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Astrophysicists have long speculated about the creation of the moon. Since the late twentieth century, the dominant theory has been “the giant impact hypothesis” which posits that a huge object about the size of Mars smashed into the newly coalesced Earth 4.5 billion years ago. Astronomers name this mysterious proto-planet “Theia” after the titan who was the mother of the moon is ancient Greek mythology. They speculate that the Earth and Theia melded together and the iron/heavy metal core of Theia sank into the molten Earth. A great deal of the light material was thrown into orbit around Earth where it coalesced into two moons (the smaller of which was unstable and pancaked into the dark side of the moon a few million years after formation).
These are pretty intense ideas, however they explain many of the features of the moon and Earth (you can look at a comprehensive list on Wikipedia if you like). Yet astrophysicists have not been completely satisfied by the current model of the giant impact hypothesis. The composition of the moon is suspiciously identical to that of Earth (whereas, computer models seem to indicate that it should contain more of Theia).
This week, a scientific paper suggests that the collision was somewhat different than envisioned in the giant impact hypothesis. The paper’s main author is Natsuki Hosono, and he has a revised version of how Theia hit Earth. According to this new hypothesis, the freshly formed Earth was still piping hot and its surface was covered with a lava ocean. Theia banged into Earth and careened off into space like a pool ball but the impact knocked the liquid ocean of lava into space, where it coalesced into one or two moons (which then ultimately amalgamated together). The new hypothesis answers critical questions about lunar composition (and about the ratios of volatile elements on the moon). Yet it does tend to beg questions such as what happened to Theia and what the nature of the Earth’s lava ocean was.
I guess we’ll keep watching the sky and the news to see how the world astronomy community reacts to the revised hypothesis. In the mean time I will see what I can dig up concerning Theia (the goddess or the proto-planet). That seems like the most intriguing part of the story yet details are weirdly exiguous.
Expanded-color image of Mercury’s 52-km Degas crater, showing an abundance of dark material (NASA)
Today brings interesting color themed news from outer space. The Messenger space craft (which was destroyed when it was deliberately crashed into Mercury in spring of 2015) spotted numerous mysterious dark spots on Mercury. Indeed the Messenger spacecraft probably now is a dark spot on Mercury. Apparently the small dense planet has a dark layer close beneath the surface. Asteroid impacts, volcanoes, space probe collisions, and other events which disturb the surface of the planet reveal this extremely dark black/gray layer.
Scientists have been analyzing the data from Messenger and it now seems that this black layer which is the color and texture of pencil lead is actually composed of…graphite, the same material as pencil lead! Apparently when Mercury formed (which featured strange geological processes unseen anywhere else in the solar system) a planet-sized ocean of lava covered the entire world. As Mercury cooled the heavier elements of this lava field crystallized and sank leaving the buoyant pure carbon at the top. This dark layer has been subsequently covered with ejecta, dust, and fragments, but any disruption shows the crystallized carbon is still there.
Credit: NASA/Johns Hopkins University Applied Physics Laboratory/Carnegie Institution of Washington
There is a lot more data from Messenger left to analyze. I wonder what other surprises the closest planet to the sun still holds.
ferrebeekeeper 