Amorphous ice on saturnian rings and on icy satellites: its formation, stability, and observability

Saturnian rings and icy satellites may be covered with amorphous rather than crystalline ice. Its likely source is water molecules sputtered by particles in the radiation belts, it may be stable, and its presence could be deduced from the rate of temperature drop in a shadow. Observation of this effect is, however, difficult, especially for the rings. A possible relation to the brightness anisotropy of ring A is pointed out.     

Venus: global surface radar reflectivity

Observations of the surface of Venus, carried out by the Pioneer Venus radar mapper at a wavelength of 17 centimeters, reveal a global mean reflectivity at normal incidence of 0.13 +/- 0.03. Over the surface, variations from a low of 0.03 +/- 0.01 to a high of 0.4 +/- 0.1 are found, with Theia Mons, previously identified as possibly volcanic, showing a value of 0.28 +/- 0.07. Regions of high reflectivity may consist of rocks with substantial inclusions of highly conductive sulfides.     

Asteroid vesta: spectral reflectivity and compositional implications

The spectral reflectivity (0.30 to 1.10 microns) of several asteroids has been measured for the first time. The reflection spectrum for Vesta contains a strong absorption band centered near 0.9 micron and a weaker absorption feature between 0.5 and 0.6 micron. The reflectivity decreases strongly in the ultraviolet. The reflection spectrum for the asteroid Pallas and probably for Ceres does not contain the 0.9-micron band. Vesta shows the strongest and best-defined absorption bands yet seen in the reflection spectrum for the solid surface of an object in the solar system. The strong 0.9-micron band arises from electronic absorptions in ferrous iron on the M2 site of a magnesian pyroxene. Comparison with laboratory measurements on meteorites and Apollo 11 samples indicates that the surface of Vesta has a composition very similar to that of certain basaltic achondrites.     

Spectral reflectivity of Mars

Analysis of data on the spectral reflectivity curves for both bright and dark areas of Mars disclosed several features not considered in previous models of the martian surface. The shape of the mean spectral geometric albedo curve between 0.3 and about 1.3 microns for Mars is defined to within a few percent. Spectral reflectivity curves based on relative reflectivity data were calculated for both a bright and a dark region between 0.4 and 1.1 microns. The curve for the dark region shows a broad, deep ( approximately 13 percent) absorption feature centered near 1 micron. The curve for the dark area crosses that of the bright area between 0.4 and 0.5 micron during some martian seasons.     

Radar reflectivity of titan

The present understanding of the atmosphere and surface conditions on Saturn's largest moon, Titan, including the stability of methane, and an application of thermodynamics leads to a strong prediction of liquid hydrocarbons in an ethane-methane mixture on the surface. Such a surface would have nearly unique microwave reflection properties due to the low dielectric constant. Attempts were made to obtain reflections at a wavelength of 3.5 centimeters by means of a 70-meter antenna in California as the transmitter and the Very Large Array in New Mexico as the receiving instrument. Statistically significant echoes were obtained that show Titan is not covered with a deep, global ocean of ethane, as previously thought. The experiment yielded radar cross sections normalized by the Titan disk of 0.38 +/- 0.15, 0.78 +/- 0.15, and 0.25 +/- 0.15 on three consecutive nights during which the sub-Earth longitude on Titan moved 50 degrees. The result for the combined data for the entire experiment is 0.35 +/- 0.08. The cross sections are very high, most consistent with those of the Galilean satellites; no evidence of the putative liquid ethane was seen in the reflection data. A global ocean as shallow as about 200 meters would have exhibited reflectivities smaller by an order of magnitude, and below the detection limit of the experiment. The measured emissivity at similar wavelengths of about 0.9 is somewhat inconsistent with the high reflectivity.     

Venus: mapping the surface reflectivity by radar interferometry

The surface reflectivity of Venus obtained by radar interferometry at a wavelength of 3.8 centimeters has been mapped for a region extending approximately from -80 degrees to 0 degrees in longitude (Carpenter's definition) and from -50 degrees to +40 degrees in latitude. The map is free from the twofold range-Doppler ambiguity because the interferometer fringe pattern makes possible the separation of two points of equal range and Doppler shift. The map presents many new features and clearly delineates features already observed. Most notably, the map shows large circular regions of significantly lower reflectivity than their surroundinigs.     

Phytoplankton bloom produced by a receding ice edge in the ross sea: spatial coherence with the density field

Measurements of chlorophyll, particulate carbon, and biogenic silica concentrations near a receding ice edge off the coast of Victoria Land, Antarctica, indicated the presence of a dense phytoplankton bloom. The bloom extended 250 kilometers from the ice edge and was restricted to waters where the melting of ice had resulted in reduced salinity. The region involved was one of enhanced vertical stability, which may have favored phytoplankton growth, accumulation, or both. Epontic algae released from melting ice may have served as an inoculum for the bloom. Ratios of organic carbon to chlorophyll and biogenic silica to carbon were unusually high, resulting in high biogenic silica concentrations despite only moderately high chlorophyll levels.     

