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by tabtab 2729 days ago
Re: first Kuiper Belt Object we've visited

Pluto may be a KBO also. Further, it's possible some moons of the gas-giant planets are captured KBO's.

The unusual thing about Ultima Thule is that it's in a nearly circular and "flat" orbit, which many experts interpret to mean it's mostly undisturbed from its point of origin. It's a prime "fossil".

Pluto has a "disturbed" orbit such that its origin is currently unknown. Same for gas-giant moons. Pluto has also been turned inside-out, perhaps multiple times, by a still unidentified force. Ultima Thule is probably mostly as-is since formation.
1 comments
weberc2 2729 days ago
> Pluto has also been turned inside-out, perhaps multiple times, by a still unidentified force. Whoa, come again? How do we know this? What does it mean for a planet-like object to be turned inside out?
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deeths 2729 days ago
Inside out just means that the local equivalent of Earth's geologic plates seem to be subducting below the surface and fresh geologic plate equivalents are abducting back up. You can estimate how long a planetary surface has been exposed to the elements by the statistical distribution of number and size of craters. We have a reasonable idea of the likelihood of objects impacting Pluto, and it should have a lot of craters. Pluto's moon Charon has many more visible craters but Pluto has large areas with no large craters, so some sort of geologic process must be going on that's recycling areas of the landscape and covering or subducting the craters.

Given lack of fast erosion from liquid water or significant wind (in a very thin atmosphere), whatever geologic process is smoothing out Pluto is happening quickly compared to other points of reference we have.

I use the example of Earth's geologic plates, but that's only one possible explanation. There's also speculation about freeze/thaw cycles of the planetary material and atmosphere (since it has an irregular orbit), movement of water-ice mountains, cryovolcanos, and other theories.

Some examples: https://www.nasa.gov/sites/default/files/thumbnails/image/du... https://www.nasa.gov/sites/default/files/thumbnails/image/nh... https://www.nasa.gov/sites/default/files/thumbnails/image/co... https://www.nasa.gov/sites/default/files/thumbnails/image/nh...

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