Snowballs in Hell

The Small Comet Hypothesis and Mercury

The more we look at Mercury, the more water we find. Glacial water-ice is extensive at both poles. Current estimates are perhaps a trillion tons of glacial ice in the shaded craters. Everyone is careful not to over-estimate because any water on Mercury’s surface is unexpected. The average surface temperature of Mercury is measured at 320° Fahrenheit - daytime highs are in excess of 800°F. Without an atmosphere its always cold in the shade. It’s understandable why glacial water-ice formations are confined to the interiors of craters. The question is: How does the ice get there, and how long has it been there? I want to try and answer those questions by looking at another feature of Mercury’s surface: Something called, “the hollows.”

“Mercury’s hollows cover .08% of the planet’s surface... Everyone seems to agree that hollows form when something in Mercury's surface sublimates (turns from solid to gas).” What are Mercury’s Hollows? The Planetary Society, Emily Lakdawalla, Feb. 18, 2014

The hollows are recent formations. They are forming in the present. They are described in sub-kilometer scale. They are not impact craters. “The hollows look kind of like the surface is being eaten away, like someone splashed acid inside the crater.” ibid. The surface of Mercury is rich in the volatiles, Carbon, Sodium, Potassium and Chlorine. C, Na, K, Cl. We know all this because NASA’s spacecraft Messenger sent back over 250,000 images as it orbited Mercury between 2011 and 2015.

The small comet hypothesis: In 1982 Professor Louis A Frank, and his team at the University of Iowa, discovered ‘atmospheric holes’ in the ultraviolet dayglow images of earth’s ionosphere that were being generated by imaging equipment aboard the Dynamics Explorer spacecraft. Frank concluded these ‘atmospheric holes’ were caused by dense clouds of water vapor which were the remains of meteoric objects made almost entirely of water-ice. He called these meteoric objects, “small comets.” There would have been nothing controversial about his discovery, except the images indicated an estimated in-fall of ten million, thirty-ton, snowball-like, diminutive comets, annually. His discovery has never been refuted. It has been denied. His proofs have been considered insufficient. Frank was forced, in defense of his discovery, to determine the composition of the small comets. He posited a thin mantle consisting of carbon and other volatiles protected the small water comets from the solar heat; very similar, but not exactly similar to large comets. Frank suffered a stroke twenty years ago. He died in 2014. Since Frank’s time we have discovered vast quantities of water on all the planetary objects in our solar system - including Mercury.

Frank never estimated small comet impacts on Mercury. He dismissed the possibility the small comets would reach the inner most part of the solar system intact. He estimated the small comet flux would deliver 500,000 impacts to Pluto every Earth-year. Pluto is not as large and dense as Mercury. Frank’s estimate for Pluto extrapolated for Mercury would mean approximately sixty small comets would impact the surface every hour at an average speed of 30,000 miles per hour.

The small comet composition and impact speed combined with the planet’s very high surface temperatures would react with the surface volatiles explosively, one can imagine what was described in the above quotation. “The hollows look kind of like the surface is being eaten away, like someone splashed acid inside the crater.” ibid The very thin exosphere of Mercury could not exist more than two or three days without the constant resupply of particles. The small comet hypothesis has these explosive, volatilizing, impacts continually supplying the exosphere with it’s principal components, hydrogen and oxygen, sodium and potassium, and interestingly, trace water vapor. The surface volatiles have been delivered to the planet by the small comets in the first place and concentrated through the dissipation of the water molecules. Much of the exosphere returns to the planet’s surface by gravity, although obviously a substantial quantity is lost to space.

Inside the shaded craters, the impacts are not explosive. In those craters where direct sunlight never shines, the water accumulates as glacial ice. The small comet hypothesis is consistent with the distribution of both the hollows and the water-bearing polar craters. It’s a simple, even elegant, hypothesis. It’s proven true - if the small comets are in-falling. I suspect they are. I will discuss water and water-ice on other planetary objects in the future.

I know the small Mercury Comets are history. It doesn’t mean we shouldn’t consider the possibilities. If we look we might see them.