Lunar samples returned by Chang’e-5 speak of recent volcanism

Chang’e-5 was a major breakthrough for China’s lunar program, as it was the first of the country’s missions to return samples to Earth. Now the results of the dating of the samples are available, and it is clear that while the deposits are old, they are young enough to be a little weird.

To the moon and back

China has now successfully landed several probes and rovers on the lunar surface as part of a larger exploration program. Chang’e 5 was the next step, as it had a drill rig to get underground samples and a return vehicle that could bring them back to Earth. The spacecraft successfully landed almost two years ago, and a few weeks later sent about 2 kilograms of rock back to Earth from the moon.

China benefits from decades of knowledge gained from the Apollo missions, as well as a better theoretical understanding of how the moon was formed. And he carefully chose the Chang’e 5 landing site, a large volcanic deposit called Oceanus Procellarum, considered one of the youngest areas on the Moon’s surface. But “younger” covers a lot of ground, as estimates based on the number of craters ranged from 3.2 billion years to just 1.2 billion.

Getting a specific age on the deposit would offer a number of advantages. To begin with, the Moon presents a “clock” for the crater of our entire solar system. This is the only place where we can match the number of craters to the ages obtained from rock samples; we then use those numbers to determine the age of other bodies based on their number of craters. The date range for the Chang’e 5 landing site includes periods for which we do not have radioactive decay dates.

Additionally, understanding when the Moon was volcanically active to produce large deposits like Oceanus Procellarum can help us build more accurate models of the Moon’s formation and evolution. Volcanic activity requires heat, and this heat comes from a combination of the body’s formation and its makeup, which will include radioactive isotopes that produce additional heat.

Marking time

The samples used for the new study were remarkably small: two cubes, each about three to four millimeters across. Yet this small volume contained a remarkable blend of minerals (clinopyroxene, plagioclase, olivine, quartz, cristobalite, and ilmenite all make appearances). The overall composition is, however, consistent with other volcanic deposits on the Moon, and most of the differences between the two samples can be attributed to the rate at which they cooled.

The research team, representing a large international collaboration, used isotopes of lead to estimate the dates of several sites within each sample. (Volcanic rocks often contain material that solidified at different times and therefore can produce a range of ages.) The results indicate that one of the samples was 1.893 ± 0.280 billion years old and 1.966 ± 0.059 billion years old. years. Combining all the data from the two samples produces an age of 1.963 ± 0.059 billion years.

Fortunately, all the ranges of uncertainty overlap. And while radioactive dating can be skewed by certain factors, there is no indication that those factors are at play here. There is no sign of contamination from other materials, present at the eruption site or set up by impacts nearby.

The overall makeup matched well with the remote sensing done both from orbit and by the material that China landed. So overall, the evidence supports that the entire Oceanus Procellarum would probably have been deposited around two billion years ago.

Implications

Regarding the solar system crater clock, the new Oceanus Procellarum date excludes a few proposed timelines, as it indicates fewer impacts since its formation than those timelines would have predicted. However, other models are consistent with the date, so the data does not call for major revisions.

The biggest surprise is to explain the evolution of the Moon. The age “implies that nearly 2,000 cubic kilometers of basaltic magma erupted near the landing site nearly a billion years later than the placement of any previously measured lunar basalt.” While these dates represent the deposit as a whole, the Moon had more active volcanism than one would expect otherwise.

In other regions of the Moon, younger volcanic deposits are associated with elements like potassium and thorium, which provide heat through radioactive decay. The presence of these elements seemed suggestive, as it might indicate that higher radioactivity was providing the heat needed to keep things molten later. 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 volcanism does not hold. not.

So we have quite a bit of uncertainty left over how the Moon managed to withstand such large-scale eruptions so late in its history. Smaller-scale eruptions clearly occurred later, caused by local variations in areas where hot material was trapped. But it’s not clear if these processes could support something like Oceanus Procellarum eruptions.

Science, 2021. DOI: 10.1126 / science.abl7957 (About DOIs).