Hampton University planetary scientist William Moore and colleagues have proposed an alternative theory for how planets and moons with Earth-like interiors formed.
Mercury, Venus, Earth, Mars. At their core, and immediately surrounding it, they’re quite similar. Internally, our Moon looks much like these terrestrial planets—containing an iron-rich core and a silicate mantle—although the Moon’s core is smaller and its mantle contains more iron than Earth’s.
Interestingly, two of Jupiter’s moons—Io and Europa—also possess a similar internal structure, even though their home planet is a gas giant. Io is believed to be the densest satellite in the Solar System, followed by the Moon.
So what do Io, the Moon, and Earth have in common? And what can the composition of their outer shells tell us about the way planets form and evolve? Those are questions being explored by Justin Simon at NASA’s Johnson Space Center in Houston, William Moore at HBCU Hampton University, and Alexander Webb at Louisiana State University. In a paper presented at the 46th annual Lunar and Planetary Science Conference in March, the scientists present a modified model for how the Moon’s lithosphere (the mantle and the crust) formed.
There’s a good deal of scientific consensus that the Moon itself likely formed from the debris produced when an astronomical body the size of Mars collided with the Earth some 4.5 billion years ago. That so-called Big Splash initially created a lunar ocean of molten rock (also known as magma when its underground and lava when it surfaces). Some models for lithosphere formation state that material in the magma ocean then separated by density and cooled and thickened over time.
(See this video for a conventional guided tour of how the Moon evolved)
But Simon, Moore and Webb argue that density-driven separation and monotonic thickening of the lithosphere do not sufficiently explain the specific chemical makeup of certain moon rocks, known as anorthosite. Instead, the researchers suggest an additional process similar to one occurring now on Io, where, in some spots, heat travels from the molten interior to the surface through the more than 400 active volcanic “pipes” —which also spew lava onto the surface—while the cooler portions of the lithosphere thicken. In the lunar case, the early surface rocks may have been repeatedly remelted by lava flowing up through lunar heat pipes, each melt altering the anorthosite chemical composition.
The researchers’ “heat-pipe hypothesis” may generally explain how terrestrial planets and satellites evolve: Their lithosphere initially thickens during volcanic eruption and magma/lava cooling, then thins as volcanic activity wanes, and finally thickens again and becomes rigid. Indeed, Moore and Webb made the case for an ancient Heat-Pipe Earth in the prestigious journal Nature.
In addition to his research, Moore directs Hampton’s Center for Planetary Dynamics and is professor in residence at the Virginia-based National Institute of Aerospace. He is also part of a collaboration that’s proposing an unmanned mission to watery Europa, considered to be the Solar System’s best bet to host alien life.