A Possible Impact Basin on Pallas

In 2007, we observed Pallas in 5 filters with the Wide Field and Planetary Camera 2 (WFPC2) on Hubble Space Telescope(HST). In addition to photometry that showed Pallas" surface to have color contrast, particularly in the UV, our images were also useful for shape modeling. We arrived at a shape model of 291x278x250+/-9 km from the unprocessed images. We then used the MISTRAL deconvolution routine to reconstruct the images with knowledge of HST's point spread function. We were able to detect a sharp edge to the body in the processed images. While the shape quoted above represents the mean global shape, in the deconvolved images there is evidence for a large impression, possibly an impact basin, in the Northern Hemisphere. This is in agreement with recently presented measurements from Keck in the NIR (Carry et al., 2008 ACM meeting). As we viewed Pallas with a sub-observer point near 30 degrees S, we did not image the full extent of the basin. However, we can place limits upon its size and depth and the potential volume of ejected material. Such a result is interesting given Pallas" somewhat irregular shape and the presence of the Pallas family of small B-type asteroids that share common orbital properties with Pallas. We discuss the results of our continued study of Pallas and the implications that a large impact basin may have on our understanding of the body, its interior, its environment and history.