After the solar system formed 4.6 billion year ago, an object slammed into the Moon and formed a 620-mile-wide indentation now known as the Crisium basin. Scientists examining the region say they’ve spotted a crater within the basin that appears to contain pristine impact melt of volcanic rock, reports National Geographic.
Not only might the geologic feature yield clues about the frenzied meteor bombardment during the early history of the Earth and Moon, the basin holds a geologic blister the size of Washington, D.C., unlike any other feature seen in the the solar system. The volcanic lump appears to have been cracked open by underground magmatic activity.
Rocks recovered by U.S. Apollo missions and Russian robotic missions are estimated to be between 3.8 and 4.0 billion years old, leading scientists to theorize that there was spike in the number of impacts on the Moon during a period known as the Late Heavy Bombardment. But that conventional wisdom has come into question. Why was there a 700-million-year quiet period before then?
The Crisium basin could shed light on that mystery. The original impact that formed the basin was so powerful that it created a melt sheet up to 9.3 miles thick. Later, profuse eruptions of lava flooded the basin beginning about 3.6 billion years ago, forming a volcanic sea known as Mare Crisium that covered up much of the original melt. But “islands” of rock within the basin, known as Kipukas, survived the lava inundation. As lunar scientists examined the region, one Kipuka stuck out. Close examination showed that much of it was made of frozen volcanic rock. The research team conjectures that the lump was pushed upward by subsurface volcanic activity, but the origin remains a mystery.
Nestled within the Crisium basin is the 220-mile-wide Yerkes crater. The impact creating that crater was powerful enough to form a central peak, made from debris that flowed like a fluid and solidified into a mound. This feature kept Crisium impact melt out of reach of the lava sea that later would flood the basin’s floor.
A mission to Yerkes may definitively reveal the age of the lunar basin. Writes National Geographic: “If it is 3.9 billion years old, like [Mare] Imbrium,, that supports the idea of an ancient spike in meteor impacts. If it’s much older, that implies that the meteor impacts were spread out over a longer period of time.