Arrokoth's Formation Insights: Unraveling the "Snowman" Mystery
New research provides significant insights into how Arrokoth, the most distant and primitive object visited by a spacecraft from Earth, developed its distinctive snowman-like shape. This 4-billion-year-old body is located in the Kuiper belt, a vast ring of icy objects beyond Neptune's orbit, home to dwarf planets, comets, and planetesimals—the fundamental building blocks of planets.
Approximately 10-25% of planetesimals in the Kuiper belt, including Arrokoth, exhibit a two-lobed, or peanut/snowman-like, form.
Understanding Double-Lobed Objects
Previous expert analysis suggested Arrokoth's unique shape, composition, and minimal cratering indicated a gentle, simultaneous formation of its two lobes. This was possibly through gravitational collapse, where material slowly aggregated. However, the precise details of this process remained a subject of ongoing debate.
"Previous expert analysis suggested Arrokoth's shape, composition, and minimal cratering indicated a gentle, simultaneous formation of its two lobes, possibly through gravitational collapse."
New Simulations Illuminate Formation Mechanism
Now, researchers have utilized computer simulations to demonstrate that gravitational collapse is indeed capable of producing such double-lobed objects, shedding light on the underlying mechanism. Jackson Barnes, the lead author from Michigan State University, highlighted the significance of this breakthrough, stating it's the first time they've been able to observe and confirm this entire process from beginning to end.
The Gravitational Collapse Model
Barnes explained that the Kuiper belt is considered a remnant of the solar system's primordial protoplanetary disk, where large rotating clouds of pebbles are thought to have formed. In the gravitational collapse model, gravitational forces within these clouds cause pebbles to clump together, forming planetesimals of various sizes.
Simulation Details and Results
The groundbreaking research, published in the Monthly Notices of the Royal Astronomical Society, involved 54 simulations. Each simulation commenced with a pebble cloud containing 105 particles, each approximately 2 kilometers in radius. While a low-resolution model compared to real pebble clouds (estimated to contain 1024 millimeter-sized particles), the team observed crucial outcomes.
In some simulations, two small planetesimals would begin orbiting each other, spiraling inward. Eventually, they would touch and join at remarkably low velocities (around 5 meters per second or less) to form a double-lobed planetesimal, or "contact binary." Barnes noted that some simulated contact binaries bore a striking resemblance to Arrokoth.
Addressing Prior Limitations
Barnes pointed out a key advancement: previous simulations of gravitational collapse had not incorporated the physics of how particles rest upon each other when they make contact. This critical omission led earlier models to suggest that collisions between smaller planetesimals would result in a single, larger, spherical object. The current simulations, by including this essential physics, offer a more accurate representation of contact binaries.
Broader Support for Planetesimal Formation
Beyond Arrokoth's specific case, the new simulations also lend significant support to the long-standing theory that planetesimals in general form through gravitational collapse.
Expert Commentary
The study has garnered attention from the scientific community. Alan Stern, a planetary scientist at the Southwest Research Institute and principal investigator of Nasa's New Horizons mission, praised the study, stating it aligns with prior research and reinforces the hypothesis of gentle formation processes for Arrokoth.
Alan Fitzsimmons, an emeritus professor of astronomy at Queen’s University Belfast, offered a nuanced perspective. He observed that the simulations indicated only 4% of objects formed as contact binaries, a figure lower than what telescopic surveys suggest. Fitzsimmons proposed that other formation methods might exist, or that more complex future simulations could bridge this discrepancy between modeled and observed occurrences.