Yutu finds Moon still active in old age

Apr. 14, 2015 10:09:00

Findings from China’s first moon rover Yutu (Jade Rabbit) – the only scientific probe to land on the Moon in almost 40 years – are rewriting lunar history.

Analyzing data sent back from Yutu, Chinese scientists find that a large volcanic eruption happened at its landing site, a basin called Mare Imbrium, about 2.5 billion years ago. Previously it was believed the Moon had seen little geological activity since 3.1 billion years ago.

“It’s like an old man in a wheelchair who can suddenly run again,” says Lin Yangting, a researcher at the Institute of Geology and Geophysics, Chinese Academy of Sciences.

When there's little geological movement and a planet or moon is an icy rock, the planet is in its old age or already dead, say scientists.

The Moon is a case in point, explains Lin,while the Earth is now in its adulthood, and quite active.

On December 14, 2013, the Chang’e 3 lunar craft and Yutu rover performed a soft landing in the northeast of the Mare Imbrium, on the nearside of the Moon. Though Yutu encountered operational difficulties after moving just 114 meters, it gathered some useful data.

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Chang'e-3 landing site and the rover Yutu's track. The image was composed from the series images taken by the Chang'e-3 landing camera. (Provided by Lin Yangting)

The Chinese Academy of Sciences assigned five interdisciplinary teams to study the data. Lin leads one of the teams, and its findings have been published as the cover story in the latest issue of the U.S. journal, Proceedings of the National Academy of Sciences.

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The cover picture of the latest issue of the U.S. journal, Proceedings of the National Academy of Sciences. (Provided by Lin Yangting)

VOLCANIC MYSTERY

The surface of the Moon features numerous large basins caused by bombardments of asteroids about 3.9 billion years ago. They were filled with dark basalt lava flows from volcanic eruptions, mainly during a period from 3.8 billion years to 3.1 billion years ago.

The eruptions produced layer after layer of basalt stacked in a vertical sequence, which, scientists believe, hides the history of the Moon.

Among the sophisticated instruments Yutu carries, the lunar penetrating radar (LPR) helps to see below the surface.

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The photo of the Yutu moon rover taken by the camera on the Chang'e-3 moon lander during the mutual-photograph process on Dec. 15, 2013.  (Xinhua/ China Features)

The LPR data indicates three underlying basalt layers measuring 195 meters, 30 meters and 130 meters from the top at the Mare Imbrium. “That shows the upmost layer of the basalt is quite thick,” Lin says.

Satellite images show the upmost layer of basalt on which Yutu landed is large and dates back 2.5 billion years.

“For the Moon, 2.5 billion years ago was its late years in terms of geological activity, but the facts show a very large volume of basalt lava erupted at that time,” Lin says.

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The photo of the Chang'e-3 moon lander taken by the camera on the Yutu moon rover during the mutual-photograph process on Dec. 15, 2013. (Xinhua/ China Features)

THICKER LUNAR SOIL

Using LPR, Yutu for the first time carried out an in-situ measurement of the thickness of the lunar regolith layer, also called lunar soil, which was about 5 meters deep.

“The figure is significantly deeper than what we expected for a young mare site, ” Lin says.

The thickness of the lunar soil was estimated to be 2 to 4 meters in the mare and up to 8 to 16 meters in the highland areas.

“We think the thickness of the lunar regolith layer was underestimated,” Lin says.

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The landing site of Chang'e-3 (red cross). The background image was taken by Chang'e-1. (Provided by Lin Yangting)

Scientists believe many important resources such as helium-3, believed to be a promising future energy source, can be found in the lunar soil.

Spectral analysis showed the soil beneath Yutu has similar mineral contents to the lunar soil samples taken back to earth by the U.S. Apollo missions.

Chinese scientists detected 12 chemical elements, including four trace elements in the lunar soil. They also found higher iron oxide (FeO) and titanium dioxide (TiO2) concentrations and lower aluminum oxide (Al2O) concentrations than in the samples collected by the lunar missions of the United States and the former Soviet Union. These findings suggest a new type of basalt beneath, which has not yet been sampled, Lin says. 

NEW EVIDENCE

Another important finding of Lin’s team related to KREEP, a special geochemical component on the Moon. KREEP is an acronym formed from the atomic symbol for potassium (K), rare earth elements (REE) and phosphorus (P). Scientists believe KREEP is related to the origins and early history of the Moon.

It is now believed that a rocky object about the size of Mars struck the Earth about 4.5 billion years ago. This collision threw a large amount of melt and vapor into orbit around the Earth. These high temperature materials ultimately formed the Moon.

Given the energy generated by this collision, it has been deduced that a large part of the Moon would have been liquefied, and this formed a lunar magma ocean. As the melt crystallized, minerals such as olivine and pyroxene precipitated and sank to the bottom to form the lunar mantle. And the material anorthosite plagioclase floated because of its low density, forming a solid crust. Elements that are usually incompatible would have been progressively concentrated into the residual magma. Thus a "KREEP"-rich magma was formed between the crust and mantle.

Lin says data from Yutu show that the basalt beneath the rover contains 10 percent to 20 percent KREEP substances.

“We think the substances were added to the basalt when the lava went through the KREEP-rich layer during volcanic eruptions, ” Lin says. The finding is new evidence supporting the lunar magma ocean hypothesis.

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Photo taken by the landform camera on the Chang'e-3 moon lander on Dec. 22, 2013 shows the Yutu moon rover during Chang'e-3 lunar probe mission's first lunar day circle. (Xinhua/ China Features)

Lin believes that the Moon probe will help in understanding the early history of the Earth. Moreover, the Moon could be a base for deep space exploration in the future.

“The Earth and the Moon share similar experiences in their origins. The early history of the Earth, which has been erased in the frequent geological activity, can be studied on the Moon,” he says.  

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