The theory the Moon was formed by a Mars-sized object, named Theia, smashing into Earth is widely accepted among astronomers, to the extent that it is often taken for granted. However, some continue to champion alternatives, pointing to observations that fit the Theia hypothesis poorly. While looking for ice on the floor of lunar craters planetary scientists have found something surprising that may increase the uncertainty, or help settle it.
A single collision theory explains much of what we know about the Moon, but doesn't seem consistent with a few aspects. Among these are measurements indicating the Moon is 13 percent iron oxide (FeO). The Earth's core is mostly iron, but the combined crust and mantle is just 8 percent FeO. If the Moon is composed of a mix of ancient Earth's and Theia's outer layers, the extra iron needs explaining.
The Apollo astronauts brought back samples of lunar “seas” and highlands but did not explore the deeper craters. Such craters, particularly close to the poles, are thought to be the best place on the Moon to find ice (water), and therefore site future colonies. Dr Essam Heggy of the University of Southern California sought to test this theory using the Lunar Reconnaissance Orbiter's radar and in the process noticed something odd about the composition of the crater floors.
In Earth and Planetary Science Letters, Heggy reports the larger a crater is, up to a point, the greater the dielectric constant, or capacity to transmit electric fields, of its floor. Above about 5 kilometers (3 miles) in diameter the value stabilizes. "It was a surprising relationship that we had no reason to believe would exist," Heggy said in a statement.
The paper attributes this to additional iron and titanium at the bottom of larger craters. To explain those metals' presence the authors propose the Moon is enriched in them at depth, and large impacts bring them to the surface. If so, the true lunar iron concentration may be substantially higher than the estimated 13 percent.
That will require some rejigging of theories of the Moon's formation, but it is not clear whether it makes the overall picture easier or harder to explain. One possibility is that Theia was very iron-rich, and is the source of the extra metal. However, this contradicts observations suggesting most of the Moon's material came from Earth, not Theia.
Alternatives include the possibility Theia's impact was even greater than modeling has suggested, releasing more metal-rich material from the core. Alternatively, the impact may have occurred when Earth was still covered by a magma ocean, rather than having formed a solid crust, as has been imagined.
The results pose an even bigger problem for the alternative theory the Moon's material was thrown up in a series of smaller collisions, rather than one big one.
As to the lunar ice, Heggy and colleagues think smaller craters, with consequently lower dielectrics, in the polar regions should be the priority in future studies.