“Blood Stains” on Tethys: Evidence for Recent Activity?

Abstract:

Distinctive set of arcuate, reddish-colored lineaments has been identified on Tethys. These markings are slightly darker than adjacent cratered terrains but have a flatter green–IR spectral slope. There are at least three prominent sets in the northern hemisphere centered on the anti-Saturn meridian. Each set consists of ~5–10 parallel lineations a few kilometers across and 50–250 km long. The lineations are remarkably curvilinear (i.e., non-sinuous) and not deflected by major impact structures: they cross uninterrupted the floor of 400-km-diameter, 10-km-deep Odysseus impact basin. In one area (~25° N, 185°W), high-resolution mapping at ~90–125 m/pixel shows no discrete scarp, ridge, or other tectonic manifestation along the feature. Instead, only a faint discoloration and ~25 small dark spots 200–800 m in diameter have been identified. These spots are characterized by very low albedos, sharp boundaries, and no evidence of raised rims expected with impact origin. Of these, ~60% (15) are at the bottom of small craters. The IR lineaments show remarkable symmetry centered on the tidal axis with Saturn. Exogenic mechansims are ruled out by geometry and geology, hence we explore possible endogenic origins. Stress mechanisms being tested include non-synchronous rotation (considered unlikely for a cold, triaxial body like Tethys), tidal recession and true polar wander. The lack of obvious tectonic deformation despite the strong color signature is unusual (although structures may exist below the current resolution limit). Though unlikely, the lineaments could be reactivated ancient fractures, producing a temporal discoloration. If tectonic, the lineaments might be still forming, with deformation only on a scale below that which we can resolve. The coloration, global pattern and collocated dark spots are consistent with recent/active alteration of the surface, given that E-ring accumulation is expected to remove intrinsic color signatures in a geologically short time period. Low-volume but persistent outgassing/deposition of volatiles from the interior, the colors of which are distinct from the evolved surface and/or result from exposure to space, may be responsible. High-resolution color and IR mapping is planned for November.