Measurements of potassium in lunar and terrestrial rocks have disproved the leading hypotheses for the origin of Earth’s sole natural satellite.
Moon. Image credit: Gregory H. Revera / CC BY-SA 3.0.

Moon. Image credit: Gregory H. Revera / CC BY-SA 3.0.

A duo of researchers reports isotopic differences between Moon and Earth rocks that provide the first experimental evidence that can discriminate between the two leading models for the Moon’s origin.

In one model, a low-energy impact leaves the proto-Earth and Moon shrouded in a silicate atmosphere.

In the other, a much more violent impact vaporizes the impactor and most of the proto-Earth, expanding to form an enormous superfluid disk (more than 500 times bigger than today’s Earth) out of which the Moon eventually crystallizes.

The team’s findings, which support the high-energy model, will appear today in the journal Nature.

“Our results provide the first hard evidence that the impact really did (largely) vaporize Earth,” said co-author Dr. Kun Wang, of Washington University in St. Louis.

Dr. Wang and his colleague, Prof. Stein Jacobsen of Harvard University, examined seven lunar rock samples from different lunar missions and compared their potassium isotope ratios to those of eight terrestrial rocks representative of Earth’s mantle.
Two recent models for the formation of the Moon, one that allows exchange through a silicate atmosphere (top), and another that creates a more thoroughly mixed sphere of a supercritical fluid (bottom), lead to different predictions for potassium isotope ratios in lunar and terrestrial rocks (right). Image credit: Kun Wang.

Two recent models for the formation of the Moon, one that allows exchange through a silicate atmosphere (top), and another that creates a more thoroughly mixed sphere of a supercritical fluid (bottom), lead to different predictions for potassium isotope ratios in lunar and terrestrial rocks (right). Image credit: Kun Wang.

Potassium has three stable isotopes, but only two of them, potassium-41 and potassium-39, are abundant enough to be measured with sufficient precision for this study.

The team found that the lunar rocks were enriched by about 0.4 parts per thousand in the heavier isotope of potassium, potassium-41.

“The only high-temperature process that could separate the potassium isotopes in this way is incomplete condensation of the potassium from the vapor phase during the Moon’s formation,” Dr. Wang explained.

Compared to the lighter isotope, the heavier isotope would preferentially fall out of the vapor and condense.

Calculations show, however, that if this process happened in an absolute vacuum, it would lead to an enrichment of heavy potassium isotopes in lunar samples of about 100 parts per thousand, much higher than the value the team found.

“But higher pressure would suppress fractionation,” Dr. Wang said.

For this reason, the researchers predict the Moon condensed in a pressure of more than 10 bar, or roughly 10 times the sea level atmospheric pressure on Earth.

Their finding that the lunar rocks are enriched in the heavier potassium isotope does not favor the silicate atmosphere model, which predicts lunar rocks will contain less of the heavier isotope than terrestrial rocks, the opposite of what the scientists found.

Instead it supports the mantle atmosphere model that predicts lunar rocks will contain more of the heavier isotope than terrestrial rocks.