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Kuiper Belt Oddity? Explaining Arrokoth’s Shape

Now and then people mention that our Pioneers and Voyagers made it through the Kuiper Belt on their long journey toward system’s edge, though unfortunately without operational cameras to record what they saw. A missed opportunity? Not really. Think about how long it took to find a Kuiper Belt Object like Arrokoth, the first one ever seen close up thanks to the mighty work of the New Horizons team. Without a major search to find a target, a craft passing through the Kuiper Belt is almost certainly going to encounter no objects whatsoever within range to record the details.

For now, Arrokoth, the object once known as Ultima Thule before running afoul of our times, has to serve as our example of what can emerge in this distant region, and an odd object it is. When its shape is compared to a flattened snowman, as it often is, the real story is in the word ‘flattened.’ How does this roughly 30-kilometer object emerge in the shape it’s in, and under what conditions was it spawned out there in its 298 year orbit around the Sun?

Two connected lobes are involved, as the famous image below makes clear, making this an evident contact binary, where two formerly discrete objects nudged into each other at low velocity. That part of the origin story is easy, and in fact the bi-lobed shape is not unique. We can find it in certain comets as well — consider 67P/Churyumov–Gerasimenko.

But both lobes of Arrokoth are flattened, not just one. A new paper in Nature Astronomy goes to work on the question of how this happened. Ladislav Rezac (Max Planck Institute for Solar System Research), one of the two first authors of the paper, comments:

“We like to think of the Kuiper Belt as a region where time has more or less stood still since the birth of the Solar System. There is as yet no explanation as to how a body as flat as Arrokoth could emerge from this process.”

Exactly so, because four billion kilometers out from the Sun, KBOs should have remained largely unchanged, frozen into their form in ways that give us information about that era. Yet here we have Arrokoth, whose surface seems smooth, uncratered, and again, flat. Is this shape primordial, or are we actually looking at an evolution of a cold classical Kuiper Belt Object that should have formed where we observe it today?

Image: Arrokoth’s flattened shape can only be seen from a certain perspective. The first images returned by NASA’s New Horizons spacecraft gave the impression of a “normal” snowman-shaped object. Arrokoth’s surface is surprisingly smooth and displays only few craters. Credit: © NASA/Johns Hopkins University Applied Physics Laboratory/Southwest Research Institute.

To get from there to here, Rezac and co-author Yuhui Zhao (Purple Mountain Observatory, Chinese Academy of Sciences) ran a ‘mass-loss-driven shape evolution model’ called MONET that suggests a process driving the answer, one developed in another paper earlier this year (citation below).

At the heart of the discussion is Arraokoth’s rotational axis, which is close to alignment with the orbital plane; the obliquity is about 99°. One polar region faces the Sun while the other faces away, while equatorial regions and the lower latitudes are dominated by daily variations in flux. The poles heat and release frozen gases, to produce a loss of mass which, the authors argue, produces the observed flattening over time. Thus “…the polar regions reach higher peak temperatures than the equator, due to the fact that the thermal timescales are longer than the rotational timescales, and the poles experience more sublimation than the equatorial regions.”

We wind up with symmetric erosion between northern and southern hemispheres. The process is thought to have occurred early on, perhaps in 1 to 100 million years, as volatile ices sublimated. The authors assume methane as the driving volatile for the evolution of Arrakoth’s precursor body, but add that N2, CO and CH4 ices all show active sublimation and condensation on large KBOs.

The escape of volatiles early in the formation process would not be particularly surprising, in comparison to what New Horizons found at Pluto, where larger size and stronger gravity allowed the dwarf planet to retain carbon monoxide, nitrogen and methane gases to the present era. Small bodies like Arrokoth would hardly have the gravitational attraction to hold on to these volatiles. Moreover, this may be a common part of the evolution of Kuiper Belt Objects:

This process most likely occurred early in the evolution history of the body, during the presence of supervolatile ices in the near subsurface layers. This could be the dominant process shaping the structure of KBOs if there were no catastrophic collision reshaping the body in their later history. Furthermore, while cold classical KBOs reserve their shape sculptured by early out-gassing, the structure of Centaurs and Jupiter-family comets will be further modified by the same scenario once they enter their current orbit configuration from the Kuiper belt, under sublima- tion of different volatile species. We suggest that this mechanism should be taken into account in models studying planetesimal formation and the shape evolution of KBOs, as well as other small icy bodies in regions where supervolatile ices are expected to be present.

