According to the latest estimates by a team of researchers with the Technische Universität Berlin, it might have taken ten times longer than previously thought for the early Moon to transform from a ball of super-heated magma into its current form.
The oldest rock found on the Moon, brought to Earth by the Apollo 14 mission, has been dated to 4.51 billion years old, comparing to the estimated 4.54 billion years estimated for the age of the Earth. But minerals can go back only as far as the moment when those minerals formed. To date the Moon, scientists need to know how much time elapsed until the magma ocean solidified, explains Sky & Telescope.
Maxime Maurice and his colleagues at the German university have developed a new thermal evolution model — a detailed computer simulation — to reconstruct the first 200 million years of lunar evolution. Their studies identified two previously underappreciated dynamics: the insulating effect of the primordial lunar crust, and mantle convection that probably started even before the magma ocean completely solidified.
It has been long known that the early Moon formed a crust made of a light mineral called pagioclase, which floated atop the magma ocean. That crust turned out to be an excellent insulator. While previous studies had accounted for that effect, Maxime concluded the insulating effect had been under-estimated.
The other factor was mantle convection. Summarizes Sky & Telescope:
The lunar magma ocean solidified from the bottom up because high pressure at depth forced the magma to solidify even at high temperatures. This process likely solidified 80% of the magma ocean within 1,000 years of the Moon’s formation. If mantle convection started at this point, it could have allowed heat to continue flowing from the depths toward the surface, keeping the magma ocean hot and molten.
On Earth, mantle convection creates the magma that feeds volcanoes. “In this case, it would be exactly the same, but the volcanoes would have spilled their lava into the magma ocean,” Maurice explains.
Until now, van Westrenen says, most lunar evolution scenarios assumed that mantle rocks didn’t start moving until the magma ocean had completely solidified.
According to van Westrenen, the combination of these two processes makes a huge difference for the longer-term survival of the magma ocean.
A slowly crystallizing magma ocean could require a reinterpretation of mineral isotope dating of many lunar samples, and thus for the aging of the Moon-forming impact. The bottom line, the Moon may be 100 million years younger than commonly postulated.