A duo of planetary scientists from Brown University and the SETI Institute has found that the Kuiper Belt object (486958) Arrokoth, which was the target of the January 1, 2019 flyby by NASA’s New Horizons mission, may have ancient ices stored deep within it from when the object first formed billions of years ago. Using a new model they developed to study how comets evolve, the researchers suggest this feat of perseverance isn’t unique to Arrokoth but that many objects from the Kuiper Belt may also contain the ancient ices they formed with.
This composite image of Ultima Thule was compiled from data obtained by NASA’s New Horizons spacecraft as it flew by the object on January 1, 2019. The image combines enhanced color data (close to what the human eye would see) with detailed high-resolution panchromatic pictures. Image credit: NASA / Johns Hopkins University Applied Physics Laboratory / Southwest Research Institute / Roman Tkachenko.
“We’ve shown here in our work, with a rather simple mathematical model, that you can keep these primitive ices locked deep within the interiors of these objects for really long times,” said Dr. Sam Birch, a planetary scientist at Brown University.
“Most of our community had thought that these ices should be long lost, but we think now that may not be the case.”
Until now, planetary scientists had a hard time figuring out what happens to ices on these space rocks over time.
The new study challenges widely used thermal evolutionary models that have failed to account for the longevity of ices that are as temperature sensitive as carbon monoxide.
The model Dr. Birch and SETI Institute researcher Orkan Umurhan created for the study accounts for this change and suggests that the highly volatile ices in these objects stick around much longer than was previously thought.
“We are basically saying that Arrokoth is so super cold that for more ice to sublimate — or go directly from solid to a gas, skipping the liquid phase within it — that the gas it sublimates into first has to have travel outwards through its porous, sponge-like interior,” Dr. Birch said.
“The trick is that to move the gas, you also have to sublimate the ice, so what you get is a domino effect: it gets colder within Arrokoth, less ice sublimates, less gas moves, it gets even colder, and so on.”
“Eventually, everything just effectively shuts off, and you’re left with an object full of gas that is just slowly trickling out.”
The study suggests that Kuiper Belt objects can act as dormant ‘ice bombs,’ preserving volatile gases within their interiors for billions of years until orbital shifts bring them closer to the Sun and the heat makes them unstable.
This new idea could help explain why these icy objects from the Kuiper Belt erupt so violently when they first get closer to the Sun.
All of a sudden, the cold gas inside them rapidly gets pressurized and these objects evolve into comets.
“The key thing is that we corrected a deep error in the physical model people had been assuming for decades for these very cold and old objects,” Dr. Umurhan said.
“This study could be the initial mover for reevaluating the comet interior evolution and activity theory.”
Altogether, the study challenges existing predictions and opens up new avenues for understanding the nature of comets and their origins.
D. Birch and Dr. Umurhan are co-investigators in NASA’s Comet Astrobiology Exploration Sample Return (CAESAR) mission to acquire at least 80 grams of surface material from the comet 67P/Churyumov-Gerasimenko and return it to Earth for analysis.
The results from this study could help guide CAESAR’s exploration and sampling strategies, helping to deepen our understanding of cometary evolution and activity.
“There may well be massive reservoirs of these primitive materials locked away in small bodies all across the outer Solar System — materials that are just waiting to erupt for us to observe them or sit in deep freeze until we can retrieve them and bring them home to Earth,” Dr. Birch said.
The study appears in the journal Icarus.
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Samuel P.D. Birch & Orkan M. Umurhan. 2024. Retention of CO ice and gas within 486958 Arrokoth. Icarus 413: 116027; doi: 10.1016/j.icarus.2024.116027
