Local lunar topography from the Apollo 17 Alse radar imagery and altimetry

The Apollo 17 ALSE VHF radar provided imagery and continuous profiling data around the Moon during two revolutions. The imagery data are used to derive depth and diameter measurements of small craters (diameter <30 km). The profiling data are used to study the topography of a few large craters: the bulged floors in Hevelius, Neper, and Aitken; central peaks in Neper and Buisson; and the depressed floor of Maraldi. The same data provided accurate (better than 25 m) profiles of Mare Crisium and Mare Serenitatis.     

Results of recent COCORP profiling in the southeastern United States

Summary. Recently acquired COCORP profiles in the southeastern United States show that: 1) Reflections associated with the Appalachian detachment are prominent beneath the Inner Piedmont of western Georgia, but do not extend further southeast beneath the Pine Mountain belt. 2) The Brunswick magnetic low is associated with a broad zone of crustal-penetrating dipping reflections that probably marks the Alleghanian suture in the southeastern U.S. 3) The South Georgia basin is a composite feature consisting of several half-graben, locally containing >5 km of Triassic - E. Jurassic basin fill. These basins occur within the interior of the Alleghanian orogen, but are not specifically associated with Alleghanian suture. 4) Across-strike crustal thickness variation, and distribution and character of lower-crust and Moho reflections in the Southern Appalachians is grossly similar to that observed in other parts of the Appalachian/Hercynian orogenic belt. Global comparisons suggest that these regional variations are a consequence of post-collisional extensional tectonics, rather than a primary (Palaeozoic or older) feature of the orogenic belt.     

On the ionisation of hydrogen in optical flares

Non-steady state and non-LTE effects on the ionisation equilibrium of hydrogen in optical flares are considered in terms of a two-level hydrogen atom. It is shown that, just as in the quiet low chromosphere, the ionisation equation is controlled by spontaneous recombination to the second level and by photoionisation from this level by photospheric radiation, and is independent of the nature of the flare energy input mechanism.Adjustment of the ionisation then occurs on the recombination time scale which is short compared to the flare heating time scale. Consequently the ionisation is given by a simple LTE-modification of Saha's equation at the instantaneous electron temperature obtained from the energy equation. These conclusions are contrasted with those of previous authors who, by using one-level atoms only, have omitted the optically thin Balmer continuum which dominates the optically thick Lyman transitions.     

The possible protogalactic role of eddies in a turbulent early universe

It is shown that eddies, present in a turbulent early universe, can form excellent galaxies upon collapse of the eddies and a small amount of viscous evolution. The resulting model galaxies display realistically flat rotation curves as well as exponential-like luminosity profiles. The dark mass is allowed to be non-baryonic in this model.Work performed under the auspices of the U.S. Department of Energy.     

Distribution of bottom stress and tidal energy dissipation in a well-mixed estuary

Estimates of area-averaged tidal bottom stress are made for four channel segments of the Great Bay Estuary, N.H. Current and sealevel measurements are used to estimate acceleration and pressure gradient terms in the equation of motion, while the equation of motion itself is used to infer the remaining stress term. Dynamic terms, bottom stress values, friction coefficients and energy dissipation rates are estimated for each site. The analysis shows that while throughout the estuary the principal force balance is between the frictional stress and the pressure gradient forcing, RMS values of total bottom stress range from 2·67 to 10·38 Nm^?2 and friction coefficients vary from 0·015 to 0·054. Both stress and energy dissipation are largest in the seaward portion of the estuary with an order of magnitude decrease in dissipation at the most inland site.These distributions of stress and energy dissipation are consistent with cotidal charts of the principal semi-diurnal tidal constituent (M₂) which indicate that the estuary is composed of a highly dissipative more progressive tidal wave regime seaward and a less dissipative standing wave regime landward.     

G-MODE CLASSIFICATION OF SPECTROSCOPIC DATA

We present the results of the application of the G-mode method to the spectral classification of the icy satellites of the giant planets. G-mode is a multivariate statistical technique for the classification of samples depending on many variables. Here this method is tested on the infrared spectra acquired by the Cassini/VIMS instrument onboard the Cassini spacecraft. This work demonstrates the suitability of automatic spectral classification methods for the study of fair resolution spectra, such as those from VIMS. Our data set is composed by two different kinds of data: observations of point targets (Galilean satellites data) and observations with medium spatial resolution (Phoebe data). In both situations, the G-mode classification performed well. In the first case, of a large number of subpixel observations of the Galilean satellites, through the G-mode it was possible to find statistically meaningful spectral groups of observations. In the case of Phoebe, of some spatially resolved observations, the G-mode classification of␣the infrared spectra of the surface led to several types, dominated by the different illumination geometry of the pixels, because, due to the irregular shape of the satellite, a proper illumination correction was not trivial to apply. Nevertheless, the decrease of the confidence level of the test as well as the re-application of the G-mode on the main type found, led to further types, whose statistical distance can be related to different chemical abundances. We plan to use the G-mode also on the data coming from ongoing and future observations of the icy Saturnian satellites.In the helioseismology literature, G-modes are gravity wave modes of the frequencies of oscillations of the Sun. Here we are dealing with a clustering method, which is essentially different.*E-mail: federico.tosi@rm.iasf.cnr.it     

Nitrogen on Triton

The near-infrared spectrum of Triton is characterized by strong absorption bands of methane, probably in the solid state. An additional absorption band at 2.16 μm is tentatively identified as the density-induced (2-0) band of molecular nitrogen in the liquid state. The fundamental overtones of this band system cannot presently be observed because of limitations of the terrestrial atmosphere or spectral signal precision. Using the absorption coefficient for this band derived from laboratory observations and from the literature, it is calculated that Triton must have a layer of nitrogen at least tens of centimeters deep over much of its surface; this quantity is plausible in terms of the cosmic abundance of nitrogen and by comparison with Titan where a massive atmosphere of nitrogen exists. The Triton spectrum has been modeled with liquid nitrogen and solid methane, and it is found that the shape of the continuum in two spectral regions can be properly accounted for by adding a spectral component corresponding to fine-grained water frost. It is speculated that yet another component, a dark, solid, photochemical derivative of methane, may occur as a trace contaminant of the surface materials. If much of the surface of Triton is liquid, the radiometric observations of the satellite must be reinterpreted to derive the radius and surface albedo. If there is liquid nitrogen exposed on the surface, the atmosphere of Triton is probably dominated by nitrogen rather than methane because of the much higher vapor pressure of the former. At the calculated subsolar temperature of Triton, the vapor pressure of nitrogen implies a surface atmospheric pressure in the range 0.13 to 0.30 atm.