Jeffrey N. Cuzzi

Voyager 2 at Neptune: Imaging Science Results

Voyager 2 images of Neptune reveal a windy planet characterized by bright clouds of methane ice suspended in an exceptionally clear atmosphere above a lower deck of hydrogen sulfide or ammonia ices. Neptunes atmosphere is dominated by a large anticyclonic storm system that has been named the Great Dark Spot (GDS). About the same size as Earth in extent, the GDS bears both many similarities and some differences to the Great Red Spot of Jupiter. Neptunes zonal wind profile is remarkably similar to that of Uranus. Neptune has three major rings at radii of 42,000, 53,000, and 63,000 kilometers. The outer ring contains three higher density arc-like segments that were apparently responsible for most of the ground-based occultation events observed during the current decade. Like the rings of Uranus, the Neptune rings are composed of very dark material; unlike that of Uranus, the Neptune system is very dusty. Six new regular satellites were found, with dark surfaces and radii ranging from 200 to 25 kilometers. All lie inside the orbit of Triton and the inner four are located within the ring system. Triton is seen to be a differentiated body, with a radius of 1350 kilometers and a density of 2.1 grams per cubic centimeter; it exhibits clear evidence of early episodes of surface melting. A now rigid crust of what is probably water ice is overlain with a brilliant coating of nitrogen frost, slightly darkened and reddened with organic polymer material. Streaks of organic polymer suggest seasonal winds strong enough to move particles of micrometer size or larger, once they become airborne. At least two active plumes were seen, carrying dark material 8 kilometers above the surface before being transported downstream by high level winds. The plumes may be driven by solar heating and the subsequent violent vaporization of subsurface nitrogen.

Voyager 2 in the Uranian system: imaging science results

Voyager 2 images of the southern hemisphere of Uranus indicate that submicrometersize haze particles and particles of a methane condensation cloud produce faint patterns in the atmosphere. The alignment of the cloud bands is similar to that of bands on Jupiter and Saturn, but the zonal winds are nearly opposite. At mid-latitudes (-70� to -27�), where winds were measured, the atmosphere rotates faster than the magnetic field; however, the rotation rate of the atmosphere decreases toward the equator, so that the two probably corotate at about -20�. Voyager images confirm the extremely low albedo of the ring particles. High phase angle images reveal on the order of 102 new ringlike features of very low optical depth and relatively high dust abundance interspersed within the main rings, as well as a broad, diffuse, low optical depth ring just inside the main rings system. Nine of the newly discovered small satellites (40 to 165 kilometers in diameter) orbit between the rings and Miranda; the tenth is within the ring system. Two of these small objects may gravitationally confine the e ring. Oberon and Umbriel have heavily cratered surfaces resembling the ancient cratered highlands of Earths moon, although Umbriel is almost completely covered with uniform dark material, which perhaps indicates some ongoing process. Titania and Ariel show crater populations different from those on Oberon and Umbriel; these were probably generated by collisions with debris confined to their orbits. Titania and Ariel also show many extensional fault systems; Ariel shows strong evidence for the presence of extrusive material. About halfof Mirandas surface is relatively bland, old, cratered terrain. The remainder comprises three large regions of younger terrain, each rectangular to ovoid in plan, that display complex sets of parallel and intersecting scarps and ridges as well as numerous outcrops of bright and dark materials, perhaps suggesting some exotic composition.

Scattering by Nonspherical Particles of Size Comparable to a Wavelength: A New Semi-Empirical Theory and Its Application to Tropospheric Aerosols

Abstract We propose an approximate method for evaluating the interaction Of randomly oriented. nonspherical particles with the total intensity component of electromagnetic radiation. When the particle size parameter x (the ratio of particle circumference to wavelength) is less than some upper bound xO (∼5) theory is used. For x>xO the interaction is divided into three components: diffraction, external reflection and transmission. Physical optics theory is used to obtain the first of these components; geometrical optics theory is applied to the second; and a simple parameterization is employed for the third. The predictions of this theory are found to be in very good agreement with laboratory measurements for a wide variety of particle shapes, sizes and refractive indices. Limitations of the theory are also noted. As an application of the theory, we consider the influence of the shape of tropospheric aerosols on their contribution to Earths global albedo. Irregularly shaped tropospheric particles generall...

