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Ice on the Moon: Blazing a Trail: The Origins of Ice"The moon was born hot," says Principal Investigator Dr. Alan Binder, "all the atmosphere and water boiled away." Any substance on the lunar surface is exposed directly to vacuum. For water ice, this means it will rapidly sublime directly into water vapor and escape into space, as the Moons low gravity cannot hold gas for any appreciable time. Over the course of a lunar day (~28 Earth days), all regions of the Moon are exposed to sunlight, and the temperature on the Moon in direct sunlight reaches about 395 degrees K (250 degrees above zero F). So any ice exposed to sunlight for even a short time would be lost. The only possible way for ice to exist on the Moon would be in a permanently shadowed area. The Moon's surface is continuously bombarded by meteorites and micrometeorites. Many, if not most, of these impactors contain water ice, and the lunar craters show that many of these were very large objects. When a body hits the moon,most of it is completely obliterated. "The few remaining molecules," Binder said, "will then jump as a function of the thermal energy of its current location." The molecule takes off like a rocket across the surface, and the hotter it is the further the molecule will jump. These molecules continue to jump around randomly in a process aptly called, 'random walk'. When they reach an area the is colder than 80 degrees Kelvin. The molecules cease to move at all. The migration naturally brings the molecules to cold traps in permanently shadowed craters because as it gets colder the molecules jump less, eventually slowing down to nothing. However, loss of ice due to photodissociation, solar wind sputtering, and micrometeoroid gardening is not well quantified. If the moon's inclination had been zero degrees, or near zero for the life of the solar system, then any ice found could have been there for just as long. However, while the moon was approximately 35 earth radi away from us (about 2 Billion years ago) we know it entered its second Casseni state. In this state, the moon's axis was not fixed and the poles swung around wildly, exposing themselves to sunlight. Any ice that was in the poles at that time would have been evaporated. Now, in its third Casseni state, the moons inclination is about 1 1/2 degrees allowing the craters of the moon's poles to lie in permanent shadow. Another suspicion is that maybe not ALL of the water boiled away. Some intrinsic water may remain and surface through volcanoes. This wouldn't account for much, however, and it is believed that most of the water would come from external forces. ICE ON THE MOON |
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