February 18, 1998
Lon Hood, a senior research scientist at The University of Arizona's lunar and planetary laboratory, thinks data from the Lunar Prospector satellite might indicate whether the moon originated from debris orbiting Earth or from material ejected in a colossal impact between Earth and some other massive body. As a member of the magnetometer team, Hood will examine data which should reveal the presence of a lunar metallic core. The existence of a lunar core could provide important evidence for one of the competing theories of the moon's origin.
Planetary scientists know that the moon has a magnetic surface and lacks a global magnetic field. What they don't know is if that surface magnetism was caused by ancient impacts from asteroids or comets, or by a metallic core. Lunar Prospector's magnetometer instruments will map magnetic anomalies as they correlate with impact characteristics on nearly the entire lunar surface, and possibly show if impacts caused the magnetization.
The instruments will also search for external magnetic fields near the moon caused by electric currents flowing through a metallic core. Some analyses have suggested that the moon may have had, at some point, a global magnetic field caused by a metallic core. If the moon does have a core, data from the magnetometer may provide information on its size and degree of electrical conductivity, and possibly explain the source of the surface magnetization.
Verification of a lunar core may be a step toward resolving the long-standing question among planetary scientists of whether an airless, celestial body can become magnetized by impact. But it is especially important because it may be useful in understanding the moon's origin.
An indication of the size of a lunar metallic core could possibly lend support to the less-favored co-accretion model of the moon's formation. According to the co-accretion account, the moon formed from a collection of debris orbiting the Earth. While the Earth formed simultaneously, drawing most of the iron among the orbiting debris into itself, the moon would have absorbed enough iron to have a substantial metallic core. If the Lunar Prospector's magnetometer detects a substantial core, it would validate one of the co-accretion model's predictions.
Currently, however, the more dominant theory of the formation of the moon is the giant impact theory. On this account, the moon formed from the material ejected by a tremendous collision between a huge impactor and the Earth. Since this model assumes that the ejected material was from the mantle of both the Earth and the impactor, a very small or nonexistent metallic core is predicted.
It is important to note that although information provided by Lunar Prospector's magnetometer might be evidence for one model or the other, it would not conclusively prove either theory. The discovery of a lunar metallic core could favor the co-accretion model, but as Hood notes, there is a small class of giant impact models which suppose that a grazing impact could just as easily explain the existence of a lunar metallic core.
Hood is developing software which will analyze the data and minimize interferences like external solar wind fields. Given the limited value of all of the magnetometer data, data analysis will be a long, "difficult and delicate" process.
Does Hood think the moon has a core?
"I think it does. The question is: how big is it? It could be as large as 500 km in radius."
A metallic core that big would be a little less than a third of the moon's total radius.
After the one year nominal mission is complete, there are plans to lower the spacecraft altitude to ten miles above the surface. At this distance, the magnetometer's measurements of lunar crustal magnetism will improve considerably. In the meantime, magnetometer data can be analyzed to investigate fields resulting from electrical currents in a lunar metallic core.