Barren, desolate, and geologically dead … Our view of the Moon ever since the first humans set foot on the soft lunar soil almost half a century ago has been that of a still and unchanging world which, apart from the occasional moon quake, is only disturbed by the random meteoroid that strikes its surface. Despite the apparent timelessness of the Moon, a growing body of evidence in recent years has started to reveal that our nearest celestial neighbor has been geologically active much more recently than previously thought. The latest such evidence come from NASA’s Lunar Reconnaissance Orbiter, or LRO, which has discovered many dozens of geologic features across its surface that are indicative of very young volcanic activity, as recently as a few million years ago.
“Magnificent desolation”—These were the first words that came to Edwin “Buzz” Aldrin’s mind when he graced the stark beauty of the ancient lunar landscape, while becoming the second man to walk on the Moon after Neil Armstrong during the historic flight of Apollo 11 in July 1969. The lunar rock samples that the two astronauts brought back with them to Earth, as well as those that were returned during the rest of the Apollo missions, helped scientists to determine that our natural satellite had most probably been the end product of a collision between the newly formed Earth and a Mars-sized object approximately 4.5 billion years ago. In the next one billion years following this cataclysmic event, the rapidly cooling Moon experienced extensive volcanism due to the constant battering of its surface from asteroid impacts, as well as internal geologic processes that gave rise to the vast plains of dark, solidified magma known as the “maria,” eventually shaping the surface of the Moon as we know it today. Conventional wisdom held that due to its smaller size, the Moon cooled down much sooner than the Earth with most of the lunar volcanism occurring from 3.8 to 3 billion years ago, until eventually the last remaining episodes of volcanic activity died out abruptly approximately 1 billion years ago. Yet a series of volcanic deposits on the Moon’s surface, informally called Irregular Mare Patches, or IMPs, that have been an object of study ever since the Apollo missions, paint a different picture by showing that our natural satellite has been geologically active as recently as a few million years ago and might even be so today.
The first of these enigmatic surface features ever to be discovered was a D-shaped, 2-km-wide depression named Ina, that had been observed in Lacus Felicitatis, or Lake of Happiness, by the Apollo 15 astronauts in 1971 during their orbital reconnaissance of the Moon. Ina comprised a mixture of two distinct, overlapping deposits of smooth rounded dark material superimposed on a rougher light-colored one, with the latter forming steep, sharp edges at the borders of the former, resulting in an irregular sparsely cratered topography which indicated that Ina had a relatively young age. Scientists at the time had speculated that the intriguing structure was a collapsed caldera of a low-shield volcano, and, even though a handful more IMPs had also been detected in subsequent years, they were largely perceived as unique peculiarities that were unlike anything else seen on the lunar surface.
Ina’s nature became more apparent in 2006, when a U.S. research team studied the multi-spectral images of the Moon taken by the Clementine spacecraft in the early 1990s. These images revealed that the coloring of Ina’s terrain resembled that of very young impact craters (even though Ina wasn’t a crater itself) and was brighter than other darker and much older areas of the Moon. This indicated that its topography was too young to have been effectively darkened by the eroding effects of space weather, namely the meteoroid collisions and solar and cosmic radiation that constantly bombard the lunar surface. This line of evidence led the researchers to speculate that the mysterious feature was probably the result of outgassing from the Moon’s interior or volcanic eruptions of basaltic lava flows, which had occurred as recently as 10 million years ago—a mere blink in geologic time.
A new era of lunar exploration began in June 2009 with the launch of NASA’s Lunar Reconnaissance Orbiter, or LRO. With the help of its high-resolution LROC camera, the spacecraft has been constantly studying the Moon ever since while also mapping its surface at a resolution of up to 50 cm/pixel, providing us with images of the lunar landscape of unprecedented detail and clarity. By analysing the treasure trove of data that has been returned by LRO, an international team of planetary scientists revealed the presence of 70 Irregular Mare Patches, ranging in size from 100 m to 5 km across, that were too small to be detected from Earth. The newly discovered geologic features that were announced in a new study, published in the Oct. 12 issue of the journal Nature Geoscience, display an irregular morphology and terrain textures that are similar to that of Ina and are widely dispersed across the Moon’s near side.
In order to estimate their age, the researchers counted the number and sizes of craters inside the IMPs’ smooth terrain, using a technique called crater size–frequency distribution. Their results showed that some of the newly found volcanic formations were probably younger than 50 million years old, indicating that the Moon was volcanically active over a far greater time span than that predicted by theoretical models. “The basaltic flows of the IMPs (smooth deposits) are significantly smaller in volume than the lunar maria, consistent with short-duration, late-stage eruptions,” write the researchers in their study. “The existence of Ina and other IMPs provides evidence for a period of young basaltic volcanism more recent than the commonly accepted cessation of basaltic volcanism (approximately 1 billion years ago). Young, small-volume extrusions of mare basalt imply a thermal history where mare volcanism did not end abruptly, but rather decreased gradually over time. The existence and inferred late ages of the IMPs have implications for models of lunar thermal evolution, which must provide enough heat to account for small-volume eruptions [during the late lunar geologic history].”
These results from LRO helped to showcase that rather than being a peculiar oddity of the lunar landscape, as was previously thought, Ina turned out to be a representative of the Moon’s more volcanically active behavior instead. Furthermore, the similarity of the newly found IMPs to Ina and their geologically very young age indicate that the Moon could still be kicking inside even today. If that turns out to be the case, it could also potentially help to explain other intriguing enigmas surrounding our celestial neighbor, like the fleeting and mysterious transient lunar phenomena that have baffled observers for centuries. More importantly, LRO’s recent findings serve as a window to the Moon’s geologic past, allowing scientists to gain a much better understanding of its evolution over time to the present day. “This finding is the kind of science that is literally going to make geologists rewrite the textbooks about the Moon,” says John Keller, deputy project scientist for the LRO mission at NASA’s Goddard Space Flight Center in Greenbelt, Md.
To that end, most scientists agree that a sample return from these sites is necessary in order to further constrain theoretical predictions regarding their age and chemical composition. Future manned missions to these fascinating and mysterious locations on the lunar surface would allow astronauts to sample and characterise the IMPs’ rock and soil and determine whether its composition and overall properties is similar to that of the samples that were returned from the Apollo landing sites, greatly enriching our geologic knowledge of the Moon in the process. “These young volcanic features are prime targets for future exploration, both robotic and human,” says Mark Robinson, principal investigator for the LROC camera at the Arizona State University.
In our continuing study of the Moon, we find that every discovery highlights a series of even more intriguing secrets that are in turn waiting to be uncovered. The riches and the knowledge to be gained on our nearest celestial neighbor in space beckon, if only we had the ears to listen.