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Nicholson, P. D.

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

At least 19 records
All sourcesNASA NTRSDOE OSTI
NASA NTRS2012

Low-Latitude Ethane Rain on Titan

Cassini ISS observed multiple widespread changes in surface brightness in Titan's equatorial regions over the past three years. These brightness variations are attributed to rainfall from cloud systems that appear to form seasonally. Determining the composition of this rainfall is an important step in understanding the "methanological" cycle on Titan. I use data from Cassini VIMS to complete a spectroscopic investigation of multiple rain-wetted areas. I compute "before-and-after" spectral ratios of any areas that show either deposition or evaporation of rain. By comparing these spectral ratios to a model of liquid ethane, I find that the rain is most likely composed of liquid ethane. The spectrum of liquid ethane contains multiple absorption features that fall within the 2-micron and 5-micron spectral windows in Titan's atmosphere. I show that these features are visible in the spectra taken of Titan's surface and that they are characteristically different than those in the spectrum of liquid methane. Furthermore, just as ISS saw the surface brightness reverting to its original state after a period of time, I show that VIMS observations of later flybys show the surface composition in different stages of returning to its initial form.

Cassini↗
NASA NTRS2005

The Atmospheres of Saturn and Titan in the Near-Infrared: First Results of Cassini/VIMS

The wide spectral coverage and extensive spatial, temporal, and phase-angle mapping capabilities of the Visual Infrared Mapping Spectrometer (VIMS) onboard the Cassini-Huygens Orbiter are producing fundamental new insights into the nature of the atmospheres of Saturn and Titan. For both bodies, VIMS maps over time and solar phase angles provide information for a multitude of atmospheric constituents and aerosol layers, providing new insights into atmospheric structure and dynamical and chemical processes. For Saturn, salient early results include evidence for phosphine depletion in relatively dark and less cloudy belts at temperate and mid-latitudes compared to the relatively bright and cloudier Equatorial Region, consistent with traditional theories of belts being regions of relative downwelling. Additional Saturn results include (1) the mapping of enhanced trace gas absorptions at the south pole, and (2) the first high phase-angle, high-spatial-resolution imagery of CH4 fluorescence. An additional fundamental new result is the first nighttime near-infrared mapping of Saturn, clearly showing discrete meteorological features relatively deep in the atmosphere beneath the planet's sunlit haze and cloud layers, thus revealing a new dynamical regime at depth where vertical dynamics is relatively more important than zonal dynamics in determining cloud morphology. Zonal wind measurements at deeper levels than previously available are achieved by tracking these features over multiple days, thereby providing measurements of zonal wind shears within Saturn's troposphere when compared to cloudtop movements measured in reflected sunlight. For Titan, initial results include (1) the first detection and mapping of thermal emission spectra of CO, CO2, and CH3D on Titan's nightside limb, (2) the mapping of CH4 fluorescence over the dayside bright limb, extending to approximately 750 km altitude, (3) wind measurements of approximately 0.5 ms(exp -1), favoring prograde, from the movement of a persistent (multiple months) south polar cloud near 88 deg S latitude, and (4) the imaging of two transient mid-southern-latitude cloud features.

Cassini Mission↗
NASA NTRS2001

Saturn's Rings: Pre-Cassini Status and Mission Goals

Theoretical and observational progress in studies of Saturn's ring system since the mid-1980s is reviewed, focussing on advances in configuration and dynamics, composition and size distribution, dust and meteoroids, interactions of the rings with the planet and the magnetosphere, and relationships between the rings and various satellites. The Cassini instrument suite of greatest relevance to ring studies is also summarized, emphasizing how the individual instruments might work together to solve outstanding problems. The Cassini tour is described from the standpoint of ring studies, and major ring science goals are summarized.

Cuzzi, Jeffrey N.↗
NASA NTRS1995

Palomar observations of the collision of comet Shoemaker-Levy 9 with Jupiter

We present two papers of the near-infrared observations from Palomar observatory of the impact of fragment R of comet Shoemaker-Levy 9 with Jupiter on July 21, 1994. The first paper is concerned with the lightcurves found from these observations: Two instruments were used to image the event at 3.2 and 4.5 microns simultaneously. The lightcurves from these image sequences both show two faint precursor flashes, a bright main peak, and several oscillations over the following hour. We identify the precursor flashes with the entry of the bolide into Jupiter's upper atmosphere, and with the post-impact ejecta plume rising above the planet's limb. The main peak is due to the re-entry of the collapsing plume in Jupiter's atmosphere and the resultant shock heating. The second paper is concerned with the low-resolution spectra from the lightcurves: Low-resolution 8-13 micron spectra taken near the peak of the lightcurve show a broad emission feature that resembles the delicate silicate feature commonly seen in comets and the interstellar medium. We use this feature to estimate the dust content of the impact plume. The overall infrared spectral energy distribution at the time of peak brightness is consistent with emission from an optically-thin layer of small particles at approximately 600 K. Integrating over the spectrum and the lightcurve, we obtain a total radiated energy from the R impact of greater than or equal to 10(exp 25) ergs and a plume mass of greater than or equal to 3 x 10(exp 13) g.

