Joel Mosher

Europa: Characterization and interpretation of global spectral surface units

Abstract The Voyager global multispectral mosaic of the Galilean satellite Europa ( T. V. Johnson, L. A. Soderblom, J. A. Mosher, G. E. Danielson, A. F. Cook, and P. Kupferman, 1983, J. Geophys. Res. 88, 5789–5805 ) was analyzed to map surface units with similar optical properties ( T. B. McCord, M. L. Nelson, R. N. Clark, A. Meloy, W. Harrison, T. V. Johnson, D. L. Matson, J. A. Mosher, and L. Soderblom, 1982, Bull Amer. Astron. Soc. 14, 737 ). Color assignments in the unit map are indicative of the spectral nature of the unit. The unit maps make it possible to infer extensions of the geologic units mapped by B. K. Lucchitta and L. A. Soderblom (1982, in Satellites of Jupiter, pp. 521–555, Univ. of Arizona Press, Tucson) beyond the region covered in the high-resolution imagery. The most striking feature in the unit maps is a strong hemispheric asymmetry. It is seen most clearly in the ultraviolet/violet albedo ratio image, because the asymmetry becomes more intense as the wavelength decreases. It appears as if the surface has been darkened, most intensely in the center of the trailing hemisphere and decreasing gradually, essentially as the cosine of the angle from the antapex of motion, to a minimum in the center of the leading hemisphere. The cosine pattern suggests that the darkening is exogenic in origin and is interpreted as evidence of alteration of the surface by ion bombardment from the Jovian magnetosphere.

CCD photometry of the Uranian satellites

Broadband V and R CCD observations of the Uranian satellite system have been obtained over the full range of solar phase angles observable from Earth. These first visual observations of the phase curves of Miranda, Ariel, and Umbriel show that Ariel and Miranda exhibit the large opposition surges previously seen on the two outer Uranian satellites. Umbriel, however, lacks an appreciable opposition surge; its surface is either extremely compact or consists of small particles which lack a backscattered component. The tenuous structure of the other satellites is most likely due to the effects of eons of meteoritic gardening

The spatial distribution of UV-absorbing regions on Triton

Abstract Using mosaic maps of Tritons surface made with images taken through the Voyager 2 UV and Clear filters, we have sought to: (1) distinguish between surface units containing UV-absorbing (UVA) materials and fresh volatile frosts; (2) place any such identified regions into a geological context; (3) suggest possible candidate materials for the UVAs; and (4) to interpret these regions in terms of seasonal volatile transport scenarios. In pursuit of these goals, we have identified both a region possibly containing involatile UVA materials and an adjoining region possibly covered by uncontaminated, fresh frosts. The UVA region correlates well with a distinct geological unit on Triton containing maculae and wind streaks. The location of these two regions suggests that they might be linked through the dynamical transport of volatiles between them. Hydrocarbon deposits may explain the UVA region. However, they cannot be ruled out in regions darker than average in the Clear filter. Also, we cannot rule out other UVA materials as a visible component of the UVA region, such as salts or sulfur-bearing compounds, but note that such material has not yet been detected and may be largely obscured by hydrocarbon deposits. Recent images of Triton taken by us with the post-COSTAR HST FOC in bandpasses centered near 4100 and 2780 A will be analyzed to provide further information on the possible UVA and adjoining fresh frost regions. With resolution elements of ⋍400–700 km (depending on wavelength) at Tritons distance, the FOC images will provide the first mapping of these regions in the mid-UV (2780 A). This wavelength corresponds to the extreme of the broad-band absorption feature seen in Tritons UV spectrum (Stern et al., 1995), suggesting that the contrast between the UVA region and frost regions may be enhanced in the HST images.