Mars: retention of ice

Water in the form of ice can exist on Mars as permafrost that is either in equilibrium with the water content of the atmosphere or gradually evaporating through a protective layer of soil. The latter situation is evaluated quantitatively, and the required thicknesses of the protective layers are estimated. The presence of subsurface ice may explain the higher radar reflectivity of the dark areas than of the bright areas. Observation of its seasonal variations is suggested.     

Spectral reflectivity of lunar samples

Twelve rock chips and two samples of fines all have electronic absorption bands in diffuse reflected light between 0.32 and 2.5 micrometers. Major bands occur between 0.94 and 1.00 micrometer and at 2.0 micrometers, and arise from Fe(2+) in clinopyroxene and to a lesser extent in olivine. A band at 0.95 micrometer and other details of curve slope and shape for the lunar surface fines match McCord's telescopic curve for an 18-kilometer area that includes the Apollo-il site. Results confirm mineralogical predictions based on telescopic data and support the feasibility of obtaining mineralogical information by remote and in glass content. reflectivity measurements.     

Amorphous solid water: a neutron diffraction study

The structure factor of amorphous solid D(2)O deposited from the vapor at 10 degrees K has been obtained by measuring the neutron diffraction spectrum in the wave vector transfer from 0.8 to 12.3 reciprocal angstroms. The results indicate that the phase investigated is amorphous and has a liquiid-like structure factor. The Fourier-transformed structure e factor yields a real space pair distribution function consistent with local tetrahedral coordination and hydrogen bonding, as in other condensed phases of water. The intramolecular OD separation is 1.00 angstrom; the lack of data for very large wave vector transfer and the expected near equality of the intramolecular DD separation and intermolecular O . . . D separation make it impossible to determine the intramolecular DOD angle with precision. The neutron scattering data presented are complementary to the x-ray diffraction studies of Venkatesh, Rice, antd Narten.     

Devon island ice cap: core stratigraphy and paleoclimate

Valuable paleoclimatic information can be gained by studying the distribution of melt layers in deep ice cores. A profile representing the percentage of ice in melt layers in a core drilled from the Devon Island ice cap plotted against both time and depth shows that the ice cap has experienced a period of very warm summers since 1925, following a period of colder summers between about 1600 and 1925. The earlier period was coldest between 1680 and 1730. There is a high correlation between the melt-layer ice percentage and the mass balance of the ice cap. The relation between them suggests that the ice cap mass balance was zero (accumulation equaled ablation) during the colder period but is negative in the present warmer one. There is no firm evidence of a present cooling trend in the summer conditions on the ice cap. A comparison with the melt-layer ice percentage in cores from the other major Canadian Arctic ice caps shows that the variation of summer conditions found for the Devon Island ice cap is representative for all the large ice caps for about 90 percent of the time. There is also a good correlation between melt-layer percentage and summer sea-ice conditions in the archipelago. This suggests that the search for the northwest passage was influenced by changing climate, with the 19th-century peak of the often tragic exploration coinciding with a period of very cold summers.     

Fabrics in polar ice sheets: development and prediction

Fabrics in polar ice sheets provide a record of deformational history and control the viscosity of ice during further deformation; they affect geophysical sensing of ice sheets and provide an accessible analogue to fabric development during deformation of other geological and engineering materials. A new synthesis of experimental and theoretical results shows that c-axis fabrics are quantitatively related to cumulative strain and stress state in ice sheets for the full range of likely flow patterns. Basal shear, divergent flow, and parallel flow cause c axes to rotate toward the vertical axis, whereas convergent flow causes c axes to rotate toward a vertical plane transverse to flow.     

Catch a falling star: meteorites and old ice

A model for the process of meteorite concentration in blue ice regions of the Antarctic ice sheet is proposed based on data from near the Allan Hills and the assumptions that both meteorite influx and glacial flow have been constant. The meteorite influx is calculated to be 60 x 10(-6) kilogram per square kilometer per year, and the age of the exposed ice to be 0 to 600,000 years, varying with distance from the Allan Hills. These results are in line with other estimates of influx rate and with measurements of the terrestrial ages of the meteorites, providing support for the assumption of steady flow and meteorite influx. This may be the oldest sequence of ice in stratigraphic order yet discovered, and the results imply that this part of the east Antarctic ice sheet has been approximately steady during this time interval.