It’s one of the most prominent features on the surface of our natural satellite, readily identifiable by anyone who has ever looked up at a Full Moon: a series of dark patches that create the illusion of a human face that smiles back at the us. The dark areas that create this pareidolic image of the so-called “Man in the Moon” however, in reality compose a vast 3,000-km-wide (1,860-mile) “mare,” or “sea,” called Oceanus Procellarum, or Ocean of Storms, an expansive flat basaltic basin that covers approximately 17 percent of the Moon’s near side, filled with solidified magma that welled up from the Moon’s interior billions of years ago. It was long thought that its flat plains constituted an ancient impact basin that was probably formed when a giant asteroid slammed onto the lunar surface during the Late Heavy Bombardment, a period of cataclysmic asteroid collisions early in the history of the Solar System. Yet a recently published research based on data from NASA’s GRAIL mission is casting doubt on this long-held hypothesis, while providing new evidence which indicate that Oceanus Procellarum was likely formed by internal geologic processes instead.
NASA’s Gravity Recovery and Interior Laboratory, or GRAIL mission, consisted of two identical spacecraft, named Ebb and Flow, which orbited the Moon for a full year between Dec. 31, 2011, and Dec. 17, 2012. Flying in tandem in a nearly circular polar orbit, the two spacecraft continuously measured the distance between them as they raced above the Moon’s surface, detecting even the tiniest changes to their relative speed to an accuracy of 1 micrometer per second, which were caused by the uneven gravity field of our natural satellite. This precision allowed scientists to construct the highest-resolution gravity field map ever of the entire Moon and study the geophysics and overall internal structure of our nearest celestial neighbor in unprecedented detail. The ongoing analysis of the wealth of data beamed back by the twin spacecraft has already yielded important findings, like the discovery that the lunar crust is substantially thinner than previously thought, exhibiting an average thickness of approximately 34 km, and the discovery of gravity anomalies which indicated the presence of linear cracks, or dikes, beneath the Moon’s surface that didn’t correspond to any visible topographic features on the surface. Now, a new paper that was published on 2 October at the journal Nature by a U.S. team of astronomers, led by Dr. Jeff Andrews-Hanna, an associate professor at the Colorado School of Mines and co-investigator for the GRAIL mission, argues for a complete rethinking of the formation mechanism for Oceanus Procellarum, challenging the long-held notion regarding the impact origin of the giant lunar basin.
The researchers used the high-resolution gravimetric data that had been gathered by the twin GRAIL spacecraft during their year-long mission in order to study the underground structure of Oceanus Procellarum. Their analysis revealed the presence of a distinct pattern of underground long linear gravity anomalies—narrow regions of a slightly higher gravity acceleration which, contrary to the dike-like features that had previously been discovered by GRAIL across the lunar subsurface, were exactly aligned with the borders of the vast lunar basin when compared to topographic data of the same region. These gravimetric data from GRAIL showed that these observed gravity anomalies that surrounded Oceanus Procellarum formed a series of narrow straight lines which run for hundreds of kilometers and intersected at the borders of the basin forming angles between 109 and 125 degrees, similarly to a rectangle. “The rectangular pattern of gravity anomalies was completely unexpected,” says Dr. Andrews-Hanna. “Using the gradients in the gravity data to reveal the rectangular pattern of anomalies, we can now clearly and completely see structures that were only hinted at by surface observations.”
In order to shed more light to the mystery, Andrews-Hanna’s team run a series of computer models under the assumption that the former were caused by solidified lava which erupted from the lunar interior between 3 and 4 billion years ago. Oceanus Procellarum has an enhanced concentration of heat-producing, radioactive elements like uranium, thorium, and potassium that would have heated the region more compared to the rest of the lunar surface during the Moon’s early history. According to the simulations, this excess heat would eventually melt the lunar crust, producing plumes of magma that would flow toward the surface. The great thermal differential between the hot lava and the surrounding lunar crust would cause the latter to stretch, contract and crack open, leading to the formation of polygonal rift-like features that bore a striking similarity to those surrounding Oceanus Procellarum. Based on these results, the researchers concluded that the gravity anomalies which GRAIL observed in the subsurface of the vast lunar basin, are likely the frozen remnants of such an ancient rift valley that acted as a “plumbing system” through which lava from the Moon’s interior would later fill the rest of the smaller basins on the Moon’s near side. “How such a plume arose remains a mystery,” says Dr. Maria Zuber, principal investigator for the GRAIL mission, at the Massachusetts Institute of Technology in Cambridge. “It could be due to radioactive decay of heat-producing elements in the deep interior. Or, conceivably, a very early large impact triggered the plume. But in the latter case, all evidence for such an impact has been completely erased. People who thought that all this volcanism was related to a gigantic impact need to go back and think some more about that.”
This conclusion was strengthened by the fact that the observed polygonal features surrounding Oceanus Procellarum weren’t seen on other impact basins, either on the Moon or other terrestrial bodies in the Solar System. “This quasi-rectangular pattern is in contrast with the circular or elliptical shapes of all other large impact basins, including the ancient hemisphere-scale Borealis basin on Mars, for which a continuous elliptical basin rim can be traced in topography and gravity data,” write the researchers in the study. “The interpretation of the [Oceanus Procellarum] border structures as the rim of an impact basin, would require hundreds of kilometers of horizontal deformation with large strain gradients to produce the angular corners, but there is no evidence for such large-magnitude strain on the Moon. Furthermore, the negative gravity gradients of the border structures, do not match the signatures of known impact basins such as the Imbrium and South Pole-Aitken basins … Although it is not possible to disprove the existence of an ancient degraded Procellarum basin that lacks a clear geophysical signature, the geometry and gravitational signature of the structures bordering [Oceanus Procellarum], do not support the interpretation that they mark the rim of an [impact] basin.”
Despite their apparent uniqueness in the interior of our natural satellite, similar rectangular patterns have nevertheless been observed in other Solar System bodies as well, like Saturn’s moon Enceladus. “We note a similarity in the pattern of structures to the south polar terrain of Saturn’s icy moon Enceladus,” conclude the researchers. “Both the [latter] and [Oceanus Procellarum] are bordered by quasi-rectangular sets of tectonic belts with angular intersections that sometimes take the form of triple junctions … The gross morphological and geophysical similarities between Oceanus Procellarum on the Moon and the south polar terrain on Enceladus suggest the possibility of broad parallels in their geodynamic evolution and that similar parallels may exist with other magmatic-tectonic centers, like the northern lowlands of Mercury.”
NASA’s tremendously successful GRAIL mission helped to showcase the fact that even after many decades of extensive study, the Moon remains essentially a mysterious, largely unexplored world. “Our gravity data are opening up a new chapter of lunar history, during which the Moon was a more dynamic place than suggested by the cratered landscape that is visible to the naked eye,” says Dr. Andrews-Hanna. “More work is needed to understand the cause of this newfound pattern of gravity anomalies, and the implications for the history of the Moon.”
Far from being a “been there, done that” destination, our nearest celestial neighbor holds many more secret wonders that are just waiting to be discovered by the next wave of human and robotic explorers.