Mercury:: The image of the planet in the 210°-285° W longitude range obtained by the short-exposure method

For the purpose of obtaining images of the unknown portion of Mercury, we continued the previously started series of observations of this planet by the short exposure method. Several thousand electronic images of Mercury have been acquired on 1-2 May 2002 under good meteorological conditions at the high-altitude Skinakas Astrophysical Observatory of Iraklion University (Crete, Greece, 35degrees13' E, 24degrees54' N) during the evening elongation. The phase angle of Mercury was 95degrees-99degrees and the observed range of longitudes was 210degrees-285degrees W. Observations were carried out using Ritchy-Chretien telescope (D = 1.29 m, F = 9.857 m) with the KS 19 filter cutting wavelengths shorter than about 700 nm. The planet's disk was seen, on average, at an angle of 7.75" arcsec. The image scale was equal to 47.8 mum/arcsec. We used a CCD with a pixel size of 7.4 x 7.4 mum in the regime of short exposures. By processing a great number of electronic images, we succeeded in obtaining a sufficiently distinct synthesized image of the unknown portion of Mercury's surface. The most prominent formation in this region is a giant basin (or cratered "mare") centered at about 8degrees N, 280degrees W, which was given a working name "Skinakas basin" (after the name of the observatory where observations were made). By its size, the interior part of this basin exceeds the largest lunar Mare Imbrium. As opposed to Mare Imbrium, the Skinakas basin is presumably of impact origin. Its relief resembles that of Caloris Planitia but the size is much larger. A series of smaller formations are also seen on synthesized images. The resolution obtained on the surface of Mercury is about 100 km, which is close to the telescope diffraction limit. Also considered are the published theoretical estimations of the possible advantages offered by the short exposure method. Some results obtained by other research groups are discussed.