Toward Planetesimals: Dense Chondrule Clumps in the Protoplanetary Nebula

We outline a scenario that traces a direct path from freely floating nebula particles to the first 10-100 km sized bodies in the terrestrial planet region, producing planetesimals that have properties matching those of primitive meteorite parent bodies. We call this primary accretion. The scenario draws on elements of previous work and introduces a new critical threshold for planetesimal formation. We presume the nebula to be weakly turbulent, which leads to dense concentrations of aerodynamically size-sorted particles that have properties similar to those observed in chondrites. The fractional volume of the nebula occupied by these dense zones or clumps obeys a probability distribution as a function of their density, and the densest concentrations have particle mass densities that are 100 times that of the gas. However, even these densest clumps are prevented by gas pressure from undergoing gravitational instability in the traditional sense (on a dynamical timescale). While in this state of arrested development, they are susceptible to disruption by the ram pressure of the differentially orbiting nebula gas. However, self-gravity can preserve sufficiently large and dense clumps from ram pressure disruption, allowing their entrained particles to sediment gently but inexorably toward their centers, producing 10-100 km sandpile planetesimals. Localized radial pressure fluctuations in the nebula, as well as interactions between differentially moving dense clumps, will also play a role that must be accounted for in future studies. The scenario is readily extended from meteorite parent bodies to primary accretion throughout the solar system.

The evolution of the water distribution in a viscous protoplanetary disk

Astronomical observations have shown that protoplanetary disks are dynamic objects through which mass is transported and accreted by the central star. This transport causes the disks to decrease in mass and cool over time, and such evolution is expected to have occurred in our own solar nebula. Age dating of meteorite constituents shows that their creation, evolution, and accumulation occupied several Myr, and over this time disk properties would evolve significantly. Moreover, on this timescale, solid particles decouple from the gas in the disk and their evolution follows a different path. It is in this context that we must understand how our own solar nebula evolved and what effects this evolution had on the primitive materials contained within it. Here we present a model which tracks how the distribution of water changes in an evolving disk as the water-bearing species experience condensation, accretion, transport, collisional destruction, and vaporization. Because solids are transported in a disk at different rates depending on their sizes, the motions will lead to water being concentrated in some regions of a disk and depleted in others. These enhancements and depletions are consistent with the conditions needed to explain some aspects of the chemistry of chondritic meteorites and formation of giant planets. The levels of concentration and depletion, as well as their locations, depend strongly on the combined effects of the gaseous disk evolution, the formation of rapidly migrating rubble, and the growth of immobile planetesimals. Understanding how these processes operate simultaneously is critical to developing our models for meteorite parent body formation in the Solar System and giant planet formation throughout the galaxy. We present examples of evolution under a range of plausible assumptions and demonstrate how the chemical evolution of the inner region of a protoplanetary disk is intimately connected to the physical processes which occur in the outer regions.

Structure and particle properties of Saturn's E Ring

We have systematically reanalyzed a large subset of the available photometric data on Saturns E Ring, including Voyager images and star tracker data, Earth-based CCD images, photometry, and spectrophotometry. In order to compare data sets with vastly different vi viewing geometries on this broad, vertically extended ring, we have calibrated every observation based on a common three-dimensional model. The ring shows a density peak at a location indistinguishable from the Enceladus orbit, with a radial offset of <3000 km. We have found that a very simple power-law model describes the rings normal optical depth profile with orbital radius. The ring shows a general increase in vertical thickness with distance from the planet, ranging from 6000 km FWHM at its inner bound to nearly 40, 000 km at its outer. However, this trend is violated near the density peak, where a localized decrease in thickness by ∼30% is observed. The ring is also displaced northward from the planets equatorial plane by 310 ± 25 km in one set of Voyager images, although the overall nature of the rings vertical asymmetry is unclear. No power-law size distribution is compatible with the available photometry. A narrow distribution of slightly nonspherical particles of radius 1.0 ± 0.3 μm provides the best fit to the data. This highly peculiar size distribution clearly indicates that the ring does not originate from collisional or disruptive processes, and is therefore unlike any other known ring. Hence, we can give some credence to the possibility that the E Ring originates in “geyser-like” eruptions from the surface of Enceladus. Depending on the precise particle size chosen, the rings peak normal optical depth is found to be 1.5(±0.4) × 10−5, corresponding to a geometric cross-section per unit area of 5.3(±1.3) × 10−5. Macroscopic bodies comprise less than 1% of the rings optical depth.

