[Loud cries of "huh?" from the audience members who notice the difference in Subject line... This is a one-shot.]
The 23 Oct issue of Science has three very interesting papers, the formal scientific announcement of the radar discovery of ice at Mercury's poles. Herewith a short summary...
The first paper discusses the Goldstone/VLA observations that first spotted radar-reflection anomalies at Mercury's north pole. Viewed with circularly- polarized radar, looking at "same sense" echos, a small region near the north pole is the brightest feature in the image. It's visible, although fainter, in "opposite sense" images. In an image showing SS/OS ratios, it blazes brilliantly. No ordinary feature or material will do this; the only cause that is at all likely is "coherent backscatter" from particles imbedded in ice. The reflection is very similar to that of Mar's south polar cap, except that the radar reflectivity is too low. Possible causes of the poor reflectivity are patchy ice, ice underneath 0.5-1m of soil, and relatively thin ice; there is limited evidence pointing to the second explanation.
The second paper discusses observations from Arecibo that confirmed the north-pole anomaly at a different wavelength, found a similar one at the south pole, observed both through a substantial fraction of Mercury's rotation, and as a bonus, pinned down the location of Mercury's poles rather more precisely.
The third paper discusses thermal stability of ice at Mercury's poles.
Even assuming a simple spherical planet, Mercury's poles are rather cold places, with an average temperature of 138K and a peak of 167K. While the Sun is big and close, it also never gets much above the horizon (the Arecibo observations having confirmed theoretical predictions that Mercury's axis is almost precisely perpendicular to the plane of its orbit). However, this isn't really cold enough for ice to be stable on the surface for long periods, unless it has, and maintains, a highly reflective surface. This is perhaps not impossible, since reflectivities at glancing angles are higher than at more normal angles, and there are some areas of quite high reflectivity elsewhere on Mercury.
More to the point, local topography can cool things down further, by creating permanently-shadowed areas, e.g. in crater bottoms. It turns out that large shallow craters work best, because the bottoms of smaller (deeper) craters tend to be kept warm by light scattered from the sunlit rims. Large, shallow craters within 5 degrees of the poles should have floors cold enough to retain exposed ice over geological time. This fits the observations. The south-pole radar anomaly appears to be contained within the crater Chao Meng-Fu, 155km across and practically at the pole. Mariner 10 images of the north-pole area aren't as good, but there are several substantial craters in the imaged part of the radar anomaly.
Issues of possible sources, migration, etc. of the ice are punted to another paper, listed as "in preparation".
[Oh yes, this issue also has a feature on "Science In Japan", with some nice solar-flare images from the Yohkoh solar-astronomy satellite.]
MS-DOS is the OS/360 of the 1980s. | Henry Spencer @ U of Toronto Zoology -Hal W. Hardenbergh (1985)| henry@zoo.toronto.edu utzoo!henry