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No. 95: Sep-Oct 1994

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Snowballs in hell?

Arecibo radar image of Mercury's morth pole
Arecibo radar image of Mercury's morth pole showing several craters.
In SF#79, we revealed that anomalous radar reflections from Mercury's polar regions might be due to residual deposits of water ice. At first, this possibility seems most unlikely given Mercury's proximity to the sun. Where the sun's rays beat directly on Mercury's surface, the temperature can reach 700�K. Even glancing sunlight, occurring when the sun is perched on Mercury's horizon, should heat the surface to 170�K. At this temperature, water ice would evaporate quickly in Mercury's near-vacuum atmosphere. But any permanently shaded areas at the planet's polar caps -- say, deep in a crater -- would remain below 100�K. This is cold enough to retain ice, even in a vacuum.

Radar topographic studies of Mercury's polar regions, using the Jet Propulsion Laboratory's Goldstone antenna with the VLA (Very Large Array) plus the big Arecibo antenna in Puerto Rico, have been able to confirm that there are indeed craters in the polar regions of Mercury. These craters match up well with the radar reflectivity anomalies recorded earlier. So, it now seems likely that ice does exist on Mercury. And, since our moon also boasts permanently shadowed crater areas, ice probably survives there, too. This is good news for future lunar colonists.

But where could the ice on Mercury and the moon have come from? One source might have been the gases seeping out from the bodies' interiors. Also, cometary impacts could have added water vapor to the atmospheres. This would then have been deposited as frost in cold crater bottoms, just like the frost seen on winter window panes.

(Harmon, J.K., et al; "Radar Mapping of Mercury's Polar Anomalies," Nature, 369:213, 1994.)

Comment. But are comets really the water bearers the astonomers say they are? See the item under ASTRONOMY.

From Science Frontiers #95, SEP-OCT 1994. � 1994-2000 William R. Corliss