A team of researchers has discovered a new mechanism that may explain the small difference in composition between Earth and the Moon. Lunar rocks are strikingly similar in composition to those on Earth, but they lack a certain amount of volatile elements such as zinc, potassium, and sodium. These elements have a relatively low boiling point, compared to other substances, and they vaporize quickly.
The discrepancy has puzzled scientists for many years. Now, a team led by Dr. Robin Canup from the Southwest Research Institute in Boulder, Colorado, has used a dynamical simulation to model the formation of our natural satellite. They showed that the variation in composition is perfectly explained by how the Moon came to be. The volatiles were too hot to condense on the newly born Moon, so they rained back down on Earth.
Their paper is published today in Nature Geophysics.
The Moon was most likely formed when a Mars-sized object impacted the newly formed proto-Earth over 4.4 billion years ago, known as the giant impact hypothesis. The impactor is called Theia, the mother of the Moon in Greek mythology. It blew a chunk of Earth’s material into space, which formed a debris disk around our planet.
The study shows that the debris disk coalesced into the Moon in two phases. The outer portion of the disk cooled down quickly, with materials including volatiles condensing into a proto-Moon. After the proto-Moon formed, it started attracting material from the still-hot inner disk. Gravitational interactions with the disk itself pushed the Moon into an outer orbit until the accumulation of material stopped.
The inner disk is still hot throughout this phase, meaning the volatiles are still vaporized, and they never condense on the outer layers of the Moon. And according to Dr. Canup, there are several reasons why the inner disk remained hot.
“First, the inner disk material is initially more highly heated by the impact,” he told IFLScience. “Second, it continues to self-heat: the disk clumps due to its self-gravity, but close to the planet these clumps are disrupted by planetary tides. This provides an ongoing source of energy dissipation that heats the inner disk that does not occur in the outer disk. And third, the outer disk has a larger surface area over which it can radiatively cool compared to the inner disk.”
According to the team's model, the Moon is covered by a deep layer of volatile-poor material 100 to 500 kilometers (62 to 310 miles) thick. This discovery helps to confirm how we think the Moon formed.
“How the Moon came to be lacking in volatile elements compared with the Earth has been a lingering question for decades, and we now have a plausible answer to that question in the context of the favored ‘great impactor’ hypothesis," added Dr. Canup. "So I would say that this is another strong argument in favor of the giant impact theory.”