NH Arrokoth (formerly Ultima Thule) Encounter Observations & Results, post-flyby discussion as the data arrives Options

[tty]

It seems to me that an impact as large as the one on Thule would create enough momentum to separate the lobes, at least temporarily. Could the "ring" be a previous "attachment point"?

[Marcin600]

Small round pits are not necessarily impact craters. They are best seen at the terminator, but they seem to cover large surfaces - they appear as white dots. They can be "internal" and not "external" origin. They have similar size and lack of size distribution typical of craters. (I'm not talking about a big hole at Thule - it can be a crater, but it does not have to be!)

[Rittmann]

Some speculation based on the last image two-frame rotating image...

In the rotational image it seems as if there was not one, but two rings of bright material: the central one, and another one to the lower left side of the image. If this is a remnant of an ancient contact point between Ultima and Thule, I speculate that one of them was an earlier contact point and a later impact made Thule roll over Ultima creating the second ring. The exact place where the rolling happened could be the bright patch of material that is where both possible rings find each other. Since the lower left area apparently has two small craters / sinkholes, it would be the most ancient area.

In Thule there appears to be two different types of terrain. Right on the top we see some possible cratering, as well as on the right side of the depression / crater. But the central left side of the depression / crater appears smooth to a certain degree. This may be due to lightning conditions, but I think that an impact has enough kinetic energy as to melt partially the material in Thule and make it flow over older terrain. With such a low gravity, most of the splash would be lost but the melting could flow that way. The "melting" also seems to hide a bright line, that could be the ancient contact point between Ultima and Thule: it appears to be an arc of roughly the same size of the other rings in the current contact area and in Ultima. If so, stretching the hypothesis the V in Thule could correspond to the marking of the rolling: in Ultima we see two well-defined arcs touching, and the V in Thule would be the remnant of one of the sides of the touching arc. Also, the crater / depression has no clean border which could be explained by the melting hypothesis.

So the story of these bodies, according to these speculations, would be:

1. Formation of both bodies and for some unknown mechanism they eventually enter in contact. Original contact point is the central circle on Ultima.

2. Some moderate impact causes Thule to roll over Ultima, possibly from a not strong contact equilibrium position. This creates a second neck, the lower-left circle that seems more visible in the rotation image.

3. A bigger impact creates the crater / depression seen in Thule, splitting the contact binary and causing both bodies to change rotation axis. Eventually both bodies came into contact again, in the position we see nowadays. Impact creates enough cynetic energy as to partially melt the surface of the bodies, causing the flows we see and partially erasing the original contact rings in Thule, as well as melting the crater borders. Ejecta orbits the plane of the impact until collisioning with both bodies, littering the craters we see near the border of both bodies. Since ejecta impactors are small, melting is on a smaller scale and don't cause flows.

Against these hypothesis: there appears to be some sort of flow on Ultima, or at least similarities in the surface features, yet the rings are dimmed but not erased. This could mean that the flow is not such, but the rain of fine debris from the impact. The original impactor would then be the source of most impact craters we see in the border: we can see in the high-resolution image at least a couple of craters of a similar size in the (2) ancient neck, so at least from current data it seems that craters have a clear distribution over both surfaces.

As for the mechanism of making both bodies end up contacting, the only idea that comes into my mind is that since both bodies are fairly irregular in shape, the micro-gravity field should be far from homogeneous causing losses over time as mutual rotations cause shifts in the gravity field, so orbit would degrade over time. But I have no expertise in orbital mechanics, so I don't know if I just said something stupid here.

[wildespace]

Exciting!

Here's a version with some colours plugged in from a reprocessed colour image:

[tty]

As for the mechanism bringing the bodies back together after a major impact, tidal dissipation immediately comes to mind. If the impact energy was enough to make one body at least partially plastic the dissipation would be greatly increased.

However explicitly calculating the effect on two highly irregular (and possibly inhomogenous) rotating bodies in probably very eccentric orbits would be quite challenging. Qualitatively I would guess that the tidal effects would first slow down rotation of both bodies until they became tidally locked, and probably simultaneously decrease eccentricity of the orbits. Once tidal rotational lock is reached any further dissipation would very slowly bring the bodies together.

Of course they could also have re-collided physically long before this. It all depends on the orbits.

[Spock1108]

An attempt to add colors ...

[Roman Tkachenko]

My attempt to improve the image.
Not the best way to do such things when you have no raw image, but anyway

[ronatu]

[jasedm]

Following on from discussions about the surface geology, some of the shapes seem to be somewhat reminiscent of 'patterned ground' - polygonal structures caused by freeze/thaw and subsequent differentiation of surface material. I'm not sure if UT has enough of an eccentric orbit to cause these processes, but it seems a possibility.

[PhilipTerryGraha...]

I'm assuming the raw MVIC data downlinked thus far hasn't been thrown up onto the PDS or any other place just yet?

[Spock1108]

It reminds me of someone ...

[mcaplinger]

I'm assuming the raw MVIC data downlinked thus far hasn't been thrown up onto the PDS...

One does not simply "throw stuff" onto the PDS. https://pds-smallbodies.astro.umd.edu/data_...AP-01_R0_C0.pdf doesn't describe the schedule for the extended mission but Table 3 says the first delivery of Pluto encounter data took about 9 months from encounter.

[JRehling]

re: resurfacing, we're certainly looking at a world where geology doesn't apply in the usual sense, but there are still mechanisms at this scale that can blank the surface:

• The shaking that occurs with a straight-on impact could cause material to flow down slopes.
• An impact could shower the surface with material causing a new surface to overlie the old one.
• Porous portions of the body could collapse inward to a new configuration at higher density.
• Electrostatic accumulation of fine-grained material burying the old surface.

More?

[Gladstoner]

• Porous portions of the body could collapse inward to a new configuration at higher density.

The feature marked here does resemble a line of collapse pits along a fracture:

[Rittmann]

Another mechanism comes to my mind for the white lines we see. 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. Any energetic impact that could have partially melted the surrounding material of the impact area could cause a melting, which would naturally flow through these discrete boundaries filling them up and causing contrast of materials.

As a result, even if there was a full melting of the clump surface, boundary would still differentiate. And the big circle in Última seems exactly that: melted surface which slightly seems a done with a surrounding ring, but the flowing material escapes this boundary - this seems apparent on the right side.

Against this, is where would be the impact crater causing any such melting. Thule has a crater, but Ultima seems devoid of any such originator in the area.