The moon hasn’t had it easy over the years. Since the dawn of the solar system 4.5 billion years ago, its gray and lifeless surface has been repeatedly pummeled by incoming space rocks, leaving behind a pockmarked landscape strewn with rubble. Beneath this surface, however, hide the moon’s most tantalizing secrets for human explorers, from possible reservoirs of ice for producing potable water and rocket fuel to hollow lava tubes that are suitable for harboring habitats. More fundamentally, mapping the moon’s subsurface can reveal otherwise-hidden epochs of solar system history written by impacts, buried craters and associated debris—as demonstrated by fresh results from a Chinese rover on the little-explored lunar far side.

In a paper published in the journal Science Advances today, a collaboration of Chinese and European researchers describes the latest results from the Chang’e-4 mission, run by the China National Space Administration. Launched in December 2018 and reaching the moon in early January 2019, the mission became the first to land on the far side of the natural satellite, targeting an intriguing region near the lunar south pole called the South Pole–Aitken Basin. Formed 3.9 billion years ago and stretching some 2,500 kilometers across, it is the biggest impact basin in the solar system—and perhaps a key to understanding how great impacts have shaped Earth and other inner planets. The Chang’e-4 rover is still operational today and has been slowly trundling across this region, traveling a few hundred meters since it landed.

Chang’e-4’s landing site is within the 186-kilometer-wide Von Kármán crater, which lies inside the basin. Nearby are several other craters, such as the 72-kilometer-wide Finsen crater, thought to be about 3.2 billion years old. Using a ground-penetrating radar instrument on Chang’e-4, researchers have found that the rover is likely sitting on different layers of ejecta—debris from multiple impacts over time that rained down at high velocities to blanket the lunar surface and now fill the crater.  “[We] see a very clear sequence of [layers],” says Elena Pettinelli of Roma Tre University in Italy, one of the paper’s co-authors.

The rover’s radar instrument was able to penetrate up to 40 meters below the surface of the moon, more than twice the distance achieved by its predecessor, the Chang’e-3 mission, which landed on the lunar near side in December 2013. Data from the latest mission show three distinct layers beneath the rover: one made of lunar regolith, or soil, down to 12 meters; another made of a mix of smaller and larger rocks down to 24 meters; and a third with both coarse and fine materials extending the rest of the 40-meter depth.


Schematic illustration of the Chang’e-4 rover’s traverse across the lunar surface and the three distinct layers of subsurface debris revealed by its ground-penetrating radar. Credit: “The Moon’s Farside Shallow Subsurface Structure Unveiled by Chang’E-4 Lunar Penetrating Radar,” by Chunlai Li et al., in Science Advances. Published online February 26, 2020

It is not currently possible to definitively date the layers under Chang’e-4 and assign them to nearby craters. But they do provide some clues into lunar history stretching back about four billion years. Pettinelli notes that smaller rocks in the layers likely come from more distant craters, because they would have been able to travel farther across the moon, while the larger rocks hint at closer impacts. “If the blocks are big, you’re probably close to the source of the ejecta,” she says. Debris from at least four or five impacts is thought to be beneath the rover, extending down perhaps 80 meters or more to the basin’s floor.

While the moon was the focus of the American Apollo and Soviet Luna missions in the 1960s and 1970s, they mostly lacked the ground-penetrating-radar capabilities of the Chang’e-3 and Chang’e-4 missions—and of course, none of those earlier efforts ventured to the surface of the far side. As such, China’s two rovers have provided some of our first glimpses into the upper reaches of the moon’s subsurface. Other missions—such as NASA’s twin GRAIL (Gravity Recovery and Interior Laboratory) spacecraft, which orbited the moon from 2011 to 2012—have been able to peer much deeper beneath the surface but only in a limited way: using lunar-gravity data, they have provided relatively low-resolution glimpses of large features at depths of hundreds of kilometers.

Lunar scientist Daniel Moriarty of NASA’s Goddard Space Flight Center, who was not involved in the new paper, says the researchers’ results are interesting because those findings provide a look at how the moon has evolved over time. “The surface of the moon is very different from Earth,” he says. “The only two real large-scale processes that occur on the surface of the moon are impact cratering and volcanic activity, and they’re seeing evidence for both of those things here. The place they landed is a big volcanic floodplain. And then that floodplain was affected by impacts itself.”

