The Moon is a common sight in our night time (and sometimes daytime) skies but it hasn’t always been there. The widely accepted theory of lunar formation involves a Mars-sized planet crashing into the Earth, creating a cloud of debris that eventually that eventually coalesced to form the Moon. Estimates of this cataclysmic event that gave us the Moon range from between 4.52 to 4.35 billion years ago however a new presentation at the Lunar and Planetary Science Conference have pushed that timeline back further!
The theory that describes the formation of the Moon is known as the Giant Impact Hypothesis and it proposes the protoplanet called Theia collided with the early Earth in the collision to end all collisions, at least as far as Earth is concerned. The impact ejected an enormous amounts of molten rock and debris into space which scattered into orbit around Earth. Gradually over time, the material condensed and cooled, eventually forming the Moon that we see today. The tremendous energy of the collision melted both the impactor and the early Earth, explaining why the Moon’s composition is similar to Earth’s and why it lacks a substantial iron core.
Artist impression of Theia’s impact with Earth
The theory is sound and has stood firm despite significant analysis. However what does remain uncertain is the exact time of the event. Some evidence suggests a formation around 4.35 billion years ago, other research however points to an earlier date of about 4.5 billion years ago. The difference might be explained by a secondary geological event, such as the formation of the South Pole’s Aitken Basin or changes in the Moon’s orbital dynamics. Because of these uncertainties, researchers have been looking for alternative methods to refine estimates of the formation.
Building on previous research, the team employed a geological dating technique using the radioactive decay of rubidium-87 into strontium-87 isotopes. They are found in lunar rocks in the lunar highlands and are known as ferroan anorthosites (FANs). They are thought to be among the oldest lunar rock so preserve information about the Moon’s earliest geological history. By taking measurements of the isotope ratios in the rocks, the team hope to construct a more accurate timeline of the Moon’s formation.
The researchers studied eight samples using thermal ionisation mass spectrometry which involves heating samples, typically deposited on a metal filament, to temperatures exceeding 1000°C to cause ionisation. Most of the samples provided reliable data about their initial strontium composition. Five of these rocks, including one dated at 4.360 billion years old, formed a consistent group that helped define a precise initial strontium ratio. Three other samples showed different strontium ratios, suggesting they either formed later or experienced chemical changes after their initial crystallisation.
The Moon
Modelling the evolution of the rubidium-strontium isotope under four different impact scenarios allowed the team to calculate a formation age approximately 65 ± 21 million years after the formation of the Solar System, a mere 4.502 ± 0.021 billion years ago! To account for uncertainties, they ran calculations varying different parameters like the isotope compositions of proto-Earth and Theia, and the size and mass of Theia too. By exploring different scenarios and analysing isotopic ratios, they hope in time, to be able to develop a revised model for determining an accurate value for the age of the Moon.
Source : Formation of the Moon ~65 Million Years After Solar System Formation