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| title | chunk | source | category | tags | date_saved | instance |
|---|---|---|---|---|---|---|
| 486958 Arrokoth | 5/8 | https://en.wikipedia.org/wiki/486958_Arrokoth | reference | science, encyclopedia | 2026-05-05T13:09:12.451441+00:00 | kb-cron |
Arrokoth is thought to have formed from two separate progenitor objects that formed over time from a rotating cloud of small, icy bodies since the formation of the Solar System 4.6 billion years ago. Arrokoth had likely formed in a colder environment within a dense, opaque region of the early Kuiper belt where the Sun appeared heavily obscured by dust. Icy particles within the early Kuiper belt experienced streaming instability, in which they slowed down due to drag against the surrounding gas and dust, and gravitationally coalesced into clumps of larger particles. Because there have been few to no disruptive impacts on Arrokoth since it formed, the details of its formation have been preserved. From the differing present appearances of the lobes, each is thought to have accreted separately while in orbit around each other. Both progenitor objects are believed to have formed from a single source of material as they appear to be homogeneous in albedo, color, and composition. The presence of rolling topography units on the larger object indicates that it had likely formed from the coalescence of smaller planetesimal units prior to merging with the smaller object. The larger lobe Wenu appears to be an aggregate of 8 or so smaller components, each approximately 5 km (3 mi) across.
=== Flattening and merging ===
It is unclear how Arrokoth has attained its present flattened shape, though two leading hypotheses have been postulated to explain the mechanisms leading to its flattened shape during the formation of the Solar System. The New Horizons team hypothesizes that the two progenitor objects formed with initially rapid rotations, causing their shapes to become flattened due to centrifugal forces. Over time, the rotation rates of the progenitor objects gradually slowed down as they experienced impacts by small objects and transferred their angular momentum to other orbiting debris left over from their formation. Eventually, loss of momentum, caused by impacts and momentum shifting to other bodies in the cloud, caused the pair to slowly spiral closer until they touched—where over time the joints fused together, forming its present bilobate shape. In an alternative hypothesis formulated by researchers of the Chinese Academy of Sciences and the Max Planck Institute in 2020, the flattening of Arrokoth may have resulted from the process of sublimation-driven mass loss over a timescale of several million years after the merging of the lobes. At the time of formation, Arrokoth's composition had a higher volatile concentration from the accretion of condensed volatiles within the dense and opaque Kuiper belt. After the surrounding dust and nebula subsided, solar radiation was no longer obstructed, allowing for photon-induced sublimation to occur in the Kuiper belt. Due to Arrokoth's high rotational obliquity, one polar region faces the Sun continuously for half of its orbital period, resulting in extensive heating and consequent sublimation and loss of frozen volatiles at Arrokoth's poles. Regardless of the uncertainty surrounding the mechanisms for the flattening of Arrokoth, the subsequent merging of the bodies ancestral to the lobes appeared to be gentle. The present appearance of Arrokoth does not indicate deformation or compression fractures, suggesting that the two progenitor objects had merged very slowly at a speed of 2 m/s (6.6 ft/s)—comparable to the average walking speed of a person. The progenitor objects must have also merged obliquely at angles greater than 75 degrees in order to account for the present shape of Arrokoth's thin neck while keeping the lobes intact. By the time the two progenitor objects merged, both of them had already been tidally locked in synchronous rotation. The long-term frequency of impact events occurring on Arrokoth was low due to the slower speeds of objects in the Kuiper belt. Over a period of 4.5 billion years, photon-induced sputtering of water ice on Arrokoth's surface would minimally reduce its size by 1 cm (0.39 in). With the lack of frequent cratering events and perturbations of its orbit, the shape and appearance of Arrokoth would remain virtually pristine since the conjoining of two separate objects that formed its bilobate shape.
== Observation ==
=== Discovery ===