Image: Snapshots from numerical simulation of shape evolution of Arrokoth’s analogue due to sublimation driven mass loss. The bottom most shape is a digital terrain model derived from New Horizon observations. The color represents single orbit averaged temperatures. Red stands for warm and blue for cooler regions. Credit: © PMO/MPS.

The paper is Zhao, Rezac et al., “Sublimation as an effective mechanism for flattened lobes of (486958) Arrokoth,” Nature Astronomy 5 October 2020 (abstract). The earlier paper referenced above is Zhao et al., “The phenomenon of shape evolution from solar-driven outgassing for analogues of small Kuiper belt objects,” Monthly Notices of the Royal Astronomical Society 492, 5152–5166 (2020). Abstract.

{ 14 comments… add one }
  • charlie October 7, 2020, 14:18

    Such an extremely “ugly” looking body; there is something about the shape of this binary object that unpleasantly has an association with the process of what we may call ‘removal’ (I won’t go any further into that). Perhaps the name ‘Arrokoth’ is highly appropriate to this particular object.

    With regards to whether it is contact binary or not my first (contrarian) viewpoint was that it may not be a contact binary body at all. If in fact the rotational axis is parallel to basically the Plutonian orbital plane (I know, I know it’s not part of the Plutonian system) then I began to wonder whether or not that band we see between the two bodies is the result of erosion over billions of years through dust or solar radiation or what have you. I can’t explain it but there’s something that suggests that what we are seeingContrarian as that waist region might not be a contact place but rather a place that was eroded into the narrowness of what is a unitary body. There’s just something about the appearance that makes me believe it’s actually all one body.

  • charlie October 7, 2020, 14:18

    Nickname and designation
    When Arrokoth was first observed, it was labelled 1110113Y in the context of the Hubble Space Telescope’s search for Kuiper belt objects,[27] and was nicknamed “11” for short.[28][29] Its existence as a potential target of the New Horizons probe was announced by NASA in October 2014[30][31] and it was unofficially designated as “Potential Target 1”, or PT1.[29] Its official designation, 2014 MU69, was assigned by the Minor Planet Center in March 2015, after sufficient orbital information was gathered.[29] The provisional designation indicates that Arrokoth was the 1745th minor planet discovered during the second half of June 2014.[e] After further observations refining its orbit, it was given the permanent minor planet number 486958 on 12 March 2017.[33]

    Before the flyby on 1 January 2019, NASA invited suggestions from the public on a nickname to be used.[34] The campaign involved 115,000 participants from around the world, who suggested some 34,000 names. Of those, 37 reached the ballot for voting and were evaluated for popularity – this included 8 names suggested by the New Horizons team and 29 suggested by the public. Ultima Thule,[c] which was selected on 13 March 2018,[6] was proposed by about 40 different members of the public and obtained the seventh highest number of votes among the nominees.[35] Θούλη Thoúlē (Latin: Thūlē) is the farthest north location mentioned in ancient Greek and Roman literature and cartography, while in classical and medieval literature, ultima Thule (Latin, “farthermost Thule”) acquired a metaphorical meaning of any distant place located beyond the “borders of the known world”.[36][6] Once it was determined the body was a bilobate contact binary object, the New Horizons team nicknamed the larger lobe “Ultima” and the smaller “Thule”.[37]

    The nickname was criticized due to its use by 19th century racists as the mythical homeland of the Aryan race, a belief which was later adopted by Nazi occultists including the Thule society, which was a key sponsor of what became the Nazi Party. The phrase is used by some modern-day neo-Nazis and members of the alt-right.[38] In more recent times it was used to refer to the historical Inuit culture of the Thule people.[39]

    A few members of the New Horizons team were aware of that association when they selected the nickname, and have since defended their choice. Responding to a question at a press conference, Alan Stern said, “Just because some bad guys once liked that term, we’re not going to let them hijack it.”[40]

    • Michael October 8, 2020, 11:54

      Thanks for the info but I am not entirely sure why you brought the racist bit up, there are many names in history which are connected with heinous acts. Joseph Stalin to name one, should we stop naming anyone Joseph because of that heinous individual.

      • charlie October 8, 2020, 15:48

        “Joseph Stalin to name one, should we stop naming anyone Joseph because of that heinous individual.”

        I’m actually in total agreement with you. The entry that I posted was from the Wikipedia website on Ultima Thule and I was just passing it on. I absolutely don’t see anything wrong with the name Ultima Thule, and in fact I’m a person who thinks we shouldn’t bow down to political correctness just because some other person gets a little bit offended and gets their dander up. It’s nice to see that some members of the new horizon team actually stood up for retaining the naming of the object as it was originally named.

