January 31 will be an early morning show for Moon lovers. Starting about 2.51 a.m. Pacific Time will be a lunar eclipse, or “blood moon” as the Moon passes through Earth’s shadow and picks up a reddish tint. At the same time, the full Moon of Jan. 31 is also a “supermoon” when the Moon is relatively close to Earth and looks bigger and brighter, and a “blue Moon” because it is the second full Moon in one month.
Where did the Moon come from?
All of these phenomena are well understood, but there is still a great deal we don’t know about our nearest neighbor. Luna is much larger compared to the planet it orbits than any other moon in our Solar System, and we know from the Apollo missions that it is made of the same stuff as Earth. How exactly the Moon came to be where and what it is continues to fascinate planetary scientists like Sarah Stewart at UC Davis.
The general theory of how the rocky planets form is that when the Solar System formed there were a number of protoplanets orbiting the Sun. These objects collided with each other until we ended up with the four inner planets we have today. These collisions would have been enormously energetic, converting planets into vaporized rock before slowly cooling into something more solid.
In the case of Earth, it’s thought that proto-Earth collided with another large body (sometimes called Theia) knocking out a chunk of material that condensed into the Moon.
Stewart’s lab studies these problems through computer modeling and by recreating high-energy collisions on a small scale by shooting materials into each other with what are effectively gas-powered cannons.
In a recent publication, Stewart, Matija Ćuk (now at the SETI Institute in Mountain View), Douglas Hamilton at the University of Maryland and Simon Lock, Harvard University, proposed a new spin on the “giant impact” theory of Earth/Moon origins.
The initial impact, they argue, was even more energetic than thought – but much of that energy, preserved as angular momentum in the Earth/Moon system, was later transferred to the Earth/Sun system through tidal forces.
The new theory – in which the Earth initially had a two-hour day – explains some of the odd features of the Moon, including its composition and why its orbit is tilted a bit off the ecliptic.
Last year, Stewart and Lock published another paper proposing a new type of planetary object they call a “synestia.” A synestia is a giant spinning donut of vaporized rock formed from planetary collisions. The Earth and Moon could have formed out of such an object and the idea opens up new ways to think about planet formation.
Three Minute Egghead podcast about synestias here.
Read more about the Stewart group’s research.
What Made the Moon? (Quanta magazine)