Paul Helfenstein

Patterns of fracture and tidal stresses due to nonsynchronous rotation: Implications for fracturing on Europa

This study considers the global patterns of fracture that would result from nonsynchronous rotation of a tidally distorted planetary body. The incremental horizontal stresses in a thin elastic or viscous shell due to a small displacement of the axis of maximum tidal elongation are derived, and the resulting stress distributions are applied to interpret the observed pattern of fracture lineaments on Europa. The observed pattern of lineaments can be explained by nonsynchronous rotation if these features formed by tension fracturing and dike emplacement. Tension fracturing can occur for a small displacement of the tidal axis, so that the resulting lineaments may be consistent with other evidence suggesting a young age for the surface.

Patterns of fracture and tidal stresses on Europa

Abstract the hypothesis that lineaments on Europa are fractures produced by tidal distortion and planetary volume change is examined by comparing the orientations of dark bands, triple bands, and cuspate ridges to fracture patterns predicted for tidal distortion due to orbital recession and orbital eccentricity. If short, reticulate dark band nnear the anti-Jove point are tension cracks which formed in response to tidal distortion, they could only have been produced by orbital eccentricity. Long, arcuate dark band and triple bands peripheral to the anti-Jove point orientations which suggest that they are strike-slip faults which formed in response to orbital recession. If cuspate ridges are compressional features, their orientations and distribution suggest that they formed in response to combined orbital recession and a decrease in planetary volume. Stresses due to orbital eccentricit could have produced tension cracks near the anti-Jove point only if tensile failure occurred either prior to the accumulation of orbital recession stresses or after they had relaxed. Surface fracturing, if a consequence of tidal deformation, places important constraints on the orbital evolution of Europa.

Voyager photometry of Triton: Haze and surface photometric properties

We have analyzed Voyager whole-disk observations of Triton at violet (0.41 μm), blue (0.48 μm),and green (0.56 μm) filter wavelengths, using a model which combines an improved version of Hapkes photometric equation with a thin atmospheric haze layer in the appropriate spherical geometry. The model accurately describes the phase curves over a wide range of phase angles (10° ≤ α ≤ 159°) and agrees well with disk-resolved brightness scans along the photometric equator and mirror meridian. An upturn in the phase curves seen at the highest phase angles observed (Smith et al., 1989) can be explained by including scattering in a thin atmospheric haze layer with optical depths ranging from 0.066 in the violet to 0.036 for the green filter data. The haze is forward scattering with g ∼ +0.6, requiring particle sizes of ≳0.2 μm. The haze single-particle scattering albedo is near unity at all three wavelengths, suggesting the haze particles are relatively neutral ice condensates. The geometric albedo, phase integral, and spherical albedo of Triton in each filter corresponding to our best fit Hapke and atmospheric parameters are derived. The bolometric Bond albedo of Triton calculated from our results is 0.85 ± 0.05. If the 16-μbar N2 atmosphere detected by the Voyager radio occultation experiment (Tyler et al., 1989) is in vapor equilibrium with the surface (therefore implying a surface temperature of 37.7 K), then our Bond albedo implies a surface emissivity of 0.46 ± 0.16.

The wavelength dependence of Triton's light curve

Using Voyager observations, we demonstrate that Tritons orbital light curve is strongly wavelength dependent, a characteristic which readily explains some of the apparent discrepancies among pre-Voyager telescopic measurements. Specifically, we find a light curve amplitude (peak to peak) that decreases systematically with increasing wavelength from about 0.08 magnitude (peak to peak) near 200 nm to less than 0.02 magnitude near 1000 nm. Peak brightness occurs near 90° orbital longitude (leading hemisphere). The brightness variation across this hemisphere is close to sinusoidal; the variation across the darker hemisphere is more complex. The decrease in light curve amplitude with increasing wavelength appears to be due to a decrease in contrast among surface markings, rather than to atmospheric obscuration. Our model also explains the observed decrease in the amplitude of Tritons light curve at visible wavelengths over the past decade, a decrease related to the current migration of the subsolar latitude toward the south pole: we predict that this trend will continue into the 1990s.