A Fossil from the Solar System's Dawn
In the cold, dark reaches of space beyond Neptune, a cosmic snowman holds clues to our solar system's birth.
The Kuiper Belt object 486958 Arrokoth, the most distant and primitive object ever visited by a spacecraft, captivated the world with its unusual appearance—a flattened, two-lobed structure resembling a partially flattened snowman. This article explores the compelling scientific theory that the sublimation of dry ice may be responsible for sculpting Arrokoth's unique shape and altering its spin, reshaping our understanding of planet formation.
Discovered in 2014 and visited by NASA's New Horizons spacecraft on January 1, 2019, Arrokoth is a contact binary located in the Kuiper Belt, a region beyond Neptune swarming with small, icy, and ancient objects1 5 . Its official name, "Arrokoth," means "sky" in the Powhatan/Algonquian language, a name chosen with consent from Powhatan Tribal elders1 .
As the most primitive object ever explored, Arrokoth is considered a planetesimal—a preserved relic from the era of planet formation over 4.5 billion years ago5 . Its double-lobed, flattened shape and very red color have made it a subject of intense study, offering scientists a window into the conditions and processes that built the planets1 .
Initial images from New Horizons showed Arrokoth as a "snowman," but additional data revealed a surprising truth: the two lobes were actually far flatter than first thought. The larger lobe, "Wenu" (formerly "Ultima"), was described as a "pancake," while the smaller lobe, "Weeyo" (formerly "Thule"), was likened to a "walnut"4 .
This flattened appearance presented a puzzle: how did a cosmic object formed in the near-vacuum of space acquire such a shape? The answer may lie in the behavior of exotic ices, particularly dry ice, under the extreme cold of the outer solar system.
"Snowman" shape
Flattened "pancake" and "walnut" lobes
In 2019, a compelling theory was presented at an American Geophysical Union meeting, proposing that the sublimation of dry ice (solid carbon dioxide, CO₂) could explain both the flattened, bilobate structure and the slow spin rate of Arrokoth3 .
The theory proposes that dry ice sublimation generated sufficient internal pressure to cleave Arrokoth into two parts, which then drifted apart before gently re-merging into the flattened structure we observe today3 .
To test the dry ice sublimation hypothesis, researchers constructed a detailed thermal evolution model of proto-Arrokoth. This experiment was crucial for determining whether the proposed sequence of events was physically possible given the environmental conditions in the early Kuiper Belt.
The model assumed an initially rough spheroid with a mean diameter of 21 km, composed of the specific mixture of ices and refractories mentioned previously3 .
The primary heat source was set to be the decay of ²⁶Al, a radioactive isotope known to have been present in the early solar system. The simulation began at an initial temperature of 35 K3 .
The model tracked the internal temperature rise and monitored when and where different ices would undergo phase changes (melting and sublimation) as heat accumulated3 .
The simulation calculated the pressure build-up from sublimated ices, particularly CO₂, and modeled the mechanical stresses this would place on the object's structure3 .
The modeling yielded a precise timeline for Arrokoth's potential transformation3 :
| Time Since Formation | Significant Event | Resulting Change |
|---|---|---|
| ~24,000 years | ~50% of inner CH₄ ice melted and vaporized | Formation of outer methane shell (~0.6 km thick) |
| ~60,000 years | Central CO₂ ice sublimation begins | Gas pressure starts building in interior |
| ~64,400 years | CO₂ ice sublimed out to 0.5 radius point | Critical pressure reached, object cleaves in two |
The core result of this experiment was the demonstration that sublimation-driven cleavage was mechanically feasible within a specific, relatively short timeframe in Arrokoth's early history3 .
The scientific importance of these findings lies in offering a natural explanation for two puzzling observations: Arrokoth's unusually flattened shape and its slow rotation rate. If the two lobes were once part of a single body that split and then slowly re-merged, it would explain both characteristics simultaneously3 .
