Dark Terrain Research Articles

Crater numbers and geological histories of Iapetus, Enceladus, Tethys and Hyperion

Tethys, Iapetus and Enceladus, satellites of Saturn, display surfaces which indicate that geological processes have been active suggesting a certain degree of internal evolution. On Iapetus activity seems to have been confined to the dark terrain and its timing and extent remain unknown. The widest variation of terrains and crater numbers occurs on Enceladus and indicates the most prolonged geological activity of any satellite. Activity on Tethys seems confined to the first few hundred million years. The satellite Hyperion and the co-orbitals 1980S1 and 1980S3 show no geological activity and apparently are fragments of once larger bodies.

The tectonics of Ganymede

The surface of Ganymede, the largest icy jovian satellite, is composed of two distinct terrain types showing abundant evidence of global-scale tectonics. There is geological evidence consistent with the formation of younger bright terrain by the emplacement of relatively clean ice into rift zones developed in the more silicate-rich dark terrain.

Size Frequency Distribution of Martian Craters and Relative Age of Light and Dark Terrains

Light and dark terrains in and around Meridiani Sinus, mapped on the imagery from Mariner 6 and Mariner 7, were found to have significantly different cumulative size frequency distributions of craters. The light terrain on a mosaic of frames 6N11, 6N13, and 6N19 has a greater proportion of large craters and a lesser proportion of smaller craters than the dark terrain on the same frames. The light terrain is interpreted to be generally older than the dark terrain. The filling or partial filling of the smaller craters on the light terrain by surface detritus is suggested. Several wide-angle frames have remarkably similar cumulative crater size frequency distributions that may be representative of a large portion of the martian surface.

Airglow of Venus: a re-examination

Rhea is the second largest icy satellites of Saturn and it is mainly composed of water ice. Its surface is characterized by a leading hemisphere slightly brighter than the trailing side. The main goal of this work is to identify homogeneous compositional units on Rhea by applying the Spectral Angle Mapper (SAM) classification technique to Rhea’s hyperspectral images acquired by the Visual and Infrared Mapping Spectrometer (VIMS) onboard the Cassini Orbiter in the infrared range (0.88–5.12 μm). The first step of the classification is dedicated to the identification of Rhea’s spectral endmembers by applying the k-means unsupervised clustering technique to four hyperspectral images representative of a limited portion of the surface, imaged at relatively high spatial resolution. We then identified eight spectral endmembers, corresponding to as many terrain units, which mostly distinguish for water ice abundance and ice grain size. In the second step, endmembers are used as reference spectra in SAM classification method to achieve a comprehensive classification of the entire surface. From our analysis of the infrared spectra returned by VIMS, it clearly emerges that Rhea’ surface units shows differences in terms of water ice bands depths, average ice grain size, and concentration of contaminants, particularly CO2 and hydrocarbons. The spectral units that classify optically dark terrains are those showing suppressed water ice bands, a finer ice grain size and a higher concentration of carbon dioxide. Conversely, spectral units labeling brighter regions have deeper water ice absorption bands, higher albedo and a smaller concentration of contaminants. All these variations reflect surface’s morphological and geological structures. Finally, we performed a comparison between Rhea and Dione, to highlight different magnitudes of space weathering effects in the icy satellites as a function of the distance from Saturn.