New study shows Moon was volcanically active for longer
Sen—The Moon is generally considered to have long been a cold and dead world, but new research suggests that volcanic activity occurred within the relatively recent past.
While the major lava plains were formed more than three billion years ago, it appears that some smaller flows were produced as recently as 18 million years ago.
The ancient outpourings of lava left a clear mark by forming the dark lunar “seas”, or maria, that make up the face of the Man in the Moon. Of course, these seas are dry, and the floods of basalt that produced them was thought to have ceased completely around one billion years ago.
However, small but widespread eruptions have occurred in geologically recent times according to a new study by a group of scientists at Arizona State University’s School of Earth and Space Exploration (SESE), and announced yesterday in the journal Nature Geoscience.
The team, led by Sarah Braden of SESE, included other members of that school plus others from the Westfälische Wilhelms-Universität Münster in Germany. They examined images taken by NASA’s Lunar Reconnaissance Orbiter (LRO) spacecraft and identified 70 small volcanic features scattered across the maria.
The feature called Ina that was first recorded by Apollo 15 astronauts. Measuring 2km by 3km (1.2 miles by 2 miles) and 50 metres deep (160ft), its floor is covered with many small, low lava mounds. Image credit: NASA/GSFC/Arizona State University
The features, which appear as smooth mounds near patches of rougher terrain, have been dubbed irregular mare patches (IMPs). Though too small to be seen from Earth, with an average width of 500 metres, they have been recorded from orbit, and the first, which is known as Ina, was imaged by Apollo 15 astronauts in 1971.
The high-resolution imagery from two Narrow Angle Cameras aboard LRO, which was sent to the Moon in June 2009, has allowed the geologists to appreciate the significance of the IMPs. By counting the numbers of craters of different sizes on these features, compared to other regions of the Moon, they were able to work out how old the features were.
This new evidence shows that volcanism on the Moon did not stop abruptly a billion years ago, as had been imagined. Instead there were still widespread eruptions during the last 50 million years. Activity at Ina ended about 33 million years ago and at another IMP called Sosigenes, it only ceased as recently as 18 million or so years ago.
Announcing the discovery, Braden said: “Finding previously unknown geologic features on the lunar surface is extremely exciting.
“Our understanding of the Moon is drastically changed by the evidence for volcanic eruptions at ages much younger than previously thought possible, and in multiple locations across the lunar maria.”
She added: “The existence and young age of the irregular mare patches provides a new constraint for models of the lunar interior’s thermal evolution. The lunar mantle had to remain hot enough for long enough to provide magma for the small-volume eruptions.”
Geologically recent eruptions of basaltic lava made smooth, mounded patches on the floor of this shallow depression labelled Sosigenes IMP. The oval feature is about 300 metres (1,000ft) deep and roughly 3km wide by 7km long (2 miles by 4.5 miles. Image credit: NASA/GSFC/Arizona State University
Sen asked Braden if she was confident that volcanic activity has now ceased completely in all areas of the Moon. She replied: “That is not a question answered by the study presented in the paper. The work on irregular made patches does tell us that the Moon has not been volcanically inactive for as long as previously thought.
“The young irregular mare patches are empirical evidence for small volume volcanic eruptions much later in lunar history (less than 100 million years ago) than previously thought. Lunar volcanism definitely declined gradually after the peak eruption period during the formation of the lunar maria 3.5 to 3.0 billion years ago. The irregular mare patches are the latest chapter of lunar volcanic history.”
Sen noted that in the latter part of the 20th century, there were reports from some lunar observers of brief glows on the Moon that they called Transient Lunar Phenomena (TLPs) but there was never any clear evidence confirming these.
Nowadays it is being suggested that if the glows were other than illusory, they might have been examples of meteor impacts on the lunar soil, or regolith, similar to those that have more recently been imaged during major meteor showers.
Braden told us: “What you said about TLPs is true. My opinion is that TLPs and irregular mare patches are unrelated.”
LRO project scientist John Keller, of NASA’s Goddard Space Flight Center, said of the discovery: “This finding is the kind of science that is literally going to make geologists rewrite textbooks about the Moon.”