Yutu Peers Inside the Moon

Although China’s lunar lander, Chang’E 3, landed on the Moon over two years ago, scientific results from its small rover Yutu are just now being published. A new paper out this week by Long Xiao and colleagues gives us a first look at the geology of a new location on the Moon. The site is on the Moon’s near side in northern Mare Imbrium, far from the Apollo and Soviet Luna landing sites. This new information (giving insights into the late volcanic history of the Moon) is surprisingly detailed.

It has been known for many years that the maria of the Moon consist of dark, smooth basaltic lava flows. Basalt (a common lava) is rich in iron and magnesium, and typically erupted as a low viscosity (runny) liquid. The lava flows rapidly across the Moon’s landscape and is known to have covered large (tens of kilometers in extent) regions. Mare volcanism on the Moon occurred mostly between 3 and 4 billion years ago, with some minor eruptions occurring both before and after this time frame (exactly how much longer after 3 billion years ago is disputed). The Chang’E 3 landing site is in an area made up of relatively young lavas (inferred from the density of superposed impact craters, which suggest an age between 2.5 and 3.0 billion years old). These late Imbrium flows are remarkably well preserved and show spectacular lobate (toe-like) morphology in low-sun illumination images.

The Yutu rover was equipped with four instruments that characterized the geology of the landing site: an imaging camera (to map geological features), an X-ray spectrometer (to measure chemical composition), a visible and near-infrared spectrometer (to measure mineral composition) and a surface penetrating radar (to probe the subsurface). All four instruments returned data and from what I have seen of it, the quality appears to be excellent. The biggest limitation is that the Yutu rover failed to move on its second command sequence after a lunar night; it only traversed a total of 114 meters, a bit more than the length of a football field. Repeated attempts to revitalize the rover failed, so the data that we have comes from this small region. However, that was more than enough distance to conduct profiles of the subsurface and to image and analyze several different sites around the Chang’E 3 lander.

The most interesting feature of the landing site is a large (450 m diameter), relatively young (~30 million years) impact crater, located just 50 m west of the landing site. This feature was seen during the descent of Chang’E and it is apparent in the landing video that active hazard avoidance software allowed the lander to steer clear of it. This crater is significant because it allows us to examine material from below the surface (when an impact crater forms, it excavates material from deep within the Moon). A crater this size will quarry blocks from levels as deep as 50 m below the surface; the deepest material is deposited nearest the rim crest, and ejecta comes from increasingly shallower levels in the target as one moves away from the rim. In theory, one could examine the vertical sequence of rocks by sampling surface blocks along a radial from the crater rim. This technique was used during the Apollo missions to put returned rock samples into a geological context.

The Yutu rover was not able to conduct a radial transect of the crater, but ground penetrating radar allows us to look at some subsurface units. This instrument works by transmitting radio waves that penetrate into the surface (longer wavelengths/lower frequencies penetrate more deeply) where they may be reflected by layering or mechanical interfaces at depth. Two frequencies (60 and 500 MHz) were used on the Yutu radar sounder, allowing profiles of the shallow subsurface (upper 10-15 m, where the regolith-bedrock interface is expected) and the deeper subsurface (hundreds of meters, which could display lava flow boundaries).

Results from the radar sounding show nine reflectors in the subsurface, ranging in depth from the base of the regolith to at least 360 m below the surface. The regolith (the ground-up soil layer that covers bedrock on the Moon) is about 4-5 m thick; this is in rough agreement with the 2.5-3 billion year age (estimated from crater counts) of the lavas of the site. A shallow reflector (1-4 m depth, decreasing to the east) may represent the ejecta blanket of the 450-m crater seen in the descent images. Additional subsurface structures are found at depths of 50-60 m, 140 m, 240 m, and 360 m. These reflectors probably represent the boundaries of earlier lava flows. These flows are likely buried lavas that are now exposed north and west of the Chang’E landing site; if so, they are between 3 and 3.5 billion years old.

The Yutu rover analyzed the chemical composition of the surface. This soil is moderately high in titanium dioxide (TiO₂₎, about 4 percent by weight, similar to the titanium content inferred from orbital remote sensing (5 ± 1 wt.%). Although mare soils are typically contaminated with debris from the underlying highlands (lowering the iron and titanium content), the soils here are on thick (more than 300 m) basalt flows and are unlikely to contain much highlands debris. Thus, Yutu’s chemical analysis is probably very close to the composition of the uppermost lava flow in this corner of Mare Imbrium. An image of a very large (4 m long, 1.5 m high) boulder on the rim of the big crater (a feature named by the Chang’E 3 team as Loong rock) shows linear, subparallel fractures and very coarse white crystals (probably plagioclase), up to 2 centimeters in length. Such coarse granularity is also seen in some Apollo lunar basalts and suggests relatively slow cooling, deep within the interior of a hot, extruded lava flow.

This detailed collection of information from a relatively small and simple rover is a testament to the power of such missions to provide information on the geology of a site on the Moon. Because we have abundant knowledge of the Moon and its processes from our study of the returned Apollo samples and from the remote sensing data of recent years, we can use that knowledge to interpret and better understand the limited data provided by Yutu and place it into a broader geological context. Similarly configured small missions could explore the polar regions of the Moon (where we expect to find water ice), measuring physical and chemical properties over a wide area. Such knowledge would allow us to better plan for the day when we return to the Moon to harvest its vast resources and create new space faring capabilities.

Paul D. Spudis | READ MORE

Paul D. Spudis (1952-2018) was a senior staff scientist at the Lunar and Planetary Institute in Houston, Texas, and a prolific author on the subject of the moon. His books include The Value of the Moon: How to Explore, Live, and Prosper in Space Using the Moon's Resources.