The Moon’s Dark Seas Are Scars From Ancient Strikes—Even Bigger Than We Thought

Mare Imbrium, one of the large dark basalt "seas" on the Moon's near side. (Photo: J. Major)
Mare Imbrium—the “Sea of Rains”—one of the large dark basalt lava flows on the Moon’s near side. (Photo: J. Major)

When you look up at a full or full-ish Moon you can’t help but notice the large dark spots that mark its Earth-facing side. These form the face of what many call the “Man in the Moon” (or the body of a Moon rabbit, to others) and are individually called mare (“MAR-ay”) which is the Latin word for sea. Early astronomers thought they were bodies of water, but in reality they are solidified dark lava flows from ancient lunar impact events that occurred several billion years ago.

One of the biggest lunar seas, Mare Imbrium (see above), had for a while been thought to have been created by the impact of an asteroid or meteorite somewhere around 50 miles across, based solely on computer models.

Now, research conducted by Brown University professor Dr. Peter Schultz—a specialist in lunar and planetary impacts—indicates that the object that formed Mare Imbrium was likely more massive and of much larger size than once thought…perhaps even as big as 190 miles wide.

Not satisfied with mere computer models, Dr. Schultz (a well-spoken and affable man whom I’ve had the pleasure to meet in person at Brown’s Northeast Planetary Data Center) used the Vertical Gun Range at NASA’s Ames Research Center in California to study actual impact events in miniature scale.

His research found that curious scratches and grooves on the Moon’s surface around Imbrium—previously unexplained geological features—were likely created by shattered pieces of the impactor upon point of first contact with the Moon.

Based on his observations and further calculations by David Crawford from Sandia National Laboratories, whatever object struck the Moon 3.8 to 4 billion years ago was at least 150 miles across—about half the size of Vesta, currently the second-largest world in the main asteroid belt.

“We show that Imbrium was likely formed by an absolutely enormous object, large enough to be classified as a protoplanet,” Schultz said. “This is the first estimate for the Imbrium impactor’s size that is based largely on the geological features we see on the Moon.”

In addition to identifying the source of Mare Imbrium, these findings hint at the type of environment that our planet existed within during the early years of the Solar System. If the Moon was impacted often by such large rogue objects, then it only stands to reason that Earth, four times larger, was also subject to enormous strikes.

“The large basins we see on the Moon and elsewhere are the record of lost giants,” said Schultz.

Read more here in a news release from Brown University.

Source: Brown University