It’s hard to imagine scientists failing to examine every corner, crevice, crack, and any other measurable attribute of Moon rocks returned by Apollo missions in the 50 years those rocks have sat on Earth.
But just because humans have looked doesn’t mean all the Moon rocks on Earth have given up all their secrets.
In a new paper published Tuesday in Nature Communications, researchers from the University of Hawaii at Manoa detail how they used newly available sampling and analysis technologies to probe more deeply than ever before into a well-studied Apollo-era Moon rock sample known as Troctolite 76535. The new study suggests the cooling of the Moon from its once molten state was more complicated than scientists thought — and that we haven’t gotten close to exhausting all we can learn from lunar samples a half-century old.
What’s New?— Of the lunar samples brought home by Apollo missions, a subset known as the magnesian suite has been particularly helpful for lunar scientists.
“The magnesium suite is of interest because it contains these chemical characteristics that can sample all the key stages of lunar evolution,” William Nelson, a Ph.D. candidate in Earth sciences at the University of Hawaii and first author of the paper, tells Inverse. “By studying this suite, we can put constraints on several key stages of lunar evolution.”
The most widely accepted model of lunar evolution today is the Large Magma Ocean model, which holds that the Moon was once mostly molten, with the various types of rocks and minerals found there today crystallizing out of this magma ocean over time. The rocks of the magnesian suite, for instance, have been assumed to have crystallized over 100 million years.
The ordinary-yet-alien Troctolite 76535. NASA Johnson Space Center
By carefully examining the composition of Troctolite 76535 — a rock collected by the Apollo 17 mission — Nelson and his colleagues found that the distribution of chemicals such as phosphorus was too heterogeneous to support such a prolonged cooling period. Using computer modeling, they showed that phosphorus, for instance, would have been more evenly distributed throughout the samples if they had remained hotter than 150 degrees Celsius for more than 20 million years.
Rather than the magnesian suite rocks crystallizing from a largely molten crust, Nelson says, they now conclude that the magnesian suite originated as a magma infiltration of a then-largely solidified lunar crust. These rocks crystallized later than initially thought.
“Previously, you would have had the magnesium suite liquid starting to crystallize a little bit more than 4.4 billion years ago,” he says. “We were able to cut that down to more like 4.35 billion years ago.”
How they did it— While the new understanding of the magnesian suite is exciting for lunar scientists like Nelson, it doesn’t radically alter our understanding of how the Moon evolved. At least, not yet.
But how they made the discovery just might. Nelson made use of the University of Hawaii’s electron microprobe — an instrument not all that different from the scanning …….