Irregular mare patches, like Maskelyne (seen here), are examples of multiple newly discovered young volcanic deposits on the Moon. Image Credit: NASA/GFSC/ASU

Typically when we think of the Moon, we assume that the landscape hasn't changed much over the past few billion years. However, thanks to new observations from NASA's Lunar Reconnaissance Orbiter (LRO), researchers have determined that lunar volcanic activity did not stop abruptly, but rather gradually slowed over time. LRO observed the terrain in certain areas, and found it to contain volcanic deposits dating back 100 million years -- a time that corresponds to the Cretaceous period here on Earth.  That's right, while dinosaurs were roaming the Earth, there could have been volcanoes erupting on the Moon. Other "younger" areas showed evidence of volcanic activity only 50 million years ago.

The Moon's surface is covered in dark, volcanic plains, known as lunar seas or "mares". These patches of terrain vary in textures, ranging from smooth, shallow areas to rough, and blocky terrain. The irregular mare patches are far too small to be from Earth, spanning less than half a mile across. However, one such mare, Ina Caldera, is a two-mile stretch of older, heavily cratered lunar crust surrounded by younger, volcanic terrain. This region was heavily photographed from lunar orbit by the Apollo 15 crew.

Once thought to be a unique piece of land, we now know Ina Caldera is one of approximately 70 irregular mare patches, thanks to LRO's observations. John Keller, LRO project scientist at NASA's Goddard Space Flight Center said, "This finding is the kind of science that is literally going to make geologists rewrite textbooks about the Moon."

This image of Ina Caldera, as seen from the LROC, highlights the older lunar crust (darker blobs) and the younger surface (lighter regions). Image Credit: NASA/GSFC/ASU

The discovery of such a large number of irregular patches, coupled with the fact they are spread over a large section of the lunar surface, tells us two things: The volcanic activity that created these patches is relatively recent, and that it is widespread. In order to estimate the age of lunar surface features, scientists count the numbers of craters and the size of those craters. So, the more craters we can observe in an area means, the older that patch of terrain is. Also, researchers look at the slope of the land from the smooth areas to the rougher terrain, and the steeper the slope, the "younger" the terrain.

This age determination technique has been used by astronomers for decades, even back during the Apollo era. Three of the irregular mare patches discovered have been aged by this technique. The terrain surrounding these patches is significantly older, and attributed to volcanic activity that started roughly 3.5 billion years ago and ended abruptly one billion years ago. It was believed that, at that time, all volcanic activity on the Moon ceased. However, these new observations tell a very different story, and give us new insight into the Moon's interior.

Sarah Barden, lead author on this study and recent Arizona State University graduate, stated, "The existence and age of the irregular mare patches tells us that the lunar mantle had to be hot enough to provide magma for the small-volume eruptions that created these unusual, young features."

In order to produce a lava flow, the Moon would need a very hot mantle and a core that was still producing massive amounts of heat. Researchers have believed for quite some time that the Moon's interior had cooled off, thus making the required lava flows impossible. The new data shows the Moon's interior remained quite hot for a lot longer than anyone expected. Could the strain of tidal forces between the Earth and the Moon cause enough friction to keep the lunar interior hot? Researchers agree that further studies are needed to definitively answer this question.

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