        • Mike Serfas October 9, 2020, 9:26

          In biological sciences, where almost every gene has multiple nmes, people have good reason to envy the universal, permanent designations used by astronomers. It is good for minor planet designations (and other terms) to remain stable. A big part of that is indeed rejecting political correctness – Jupiter is a rapist and Mars a mass murderer, and among the craters of the Moon I imagine one can find slave-owners and war criminals, but we don’t want to learn new names for them all. The flip side of this is that when a final name is to be first decided, it is good to make strong efforts toward consensus in advance to avoid reopening the case later. No name has a “right” to be applied to an astronomical body; they are a convention made among a group of people.

          When Nazis are an obscure footnote of history, there will still be some new furthest asteroid in need of a unique name.

          • Michael October 9, 2020, 16:13

            I suppose as well the communists that brought forth nazism…it goes on and on. They found the objects so let them name them as they wish we can’t cancel everything can we.

            • Mike Serfas October 10, 2020, 9:50

              Since you mentioned Communists again, I looked at Wikipedia’s list of people with craters of the Moon named after them … it would take a Joe McCarthy to find many on this list, but one Hero of Socialist Labor made for some fun reading: Leonid Voskresensky. There is some interesting information at https://www.airspacemag.com/space/apollo-why-the-soviets-lost-180972229/ but you’ll have to go to his Wikipedia article to see how he used one leak to block another when dealing with a missile.

              We are not poorer to learn this history.

  • Alex Tolley October 7, 2020, 16:28

    What I like is that this model is readily testable. Flattened binaries should be more common closer to the inner edge of the belt. The flattening is dependent on the KBO being a binary, with particular spin axis. Different spin axes should be modelable to predict the shape of other KBO binaries encountered.

    At some point in the future, when we are sending out swarms of tiny, beam propelled sail probes, we will acquire the numbers and data of KBOs to test this mechanism. Samples of such flattened binary KBOs should also be able to confirm this mechanism as presumably the outgassing and material loss should alter the composition of the volatiles with depth. primordial composition mightbe best determined by a sample deep within the KBO below the outgassing zone.

    • Patient Observer October 7, 2020, 22:24

      The process is thought to have occurred early on, perhaps in 1 to 100 million years, as volatile ices sublimated.

      The above suggests that things have been stable for over 4 billion years indicating all of the volatile substances able to be baked out have been baked out. Has the process ended or will this body continue sublimation until it falls apart from centrifugal force?

      And another thing! It the body was formed by collision, how did it acquire the indicated rotational axis? Was it a glancing blow yet still gentle enough to them to stick together? Or after the blow, did they orbit each other for a while with other mechanism robbing orbital velocity until they came together?

      The above conjecture is a product of a boring night:) The computer graphics did look convincing though.

      • Michael Spencer October 8, 2020, 13:33

        “The above suggests that things have been stable for over 4 billion years indicating all of the volatile substances able to be baked out ”

        A perhaps new twist on this question recently experienced by OSIRIS-REx, observing smallish objects being expelled from the surface of Bennu. No real explanation yet, though some noticed that loci appear following solar exposure as the object slowly rotates

  • Douglas Muir October 8, 2020, 8:49

    Can we please stop talking about how asteroids, or comets, or KBOs are “pristine” or “survivors of the formation of the early Solar System”.

    Everything — *everything* — has undergone a lot of change and evolution. Asteroids evolve and change over time. Comets evolve and change over time. Everything has geology. Everything has *processes*.

    And nothing is pristine. Let it go.

    Doug M.

  • Michael October 8, 2020, 11:48

    Just looks to me like Thule in obit around the less dense Ultima has pulled the weaker material towards it and then fused with it.

  • Jens Fridthjof Knudsen October 10, 2020, 13:04

    Will this tiny world disappear in the depths of time? It sublimes away? Just dust left?

  • Mike Serfas October 11, 2020, 8:46

    I couldn’t find this on Arxiv, but what surprises me is the development seems so simple. There are three kinds of angular momentum involved, one for each asteroid and another for their mutual revolution, allowing for a complex process of formation. For this example, let’s embrace the official name and call the larger lobe with “RO” embossed on its left and center Arro, and the remaining ball you might find on your lawn by the old German spelling Koth (I suppose I have at least a 50% chance of having these backwards) I might fantasize maybe Koth started with a strong rotation about its own axis, coming to rest against the flat side of Arro but displaced somewhat by their mutual revolution, grinding deeper into it leaving an O-shaped groove before finally transferring all momentum by friction and succumbing to a shift of axis of rotation. Could the simulation search through all such conceivable histories and evaluate their effects on the asteroids’ shapes?

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