DIX Planetary Science Seminar

Enceladus Spills its Guts: A Relationship Between Strike-Slip Motion and Jet Activity over the Tiger Stripes

At Enceladus, jets along four distinct fractures (i.e., 'Tiger Stripes') erupt ice crystals into a broad plume above the South Pole. Repeated, high-resolution measurements of plume brightness from the Cassini Imaging Science Subsystem (ISS) indicate that jet output varies with a period equal to that of Enceladus's orbit of 32.9 hrs. Tidally-driven normal displacement along fractures could modulate jet activity but require either viscous effects or fluid inertia to account for the 'phase lag' of ~ 5-6 hrs between maximal opening and observed peak activity at mean anomaly φ = 210°. Moreover, normal opening does not readily produce a second, smaller observed peak in activity near φ= 30°. We use a 3D numerical model of tidal deformation to demonstrate that the Tiger Stripes exhibit periodic strike-slip motion with a temporal pattern that is highly correlated with observations of plume activity. Diurnally rotating shear tractions along the Tiger Stripes naturally produce peaks in left- and right- lateral motion near φ = 30° and 210°, respectively. Large variations in tidally-driven normal traction (~ 60 kPa) along interfaces reduce the amplitude of the peak in strike-slip motion at φ = 30° relative to that at φ = 210° for Tiger Stripes which exhibit Coulomb friction. The spatial distribution of mean strike slip motion along the Tiger Stripes is also consistent with observations of heat flow which show maximal activity along longer faults (i.e., Baghdad Sulcus) and near the center of each fracture. We conclude that periodic right- and left- stepping bend opening due to strike-slip motion along the Tiger Stripes could feasibly modulate Enceladus's short-term activity and the long-term tectonic evolution of the South Polar Terrain.

Chemical Weathering Conditions in Nili Fossae as Indicated by Aluminum Phyllosilicates and Associated Mineralogy

Aluminum-phyllosilicates have been detected in upper stratigraphic units as the infrared spectrally dominant mineral in 100s–1000s of km2 exposures of Noachian (~3.7 Ga) rock across Mars. These units have been previously proposed to be analogous to terrestrial basalt weathering sequences with Fe/Mg-phyllosilicates below the Al-phyllosilicates. These minerals appear associated with jarosite (KFe3(SO4)2(OH)6) and alunite (KAl3(SO4)2(OH)6), sulfate minerals indicative of strongly acidic weathering conditions. Understanding what these and other indicator phases reveal about the origin of the Al-phyllosilicates and Mars' aqueous history requires a detailed analysis of their chemistry and context. We focus first on the Nili Fossae region. We examine visible–near-infrared hyperspectral imagery from the CRISM instrument, using Gaussian fitting to automatically identify and map these phyllosilicate units and their accompanying minerals. We combine these mineral maps with digital elevation models from the Mars Context Camera and high-resolution imagery from the HiRISE camera to understand the stratigraphic relationships between these minerals. Initial local-scale analyses show sparse jarosite closely spatially associated with Al-phyllosilicate, and these alteration products are found in sedimentary layers. Their stratigraphy suggest that Al-phyllosilicate and jarosite formed from felsic or more easily weathered material, perhaps volcanic ash, rather than from extensive leaching of basalt. However, further regional- and local-scale analysis of the stratigraphy is required to determine a preferred formation mechanism and understand the alteration processes on ancient Mars.