A dark patch on the western side of the moon dubbed Oceanus Procellarum was believed to have been caused by an ancient impact, but new research suggests it may actually be the remnant of ancient rifts in the moon's crust.

These rifts may have acted as a "magma plumbing system" that flooded the regions with volcanic lava between three and four billion years ago, Brown University reported. The mysterious feature is linked to series of linear gravitational anomalies and forms a giant rectangle nearly 1,600 miles in length

"Instead of a central circular gravity anomaly like all other impact basins, at Procellarum we see these linear features forming this huge rectangle," Jim Head, the Louis and Elizabeth Scherck Distinguished Professor of Geological Sciences at Brown and one of the authors of the new paper, said. "This shape argues strongly for an internal origin and suggests internal forces."

The moon is believed to have been covered in boiling magma early in its existence; this volcanic lava most likely hardened into the moon's crust. Procellarum containis high-levels of heat-producing uranium, thorium, and potassium contradict this idea so these radioactive elements would have caused Procellarum to solidify after the crust had already cooled, causing it to shrink and pull away forming the giant rifts seen today.

"We think this is a really good, testable alternative to the impact basin theory," Head said. "Everything we see suggests that internal forces were critical in the formation of Procellarum."

Most large circular features have been proven to be impact basins which were later filled with lava. Samples gathered during the Apollo missions and subsequent unmanned missions helped confirm this idea. Oceanus Procellarum has been largely questioned because it is shaped like a horseshoe, causing it to stand out. It also is not surrounded by mountains and radial grooves from ancient ejecta like most impact basins.

"Orbiting the Moon following our surface exploration, it was very clear that Oceanus Procellarum differed in many ways from the circular maria in terms of its volcanic and tectonic activity," said Apollo 15 Commander David R. Scott, a visiting professor in the Department of Earth, Environmental, and Planetary Sciences at Brown. "After so many years of puzzling over this, GRAIL has now provided the data to show why it is so distinctly different."

The findings were published Oct. 1 in the journal Nature.