From: henry@zoo.toronto.edu (Henry Spencer)
Subject: space news from Oct 23 Science (not AW&ST)
To: sci-space-news@news.arc.nasa.gov
Followup-To: sci.space
Organization: U of Toronto Zoology
Date: Fri, 20 Nov 1992 00:42:58 GMT

[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
Newsgroups: sci.space.news