From: butler@cluster.gps.caltech.edu (Bryan Butler)
Newsgroups: sci.space,alt.sci.planetary
Subject: Re: Lunar "colony" reality check
Date: 11 Nov 1992 04:35:21 GMT
Organization: California Institute of Technology, Pasadena
NNTP-Posting-Host: cluster.gps.caltech.edu
Keywords: mercury, ice

In article <1992Nov10.152154.9709@eng.ufl.edu> joev@sioux.eel.ufl.edu (Joseph Versagg) writes:
>Sorry for rehashing what was posted earlier, or , even worse, what may now
>be common knowladge, but what is this about ice at the poles of Mercury?
>Since Mercury rotates, although slowly, ice would be baked off the surface,
>then would leak into space due to the low gravity.
>

we have a report in a recent issue of Science (23 October, 1992)
explaining our findings:

Mercury Radar Imaging: Evidence for Polar Ice
M. A. Slade, B. J. Butler, and D. O. Muhleman

which appears with 2 companion papers:

Radar Mapping of Mercury: Full-Disk Images and Polar Anomalies
J. K. Harmon, and M. A. Slade

The Thermal Stability of Water Ice at the Poles of Mercury
D. A. Paige, S. E. Wood, and A. R. Vasavada

our paper describes a radar experiment in which we detected
an anomalous radar return from the north polar region of
Mercury, using the combined VLA/Goldstone bistatic radar.  
the polar region has 2 distinctive radar properties:
first, it is the brightest reflector on the portion of the
disk we imaged (about 80% of the surface), and second, its
"polarization ratio" is > 1.  the second property alone is
enough to indicate the presence of ices, but the interpretation
is certainly not unique.  however, the only surfaces
for which this property has been detected are icy ones (the
surfaces of the Galilean satellites, and the residual south polar
cap of Mars).  subsequently, the Harmon and Slade data from
Arecibo confirmed the presence of the north polar anomaly,
and found a similar one at the south pole (i should note here
that our experiment was at 3.5 cm, while the Arecibo experiment
was at 13 cm).  this clearly indicates that whatever is causing
the anomalous reflection is temperature related.  so, in order
to investigate the thermal regime of the mercurian surface,
we brought in Dave Paige and his co-workers to do the thermal
modelling.  their results indicate that permanently shadowed 
regions near the poles of Mercury can get as cold as about 50 
K (this is very cold, but remember that the Mariner 10 radiometer 
measured physical temperatures as cold as 90 K on the nightside 
equator).  the presence of large regions in permanent shadow is
facilitated by the fact that Mercury has a very small obliquity.
50 K is certainly cold enough for most ices to remain stable 
over solar system time scales (billions of years).  the problem 
is then how to get the ices there.  this is answered in part 
in a paper soon to be published in JGR (the paper is going
through revision right now):

Mercury: Full Disk Radar Images, and the Detection and Stability
of Ice at the North Pole
B. J. Butler, D. O. Muhleman, and M. A. Slade

where i discuss more fully the experiment, and the sources,
sinks, migration and stability of ice on the mercurian surface.

note that all of this has reopened the discussion of polar
volatiles on the Moon. 

	-bryan
	 butler@cluster.gps.caltech.edu
      or butler_b@caltech.edu

Newsgroups: sci.space