Neugebauer, G.↗
NASA NTRS1992

A observational test for the existence of a planetary system orbiting PSR1257 + 12

It is pointed out that, because of the near commensurability of the orbital periods of the recently reported planets of the nearby millisecond pulsar PSR1257 + 12, the mutual gravitational perturbations of the two planets should produce not only small secular changes, but also larger periodic changes in their orbital elements. In particular, it is found that changes in the eccentricities and orbital periods should become measurable within a few years. Such a measurement would help determine the three masses in the system and the inclinations of the orbits. More important, a detection of these changes, if they agree with theoretical predictions presented here, should provide irrefutable confirmation that the periodic residuals are caused by orbiting planets rather than some other effect.

Rasio, F. A.↗
NASA NTRS1992

The origin and evolution of the zodiacal dust cloud

We have now analyzed a substantial fraction of the IRAS observations of the zodiacal cloud, particularly in the 25 micron waveband. We have developed a gravitational perturbation theory that incorporates the effects of Poynting-Robertson light drag (Gomes and Dermott, 1992). We have also developed a numerical model, the SIMUL mode, that reproduces the exact viewing geometry of the IRAS telescope and calculates the distribution of thermal flux produced by any particular distribution of dust particle orbits (Dermott and Nicholson, 1989). With these tools, and using a distribution of orbits based on those of asteroidal particles with 3.4 micron radii whose orbits decay due to Poynting-Robertson light drag and are perturbed by the planets, we have been able to: (1) account for the inclination and node of the background zodiacal cloud observed by IRAS in the 25 micron waveband; (2) relate the distribution of orbits in the Hirayama asteroid families to the observed shapes of the IRAS solar system dustbands; and (3) show that there is observational evidence in the IRAS data for the transport of asteroidal particles from the main belt to the Earth by Poynting-Robertson light drag.

Dermott, Stanley F.↗
NASA NTRS1991

Origin and evolution of the zodiacal dust cloud

The astrophysical importance of the zodiacal cloud became more apparent. The most useful source of information on the structure of the zodiacal cloud is the Infrared Astronomical Satellite (IRAS) observations. A substantial fraction of the extensive IRAS data set was analyzed. Also, a numerical model was developed (SIMUL) that allows to calculate the distribution of night-sky brightness that would be produced by any particular distribution of dust particle orbits. This model includes the effects of orbital perturbations by both the planets and solar radiation, it reproduces the exact viewing geometry of the IRAS telescope, and allows for the eccentricity of the Earth's orbit. SIMUL now is used to model not just the solar system dust bands discovered by IRAS but the whole zodiacal cloud.

Dermott, S. F.↗
NASA NTRS1991

Analysis of IRAS solar system dust data

Data in the Infrared Astronomical Satellite (IRAS) Zodiacal History File were analyzed to extract dust band locations and peak brightness measurements from approximately 1,000 individual IRAS scans. The study had three goals. One was to show that the prominent solar system dust bands are associated with Hirayama asteroid families and thus that collisions between asteroids account for a significant fraction of the particles in the zodiacal cloud. Recent work suggests that while the Hirayama families are a major source of the dust in the bands, there may also be contributions from two or three smaller, more recently recognized asteroid families. A second goal was to show that there is evidence in the IRAS dust data for the transport of particles from asteroid belt to the Earth by Poynting-Robertson light drag and thus account for the fact that asteroid particles are collected in the Earth's stratosphere. Results of the study will confirm the location of the dust bands within the inner asteroid belt, and show conclusively that the material seen by IRAS is now spread over a wide range of distances from the sun. The third goal was to construct a model of the background zodiacal cloud that satisfies the proper dynamical constraints. Figures are provided to show the scans processed to remove zodiacal background and Galactic signals, and the resulting polynomial fits to the 25 micron scan. The latter provided objective estimates of band widths, peak locations, and peak fluxes. Modelling and analysis of the resulting band data has been presented at several conferences and is the subject of a number of forthcoming papers.

Dermott, S. F.↗
NASA NTRS1989

Secular perturbations of the Uranian satellites - Theory and practice

A simple revised secular perturbation theory which incorporates the averaged secular effect of first-order near-resonances is derived. By including the effects of these near-resonances, the largest error in the secular frequencies is reduced from 16 percent to less than 3 percent. It is concluded that the revised secular perturbation theory is adequate for the quantitative modeling of the long-term perturbations in the Uranian satellite system. If incorporated within the general theory of Laskar (1986), this theory would lead to completely analytic theory.

