New Images of Mercury Options

[Mongo]

A Side of Mercury Not Seen By Mariner 10
Gerald Cecil, Dmitry Rashkeev

http://xxx.lanl.gov/abs/0708.0146

More than 60,000 images of Mercury were taken at ~29 deg elevation during two sunrises, at 820 nm, and through a 1.35 m diameter off-axis aperture on the SOAR telescope. The sharpest resolve 0.2" (140 km) and cover 190-300 deg longitude -- a swath unseen by the Mariner 10 spacecraft -- at complementary phase angles to previous ground-based optical imagery. Our view is comparable to that of the Moon through weak binoculars. Evident are the large crater Mozart shadowed on the terminator, fresh rayed craters, and other albedo features keyed to topography and radar reflectivity, including the putative huge ``Basin S'' on the limb. Classical bright feature Liguria resolves across the northwest boundary of the Caloris basin into a bright splotch centered on a sharp, 20 km diameter radar crater, and is the brightest feature within a prominent darker ``cap'' (Hermean feature Solitudo Phoenicis) that covers much of the northern hemisphere between longitudes 80-250 deg. The cap may result from space weathering that darkens via a magnetically enhanced flux of the solar wind, or that reddens low latitudes via high solar insolation.

Bill

[gcecil]

Well, the technique is just frame selection from stacks of short-exposure images, so far batting ~1 in 1000 to get down to ~0.2" resolution. I'm restricted to twilight observations for reasons mentioned in the paper, and from our site at 2700 m elevation in Chile (accessed remotely). This means only elongations that get to ~29 deg elevation, when we bang off 30,000 images. There aren't many of those opportunities left before the MESSENGER flybys, so I don't plan any follow ups. I have other astro programs to execute in the near term, but am working on an instrument for SOAR's adaptive optics system that could image planets in narrow spectral lines (and that I may use for Mercury sodium emission mapping before MESSENGER arrives in orbit). Although we work in twilight, it takes time to install the aperture mask for imaging that would otherwise go to important calibrations of other observing programs.

Leonid Ksanfomality sent me the other day a superb image from July 2006 with "Basin S" smack on the terminator, a bull's eye. Unfortunately, that feature (no question from his data that it is real) was on the bright limb in our data, so we couldn't say much about it in our paper. I imagine that he is working on a follow up of his expanded paper in last April Icarus. Perhaps he will be at the DPS meeting in Orlando.