Quirks & Quarks

Arrokoth reveals how the solar system's building blocks were built

NASA's New Horizons spacecraft continues its exploration past Pluto

NASA's New Horizons spacecraft continues its exploration past Pluto

Arrokoth as imaged by NASA's New Horizon's spacecraft. (NASA/Johns Hopkins University Applied Physics Laboratory/Southwest Research Institute/Roman Tkachenk)
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One year after first encounter, we are now finding out what we learned about the most distant object NASA has ever visited. And it's telling us just how the building blocks of our solar system were themselves built.

Last January, New Horizons made its closest approach to the small object in the Kuiper belt, four billion kilometres from Earth. This was the spacecraft's second destination, after its historic encounter with Pluto in 2015.  

For the second encounter, scientists targeted a tiny object thought to be a preserved remnant from the era of planetary formation in our solar system, billions of years ago.

The object has since been named Arrokoth, a word in the Powhatan/Algonquian language meaning "sky." The first pictures from the spacecraft revealed Arrokoth was a strange, dumbbell-shaped object composed of two roughly spherical lobes, about 35 km long in total.

Planetary scientist Alan Stern, the New Horizons principal investigator told Quirks & Quarks host Bob McDonald it was immediately clear what it was. 

"We technically call it a contact binary, which means that it actually consists of two bodies that formerly orbited one another, but which are now touching," Stern said.

Artist's impression of the New Horizons spacecraft at Pluto, with the moon Charon in the background (Johns Hopkins University Applied Physics Laboratory/Southwest Research Institute)

Deeper understanding of Arrokoth had to wait.

"All of the data about Arrokoth were collected at the flyby, but we've been sending it back over radio link, and we took a lot of data — many gigabits of data," said Stern.

Since the spacecraft has a transmitter weaker than an ordinary light bulb, its rate of data transmission across the vast distance to Earth is relatively slow. 

In fact, the full download of Arrokoth data won't be complete until late 2021.

But Stern said they've got most of the critical information already.

"We've been able to already send back all of the juicy high-resolution images low-resolution images the composition spectra the colour data," he said.

This was enough information to allow mission scientists to release their latest results this week at the meeting of the American Association for the Advancement of Science in Seattle. 

Among their findings are an explanation for Arrokoth's vivid orange-red colour, a shade we're learning is common in the outer solar system. Arrokoth is covered with reddish methanol ice, as well as smaller amounts of complex organic molecules the researchers haven't yet been able to identify.  

Watch this NASA-produced, 3D animation fo the shape of Arrokoth based on New Horizons Data.

These are probably the result of chemical reactions between methane and water ice that make up much of the body of the object. These reactions would have been driven by a few billion years of exposure to solar and cosmic radiation.

More fascinating to the team was what they were able to learn by studying the intersection of the two lobes of Arrokoth. 

"Arrokoth has a very interesting story to tell," said Stern. "The two lobes individually came together very gently. There's no evidence of a high speed collision."

There are no fractures, or signs of compression. 

"We've deduced that these two lobes came together in a collision that wasn't much faster than walking speed which is really an important clue to their origin."

This single piece of evidence could be critical in understanding how the first stages of planetary formation happened. 

One of the puzzles of the early solar system is how tiny planetesimals — the seeds of future planets — could have joined together to form larger and larger objects on the way to full planets. When objects get large enough, gravity will hold them together after they collide.  

NASA’s New Horizons spacecraft captured this high-resolution enhanced colour view of Pluto on July 14, 2015. (NASA/JHUAPL/SwRI)

But for smaller things like the two objects that formed Arrokoth, there's a problem.

"When objects orbit the sun independently, they're moving at speeds of kilometres per second," said Stern. "If these two objects were in very different orbits, they would be expected to have collided at hundreds of metres per second or even faster which would have shattered them — really blown to smithereens from the violence of the collision."

This means the objects weren't formed very far apart. And that, in turn, means what they see in Arrokoth seems to favour what Stern said is called the "local collapse cloud" model of planetesimal formation.

When the solar system began to form, it's thought that it was not much more than a huge cloud of gas and dust. Gravitational attraction would have caused this monolithic cloud to break up into local swarms of ice and rock particles.

"If you think of a diffuse swarm as a cloud and it collapsing to its centre to form one or two dominant objects out of all of the little bits and pieces that were originally in the cloud, you'd probably follow what I'm getting at," said Stern. "These two objects must have formed closely together in something that has long been theorized about."

That close formation would have allowed two relatively large objects to slowly come together by gravitational attraction and eventually bump and fuse together. Arrokoth reveals the early solar system could have assembled its building blocks in a quite peaceful way.

Researchers will be starting their search for the next target for New Horizons to visit in the Kuiper belt this summer. (Alex Parker/Southwest Research Institute/Johns Hopkins University Applied Physics Laboratory/NASA)

"In the early stages of planet formation you want objects the size of rocks and then boulders and then buildings and then still larger things to actually come together in a way that there's accretion, in other words, objects grow," said Stern. "It's clear that it had to be a gentle process."

Of course one object can't tell the whole story. With much more life left in their distant spacecraft, the New Horizons team is now looking for the next Kuiper belt object to visit. Because of the fantastic distances involved in the outer solar system, this will take a few years — at least.

Stern thinks it'll be worth the wait.

"I just think that it's spectacular that we humans have invented the technology to be able to really go and investigate these places and learn about our origins and how the Earth came to be and the other planets of our solar system."

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