Moriarty notes that the floodplain and the impact debris likely mixed together, which could suggest that some of the larger boulder-sized objects that were observed were from volcanic material being broken down rather than the result of debris from nearby impacts. It might also be that material from the lunar mantle, exposed by the initial impact that created the South Pole–Aitken Basin, has mixed in with the debris, something hinted at in earlier results from the Chang’e-4 mission.

The rover is continuing to move across the surface, making regular stops to take measurements and use its instruments. And as it does so, researchers are hoping that it might see the subsurface layers of debris change in size, revealing more subtle details of the moon’s vast, violent and ancient impact history. “We are asking [for the rover] to go toward [places where researchers] can say the [debris] is changing in thickness,” says Pettinelli. “That will be important.”



What Lies Beneath the Moon's Farside?


Yutu 2 looks back over its tracks leading back to the Chang’e 4 lander during lunar day 7.

The Chang’e 4 mission made the first-ever landing on the farside of the Moon last year — now it’s providing a look at what lies below its surface.

The Yutu 2 rover's Lunar Penetrating Radar has revealed three discrete layers of regolith below the surface of Von Kármán crater on the Moon’s farside.

Regolith is loose and broken-up rock, dust, and other material, much of which has been subjected to space-weathering such as from micrometeorite impacts. The Apollo and Luna missions studied regolith on the Moon’s nearside. Now, Chang’e 4 is investigating regolith on the farside, which has an older, crater-filled surface compared to the side we’re used to seeing in the evening.

The results, based on data from Yutu 2’s first two synodic days (59 Earth days) of roving, appear the open-access journal Science Advances. The image coming out of the data shows that down to 12 meters (39 feet), the regolith consists of fine, loose deposits with occasional larger rocks. The layer beneath the first sees an increase in large boulders, which appear down to a depth of 24 meters. The final unit consists of alternating layers of larger, coarse deposits and fine materials. These switching layers go down to a depth of at least 40 meters. The team speculate that these layers extend downward beyond that point, but the rover’s radar can’t penetrate any deeper. Ejecta from impacts throughout the Moon’s history played a part in creating this diversity of layers, along with other processes, including shearing, mixing, excavation, and subsurface structural disturbances.

Layered regolith beneath Moon's farside
This schematic shows the farside's subsurface, which can be divided into three layers: the topmost layer is regular regolith, while the second one consists of coarser materials with embedded rocks. The bottom-most layer accessible by Yutu 2's radar consists of alternating layers of fine and coarse materials.
Chunlai Li et al. / Science Advances 2020

“This is the first measurement of the Moon’s subsurface at high resolution that allows us to understand the stratigraphy and, in particular, the characteristics of an ejecta deposit,” says paper coauthor Elena Pettinelli (Roma Tre University, Italy).

The findings provide new insight into the thickness of the regolith, she adds. Regolith thickness can change from place to place and was hypothesized to go from a few centimeters to about 100 meters.

Pettinelli adds that understanding the regolith is important for oxygen extraction, either for use by future astronauts or as propellant.

Ian Crawford (University of London) found the study impressive from a scientific standpoint: “The tentative identification of buried regolith layers developed on top of ancient crater ejecta deposits is especially interesting.”

“In principle,” he adds, “such layers may preserve ancient solar wind and galactic cosmic ray particles which could potentially provide information on the past evolution of the sun and the solar system's galactic environment.

“Accessing such buried regolith layers will be a task for future exploration,” he notes, “but locating them is clearly a first step.”

Nearside vs. Farside

Yutu 2’s predecessor on the 2013 Chang’e 3 mission was also equipped with a Lunar Penetrating Radar. This instrument transmits radio waves, which penetrate the surface and reflect off of materials in subsurface layers. The detector then times the reflections, measuring the depths and reflectivity of the layers of materials below.

“This site is particularly transparent to radio waves; for this reason, we can see as deep as 40 meters,” says Pettinelli. “The nearside at the Chang'e 3 landing site was much less transparent”. Indeed, observations with Chang’e 3’s radar, used at Zi Wei crater in Mare Imbrium on the nearside, only down to 10 meters, suggesting different geological histories for the Chang’e 3 and Chang’e 4 landing sites.

“All these observations can help us understand the geology and origin of the different areas,” Pettinelli explains.

Chang'e 4's landing site
Chang'e 4 landed in Von Kármán Crater. Other nearby impacts likely ejected deposits that formed the subsurface layers Yutu 2's radar is detecting.
Chunlai Li et al. / Science Advances 2020

Yutu 2 covered around 120 meters in the first two lunar days, but had covered more than 367 meter at the start of February. The data returned will provide further insights according to Pettinelli.

