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NH Arrokoth (formerly Ultima Thule) Encounter Observations & Results, post-flyby discussion as the data arrives
ngunn
post Jan 29 2019, 10:05 AM
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QUOTE (Rittmann @ Jan 29 2019, 06:11 AM) *
If these bodies are made of lesser balls of aggregated material, which originally may have clumped before being added to Última and Thule, they would have boundaries.


I have one observation I haven't seen mentioned explicitly here. The constituent lumps of the larger lobe Ultima all seem to be about the same size, causing it to resemble a bag of oranges. I have absolutely no suggestions as to why that might be.
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Gerald
post Jan 29 2019, 10:44 PM
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During Rosetta's 67/P mission, there have been lots of speculations about cometesimals as building blocks. This paper appears to contain some of the according discussion, especially subsection 4.3 about their hierarchical agglomeration.
The paper is referencing a paper about simulations of this mechanism. Here an excerpt of the abstract:
QUOTE
The size distribution of cometesimals growing by drag-induced collisions develops a narrow peak in the range tens to hundreds of meters.

So, there seem to exist size-dependent agglomeration mechanisms with prefered grain sizes. Above some size, gravity between cometesimals should come into relevant play, and we are getting into a rubble pile model.

This is just some clue, where to start with.
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Fran Ontanaya
post Jan 30 2019, 01:34 PM
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Ultima resembles a bit Atlas too, which would imply it accreted within a ring of particles

Attached Image


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"I can easily see still in my mind’s-eye the beautiful clusters of these berries as they appeared to me..., when I came upon an undiscovered bed of them... – the rich clusters drooping in the shade there and bluing all the ground" -- Thoreau
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alan
post Jan 30 2019, 10:22 PM
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This model assumes gravitational instabilities formed 1 km planeteseimals which them accreted leftover cm-sized pebbles. It includes a period when similar sized planetesimals accrete each other:

Forming the Cold Classical Kuiper Belt in a Light Disk
http://adsabs.harvard.edu/abs/2016ApJ...818..175S

There are also some LPSC abstracts discussing formation mchanisms:
https://www.hou.usra.edu/meetings/lpsc2019/pdf/2809.pdf
https://www.hou.usra.edu/meetings/lpsc2019/pdf/3044.pdf
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HSchirmer
post Feb 1 2019, 08:00 AM
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QUOTE (Steve5304 @ Jan 25 2019, 05:59 PM) *
I would think that you just don't have that much out that far...it unlikely to be any sort of resurfacing...how would that even happen that far out on a dead object like this?? Accretion over millions of years??


IIRC, there is a "Stria" paper The Formation of Striae within Cometary Dust Tails by a Sublimation Driven YORP like Effect
about cryo-ice sublimation torques, which estimated that CO could still exert some pressure out to 40-50AU.

Also, "think fluffy", like aerogels (solid smoke), shuttle tiles, and pink fiberglass insulation, these objects should be exceedingly bad at heat conduction, and a large internal surface area should suppress convection by any internal gasses.

I'm really curious about the results from the "radar ping" experiments, billions of years with a surface dominated by sublimation, I'll guess there may be some ice pinnacles or other "twists" that are responsible for the appearance at this resolution.
Formation of metre-scale bladed roughness on Europa’s surface by ablation of ice
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Roman Tkachenko
post Feb 1 2019, 11:44 AM
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Interpolated animation derived from CA04 LORRI and CA06 MVIC frames


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HSchirmer
post Feb 1 2019, 03:49 PM
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QUOTE (ngunn @ Jan 29 2019, 11:05 AM) *
I have one observation I haven't seen mentioned explicitly here. The constituent lumps of the larger lobe Ultima all seem to be about the same size, causing it to resemble a bag of oranges. I have absolutely no suggestions as to why that might be.


A binary KBO goes splat. It generates fragments and a gravitationally bound exosphere.
Disruption leaves 2 primaries and 6 fragments.
Thanks to the laws of orbital mechanics, all 6 fragments accrete onto one fragment, creating "the hamburger", and kicking the "the meatball" into a more eccentric orbit.
Tides and the angular momentum transfer via the exosphere lock meatball and hamburger into a spiraling synchronous orbit, tidal heating cause the crater on the meatball and the scarps on the hamburger which replenish the exosphere. They end up tidally locked, and eventually dock.

-edit- Or, you've got a situation with "snow white and the 7 dwarfs" where seven small chunks accrete and capture one larger chunk.
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alan
post Feb 5 2019, 05:18 PM
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J. I. Katz, Ultima Thule (486958; 2014 MU69): Necklace, Composition, Rotation, Formation

QUOTE
Flyby images of Ultima Thule (486958; 2014 MU69) show a comparatively bright ``necklace'' between its two lobes, in contrast to its generally low albedo. The necklace is found in the most shaded, and therefore coolest, part of its surface. It may be clean, high albedo, ``hoarfrost'' condensed from vapor evaporated from the low albedo dirty ice elsewhere. Ammonia, the likely major constituent of Ultima Thule, has the necessary vapor pressure. The rotation period of 15±1h is at least twice its breakup period, indicating either that its formation was not limited by angular momentum or that half its angular momentum was lost after formation, perhaps to surrounding gas in the proto-Solar System. The lobes of Ultima Thule must have spherized under conditions different than those encountered by its present, post-contact, configuration.
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AJAW
post Feb 5 2019, 07:56 PM
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Will our estimate of the rotation rate ever become good enough to look back at the occultation results from last year and work out exactly how many rotations have happened since?
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alan
post Feb 8 2019, 07:30 PM
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New Horizons' Evocative Farewell Glance at Ultima Thule
Images Confirm the Kuiper Belt Object's Highly Unusual, Flatter Shape

http://pluto.jhuapl.edu/News-Center/News-A...p?page=20190208

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marsbug
post Feb 8 2019, 09:00 PM
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I usually read with fascination but don't contribute for fear of denting the signal to noise ratio. However, it seems worth noting that my very un-expert brain immediately drew a connection between the weird shape of Ultima Thule and the weird shape of Omuamua. A flat, sail like, shape has been put forwards as an explanation for Omuamua's light curve I believe, and now it seems we have proof that nature can indeed craft such a flattened shape. Is it rediculous to suggest that the interstellar object could have formed in the same region of it's original solar system that Ultima Thule occupies in ours, and the two formed along approximately parallel lines under similar forces - acting in a bizarrely
pro pancake shape fashion?


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Explorer1
post Feb 8 2019, 09:07 PM
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Absolutely bizarre! A bit like the tiny moons of Saturn... but no rings anywhere nearby.

At the very least, this is good confirmation of two separate bodies being formed.
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Gladstoner
post Feb 8 2019, 11:08 PM
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A little bit of Ultima-shine is visible on Thule:

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jccwrt
post Feb 9 2019, 03:43 AM
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I realigned the MU69 departure movie on MU69 itself to make an occultation fence. It looks like the small lobe is relatively well-constrained by stellar occultations. The larger lobe is a little more difficult to pin down, with fewer stars along the occulation path.
Attached Image


Here's some chords drawn in that help show the shape of the unlit side:
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monty python
post Feb 9 2019, 07:36 AM
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Bravo for the occultation fence photo. The movie of it was too hard to follow for me.
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