These two images are from the paper “Effects of early intense bombardment on megaregolith evolution and on lunar (and planetary) surface samples” in Meteoritics and Planetary Science (November 2020) by PSI senior scientist emeritus, Bill Hartmann, and Alessandro Morbidelli, a well-known French/Italian dynamicist at the Observatoire de la Cote d’Azur, in Nice, France. The images show the best-preserved impact basin, “Orientale,” (discovered on the east “edge” of the moon by Hartmann and Kuiper in 1962). Morbidelli and co-workers in 2018 produced a new computer model of lunar cratering history, showing intense bombardment in the first few hundred million years after lunar formation about 4.5 million years ago, with declining impact rates since then (contrary to the now discarded “late heavy bombardment” theory that proposed negligible cratering in the first 500 million years after lunar formation). Hartmann and Morbidelli conclude that many giant impact basins, like Orientale, formed in the first few hundred million years, many of which have been since obliterated by later cratering.
In past years, some lunar scientists proposed that the largest visible impact scar on the Moon may be the region known as ‘Oceanus Procellarum,’ a depressed, lava-covered region visible to the naked eye. It is partly obliterated by the more recent “Imbrium Impact Basin,” dated by many Apollo mission rock samples at age 3.9 billion years. However, the gravity-mapping team using the “GRAIL” orbiting instrument in 2011-12 argued against Procellarum being impact-related, because they found linear, not circular, structures surrounding Oceanus Procellarum below the Moon’s surface. These they interpreted “as part of the lunar magma plumbing system – the conduits that fed lava to the surface during ancient volcanic eruptions.” The Hartmann and Morbidelli illustration shows, however, that linear elements alone do not disprove an impact origin. The “rings” around Orientale and other basins contain linear elements, marked in white lines in the color image below. The illustration also shows that giant impacts created parts of the “lunar plumbing system.” The innermost white line (east side) can be seen in the black and white photo to have lava extrusions all along its base. In other words, as has been known for several decades, the linear segments in the “rings” of impact basins did “feed lava to the surface.”
Hartmann and Morbidelli agree that while Oceanus Procellarum is not yet proven to be an impact scar impacts of that size would deform the spherical shape of the moon, and the physical mechanics of the readjustment of the moon to spherical shape are not well understood.
Featured image: This photo of the lunar impact basin Orientale was taken by a lunar Orbiter. © NASA
Provided by PSI