The bright side of the Moon is dominated by seas. The largest one, Oceanus Procellarum, is more than 1,500 miles across. Until now, researchers believed that the vast ocean appeared as a result of an asteroid impact. But new data provided by the Grail mission shows that this was not the case. The ocean is the result of the Moon’s own volcanic activity, according to an article published in Nature on Wednesday.
Other smaller Moon seas, like Mare Serenitatis and Mare Imbrum, are the result of asteroid impacts, which were followed by magma floods. They cooled down and the basaltic volcanic rock gives the dark aspect noticeable without any telescopes.
Procellarum does not display an entirely circular shape as the two other seas, making it a bit harder to believe that an asteroid that large has indeed hit the Moon. GRAIL satellites mapped the Moon’s gravity levels and, according to the data, the vast ocean is a result of the natural satellite’s volcanic system. The two GRAIL probes orbited the Moon at the same speed. While flying above denser Moon areas, the leading probe increased its speed slightly. After multiple trips, there was enough data to draw funded conclusions.
The incomplete circularity has been a point of debate, but the magnetic lines clearly show an area bordered by a rectangle, considered to be a “magma plumbing system.”
One paper author, Jim Head, the Louis and Elizabeth Scherck Distinguished Professor of Geological Sciences at Brown, said that “instead of a central circular gravity anomaly like all other impact basins, at Procellarum we see these linear features forming this huge rectangle.” He concludes that “This shape argues strongly for an internal origin and suggests internal forces.”
The volcanic events leading to the formation took place 3 to 4 billion years ago. Procellarum area contains higher densities of radioactive elements like uranium, thorium and potassium, keeping the area hot for longer than the rest of the Moon. The molten magma covering the whole Moon at the time cooled down in Procellarum last. The process made the crust shrink and rifts were created around it. Finally, magma filled those rifts and this is how the ocean ended up having the distinct rectangular form.
The previous theory was shaky, due to the inconsistencies between the ocean’s shape and the characteristics of its borders. Traces of impact can be signaled by elevated borders, which was not the case for Procellarum.
