Ninety Degrees South! Geology of the South Pole of Venus

As part of its second cycle of radar mapping, the Magellan spacecraft collected SAR data at 12.6 cm wavelength in a right looking configuration that provided the opportunity to image the Venus South Pole. These data were processed into Basic Image Data Record (BIDR) image strips and archived. Image mosaics that were later produced however, include data extending only as far south as ~82.5° S, resulting in a significant image gap covering over one million km2 centered on the South Pole. Subsequently, we have processed these data into a complete south polar stereographic mosaic at a resolution of 225 meters/pixel. In this paper, we provide an overview of this data set, the first geologic analysis of this previously unseen part of Venus, and an assessment of mapped units in relation to the broader regional geology. The analysis presented here covers the region extending from ~82.5° S to the South Pole. Data acquired over this part of the planet were obtained at incidence angles between ~15.2° and 12.8° where the returned radar echo strength is primarily determined by variations in surface topography with lesser contributions from scattering by sub-wavelength sized surface scattering elements. As such, radar-facing slopes are quite distinct in the resultant images, providing insight into the morphology of mapped surface features. The region centered on the south pole of Venus is a volcanic province composed of broad regional plains and shield fields. Seven intermediate scale (10s of km) volcanoes that range from ~24 to 64 km in diameter are clustered near the South Pole itself. Associated lava flows extend for distances of 100s of km, forming part of the vast volcanic plains. Located within the mapped region are five previously unobserved impact craters. These structures range from about 4 to over 17 km in diameter. Superposition relations suggest that the larger craters are relatively young as evidence of recent modification by folding, faulting, and embayment is lacking. Several of the smaller craters are irregular in shape, which may be due to post impact deformation or break up of the impactor before it hit the surface. Tectonic activity on this part of Venus is relatively limited and is characterized by faulting and fracturing associated with previously identified coronae outside the study region and the widespread formation of wrinkle ridges that suggests broad regional compression. Lava flows from some of the larger volcanoes appear to be both modified by and superposed on some of the wrinkle ridges, suggesting contemporaneous formation relations. Within a broad geologic context, the South Pole area is a distinct demarcation between the more northerly complex system of rifting, volcanism, and corona formation associated with Quetzalpetlatl Corona (region between 0° and 90° longitude) and compression forming wrinkle ridges and ridge belts covering longitudes of 180° to 315°. The center of volcanic activity at the South Pole bears similarities to other areas on Venus interpreted to be hot spot rises such as Western Eistla Regio and Juno Chasma. The presence of widespread flood volcanism, identified typically as an early stage event at other hot spots (e.g. Juno Chasma), and the lack of a rift system may indicate that the South Pole is the site of a volcanic rise in the early stages of its development.