Most people only ever encounter rubidium as the purple color in fireworks, but the obscure metal has helped two University of Chicago scientists propose a theory of how the moon may have formed.
Conducted in the lab of Prof. Nicolas Dauphas, whose pioneering research studies the isotopic makeup of rocks from Earth and the moon, the new study measured rubidium in both planetary bodies and created a new model to explain the differences. The breakthrough reveals new insights into a conundrum about the moon’s formation that has gripped the field of lunar science over the past decade, known as the “lunar isotopic crisis.”
This crisis kicked off when new methods of testing revealed Earth and moon rocks have strikingly similar levels of some isotopes, but very different levels of others. This confounds both major scenarios for how the moon formed: one being that a giant object smashed into Earth and took a chunk with it on its way to becoming the moon (in which case the moon should have a decisively different makeup, mostly the foreign object); and the other being that this object obliterated the Earth, and the two celestial bodies eventually formed out of the resulting smithereens (in which case the two makeups should be virtually identical).
“There’s clearly something missing there,” said Nicole Nie, PhD’19, first author of the study, recently published in Astrophysical Journal Letters. A former graduate student in Dauphas’ lab, Nie is now at the Carnegie Institution for Science.
To test different theories, Dauphas’ lab has a collection of moon rocks on loan from NASA, (representing every Apollo mission that recovered samples). Nie came up with a rigorous way to measure the isotopes of rubidium—an element that had never been precisely measured in moon rocks because it’s so difficult to isolate from potassium, which is chemically extremely similar.