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'Space snowman' named Arrokoth reveals how celestial objects turn into ice bombs

A new study is overturning our assumptions about icy objects lurking in the farthest reaches of the solar system. It begins with a strangely shaped object resembling a space snowman, known as Arrokoth — Kuiper Belt Object (KBO) (486958) 2014 MU69.

This space snowman, still holding its nickname for some as Ultima Thule, might hold time-capsule remnants of ancient ice dating back to the solar system‘s birth. This, according to a new study from from Brown University.

The Kuiper Belt

The Kuiper Belt, a distant realm beyond Neptune, has long intrigued astronomers. This vast expanse, teeming with icy bodies, holds clues to our solar system’s origins.

For years, the consensus was that these objects, relics from billions of years ago, had lost their ices to the vacuum of space.

NASA’s New Horizons mission kick-started the Kuiper Belt exploration. It sent back the first close-up images of Pluto and its moons. These images showed a world that is both complex and active.

Yet, the understanding of how these distant objects preserved their volatile compounds remained elusive.

Characteristics of space snowman

As mentioned above, Arrokoth, the distant Kuiper Belt object formerly known as 2014 MU69, is a celestial wonder unlike any other body we’ve ever explored.

Its unique appearance, revealed by NASA’s New Horizons spacecraft in 2019, immediately captured imaginations and earned it the nickname “the space snowman.”


Arrokoth’s most striking feature is its bi-lobed shape. Two distinct segments, a larger “head” and a smaller “body,” appear gently fused together.

Scientists believe this peculiar form tells a tale of a slow, gentle cosmic collision, where two separate bodies merged in the early solar system.


Arrokoth’s surface is a mix of surprising features. Some areas are remarkably smooth, with fewer craters than scientists anticipated.

This hints at either a young surface or processes that have reshaped it over time. Bright patches, dark spots, and areas of reddish color add complexity to its appearance.

The red hue is especially intriguing, suggesting complex organic molecules called tholins, formed through the interaction of radiation with simpler compounds.

Frozen ingredients

Many believe Arrokoth holds a treasure trove of pristine material from the solar system’s birth.

Its composition likely includes water ice, methanol ice, and other volatile compounds, along with plenty of organic material. This mix explains its dark surface, reflecting only a small portion of sunlight.


Where the two lobes meet– the “neck” region– Arrokoth displays a brighter and bluer hue.

This difference could indicate variations in particle sizes or changes in composition, possibly marking where material from the two original bodies blended long ago.


While Arrokoth shows no signs of active geology, its very formation and subtle surface variations reveal a dynamic history.

The gentle union of the lobes suggests a time when debris filled the solar system, allowing for slow collisions. It’s a frozen snapshot of how objects were built in our cosmic neighborhood.

Small object

Measuring about 22 miles (35 kilometers) long, Arrokoth is a small object even for the Kuiper Belt.

But don’t let its size deceive you — studying this peculiar “space snowman” offers a priceless window into the solar system’s formation and the secrets held within its deepest, coldest corners.

But, space snowman’s ice shouldn’t last that long

The ices found on Arrokoth and similar objects are not your everyday ice.

As discussed, substances like carbon monoxide, methane, and ammonia, which can sublime (turn from solid to gas without becoming liquid) at very low temperatures, make up their composition.

This is similar to how dry ice behaves on Earth, which sublimates in warmer conditions, skipping the liquid phase entirely and turning directly into carbon dioxide gas.

The conundrum

Given the longevity of these objects in the solar system, it was a puzzle to scientists how these volatile ices could have survived for billions of years.

The expectation was that the ices would have sublimated away, especially considering the relatively “warmer” conditions even in the distant reaches of the solar system.

This raises an intriguing question about comets, which are known to originate from regions like the Kuiper Belt: if these ices are so ephemeral, how do comets still possess them in abundance, enough to display the spectacular tails we observe when they approach the sun?

Solving the space snowman mystery

Researchers Sam Birch and Orkan Umurhan took a fresh look at an old puzzle. They designed a new simulation model. This model acts like a mini-universe, showing how celestial objects like Arrokoth behave.

“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,” they explained.

It’s as if Arrokoth and its cosmic cousins have built-in super freezers. These internal chillers keep volatile ices solid for billions of years.

“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,” 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 ‘Ice Bomb’ Theory

Now, imagine one of these objects gets knocked out of its orbit, closer to the sun.

It begins to warm up, and those frozen volatiles start to unfreeze. But deep inside, it’s still frosty. So the gases get trapped and build up pressure.

It’s like shaking a soda bottle before opening it. Eventually, BOOM! The comet erupts as the pressure escapes, and we see those iconic tails.

“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.”

Insights from the space snowman

If comets are basically sleepy ‘ice bombs’, we need to revisit how they go from quiet wanderers to spectacular sky shows.

“The key thing is that we corrected a deep error in the physical model peoplehad been assuming for decades,” Birch revealed.

The good news is this theory might help missions like NASA’s CAESAR become even more successful. CAESAR aims to collect a sample from a comet and bring it back to Earth.

“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,” Birch said.

The study is published in the journal Icarus.


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