Steven K. Croft

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.

Rheological properties of ammonia-water liquids and crystal-liquid slurries: Planetological applications

Abstract The viscosities of aqueous mixtures plausibly representing a range of cryovolcanic substances seen on the icy satellites have been measured in the laboratory or obtained from literature sources. These viscosities range from 10−2 poise (pure water) to 102 poises (ammonia-water peritectic) to about 105 poises (ammonia-water-methanol peritectic). The viscosities of the liquid mixtures examined in this work are much greater than would be expected based on the assumption that the end member molecules are noninteractive. This observation supports others based on molar volumes and vapor pressure relations indicating that strong molecular interactive forces exist and have an important bearing on the physical properties of the mixtures. With supercooling and/or partial crystallization, these substances may attain viscosities several orders of magnitude greater than those given above. The rheological effects of partial crystallization parallel the same effects in silicate lavas, so it is reasonable to interpret cryovolcanic morphologies on the icy satellites in the same ways that we interpret remotely observed silicate volcanic morphologies on the Earth and terrestrial planets, after accounting for differences in surface gravities and lava densities, and allowing for uncertainties in surface slopes and extrusion rates. Given the wide range of viscosities for simple aqueous mixtures, and the rheological effects of realistic thermal states, the characteristics of observed cryovolcanic flows and resurfaced plains on the icy satellites can be understood within the framework of conventional magmatic processes working on exotic icy substances.

Equation of state of ammonia-water liquid: Derivation and planetological applications☆

An equation of state for ammonia-water liquid has been calculated by least-squares fit for the following range of parameters: 0 to 100 wt% NH3, 170 to 300°K, and 0 to 10 kb. Measured and calculated liquid densities were used in conjunction with solid density and thermodynamic measurements to estimate the thermal expansion and density at 1 bar for the solid phases of ammonia dihydrate, ammonia monohydrate, and ammonia hemihydrate between 0°K and their respective melting points. The peritectic ammonia-water liquid near pure ammonia dihydrate in composition was found to have a density of about 0.946 g/cm3 and to be approximately neutrally buoyant relative to the corresponding solid phases. This implies that igneous activity involving ammonia-water liquids on icy satellites may be both extrusive and intrusive in nature, possibly giving rise to a wide variety of morphologic and tectonic forms.

Proteus: Geology, shape, and catastrophic destruction☆

Abstract Proteus, with a mean radius of 209 ± 8 km, is the largest irregular-shaped icy body in the Solar System. It has a primitive, heavily cratered surface. Its most prominent feature is an impact crater μ255 km in diameter and 10- to 15-km deep, relatively one of the largest craters in the Solar System. This prominent crater is similar in relative size to giant impacts on several other planets and satellites, all of which are larger than a recent theoretical limit for catastrophic disruption. Consistency is obtained by inclusion of reaccumulation in the disruption analysis. Based on this reanalysis, Proteus was not brought “to the brink” of catastrophic destruction by the giant impact, but only to the limit beyond which the surface of the impacted body and the form of the disrupting crater are modified beyond recognition. A network of large streaks, interpreted as troughs, are also discernable. This network, too extensive to be explained by strains produced in traditional thermal history calculations, may represent the onset of significant internal disruption in near-catastrophic impacts. Proteus sphericity is near unity, but it has one of the roughest surfaces known. Thus Proteus is a transitional object in the irregular-spherical shape spectrum for icy satellites: its global figure is relaxed, but its surface features are unrelaxed. Proteus intermediate relaxation state requires relatively high internal temperatures. The required temperatures and the observed radius of the shape transition for icy bodies is consistent with radiogenic heating in bodies with deep porous regoliths.

The Impact Cratering Record on Triton

Impact craters on Triton are scarce owing to the relatively recent resurfacing by icy melts. The most heavily cratered surface has a crater density about the same as the lunar maria. The transition diameter from simple to complex craters occurs at a diameter of about 11 kilometers, and the depth-diameter relationship is similar to that of other icy satellites when gravity is taken into account. The crater size-frequency distribution has a differential -3 slope (cumulative -2 slope) and is the same as that for the fresh crater population on Miranda. The most heavily cratered region is on the leading hemisphere in Tritons orbit. Triton may have a leading-trailing asymmetry in its crater population. Based primarily on the similarity of size distributions on Triton and Miranda and the relatively young surface on Triton, the source of Tritons craters is probably comets. The very peculiar size distribution of sharp craters on the cantaloupe terrain and other evidence suggests they are volcanic explosion craters.

Image processing for teaching

The Image Processing for Teaching (IPT) project provides a powerful medium to excite students about science and mathematics, especially children from minority groups and others whose needs have not been met by traditional “coded” ways of teaching these subjects. Using professional-quality software on microcomputers, students explore a variety of scientific data sets, including biomedical imaging, Earth remote sensing and meteorology data, and planetary exploration images. They also learn about the many mathematical concepts that underlie image processing, such as coordinate systems, slope and intercept, pixels, binary arithmetic, along with many others. We have developed curriculum materials in all areas of mathematics and science for the upper elementary and secondary levels, allowing this tool to be used across a variety of grade levels and student interests. Preliminary indications show image processing to be an effective and fun way to study the application of science and mathematics to “real world” applications, as represented by digital imagery. The use of image processing is also an effective method with which to engage students in inquiry and discovery learning.