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‘Arrokoth’ Hints At Solar System’s Peaceful Past

NASA/Johns Hopkins University Applied Physics Laboratory/Southwest Research Institute/Roman Tkachenko

A year ago NASA’s New Horizons spacecraft flew past a strange snowman-shaped object in the Kuiper Belt, far beyond Pluto. Named Arrokoth, it’s the most distant object ever visited by a spacecraft.  Scientists say it’s offered up several surprises, including clues into how planets formed in the early days of the solar system. KNAU’s Melissa Sevigny spoke with Lowell Observatory’s Will Grundy about the new findings published in Science.

Arrokoth is a contact binary, what does that mean exactly?

A contact binary is an object where two objects came together and are stuck together but they’re still clearly distinct objects.

Now that you have some of the data down from the Arrokoth encounter, what have you learned about it?  

It’s a weird little object, Arrokoth is. In addition to the weird shape that we were talking about, it’s composition is strange. It’s made out of a combination of complicated organics and methanol ice, which is a little weird.

Methanol, isn’t that the stuff in antifreeze?

Yeah, it’s an alcohol. It’s a simpler alcohol than ethanol which is the one in alcoholic beverages.

So it’s cold enough out there to freeze antifreeze?

It’s not only cold enough to freeze antifreeze… but it’s cold enough to freeze gases out there, things like carbon dioxide would be frozen solid out there. It’s really an extremely cold environment, the average temperature is something like 40 degrees above absolute zero.  

You can’t see any water ice on it—that’s kind of unusual, right?

Not seeing water ice was really a surprise, because water ice is such a common planetary building material in the outer solar system…. And yet on Arrokoth we see methanol ice but we can’t make out the water ice. It’s probably from a region where some kind of special chemistry is going on, at the very outer edge of the protoplanetary nebula where all the planets formed.

Were there any other surprises that popped out at you?

The shape is the biggest surprise. It’s not really like anything we’ve seen before. But everything we’ve seen before has been subject to modification since it formed. For instance in the asteroid belt things are crashing into each other and every asteroid that we’ve seen is just covered with impact craters, and pretty beat up…. This is a first glimpse at something that hasn’t been beat up much since its formation. That contact binary shape is certainly something people have talked about before, but this is the first one we’ve up close. Every time you see something for the first time, it really crystallizes your thinking. It’s like, oh, that’s what those look like.    

Do you have any idea at this point of how that contact binary shape, that snowman shape, would formed?

It must have been a very, very slow and gentle process. You should really think of it more like the docking between two spacecraft than a collision.

What’s been your favorite part of studying Arrokoth so far?

I think what’s been most exciting about this is we’ve been talking in the planetary science community for decades about how planetesimals get built…. Arrokoth is really telling us that that process doesn’t happen by a cascade of collisions. It happens instead by a gravitational collapse. Somehow or other you get enough solids concreted in a region of the nebula that they feel each other’s gravity and just stick together. Because they’re so small, the gravity is really light, they come together pretty gently.  

Will Grundy, thank you for speaking with me.

It was a pleasure.  


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