No. 79: Jan-Feb 1992
Mercury, closest planet to the sun, should be baked bone dry, seeing that equatorial temperatures reach 800°F. When Mariner 10 flew past Mercury in 1974, its camera eye reinforced the baked-cinder model. To everyone's surprise, recent radar images obtained with powerful earth-based antennas, revealed a highly reflective patch at Mercury's north pole. Could it be ice, for ice reflects radar waves well? Quite possibly, for when Mercury's polar temperatures are calculated, away from the sun's direct glare, they plunge to -235°F. This means that some of the water vapor in the planet's thin atmosphere might freeze out in the poles, creating ice or frost caps.
Wouldn't Mariner 10 have seen such a remarkable deposit? Not necessarily, for the spacecraft viewed only half the planet and, if the 640 x 300-kilometer ice patch were covered with dust, it could have been invisible to the camera. But it would still be a bright patch on terrestrial radar scopes, because radars see through thin dust layers.
So, polar ice is not physically impossible on Mercury, although it is defi nitely surprising so close to the sun. All that is needed is a little water in the planet's atmosphere. Mainstream thinking is that "passing comets and asteroids" might bequeath Mercury some of their H2O cargos. (Cowen, R.; "Icy Clues from Mercury's Other Half," Science News, 140:295, 1991.) Also: Wilford, John Noble; "Photographs by Radar Hint of Ice on Poles of Mercury," New York Times, p. A14, November 7, 1991. Cr. J. Covey)
Comment. What the above references do not mention is the possibility that the requisite water vapor for the formation of Mercury's polar caps might come from a steady rain of icy minicomets. L. Frank has suggested that 100-meter icy minicomets continuously pepper solarsystem planets. They might even have contributed to the formation of the earth's oceans. Icy comets are anathema here on earth and are equally detestable at Mercury's orbit.