Three years after the Chinese lunar rover landed to explore uncharted territory on the lunar surface for the very first time, the Yutu-2 made a major discovery Wednesday: Soil on the far side of the moon is stickier than the near side.

The finding came after scientists, led by a team at the Harbin Institute of Technology in northeastern China, found more soil residue stuck on wheels of Yutu-2 lunar rover — carried by the Chang’e 4 mission in 2019 — compared with its predecessor, the Yutu rover, which was brought to the moon’s Earth-facing near side by Chang’e 3 in 2013.

“Yutu-2 has experienced varying degrees of mild slip and skid, indicating that the terrain is relatively flat at large scales but scattered with local gentle slopes,” the paper published in Science Robotics journal said.

The latest discovery came two weeks after the Chang’e 5, the Chinese spacecraft that returned soil samples from the moon, reported it had discovered water in lunar soil and rock during its explorations. China has so far launched five Chang’e missions to examine the moon and plans to land on its south pole before 2025.

Sixth Tone spoke with Xiao Long, a prominent planetary geologist at the China University of Geosciences in the central city of Wuhan and who helped select the landing site for Chang’e 5, about the recent discoveries and what they can tell us about the Earth’s natural satellite. The interview has been edited for brevity and clarity.

Sixth Tone: Before Chang’e 4, did any other lunar probes make geological observations of the moon’s far side?

Xiao Long: Yutu-2 is the first rover to land on the moon’s far side. Before that, several lunar probes took pictures of the far side, but none of the onboard cameras had good enough resolution to retrieve information about lunar soil. So there have not been any studies about the far side’s lunar soil before Yutu-2.

Sixth Tone: Are there any conclusions as to why the soil on the far side of the moon is stickier?

Xiao: The soil is sticker than that on the near side, likely because the far side of the moon has been exposed to more space weathering, such as small meteoroids crashing into it. The intense heat from impacts melts the soil, which then fuses with nearby rock fragments and minerals to form small clumps.

Sixth Tone: What does this recent discovery tell us about the moon?

Xiao: We can use the data to compare the soil properties in different regions of the moon. It also gives us information on how to improve the design of lunar rovers and prepare us for future sample return missions from the far side.

For example, we should develop stickier lunar soil while doing test runs for future sample return missions from the far side, instead of using simulated lunar soil designed for Chang’e 5.

Sixth Tone: Two weeks ago, the Chang’e 5 team announced they detected water on the moon. Several probes have reported the presence of water molecules on the moon. How is the recent discovery different?

Xiao: It’s the first time a spacecraft has found signs of water on the lunar ground. Previous lunar rovers didn’t have instruments for detecting water, and most of the data we have are collected by probes in orbit or spacecraft flying by the moon. Previously, we knew the moon had water but didn’t know exactly where until Chang’e 5.

Lunar water detection has always been a hot topic among scientists, and we need different technical approaches to verify the finding. When we analyze how much water there is on the moon, we can’t use data from samples from one site and make generalizations of the region, much less the entire moon. Because the water content might vary by region and even by sample, we need a more comprehensive assessment.

Sixth Tone: China plans to build a lunar base, dubbed the International Lunar Research Station, together with Russia. How will the finding of lunar water support this future mission?

Xiao: Water detection is an important exploration target for the International Lunar Research Station, which would again use different approaches to look for water. Chang’e 5 landed in the mid-latitude region on the moon, where there’s expected to be only a small amount of water. The lunar polar region may have more water, and many countries have planned to explore the moon’s south pole.

Sixth Tone: Does the discovery of water suggest the moon could potentially support life?

Xiao: Not really. The moon is still a very harsh environment with no atmosphere and intense radiation from the sun. It’s also extremely cold.

Editor: Bibek Bhandari.

(Header image: Yutu-2 lunar rover on the moon, Jan. 4, 2019. CLEP/People Visual)