“We would like to follow the stratigraphy along the rover path to see how it changes in space, if the layers get thicker or thinner or if the geology changes laterally. The best [result] will be detecting the base of the ejecta deposits and image the basalt below”.

Von Kármán crater is likely to have been flooded by mare basalts following ancient volcanic eruptions.

Deeper Insights Ahead

While the dual-frequency radar instrument operates at both 60 MHz and 500 MHz, the current study only deals with the higher-frequency data. Co-author Yan Su (Chinese Academy of Sciences) says new, deeper insights are forthcoming.

“The low-frequency, 60 MHz channel is aimed at detecting the deep structure to a depth of several hundred meters,” Su says. She is confident that the lower-frequency measurements will bear out the results from the higher-frequency ones. However, the low-frequency data remains somewhat controversial, due to possible disturbances in the signal that could be related to the rover’s metallic body.

Despite these challenges, the data are revealing, according to Clive Neal (University of Notre Dame), who was not involved in the study. “Understanding the regolith at depth is one of the major questions that will inform science, exploration, and commercial potential, as the regolith represents a treasure-trove of material to inform each of these different areas.”

Ground-penetrating radar will be an important tool for future investigations, he adds, and China’s developing important expertise in this area. That informs not just our understanding of the evolution of the Moon’s surface but also resource potential.

Yutu 2 route
The Yutu 2 drive route to the end of lunar day 13, mapped by Phil Stooke.

Yutu 2 has been patrolling its home crater ever since its mesmerizing landing and subsequent deployment in early January 2019. The Chang’e 4 landing site, called Statio Tianhe lies in the relatively flat eastern region of Von Kármán Crater. The crater in turn is situated in the northwestern sector of the South Pole-Aitken Basin, the oldest and largest impact structure on the Moon.

During its first year of operations Yutu 2 has set longevity records, returned amazing images, and analyzed lunar materials with its spectrometer. The rover and lander woke up for their 15th lunar day on February 17th with all instruments in good condition.

There is more to come from ground-penetrating radar, both in this mission and beyond. NASA’s 2020 Mars rover will carry a radar instrument, as will China’s own Mars vehicle, confirms Su. Such a payload would also have great value in future Moon missions, especially in detecting water ice at the lunar south pole.

Quelle: Sky&Telescope


Update: 3.03.2020


China's lunar rover travels nearly 400 meters on moon's far side

China's lunar rover Yutu-2, or Jade Rabbit-2, has driven 399.788 meters on the far side of the moon to conduct scientific exploration of the virgin territory.

Both the lander and the rover of the Chang'e-4 probe have ended their work for the 15th lunar day, and switched to dormant mode for the lunar night, according to the Lunar Exploration and Space Program Center of the China National Space Administration.

China's Chang'e-4 probe, launched on Dec. 8, 2018, made the first-ever soft landing on the Von Karman Crater in the South Pole-Aitken Basin on the far side of the moon on Jan. 3, 2019.

As a result of the tidal locking effect, the moon's revolution cycle is the same as its rotation cycle, and the same side always faces the earth.

The far side of the moon has unique features, and scientists say Chang'e-4 could bring breakthrough findings.

The scientific tasks of the Chang'e-4 mission include conducting low-frequency radio astronomical observations, surveying the terrain and landforms, detecting the mineral composition and shallow lunar surface structure and measuring neutron radiation and neutral atoms.

Yutu-2 has worked much longer than its three-month design life, becoming the longest-working lunar rover on the moon.

The rover has helped scientists unveil the secrets buried deep under the surface on the far side of the moon, enriching human's understanding about the history of celestial collision and volcanic activities and shedding new light on the geological evolution on the moon.

Scientists used the Lunar Penetrating Radar on Yutu-2 to send radio signals deep into the surface of the moon, reaching a depth of 40 meters. It shows that the subsurface is essentially made by highly porous granular materials embedding boulders of different sizes.

The Chang'e-4 mission embodies China's hope to combine wisdom in space exploration with four payloads developed by the Netherlands, Germany, Sweden and Saudi Arabia.

China will continue its lunar exploration program, with the Chang'e-5 lunar probe, weighing about 8.2 tonnes, expected to be launched in 2020 to bring lunar samples weighing 2 kg back to Earth.

Quelle: Xinhua