Seas Of Molten Rock Created By Lunar Impacts
March 12, 2013

Moon’s Orientale Basin Formed By Lunar Impact And Contained 200-Mile Sea Of Molten Rock

April Flowers for - Your Universe Online

An ocean of molten rock covered the entire lunar surface during the early part of the Moon's history. Over millions of years, that magma ocean cooled, differentiating to form the crust and mantle. According to new analysis of NASA's Lunar Orbiter Laser Altimeter (LOLA) data, led by Brown University planetary scientists, this wasn't the last time the Moon's surface melted on such a massive scale.

Graduate student William Vaughan led the research team. The analysis shows that the impact event that formed the Orientale basin on the Moon's western edge and far side created a sea of molten rock 220 miles across and at least six miles deep. At least 30 other impact basins on the Moon's surface probably had similar seas of impact melt.

The team demonstrated that as these melt seas cooled, they differentiated in a way that was similar to the lunar magma ocean, resulting in rocks that could be mistaken for "pristine" rocks formed very early in the Moon's history.

“This work adds the concept of impact melt magma seas to the lexicon of lunar rock-forming processes,” said planetary geologist James W. Head III, the Scherck Distinguished Professor of Geological Sciences. “It emphasizes that one must consider the detailed point of origin of the rocks in order to interpret them correctly.”

The team says that this includes rocks retrieved during the Apollo program and Russia's Luna missions. It is possible, according to this new analysis, that rock samples thought to be representative of the early lunar crust formation might contain the presence of impact melt material. The amount of rock formed in melt seas is significant and the team estimates that the impacts forming the Moon´s 30 large basins produced 100 million cubic kilometers of melt, enough to make up 5 percent of the Moon´s crust.

It would help to explain puzzling findings from lunar samples if they do include melt material. A 2011 analysis of a sample assumed to have originated in the early lunar crust, for example, suggested that the sample was 200 million years younger than the estimated time when the lunar magma ocean solidified. These puzzling findings led some researchers to conclude either that the Moon is younger than previous estimates, or that the lunar magma ocean theory was flawed. If, instead, the sample actually originated from a melt sea, its young age could be explained without rewriting the history of the Moon.

On the western edge of the Moon's near side, the Orientale basin is only partially visible from Earth. It is one of the few lunar basins that has not filled with volcanic basalt, providing an excellent site to investigate the geology of melt seas and to test whether they differentiate as they cool.

The Orientale seas would have been liquid for thousands of years for differentiation to occur. To remain liquid for that long, it would have been very deep, leading the researchers to question how thick the Orientale melt was.

“In pictures, you´re just seeing the top of an impact melt body, so we have to find a way to infer how thick it was,” Vaughan said.

Vaughan and his colleagues, including scientists from Lancaster University, took advantage of the fact that a liquid shrinks when it cools and solidifies. Data from LOLA revealed that the sheet had subsided by approximately 1.25 miles from the surrounding rock. This gave the team an estimate for how much the sea shrank, allowing them to calculate volume and infer depth.

The Orientale sea must have been a little over six miles thick, according to these calculations. Melt seas from impacts on Earth are known to have differentiated at far shallower depths, making it a safe bet that the Orientale was thick enough to do so as well.

The team then wanted to understand what the differentiation might look like. They were able to determine the composition of the impact melt sea based on the compositions of the lunar crust and mantle material that melted. A model of what rocks would have formed as the melt sea cooled was made from there, showing thick layers of rocks like dunite and pyroxenite forming at the base of the sea from dense, early crystallizing minerals sinking through the melt. Layers of rocks such as norite would form as other minerals floated up through the melt. This is very similar to differentiation processes in the lunar magma ocean.

Remote sensing data from the Maunder crater, the remnant of an impact that excavated material from the melt sheet after it cooled supported the team's model, confirming a noritic composition at least four kilometers deep in the melt sheet.

The findings, when taken together, suggest that impact melt seas produce rock in a very similar manner to the lunar magma ocean, helping to clear up some lingering questions about the magma ocean paradigm.

“This is a mechanism by which the Moon was later modified to add petrologic complexity,” Vaughan said. “It helps make sense of mineralogical data that doesn´t always fit in this lunar magma ocean idea.”

The results of this study will be published in an upcoming issue of Icarus.