JGR Planets (AGU Publication) : "Large-Scale Troughs on Asteroid 4 Vesta Accommodate Opening-Mode Displacement."
Hiu Ching Jupiter Cheng and Dr. Christian Klimczak (Department of Geology, Structural Geology and Geomechanics Group, University of Georgia)
Asteroid 4 Vesta hosts two sets of enormous troughs, Divalia Fossae that encircle two-thirds of the equator and Saturnalia Fossae located in the northern hemisphere. These troughs were interpreted as grabens, thus invoking faulting. The trough sizes, their linear arrangement, and overall morphology leave no doubt that their origin is tectonic, but structures other than faults have not been considered. To test if they are fault-related or formed by accommodating opening-mode displacement (i.e., jointing) without subsequent shear, we investigate the map patterns, cross-sectional geometries, and variations of relief and width along the trough lengths. Relief and width could relate to the vertical displacement of faults and aperture of joints, respectively, and they therefore reveal differences in fracturing behavior. We analyzed six major troughs on Vesta, four belonging to Divalia Fossae, and two to Saturnalia Fossae. No map patterns are diagnostic of faulting or jointing. For each pair of trough-bounding scarps, the maximum relief does not lie at the trough center, and the two maxima occur at different positions along the trough. In contrast, troughs are widest near the centers of the troughs. These characteristics are inconsistent with graben formation but are consistent with jointing. Furthermore, rock-mechanical calculations that account for Vesta's low gravitational acceleration and degree of fracturing reveal that faulting is not favored to be initiated at depths above at least ∼3 but as much as 55 km within Vesta's lithosphere. Therefore, jointing or mixed-mode fracturing, both involving opening-mode displacements, are more plausible fracturing mechanisms for the Divalia Fossae.
- Trough morphology is inconsistent with the previously proposed fault origin and points to an opening-mode fracture origin.
- Normal faulting is not expected to be initiated above ∼3–55 km in Vesta's lithosphere.
- Opening displacements from jointing contributed to the topographic expressions of large-scale troughs.
Plain Language Summary
The camera on the Dawn spacecraft captured two sets of large linear depressions, or troughs, on asteroid Vesta. Previous studies hypothesized that these troughs are fault-bounded valleys with a distinct scarp on each side that together mark the downward drop (sliding) of a block of rock. However, cracking of rock can also form troughs, an origin that has not been considered before. Structures formed by sliding and cracking form different map patterns, have different morphologic expressions, and are controlled by different stresses acting on the rock volume. Although trough shape does not display any diagnostic map patterns, the morphology of the troughs is consistent with cracking because troughs are widest at the middle and narrow toward both ends. Calculated stresses are not favorable for sliding to be triggered within the uppermost ∼3–55 km of Vesta's rock volume, but instead, the physics shows that rocks are favored to crack apart. Therefore, the formation of these troughs must involve the opening of cracks, which is also important for understanding landforms on other small planetary bodies elsewhere in the Solar System.