MAPPING VESTA'S NORTH POLAR QUADRANGLE AV-1: A GLIMPSE INTO THE DARKNESS

NASA’s Dawn spacecraft arrived at asteroid Vesta on July 15, 2011, during northern winter. Vesta's rotational axis is tilted by 29° with respect to the orbital plane; therefore the surface at >~60° N was not illuminated during initial orbital mapping. As the one-Earth-year-long mission progressed, more of the unseen terrain became visible. The point at 90° N was captured in images during Dawn's second High Altitude Mapping Orbit (HAMO2, June 15 to July 25, 2012), though much of the surrounding area remains in shadow. Additional images at lower spatial resolution will be obtained during the spacecraft's departure from Vesta (July 26 to Aug. 26).

The Av-1 quad extends from 65° N to the pole. The region is dominated by three large depressions. One, centered at 70° N, 111° E, is clearly an impact crater of ~60 km diameter. This crater's northern wall and floor are illuminated. The wall shows evidence for downslope movement, with a hummocky floor deposit. Parts of the crater rim are sharp, while others have a rounded appearance perhaps due to the effects of impact onto pre-existing topography. The north pole lies within an ~110-km diam. depression centered at ~84° N, 233° E. The interior of this depression is largely in shadow in HAMO2 images; it is uncertain whether this structure is a degraded impact crater(s). Another large area of shadow corresponds to a somewhat elongated depression (~80 by 90 km) centered near 74° N, 50° E, possibly a degraded impact crater. The antipodes of Vesta's two large southern impact basins (Rheasilvia, 500-km diam. and Veneneia, 400-km diam.) lie in or near the Av-1 quad. Unusual "hilly and lineated" terrain, thought to have formed by converging ejecta or seismic waves, is found at the antipodes of large lunar and mercurian impact basins. Therefore it is of interest to determine if such terrain exists on Vesta. No large-scale hilly and lineated type terrain is obvious on available HAMO2 images, though shadowing and extremely low-Sun lighting make evaluation difficult. The geographical antipode of Rheasilvia is located within the 60-km crater at 70° N, 111° E. Thus the formation of this crater and emplacement of its ejecta could have destroyed or covered any antipodal landforms. We will continue to examine HAMO2 and departure images as they become available to map the geology of the Av-1 quad.