Arrokoth: Unique Witness to the Primordial Solar Nebula

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Arrokoth: Object of the Kuiper Belt

An icy inhabitant of the outer solar system

Arrokoth is a trans-Neptunian object (TNO) located in the Kuiper Belt, a region populated by icy objects beyond Neptune, such as Pluto, Haumea, Albion, or Varuna. It belongs to the population known as "cold classicals," the least dynamically perturbed objects in the solar system. Its orbit, relatively circular, is located about 44.6 astronomical units from the Sun, approximately 6.7 billion kilometers. Its orbital period around the Sun is about 298 years.

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Arrokoth means "sky", a word from the language of the Powhatan (a Native American people from the Virginia region, United States). The term "Ultima" comes from the Latin ultima, meaning "the farthest" or "the utmost". In classical literature, "Thule" was a legendary island located at the far north of the known world, often associated with the idea of an ultra-distant or mysterious place. This reflects the fact that Arrokoth was, at the time of its discovery, the most distant object ever explored.

A discovery made possible by the Hubble Space Telescope

Arrokoth was discovered on June 26, 2014, by astronomer Marc Buie, a member of the New Horizons mission team, using the Hubble Space Telescope's wide-field camera. This discovery was part of a targeted search aimed at identifying a Kuiper Belt object accessible to the probe after its Pluto flyby in 2015, the object being too faint (apparent magnitude around 27) to be detected by ground-based telescopes. Initially designated by the provisional code 2014 MU69, then informally nicknamed "Ultima Thule," the object received its official name, Arrokoth, in November 2019. This term means "sky" in the language of the Powhatan/Algonquian people, honoring the indigenous populations of the region where the Hubble Telescope and the New Horizons probe were built.

A reddish surface shaped by cosmic radiation

Arrokoth's surface is uniformly reddish across the entire object, a shade even more pronounced than Pluto's, due to the presence of tholins, complex organic compounds formed by the irradiation of simple compounds like methane and ethylene. This coloration suggests that Arrokoth's surface is the result of chemical processes caused by continuous exposure to cosmic rays and ultraviolet rays from the primitive space environment.

A preserved witness to the formation of the solar system

New Horizons observations indicate that Arrokoth is an object preserved since the formation of the solar system. Its surface is surprisingly smooth and shows few craters, suggesting it has undergone few collisions since its formation. Crater counting suggests the age of its surface dates back to the very end of the accretion phase of the protoplanetary disk, about 4.5 billion years ago. This geologically quiet history has preserved characteristics from the time of its formation.

A highly porous structure revealing a gentle birth

Having no detectable satellite or ring, Arrokoth's mass could not be directly measured by gravitational effects during the flyby. It is therefore estimated from indirect geophysical constraints, notably the resistance of its central "neck" to compression: models converge towards a reference density of approximately 0.235 g/cm³, with a range generally cited between 0.2 and 0.5 g/cm³ depending on the assumptions used. This value, much lower than that of water and incomparable to that of a rock (2.5 to 3 g/cm³), corresponds to an internal porosity that can reach about 90 %. Such a structure indicates formation under conditions of very low energy, reinforcing the hypothesis of gentle accretion in the primordial solar nebula.

Arrokoth: A Contact Binary Object

Two flattened lobes for a reassessed "snowman"

Arrokoth is composed of two distinct lobes, joined by a narrow, bright "neck," which initially gave the object the appearance of a "snowman" in the first images. The entire object measures about 36 km long, about 20 km wide, and barely 10 km thick. The larger lobe, nicknamed "Ultima" during the flyby and officially named Wenu, measures approximately 21 × 20 × 9 km. The smaller lobe, nicknamed "Thule" then named Weeyo, measures approximately 15 × 14 × 10 km.

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The very first flyby images, taken from a distance, suggested the two lobes were roughly spherical. Further analysis, published in 2020 from images taken by New Horizons after its closest approach — particularly shots where Arrokoth occulted background stars — showed that the two lobes are actually distinctly flattened, somewhat like two pebbles, rather than two snowballs.

Gentle accretion at the origin of its binary structure

Arrokoth is an example of a contact binary, formed when two distinct objects "stuck" together at a very low speed, probably on the order of a few kilometers per hour. This gentle formation contrasts with the violent collisions that form many other objects in the solar system.

Formation models indicate that this gentle accretion occurred in the primordial solar nebula, likely through the gravitational collapse of a cloud of centimeter-sized "pebbles." New Horizons data show that the rotation axes of Arrokoth's lobes are almost perfectly aligned, suggesting they formed separately but in a close orbit around each other for a long period before merging, rather than simply having "impacted" by chance. This configuration is consistent with a very low impact energy, reinforcing the idea of formation in a low-turbulence environment, well before the collapse of the primordial cloud.

Surface Composition: Methanol Ice and Complex Organics

Methanol ice detected, but no water ice

Spectroscopic analysis of Arrokoth's surface by New Horizons instruments revealed the presence of methanol ice (CH₃OH), identifiable by its characteristic absorption signatures around 2.27 and 2.34 micrometers, associated with complex organic materials likely resulting from the irradiation of simple ices. Notably, no signature of water ice was detected on the surface, whereas it is a common component of many other icy objects in the solar system. This composition suggests hydrogenation of carbon-monoxide-rich ices and energetic processing of condensed methane on ice grains, in the extremely cold environment of the outer protoplanetary disk.

An extreme surface temperature

Radiometric measurements made by New Horizons on Arrokoth's night hemisphere indicate an average brightness temperature of approximately 29 ± 5 kelvins, about -244 °C, consistent with the object's very remote position in the solar system.

Arrokoth: A Preserved Primitive Object

A time capsule of the solar nebula

Arrokoth is considered a primitive object, preserved since the formation of the solar system. The study of its physical characteristics offers a unique window into the initial conditions of the solar nebula. Unlike other objects that have undergone significant transformations, Arrokoth has remained relatively unchanged, making it particularly valuable to scientists.

A natural laboratory for understanding planetesimal formation

Arrokoth's striking features provide valuable information on the formation processes and evolution of small bodies, which contributed through their progressive aggregation to the formation of larger bodies called planetesimals. Arrokoth remains, to this day, the most distant and most primitive object ever explored up close by a space probe.