Chang’e-5 represented a major step forward for China’s lunar program, as it was the country’s first missions to return samples to Earth. Now the results of dating the samples are in, and it’s clear that although the deposits are old, they are small enough to be a bit odd.
To the moon and back
China has now succeeded in landing several probes and rovers on the moon’s surface as part of a larger exploration programme. Chang’e 5 represents the next step, as it has had an exercise to obtain subsurface samples and a return vehicle that can bring them back to Earth. The spacecraft landed successfully nearly two years ago, and after a few weeks, approximately 2 kg of rock returned to Earth from the Moon.
China has decades of knowledge gained since the Apollo missions, along with a greater theoretical understanding of the formation of the moon. It has carefully selected the Chang’e 5 landing site, a large volcanic deposit called Oceanus Procellarum, believed to be one of the younger regions on the Moon’s surface. But “younger” covers a large area of Earth, with estimates based on crater number ranging from 3.2 billion years old to just 1.2 billion.
Setting a specific age for the deposit can provide a number of benefits. First of all, the Moon offers a “clock” to dig into our entire solar system. It is the only place where we can match crater numbers with ages obtained from rock samples; We then use these numbers to put ages on other objects based on crater counts. The time range of Chang’e landing site 5 includes the periods when we have no dates of radioactive decay.
Additionally, understanding when the Moon was volcanically active to produce large deposits such as Oceanus Procellarum can help us formulate more accurate models of Moon formation and evolution. Volcanic activity requires heat, and this heat comes from a combination of body composition and composition, which will include radioactive isotopes that produce additional heat.
mark the time
The samples used in the new study were remarkably small: two cubes, each about three to four millimeters per side. However, this small one contains an impressive blend of minerals (clinopyroxene, plagioclase, olivine, quartz, cristobalite, and ilmenite all show up). The mass composition is consistent with other volcanic deposits on the Moon, however, most of the differences between the two samples can be attributed to the rate at which they cooled. Ads
The research team, which is a large international collaboration, used lead isotopes to estimate dates for multiple locations within each sample. (Igneous rocks often contain material that froze at different points in time and thus 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. Combining all data from both samples results in an age of 1.963 ± 0.059 billion years.
In the good news, all ranges of uncertainty overlap. And while radioactive dating can be skewed by some factors, there is no indication that these factors play a role here. There is no sign of contamination with other substances, whether they were at the site of the eruption or implanted there by the action of nearby impacts.
The overall configuration aligns well with remote sensing performed from orbit and through the instruments landed by China. Overall, the evidence indicates that Oceanus Brucellarum was probably completely deposited about 2 billion years ago.
When it comes to the solar system’s crater clock, the new date of Oceanus Procellarum governs some of the proposed time series, indicating fewer impacts since its formation than those time series predicted. Other models correspond to history, however, the data does not require significant revisions.
The biggest surprise lies 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 a billion years after the localization of any previously measured lunar basalt.” If these dates represent the sediments as a whole, then the Moon had more active volcanic activity than we would otherwise expect.
In other regions of the Moon, recent volcanic deposits are associated with elements such as potassium and thorium, which provide heat through radioactive decay. The presence of these elements seemed suggestive, as it could indicate that the higher radioactivity provides the heat needed to keep things molten for later. The samples obtained by Chang’e 5 also contain higher levels of these radioactive elements but not higher than other volcanic deposits, so the idea of a linear relationship between radioactivity and the age of volcanic activity does not hold.
Therefore, we have left a fair amount of uncertainty about how the Moon was able to support such large-scale eruptions so late in its history. Evidently, volcanic eruptions on a smaller scale occurred later, due to local differences in the regions where the hot material was trapped. But it’s not clear if these processes could support something like the Oceanus Procellarum eruptions.