tidal heating and eventually lead to a damping of its orbital inclination to 180 deg. Ground-based observations of Triton have tentatively found a synchronous rotational state that is consistent with despinning times of about 10,000 years, indicating that Triton is almost certainly rotating synchronously.

Jankowski, David G.↗

NASA NTRS1989

Dust bands in the asteroid belt

This paper describes the original IRAS observations leading to the discovery of the three dust bands in the asteroid belt and the analysis of data. Special attention is given to an analytical model of the dust band torus and to theories concerning the origin of the dust bands, with special attention given to the collisional equilibrium (asteroid family), the nonequilibrium (random collision), and the comet hypotheses of dust-band origin. It is noted that neither the equilibrium nor nonequilibrium models, as currently formulated, present a complete picture of the IRAS dust-band observations.

Sykes, Mark V.↗