It was a beautiful landing site, said Bradley L. Jolliff, PhD, the Scott Rudolph Professor of Earth and Planetary Sciences at Washington University in St. Louis, who is a participant in an educational collaboration that helped analyze Chang’e-3 mission data. The lander touched down on a smooth flood basalt plain next to a relatively fresh impact crater (now officially named the Zi Wei crater) that had conveniently excavated bedrock from below the regolith (the ground-up surface layer of fragmented rock) for the Yutu rover to study.
Since the Apollo program ended, American lunar exploration has been conducted mainly from orbit. But orbital sensors mostly detect the regolith that blankets the Moon, and the regolith is typically mixed and difficult to interpret.
Because Chang’e-3 landed on a comparatively young lava flow, the regolith layer was thin and not mixed with debris from elsewhere. Thus it closely resembled the composition of the underlying volcanic bedrock. This characteristic made the landing site an ideal location to compare in situ analysis with compositional information detected by orbiting satellites.
“We now have ‘ground truth’ for our remote sensing, a well-characterized sample in a key location,” Jolliff said. “We see the same signal from orbit in other places, so we now know that those other places probably have similar basalts.”
The basalts at the Chang’e-3 landing site also turned out to be unlike any returned by the Apollo and Luna sample return missions.
“The diversity tells us that the Moon’s upper mantle is much less uniform in composition than Earth’s,” Jolliff said. “And correlating chemistry with age, we can see how the Moon’s volcanism changed over time.”
Two partnerships were involved in the collection and analysis of this data, published in the journal Nature Communications [http://www.nature.com/ncomms/index.html] Dec. 22. Scientists from a number of Chinese institutions involved with the Chang’e-3 mission formed one partnership; the other was a long-standing educational partnership between Shandong University in Weihai, China, and Washington University in St. Louis.
The Moon, thought to have been created by the collision of a Mars-sized body with the Earth, began as a molten or partially molten body that separated as it cooled into a crust, mantle and core. But the buildup of heat from the decay of radioactive elements in the interior then remelted parts of the mantle, which began to erupt onto the surface some 500 million years after the Moon’s formation, pooling in impact craters and basins to form the maria, most of which are on the side of the Moon facing the Earth.
The American Apollo (1969-1972) and Russian Luna (1970-1976) missions sampled basalts from the period of peak volcanism that occurred between 3 and 4 billion years ago. But the Imbrium basin, where Chang’e-3 landed, contains some of the younger flows—3 billion years old or slightly less.
The basalts returned by the Apollo and Luna missions had either a high titanium content or low to very low titanium; intermediate values were missing. But measurements made by an alpha-particle X-ray spectrometer and a near-infrared hyperspectral imager aboard the Yutu rover indicated that the basalts at the Chang’e-3 landing site are intermediate in titanium, as well as rich in iron, said Zongcheng Ling, PhD, associate professor in the School of Space Science and Physics at Shandong University in Weihai, and first author of the paper.
Titanium is especially useful in mapping and understanding volcanism on the Moon because it varies so much in concentration, from less than 1 weight-percent TiO2 to over 15 percent. This variation reflects significant differences in the mantle source regions that derive from the time when the early magma ocean first solidified.
“The variable titanium distribution on the lunar surface suggests that the Moon’s interior was not homogenized,” Jolliff said. “We’re still trying to figure out exactly how this happened.”