Earth-based optical imaging of Mercury

We continued our earlier short exposures of Mercury to achieve resolved images of unknown parts of the surface. At the Skinakas Astrophysical Observatory (1700 m, 35°13' E, 24°54' N, on Crete, Greece) observations were made on May 1 and 2 UT of 2002; more than 20 000 electronic photographs were obtained near eastern elongation. The Ritchey-Chrétien telescope (D = 1.29 m, F = 9.857 m) was used with a KC-19 filter (cutting off below 700 nm). A CCD-camera pixel size was 7.4×7.4 mm. The Mercury phase was within 95 - 99o. The exposures were mainly 1 ms (up to 10 ms at greater zenith distance). The resolution obtained in this work fits well with the results of theoretical studies (Fried, 1978) and experimental ones (Ksanfomality, 2002); it is about 130 km on the surface of Mercury. After preliminary selection about two thousands images were processed. The surface of Mercury at longitudes 250-285oW was unknown until this time. The only possible criterion of reality of the details is therefore their presence on several independently obtained synthesized images. The most prominent formation, namely a large basin, or "crater mare" is seen at the terminator, with its center near 8°N, 280°W. This basin has a double rim with diameter of the internal rim around 25° (1060 km), while the external rim diameter exceeds 2000 km. Our nickname for this formation is "Skinakas Basin" after the observatory. The internal part exceeds the largest on the Moon, Mare Imbrium, while the external rim has scales of the Moon's Procellarum Oceanus. Skinakas Basin seems may have a central hill, implying an impact origin, while Imbrium's surface presents a lava field. The South Pole - Aitken basin may be even a closer analogue. The size of Skinakas Basin also points to very large and, probably, early impact event. The relief of Skinakas Basin may be a first manifestation that Mercury has same relief global asymmetry as is known on the Moon.