Before the age of space exploration, the size and composition of the moon's core were astronomical mysteries. Astronomers assumed that the moon's core was smaller than that of the Earth, in both relative and absolute terms -- the radius of the Earth's core is 55 percent of the overall radius of the Earth and the core's mass is 32 percent of the Earth's overall mass -- but they had no way to verify their assumption. However, data gathered by Lunar Prospector have now given astronomers the ability to determine that the moon's core accounts for 20 percent of the moon's radius and for a mere 2 percent of its overall mass.
The data have been used in two ways. In the first method, scientists measured minute variations in radio signals from Lunar Prospector as the craft moved towards or away from the Earth. These variations allowed scientists to detect even the slightest changes in the craft's velocity while the craft orbited the moon. These changes resulted from inconsistency in the gravitational pull of the moon on the craft, and permitted scientists to create a "gravity map" of both near and far sides of the moon. This map, in turn, revealed to scientists the distribution of the moon's internal mass. Scientists were then able to determine that the moon has a small, metallic core, which, if composed mostly of iron, has a radius of approximately 350 kilometers. The second method involved examining the faint magnetic field generated within the moon itself by the moon's monthly passage through the tail of the Earth's magnetosphere. This approach confirmed the results obtained through examination of the gravity map.
The size and composition of the moon's core are not academic concerns; they have serious implications for our understanding of the moon's origins. For example, if the moon and Earth developed as distinct entities, the sizes of their cores should be more comparable. In actuality, it seems that the moon was once part of the Earth and broke away at an early stage in the Earth's evolution, perhaps as the result of a major asteroid impact. The impact could have loosened iron that had not already sunk to the core of the Earth, allowing it to form the core around which the moon eventually coalesced.


The author mentioned "gravity map" in the second paragraph in order to

illustrate the method used to correct the Lunar Prospector's velocity

confirm the results obtained by examining the moon's magnetic field
detail a step in the investigation of the composition of a specific orbiting body
detail a step in the investigation of the effects of gravity on the moon and earth
explain a step in the process of confirming a new hypothesis and discarding an older theory