Water on the Moon

A number of decades ago some scientists predicted the existence of frozen water on the Moon, based on calculations they made of temperatures in the shadowed interiors of craters. The Clementine and Lunar Prospector missions and radar measurements with the Arecibo radio telescope appear to have confirmed the predictions, though not all scientists agree that the measurements made absolutely confirm the presence of water. Measurements by the Prospector neutron spectrometer show that concentrations of hydrogen are highest in permanently shadowed craters, strong evidence for water in those permanently cool places.The H may be in water ice or in some other form, and the total amount is not known. It may have come from the solar wind or from cometary impacts. Richard Vondrak of the Goddard Space Flight Center reported on a model he has been developing with Dana Crider of the Catholic University of America. They conclude that all the observed hydrogen at the poles could be derived from implanted hydrogen that has been mobilized by micrometeorite impacts and sputtering by cosmic rays, and therefore there is no need to call upon water delivery by comets. This is surprising because comets are loaded with water ice and organic compounds (hence hydrogen) and, in the natural course of solar system collisions, have been colliding with the Moon for billions of years.Where, then, is all the additional water the comets delivered to the Moon? Crider and

Vondrak suggest a comet impact might produce such a hot, gaseous plume that almost all the water escapes.

If there is water on the Moon, it will be frozen in areas of permanent shadow. A permanent shadow in a crater on the Moon is defined as an area on the surface of the Moon that never receives sunlight.This happens at the poles because the Moon's axis of rotation is nearly perpendicular to the plane of its orbit around the Sun, so the Sun is always low, close to the horizon, casting long shadows off the crater rims or any other high point. Because temperatures in these polar shadows do not exceed about —380°F (—230°C), water in them is cold enough at all times that it cannot evaporate or even sublimate (move from solid ice to gas, as happens to the ice in frostfree freezers).

In their simulations of lunar topography, Ben Bussey of the Johns Hopkins University Applied Physics Lab and colleagues superimposed the shapes of simple craters onto a sphere the size of the Moon. Simulations were run for craters ranging in diameter from 1.5 to 13 miles (2.5 km to 20 km) to see how the amount of permanent shadow varies with crater size. For each size, simulations were run on craters placed at latitudes from 70 degrees to 90 degrees at one degree increments. Seasonal variations were studied by moving the position of the Sun's direct ray 1.5 degrees above or below the equator, to represent summer and winter for the Northern Hemisphere. The researchers assumed in their simulations that the craters were fresh and that they did not lie on a regional slope (slopes would expose a crater rim to more or less sunlight, thus affecting the size and shape of the internal shadow). They found that for a given latitude, larger craters have slightly more relative permanent shadow in them than smaller craters, simply because larger craters have higher rims. More than crater size, they found that latitude is the dominant parameter affecting the amount of permanent shadow in a simple crater. Craters as far as 20 degrees away from a pole still have significant amounts (22 to 27 percent) of permanent shadow. Craters near the poles can remain up to 80 percent in shadow even in the lunar summer. Their next step was to examine images of the Moon and identify all the fresh-looking simple craters larger than one kilometer within 12 degrees of each pole. By measuring the diameters of these craters and using their calculations for permanent darkness, they calculated the total amount of permanent shadow on the Moon.

The 832 craters at the North Pole have a total surface area of approximately 4,900 square miles (12,500 km2), and contain about 2,900 square miles (7,500 km2) of permanent shadow.This value is a lower limit because it is based solely on simple craters, excluding complex craters and craters on slopes. More shadow is contributed by poleward-facing walls of complex (multi-ringed) craters. The 547 craters identified at the South Pole have about 2,500 square miles (6,500 km2) of permanent shadow. If all the permanently shadowed regions contain water ice at a concentration suggested by Lunar Prospector data, about 1.5 weight percent, then the volume of ice on the Moon might be nearly one-quarter cubic mile (one cubic kilome-ter).While reports of ice on the Moon have created excitement in the community of people who want to place permanent human habitation on the Moon, the quantity of ice and its distribution in minute amounts over large land areas makes this idea less than practical.

The ancient lunar surface changes little in the present day but it perfectly preserves the record of the processes that occurred on the Moon 4 billion and more years ago. On the Moon are written the events that undoubtedly also happened to the Earth, where their evidence was wiped away by erosion and plate tectonics: Magma ocean crystallization, the Late Heavy Bombardment, active and voluminous volcanism. The Earth has progressed to become a world of oxygen and water, but the Moon lost its magnetic field and whatever thin atmosphere it ever possessed, and now lies exposed to extremes of temperature and the battering of the solar wind.

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