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In the trace lies the truth: Halogens and the fate of the lunar crust

On a clear night, the moon you gaze upon looks the same as it looked for the first humans that walked Earth—the same black-and-white side of our nearest neighbor with large dark "seas" and white "highlands" has been facing us for billions of years.

The moon is thought to have been born in a giant impact between Earth and a Mars-sized planet, Theia, about 4.5 billion years ago. The energy associated with this impact is expected to have led to an ocean of magma covering both Earth and the young moon.

Cooling of this magma would be expected to result in a nearly homogeneous solid moon, covered with the same crust everywhere. This is not always the case. The hemisphere always facing us, called the lunar nearside, has a totally different appearance than its opposite half, the farside, which is dominated by bright, highland-dominated landscapes, with virtually no "seas."

The dark lunar "seas," or maria, in Latin, are composed of widespread basaltic magmas, mostly erupted ca. 3.5 billion years ago on the nearside, with very few eruptions on the far side. This marks a distinct evolutionary history for these two hemispheres. Why and how did this happen? The secret that shaped the moon into two worlds may well be buried within minute amounts of halogens (e.g., fluorine and chlorine), found in lunar samples.

Halogen abundances in lunar minerals provide unique insight into the moon's evolution, but incomplete knowledge of halogen incorporation in minerals and melts has limited their application. Researchers at the Geodynamics Research Center, Ehime University, collaborating with colleagues from Universität Münster (Germany) and Vrije Universiteit Amsterdam (the Netherlands), carried out high-pressure, high-temperature experiments and successfully derived unique new data on how chlorine (Cl) distributes itself between lunar minerals and co-existing magma.

The researchers coupled models of the evolution of the lunar interior to measure halogen abundances in lunar crust samples and found that most lunar nearside samples are anomalously rich in Cl. In contrast, crustal materials from the lunar farside do not show this Cl enrichment. In their new study in Nature Communications, the researchers provide evidence to link this enrichment to the incorporation of gaseous Cl-compounds by lunar nearside rocks.

This finding indicates that the existence of widespread chloride vapor (with Cl likely present as metal chlorides) was possibly limited to the lunar nearside, suggesting the metal chloride vapor appears to be tied to lunar dichotomy.

Considering Cl is highly incompatible and volatile, this vapor-phase metasomatism may be related to (impact-induced/eruption) degassing from extensive lunar mare basalts in the nearside Procellarum KREEP Terrane. Crustal rocks on the lunar farside, without Cl enrichment, are shown to be products from magma derived from the lunar interior about 4.3 billion years ago.

Based on F/Cl modeling, the researchers found that a particular type of lunar crustal rock called the Mg-suite likely originated from a deep mantle that preserves remnants of the initial lunar magma ocean that was present 4.5 billion years ago.

Chlorine-rich vapors released during eruptions (or impact-induced evaporation) played a key role in transforming the moon's nearside that humans can see. Meanwhile, the farside crust, invisible to us all, escaped from these vapor-associated volcanic activities and thus preserved more pristine information about the moon, including about the lunar magma ocean that formed right after the moon was born. This finding illustrates the scientific value of recent lunar space missions that focused specifically on studying the lunar far side.