The moon’s face can hide its age but not its past.
Data from NASA’s Lunar Reconnaissance Orbiter describe a moon with a more complex history than previously thought and highlight what could be its oldest regions, planetary scientists report in the Sept. 16 Science. Two papers present measurements of the chemical composition of the moon’s surface, and a third details the first comprehensive crater catalog. The results reveal promising targets for future missions and support previous theories about the moon’s past.
Craters interest researchers in part because their density and arrangement can be used to determine the age of a planetary body’s surface. In general, fewer craters mean a younger surface, because fewer meteorites have had time to hit it. Scientists have linked crater density to age for various locations on the moon based on radiometric dates of samples brought back by Apollo missions. To estimate ages for untested surfaces, researchers compare the number of craters on the surface to the number in dated regions.
The problem with this method is that old lunar surfaces like the highlands can reach “saturation equilibrium” — for every new crater formed by a meteorite, an old one is erased.
But LRO scientists have put saturation equilibrium to good use. Using high-resolution topographic measurements, the team cataloged lunar craters and compared their maps to theoretical models of how the impact features interact with each other as they accumulate, explains study coauthor Caleb Fassett of Brown University. The predicted saturation densities were similar to those observed on the moon in two locations: the southern nearside and the north-central farside.
These locations would be promising targets for future missions because of the possibility of finding the most ancient lunar samples available. “Absolute age dates from new missions to the ancient highlands of the Moon will be absolutely critical in tying down what the relative crater densities that we observe mean,” Fassett says.
The study also confirms previous work by Robert Strom of the University of Arizona in Tucson and his colleagues that suggested that the moon and other solar system bodies have suffered bombardment from two separate asteroid populations. The survey found that the highlands, unlike younger sealike areas called maria, have a higher density of large craters compared with small ones, suggesting that impactors tended to be larger in the moon’s early history.
The results match exactly what would be expected if the moon were hit first by objects from the main asteroid belt during a period called the Late Heavy Bombardment, roughly 3.9 billion years ago, and then later by smaller near-Earth asteroids that had been ejected from the belt, says Strom. “It’s a very good paper,” he says. “This is further confirmation to me that there are two crater populations.”
In the other two Science papers, planetary scientists report the presence of silica-rich materials that indicate past lunar volcanism. These include highly silicic materials such as quartz, with high abundances of silicon and oxygen in their composition.
Highly silicic minerals have never been directly detected on the moon before. While astronomers have known since the 1970s that the lunar highlands largely comprise silica- and calcium-rich feldspar minerals, the new data allow scientists to distinguish between different types, such as the calcium-rich feldspars they knew were there and the sodium-rich deposits they’ve only now discovered, says study coauthor Timothy Glotch of Stony Brook University in New York.
The distribution of highly silicic materials on the moon suggests there were once thick magmatic flows there, with melts spreading both above and below the surface.
“Most lunar lava flows were like Hawaii, pretty mild flowing out across the surface, though some could be explosive and form geyserlike fire fountains,” explains study coauthor Paul Lucey of the University of Hawaii at Manoa. “The silica-rich lava would have flowed out pretty undramatically, but it is very thick … so it can’t run far.” Instead, these flows built up volcano domes, which appear near some of the silicate features.
The existence of these flows was suspected for decades based on images, but the composition data positively identify them, Glotch says.
The prevalence of silicates runs counter to evidence from rock samples brought back by moon missions, in which silicates were rare, Lucey adds. The minerals’ commonness raises their importance in understanding the moon’s early history.
All three papers show that “LRO is doing exactly what it was designed to do, namely identifying new places on the Moon for human, or nonhuman, exploration that will help solve fundamental questions about the origin and evolution of the Earth-moon system,” says Benjamin Greenhagen of NASA’s Jet Propulsion Laboratory in Pasadena, Calif., a coauthor of the silicates paper.