Chang’e-5 represents a major step forward for China’s lunar program, as it was the country’s first mission to return samples to Earth. Now the results of dating the specimens are out, and it is clear that even though the deposits are old, they are young enough for something strange to happen.
to the moon and back
China has now successfully landed several probes and rovers on the lunar surface as part of a major exploration program. Chang’e 5 represented the next stage, as it contained a drill to obtain subsurface samples and a return vehicle that could take them back to Earth. The spacecraft landed successfully about two years ago, and a few weeks later, it returned about 2 kilograms of rock from the Moon to Earth.
China benefits from a greater theoretical understanding of the Moon’s formation, along with decades of knowledge gained since the Apollo missions. And it carefully selected the landing site of Chang’e 5, a large volcanic deposit called Oceanus Procellarum, thought to be one of the younger regions of the Moon’s surface. But “small” covered much more ground, as estimates based on the number of its craters ranged from 3.2 billion years to only 1.2 billion.
Achieving an accurate age on deposit will yield several benefits. First, the Moon presents a “clock” for the cratering of our entire solar system. It is the only place where we can match crater count to age obtained from rock samples; We then use those numbers to work out the ages of other objects based on their crater counts. The date range of Chang’e 5’s landing site includes periods for which we do not have dates for radioactive decay.
Furthermore, understanding when the Moon was volcanically active to produce large deposits such as Oceanus Procellarum may help us produce more accurate models of the Moon’s formation and evolution. Volcanic activity requires heat, and that heat comes from a combination of the body’s formation and its composition, which will include radioactive isotopes that produce additional heat.
The samples used for the new study are remarkably small: two cubes, each about three to four millimeters on the side. Yet that small volume contained a remarkable mix of minerals (clinopyroxene, plagioclase, olivine, quartz, cristobalite, and ilmenite all appear). The bulk composition is consistent with other volcanic deposits on the Moon, however, and most of the differences between the two samples can be ascribed to the rate at which they cooled.
The research team, representing a large international collaboration, used lead isotopes to estimate the dates of several sites within each sample. (Volcanic rocks often contain material that solidified at different time points and can therefore produce multiple ages.) The results show that one of the samples is 1.893 ± 0.280 billion years old and 1.966 ± 0.059 billion years old. was old. Combining all the data from both samples yields an age of 1.963 ± 0.059 billion years.
In the good news, all the uncertainties overlap. And while radioactive dating can be skewed by some factors, there is no indication that those factors are in play here. There is no indication of contamination by other material, either present at the blast site or from nearby impacts.
The overall structure matches well with the remote sensing done by the hardware from orbit and landed by China. So overall, the evidence argues that the entire Oceanus Procellarum was likely deposited about two billion years ago.
When it comes to the Solar System’s crater clock, the new date of Oceanus Procellarum rules out some of the proposed chronologies, as it indicates fewer impacts since its formation than those chronologies. However, other models are consistent to date, so major modifications to the data are not required.
There is a big surprise in explaining the evolution of the Moon. The age “means that approximately 2,000 cubic kilometers of basaltic magma erupted near the landing site about 1 billion years later than the displacement of any previously measured lunar basalt.” If these dates represent deposits as a whole, the Moon had more active volcanism than we might have expected otherwise.
In other regions of the Moon, young volcanic deposits are associated with elements such as potassium and thorium, which provide heat through radioactive decay. The presence of these elements seemed indicative, as it could indicate that the high radioactivity provided the heat needed to melt things later. The samples obtained by Chang’e 5 also have higher levels of these radioactive elements, but not higher than other volcanic deposits, so the idea that there is a linear relationship between radioactivity and the age of the volcano does not hold.
So we’re left with a lot of uncertainty about how the Moon managed to support such massive eruptions throughout its history. The small-scale eruption apparently occurred later, due to local variations in the regions where the hot material was trapped. But it’s not clear whether these processes could support something like an Oceanus Procellarum eruption.
Science, 2021. DOI: 10.1126/science.abl7957 (About DOI).