MERCURY RADAR-BRIGHT POLAR FEATURES

Mercury's radar-bright features at the north and south poles are likely caused by coherent backscatter from water ice deposits in shaded crater floors. The recently upgraded Arecibo 13-cm radar was used to make high-quality images of the north pole at 3-km and 1.5-km resolution. The south pole of Mercury will not be visible again from Arecibo until 2004. However, the south pole will be observable from Goldstone at the most southerly subradar locus in February 2001. The observing aspect next year will thus be as favorable as is possible, with the south pole tilted toward Earth by up to 10.7 degrees and the planet at minimum distance and hence maximum echo strength. The south pole is known to be dominated by a large circular feature coming from the floor of Chao Meng-Fu crater. A few other features have been seen at the south pole, but, because of a smaller data set, the 1994 Arecibo south polar image at 15-km resolution is somewhat lower in signal/noise than the 1994 Arecibo north polar image (and thus very degraded from the recent Arecibo coverage of the north polar features). Goldstone observations in February 2001 will be made using the so-called "long-code" technique to mitigate the aliasing effects of the overspread echo. This technique was developed primarily for Mars imaging, but the much greater overspreading at 3.5-cm requires long-code methods. In 1999, the long-code method was implemented at Goldstone for the first time, resulting in successful imaging of Mercury north polar features at 6-km resolution. This was the first time Mercury's polar features were resolved at 3.5-cm. By making similar observations in February 2001 and by integrating over several days of observation, we should obtain south polar images of quality comparable to the recent Arecibo north pole images. We also point out that, while our primary objective here is to image the south pole, these images will also give full coverage of the visible Mercury disk and will therefore be useful for reflectivity mapping of non-polar regions.