New Type Of Rock Discovered On The Moon

Yutu, photographed by its lander Chang'e-3, as it began its journey, which included sampling rocks of a composition never directly measured before. CNSA/CLEP

Yutu, the Chinese rover deployed by the lander Chang'e-3, has found a new type of lunar rock, something never before studied on the Moon or Earth.

It has been 45 years since the Apollo 11 astronauts brought back the first Moon rocks, and 39 years since Luna 24 returned further samples, but discoveries have not stopped. Two years ago, Chang'e-3 landed next to a recent impact crater within the Imbrium basin. Yutu moseyed over to sample the rim of what is now known as Zi Wei crater (Purple Palace in English).

The site was of interest to geologists because the impact that created it was so recent (relatively speaking) that the layer of fragmented rock is of local origin, unmixed with material thrown up by more distant collisions.

Zi Wei is one of a number of places where remote sensing has suggested something novel. In Nature Communications, a Chinese-American collaboration report that Yutu found a type of basalt made from approximately 41 percent silicon dioxide (SiO2), 23 percent iron oxide (FeO), 12 percent calcium oxide (CaO), 5 percent titanium dioxide (TiO2) and 10 percent alumina (Al2O3).

By lunar standards, the silicon oxide is low and the iron oxide very high, but the most unusual measure is the titanium dioxide. Samples returned from the Apollo and Luna missions had as much as 15 percent titanium dioxide or as little as 1 percent, but a large gap in middling values, which Zi Wei neatly fills.

"We now have 'ground truth' for our remote sensing, a well-characterized sample in a key location," Professor Bradley L. Jolliff of Washington University in St. Louis said in a statement. "We see the same signal from orbit in other places, so we now know that those other places probably have similar basalts."

"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. Possibly there were big impacts during the magma ocean stage that disrupted the mantle's formation."

Mare Imbrium basin with an arrow pointing to the Chang'e-3 landing site in visible light, and three other sources of radiation showing concentrations of different elements. Credit: NASA/LPI

Besides the titanium concentration, the Zi Wei basalt is interesting for its age. The Moon formed from material thrown into space when a Mars-sized object hit the proto-Earth. As on Earth, radioactive elements kept the core hot, and 500 million years later this produced an outbreak of vulcanism, filling basins and impact craters with basalt, which has become what we call seas.

The Moon's much smaller size meant that the core cooled faster than that of the Earth, and this primary volcanic period was over by 3.6 billion years ago. Yet Imbrium contains areas formed up to a billion years after most other seas. Zi Wei is far from the youngest area, estimated by density of small craters to be 2.96 billion years old. Nevertheless, it still represents lava flows that occurred long after most, offering a window into the latter stages of a satellite becoming geologically, as well as biologically, dead.  

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