MATERIAL ENHANCEMENT IN PROTOPLANETARY NEBULAE BY PARTICLE DRIFT THROUGH EVAPORATION FRONTS

Solid material in a protoplanetary nebula is subject to vigorous redistribution processes relative to the nebula gas. Meter-sized particles drift rapidly inward near the nebula midplane, and material evaporates when the particles cross a condensation/evaporation boundary. The material cannot be removed as fast in its vapor form as it is being supplied in solid form, so its concentration increases locally by a large factor (more than an order of magnitude under nominal conditions). As time goes on, the vapor-phase enhancement propagates for long distances inside the evaporation boundary (potentially all the way into the star). Meanwhile, material is enhanced in its solid form over a characteristic length scale outside the evaporation boundary. This effect is applicable to any condensible (water, silicates, etc.). Three distinct radial enhancement/depletion regimes can be discerned by use of a simple model. Meteoritic applications include oxygen fugacity and isotopic variations, as well as isotopic homogenization in silicates. Planetary system applications include more robust enhancement of solids in Jupiters core formation region than previously suggested. Astrophysical applications include differential, time-dependent enhancement of vapor phase CO and H2O in the terrestrial planet regions of actively accreting protoplanetary disks.

Statistical challenges in modern astronomy II

Despite centuries of close association, statistics and astronomy are surprisingly distant today. Most observational astronomical research relies on an inadequate toolbox of methodological tools. Yet the needs are substantial: astronomy encounters sophisticated problems involving sampling theory, survival analysis, multivariate classication and analysis, time series analysis, wavelet analysis, spatial point processes, nonlinear regression, bootstrap resampling and model selection. We review the recent resurgence of astrostatistical research, and outline new challenges raised by the emerging Virtual Observatory. Our essay ends with a list of research challenges and infrastructure for astrostatistics in the coming decade. 1. THE GLORIOUS HISTORY OF ASTRONOMY AND STATISTICS Astronomy is perhaps the oldest observational science 1 . The eort to understand the mysterious luminous objects in the sky has been an important element of human culture for at least 10 4 years. Quantitative measurements of celestial phenomena were carried out by many ancient civilizations. The classical Greeks were not active observers but were unusually creative in the applications of mathematical principles to astronomy. The geometric models of the Platonists with crystalline spheres spinning around the static Earth were elaborated in detail, and this model endured in Europe for 15 centuries. But it was another Greek natural philosopher, Hipparchus, who made one of the rst applications of mathematical principles that we now consider to be in the realm of statistics. Finding scatter in Bablylonian measurements of the length of a year, dened as the time between solstices, he took the middle of the range { rather than the mean or median { for the best value. This is but one of many discussions of statistical issues in the history of astronomy. Ptolemy estimated parameters of a non-linear cosmological model using a minimax goodness-of-t method. Al-Biruni discussed the dangers of propagating errors from inaccurate instruments and inattentive observers. While some Medieval scholars advised against the acquisition of repeated measurements, fearing that errors would compound rather than compensate for each other, the usefulnes of the mean to increase precision was demonstrated with great success by Tycho Brahe. During the 19th century, several elements of modern mathematical statistics were developed in the contextStatistics in Cosmology.- Bayesian Analysis Across Astronomy.- Data Mining and Astroinformatics.- Image and Time Series Analysis.- The Future of Astrostatistics.- Contributed Papers.

Nonlinear spiral density waves - Viscous damping

The formalism of Borderies, Goldreich, and Tremaine (1984), as simplified by Shu and Stewart (1985), is used to develop a theory for the viscous damping of nonlinear density waves in particulate disks of moderate collision frequency. The specific application is to Saturns rings, but the development is general enough to allow application to a wider context (e.g., to gas clouds in a spiral galaxy). A Krook formulation is used rather than a Boltzmann formulation to treat the statistical effects of inelastic collisions. Issues that have arisen as a result of the study include a self-induced Q barrier in the first wavelength or two of the Mimas 5:3 density wave train and the surprising discovery that Saturns B ring may behave almost as a superfluid, with hardly any viscous losses.

Bending waves in Saturn's rings

Abstract We investigate certain brightness variations seen in Saturns A ring and find them to be due to vertical corrugations of the local ring plane caused by a spiral bending wave. This wave is resonantly excited by Mimas and propagates inward via the collective gravity of the ring particles. B. A. Smith et al. [Science 212, 163–191 (1981)] had previously associated vertical relief with this feature due to its observed azimuthal variations and its proximity to an inclination resonance with Mimas. We develop the theory of forced bending waves, some aspects of which have been treated in the galactic context by C. Hunter and A. Toomre [Astrophys. J. 155, 747–776 (1969)] and by G. Bertin and J.W.-K. Mark [Astron. Astrophys. 88, 289–297 (1980)]. Our theory is in good agreement with the observations. In particular, the presence of these bending waves may resolve the conflict between ground-based estimates of 1–2 km for the global ring thickness [e.g., A. Brahic and B. Sicardy, Nature 289, 447–450 (1981)] and Voyager stellar occultation measurements of