Pascal Lee

The Exchange of Impact Ejecta Between Terrestrial Planets

Orbital histories of ejecta from the terrestrial planets were numerically integrated to study their transfer to Earth. The properties of the lunar and martian meteorites are consistent with a recurrent ejection of small meteoroids as a result of impacts on their parent bodies. Long-range gravitational effects, especially secular resonances, strongly influence the orbits of many meteoroids, increasing their collision rates with other planets and the sun. These effects and collisional destruction in the asteroid belt result in shortened time scales and higher fluxes than previously believed, especially for martian meteorites. A small flux of mercurian ejecta appears possible; recovery of meteorites from the Earth and Venus is less likely.

Longitudinal dunes on Mars: Relation to current wind regimes

Longitudinal dunes are extremely rare on Mars, but constitute a substantial fraction of terrestrial desert dunes. We report finding isolated examples of longitudinal dunes on Mars and relate their occurrence to expected sand transport regimes. Terrestrial longitudinal dunes form in bimodal and multimodal transport regimes. General circulation models and streak data indicate that bimodal and multimodal transport of sand should be very rare on Mars. Thus the dearth of longitudinal dunes on Mars is consistent with their apparent formation conditions on Earth.

Anomalous-scattering region on Triton

Abstract A broad region with unusual light-scattering properties is identified on Neptunes largest satellite Triton. Photometric analysis of Voyager 2 high-resolution images, acquired over a range of phase angles, shows that this “Anomalous-Scattering Region” (ASR) extends from at least 350°E to 60°E longitude, and 10°N to 40°N latitude. The ASR material is unusual compared to “Average Triton” regolith in that it is only weakly backward scattering at all Voyager 2 camera wavelengths (0.41 μ m ≤ λ ≤ 0.59 μ m). Fits of Hapkes model to disk-resolved photometric data yield a Henyey-Greenstein asymmetry factor g of −0.10 ± 0.02 for the ASR in the clear filter, compared to about −0.25 for Average Triton. The ASR also displays a combination of phase-dependent green/violet color ratios and clear-filter albedo that appears to be distinctive. With spatial continuity as an assumed additional property, we use these characteristics to map the global distribution of the ASR in areas for which photometric coverage is incomplete. We find that the ASR might actually form an almost continuous band of material running roughly parallel to Tritons equator and extending northward from about 10°N, at least up to the terminator (near 30°N). A likely explanation for the ASRs unusual photometric behavior and for the strong latitudinal control of its distribution is the presence of as transparent, optically thin, and seasonally controlled veneer of well-annealed solid N 2 . The relatively dark, reddish color may result from products of ex situ irradiation processing of CH 4 or other hydrocarbons present in intimate mixture within the transparent anomalous-scattering layer and/or in the substrate underlying it. Because mapping of the ASR was indirect, these various possible configurations could imply radically different areal extents for the ASR on Triton.

Large Quasi-Circular Features Beneath Frost on Triton

Specially processed Voyager 2 images of Neptunes largest moon, Triton, reveal three large quasi-circular features ranging in diameter from 280 to 935 kilometers within Tritons equatorial region. The largest of these features contains a central, irregularly shaped area of comparatively low albedo about 380 kilometers in diameter, surrounded by crudely concentric annuli of higher albedo materials. None of the features exhibit significant topographic expression, and all appear to be primarily albedo markings. The features are located within a broad equatorial band of anomalously transparent frost that renders them nearly invisible at the large phase angles (α > 90�) at which Voyager obtained its highest resolution coverage of Triton. The features can be discerned at smaller phase angles (α = 66�) at which the frost only partially masks underlying albedo contrasts. The origin of the features is uncertain but may have involved regional cryovolcanic activity.

Search for glazed surfaces on Triton

Near-infrared reflectance spectra of Triton obtained from Earth since 1978 show several absorption bands between 0.8 and 4 μm, features attributed to CH4 and N2 and inferred to represent materials in the condensed state (Cruikshank and Brown, 1986, and Spencer et al., 1990). Clark et al. (1983) had previously proposed a frost grain metamorphism model for icy satellite surfaces, according to which freshly condensed CH4 and N2crystals would rapidly reconfigure themselves into a glaze or grow to unusually large sizes under the pressure and temperature conditions prevailing on Triton. In either case, the long path lengths apparently needed to explain the prominence of the absorptions could be provided. More recently, the Voyager 2 flyby of Triton revealed the existence of topographically smooth frozen “lava lakes” and active geyserlike plumes. The presence of the former, and the solid-state greenhouse mechanism proposed by Smith et al. (1989) and Brown et al. (1990) to power the latter, both suggest the likely existence, at least as localized patches, of surficial clear ice. Thus smooth, even glazed areas of N2 and CH4ice might occur on Triton. We report the results of a search, at the pixel scale, for such glazed areas in Voyager 2 high-resolution images of Triton. Glazed surfaces, if present, are expected to produce a surge in reflectance when viewed under the geometry of specular reflection (Middleton and Mungall, 1952). Atmosphere-insensitive green and clear filter images acquired at phase angles ranging from 60° to 100°, and containing mirror points occurring over three different types of terrains (the South Polar Cap Mottled Unit, the South Polar Cap Bright Fringe, and the “Frost Band” region), have been surveyed. No smooth, glazed surfaces producing detectable specular reflection were found. The lack of evidence for glazed areas in the southern mid-latitude regions, at least at the scale of the spatial resolution (∼3–7 km per line pair), indicates that another cause may be responsible for the prominence of Tritons spectral features. We note that the southern higher-latitude regions in which active geyserlike plumes are located were not viewed under large enough incidence angles by Voyager 2 to allow conclusive searches for specular behavior.

On the global gravitational instanton and soliton that are homotopy spheres

A generalization of the Dirac string trick for higher‐dimensional manifolds is presented. As a rather unexpected result, the procedure is shown to yield homotopy spheres. Because of their relevant interest to physical theories, particular emphasis is given to even‐dimensional homotopy spheres. For instance, it is demonstrated that a nontrivial representation of 6–10‐dimensional exotic spheres in dimensions 4k+2 can be performed by using the Pontrjagin–Thom construction. The physical content of the results is consistent with Witten’s hypothesis [Commun. Math. Phys., 100, 197–229 (1985)] that n–10‐dimensional exotic spheres can be interpreted as gravitational instanton and/or soliton. It is also shown that three mutually diffeomorphic elements can be isolated from the 6–10‐dimensional exotic spheres and their spectrum is provided.