Understanding Arrokoth's physical properties is essential to appreciating the sublimation theory. Data gathered by New Horizons provides critical context for why such exotic explanations are necessary.
| Parameter | Measurement | Significance |
|---|---|---|
| Overall Dimensions | 35.95 × 19.90 × 9.75 km | Reveals flattened, elongated structure |
| Wenu (Larger Lobe) | 21.20 × 19.90 × 9.05 km | Highly flattened, "pancake-like" shape |
| Weeyo (Smaller Lobe) | 15.75 × 13.85 × 9.75 km | Less flattened, "walnut-like" shape |
| Mass | ~7.485 × 10¹⁴ kg | Very low density suggests porous, icy composition |
| Mean Density | ~0.235 g/cm³ | Much lower than water ice; indicates high porosity |
| Orbital Period | 297.67 years | Cold Classical KBO with stable, circular orbit |
| Rotation Period | 15.938 hours | Unusually slow for a Kuiper Belt Object |
| Surface Temperature | ≈ 42 K (-231 °C) | Cold enough to preserve volatile ices for eons |
| Material/Component | Function/Role |
|---|---|
| Carbon Dioxide (CO₂) Ice | Proposed agent of change in Arrokoth; its sublimation may generate internal gas pressure sufficient to cleave bodies3 |
| Water Ice | Structural backbone of many KBOs; provides mechanical strength at low temperatures3 |
| Methane (CH₄) Ice | Forms outer insulating shell in theoretical models; traps sublimated gasses within bodies3 |
| Radioactive ²⁶Al | Primary heat source in early KBOs; drives thermal processing and ice sublimation in interior models3 |
| Organic Refractory Material | Provides reddish coloration; surface materials modified by cosmic radiation over billions of years1 5 |
| Methanol | Detected on surface; indicates complex chemistry in the early outer solar system5 |
Arrokoth's extremely low density (~0.235 g/cm³) compared to water ice (~0.93 g/cm³) indicates it is highly porous, supporting the idea that internal processes could have significantly altered its structure over time.
While the dry ice sublimation theory offers an elegant explanation, science advances through debate and competing hypotheses. Other researchers have proposed different mechanisms for Arrokoth's formation.
The leading alternative theory suggests Arrokoth's two lobes formed separately and independently as distinct objects in the early solar system. These two bodies then gently merged in a slow collision within a cloud of particles1 5 .
The alignment of the lobes' axes supports this idea, indicating they may have been tidally locked before merging5 .
A groundbreaking 2024 study published in Icarus challenges fundamental assumptions about how KBOs evolve. This research suggests that KBOs like Arrokoth could retain their original volatile ices for billions of years, much longer than previously thought.
The study proposes that these objects can maintain their volatile ices, forming a kind of subsurface atmosphere that dramatically slows further ice loss.
As co-author Dr. Orkan Umurhan stated, "This study could be the initial mover for re-evaluating the comet interior evolution and activity theory".
This new perspective doesn't necessarily invalidate the dry ice sublimation theory, but it does suggest that if sublimation occurred, it would have followed a different timeline and mechanism than previously modeled.
The question of whether dry ice sublimation explains Arrokoth's unique structure remains open, a testament to the dynamic nature of scientific inquiry. The dry ice theory provides a testable, physically plausible mechanism for how this distant world acquired its distinctive flattened, bilobate shape and slow rotation.
What makes this mystery particularly exciting is that data transmission from the New Horizons flyby continued until late 2020, meaning scientists are still analyzing information that could provide crucial clues5 . Each new dataset offers the potential to validate, refine, or disprove the current theories.
Arrokoth continues to be a gift to planetary science, challenging our assumptions and pushing us to develop more sophisticated models of our solar system's birth. As Alan Stern, New Horizons Principal Investigator, noted, "There is no doubt that the discoveries made about Ultima Thule are going to advance theories of solar system formation"5 . Whether through dry ice sublimation or another process, solving the mystery of Arrokoth's shape will undoubtedly lead us to a deeper understanding of how our cosmic neighborhood came to be.