The javascript on this page is used to preload the images for links and to swap the images used for links
go to content
layout image go to introduction
go to introduction to instrumentsGo to alpha particle spectrometerGo to Electron ReflectometerGo to gamma ray spectrometergo to magnetometergo to neutron spectrometergo to doppler gravity spectrometergo to scientistsgo to ice on the moongo to Further Reading 		layout image
Layout imageGo to home
go to projectgo to resultsGo to datavisgo to historyGo to scienceGo to educationGo to archivesgo to resources


ORIGINS OF ICE

I. Background

Over the course of a lunar day (~28 Earth days), almost the entire surface of the Moon is exposed to sunlight; the temperature in these sunlit regions reaches about 395 degrees K (250 degrees above zero F). Any ice exposed to sunlight for even a short time would be lost. So, the only possible way for ice to exist on the Moon would be in a permanently shadowed area. Clementine confirmed (as many had believed) that there are areas of the Moon's polar regions that lie in constant shadow. It also suggested that these areas might contain ice. ICE AT SOUTH POLEBut where did the ice come from?

As Principal Investigator Dr. Alan Binder said "The moon was born hot, and all the atmosphere and water boiled away." Although, he added, it is possible that not all of the water boiled away, that some intrinsic water may remain and surface through volcanoes, it wouldn't account for much. Most of the ice that has been found is thought to have come from external forces.

II. How did the ice get there?

 The craters of the lunar surface are a testament to the meteorites and micrometeorites that continuously bombard the Moon's surface. COMET IMAGEMany, if not most, of these impactors contain at least some amount of water ice. Unfortunately, most of it is completely obliterated by the energyof the impact, which can sometimes amount to several megatons.

One might then conclude that the only way for the ice to "stick" would be for one such body to impact directly on the South Pole. Not so, says Dr. Binder. In fact, this would be the "worst possible scenario." In this case the impact would not only destroy most of the impactor, but also any ice that was already there!

III. How does the ice stay there?

"After the impact," Binder said, "the few remaining molecules will then jump as a function of the thermal energy of their current location." These molecules take off like rockets from the surface, and the hotter it is the further the molecules will jump. Quite a few of the remaining molecules jump straight out into space, but some jump across the surface only to land and shoot off again. Others might try to jump into space, only to find themselves unable to escape the moon's gravitational field. These molecules continue to move around randomly in a process aptly called, 'random walk', slowing down as they jump into cooler areas until they reach an area that is below 80 degrees Kelvin. Once inside these 'cold traps', the molecules cease jumping altogether and could lie trapped for billions of years. Some might be lost over time due to photodissociation, solar wind, sputtering, and micrometeoroid gardening, but this has not been well quantified.

Further, we know that before about 2 Billion years ago, while the moon was approximately 35 earth radii away from Earth, its rotational axis was not fixed and the poles swung around wildly. In this state the poles, like the rest of the surface, were often exposed to sunlight; any ice that was trapped before then would have been evaporated. The moon eventually stabilized to its current inclination of about 1 1/2 degrees, which allows some craters of the moon's poles to lie in permanent shadow.

[CONTINUE]