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Artist's_concept_of_collision_at_HD_172555

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.

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Today’s post takes us back, once more, to Ultima Thule/2014 MU69, the distant snowman shaped planetoid at the edge of the solar system which was visited by the New Horizons space probe as it flies through the Kuiper Belt on its way out of the solar system (since that time, we also blogged about the color Thulian pink–which is based on the fantasy land at the northern edge of the medieval map).  Well, space can be a confusing place, and, even with digital cameras, the way we see objects tumbling through the void can be misleading.  As New Horizons flew away from Ultima Thule, it turned its cameras around and took the following shot (which hopefully shows up in all of its glory as an animated gif below).

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Holy hemispheres! What is with that bright edge?  Spheres certainly don’t have those! It turns out that Ultima Thule may not be a snowman as originally billed.  Instead it seems more like a double pancake.

ultima-thule.jpg

This news will please flat earthers (on the off chance they believe that New Horizons actually exists), but they shouldn’t read too much into it.  Planets are spherical because they are so massive that the force of gravity causes them to collapse into the most efficient shape – a sphere. This is broadly true for objects with a diameter greater than 1000 kilometers (621 miles) and Ultima Thule was not even remotely that big (indeed we didn’t think it was a sphere before).  I do wonder how these two smushed snowballs formed and came together though.

Enthusiasts of Kuiper Belt objects will have to discard the snowman analogy and look for an object which is a lumpy disk stuck to a smaller lumpy disk.  It sounds like a hogchoker to me (see a picture of the flatfish below), but this may merely be a shallow pretext to link to my flounder art on Instagram.  It might be a while before we discover anything even remotely shaped like a flatfish in space though so I am going to take what the universe offers.  If you have better topological analogies feel free to share in the comments (after you follow my Instagram).

hogchoker.jpg

 

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