Malhotra, R.↗
NASA NTRS1989

Tidal evolution in the Neptune-Triton system

Triton, which is currently spiralling toward Neptune due to tides raised on both bodies, possesses an obliquity which may lie close to either a zero-deg 'state 1' or a 100-deg 'state 2' which correspond to the two stable Cassini extrema of its rotational Hamiltonian. The Kaula (1966) tidal formalism is presently used to model the past and future evolution of the system in both states. For nominal parameters in state 1, Triton will reach Neptune's Roche limit in about 3.6 Gyr with a decrease in orbital inclination to 145 deg from the current 159 deg; in the case of state 2, Triton's inclination will increase to 180 deg in 10-100 million years and then transition to state 1, subsequently reaching the Neptune Roche limit in about 1.4 Gyr.

Chyba, C. F.↗
NASA NTRS1987

Stellar occultation probes of the Uranian rings at 0.1 and 2.2 microns - A comparison of Voyager UVS and earth-based results

A comparison between Voyager 2 UV spectrometer data for stellar occultations of the Uranian ring system obtained at 0.11 microns and 2.2-micron earth-based occultation data reveals the anticipated factor-of-two reduction in observed optical depths relative to those observed from earth. This is due to Voyager's proximity to the rings, which allows light diffracted out of the direct beam by ring particles to be replaced by light diffracted into the direct beam light from other particles, and further permits the placing of firm lower limits on typical particle sizes of 0.3 and 1 cm for the epsilon and delta rings, respectively. As a function of true anomaly, the epsilon ring profile is noted to remain very similar in shape and essentially constant in equivalent depth, even very near periapse.

Holberg, J. B.↗
NASA NTRS1986

Preliminary analysis of the IRAS solar system dust data

The structure of the solar system dust cloud as revealed by IRAS all-sky survey data is discussed. Fourier analysis was used to separate the smooth, large-scale zodiacal background from the narrower dust lanes. The geometry of the background zodiacal cloud appears to show features associated with both the forced inclinations and the forced eccentricities of the dust particle orbits. The ecliptic latitude of the peak of the background zodiacal emission varies nearly sinusoidally with ecliptic longitude with an amplitude of 2.1 + or -0.2 degrees. However, there are marked displacements, both in latitude and longitude, between the leading or ascending curve and the trailing or descending curve, implying that the curves exhibit significant deviations from a simple sinusoidal variation. It is suggested that the plane of symmetry of the background cloud is inclined to the ecliptic by 1.47 + or -0.10 degrees with a descending node of 230 + or -4 degrees, and that the sun is not at the center of symmetry of the cloud.

Dermott, S. F.↗
NASA NTRS1985

Dynamical Studies of Saturn's Rings

The data returned by the Voyager on the Saturnian system has revealed the structure of Saturn's ring system to be dominated by many abrupt transitions in optical depth, narrow well-defined ringlets and gaps, and spiral density and bending waves. A handful of these features were attributed with reasonable certainty to the resonant influence of external satellites, but the vast majority of the structure remains unexplained. The entire C ring was studied with particular emphasis on the search for unrecognized eccentric or inclined features, and the role, if any, played by satellite resonances in controlling C ring structure.

Nicholson, P. D.↗
NASA NTRS1985

An analysis of IRAS' solar system dust bands

Measurements of longitudinal variations in the brightness and in the latitude of the solar system dust bands recently discovered by IRAS will determine the orbital elements of the particles involved and may discriminate between cometary and asteroidal models of the origin of these bands. The expected variations for bands of dust particles with common orbital elements and small eccentricity and inclination are calculated as functions of semimajor axis.

Dermott, S. F.↗
NASA NTRS1984

Origin of the solar system dust bands discovered by IRAS

It is shown that distinctive longitudinal variations in thermal flux and mean latitude can be used to determine the typical orbits of the grains comprising the Solar System bands. In particular, how the bands should vary if they are debris associated with the three principal asteroid families is predicted. Based on these ideas, IRAS observations may allow discrimination between asteroidal and cometary origins of the dust bands and, perhaps, of the entire zodiacal cloud.

Dermott, S. F.↗
NASA NTRS1984

Infrared Images of the Uranian Rings

Images of the Uranian rings at a wavelength of 2.2 micrometers, chosen to reduce the problem of scattered light from the planet, have been acquired on several occasions spanning the period 1978-1983. Although individual rings are not resolved in these images, the integrated brightness of the system is observed to vary with azimuth. The phase and amplitude of this variation is found to be consistent with the precession rate and variable width of the epsilon ring, as deduced from stellar occultation data. Quantitative analyses of the brightness variations permit an upper limit of approx. 0.0015 to be pladed on the average normal optical depth of any diffuse inter-ring material. Preliminary absolute calibration of the 1983 data yields an average ring geometric albedo, at 2.2 micrometers, of 0.22 + or - 0.002, consistent with previous estimates. Lastly, the potential use of near-infrared images in searches for faint inner satellites of Uranus is discussed.

Nicholson, P. D.↗