New Arrokoth Studies Throw Open A Window Into The Early Solar System

The uniform color and composition of Arrokoth’s surface. ASA/Johns Hopkins University Applied Physics Laboratory/Southwest Research Institute

On New Year’s Day 2019, NASA’s New Horizons visited Arrokoth (2014 MU69), the furthest world ever explored by a human spacecraft. Three new papers published this week in Science detail the properties of Arrokoth.

Using data slowly downlinked from the spacecraft and combining it with sophisticated computer models, the team has learned much more about this distant object. The researchers believe that this world formed slowly, with the two lobes that formed it undergoing a delicate merger in the distant past. They were born together from a cloud of solid particles that collapsed on itself creating the two objects, which over time spiraled gently towards each other.

"Just as fossils tell us how species evolved on Earth, planetesimals tell us how planets formed in space," said William McKinnon, a New Horizons co-investigator from Washington University in St. Louis and lead author of one of the papers, in a statement. "Arrokoth looks the way it does not because it formed through violent collisions, but in more of an intricate dance, in which its component objects slowly orbited each other before coming together."

There are two other pieces of evidence that support this formation scenario. First, Arrokoth's surface composition and color are extremely uniform. It is covered in organic molecules called tholins, which gives the world its peculiar red coloration. Secondly, it has very small craters, suggesting that it has remained mostly unchanged for billions of years.

"Arrokoth has the physical features of a body that came together slowly, with 'local' materials in the solar nebula," added Will Grundy, New Horizons composition theme team lead from Lowell Observatory in Flagstaff, Arizona, and the lead author of another Science paper. "An object like Arrokoth wouldn't have formed, or look the way it does, in a more chaotic accretion environment."

The first results were published last May and this new analysis expands on them with 10 times more data. It takes time to get everything collected by New Horizons as the dataset is slowly being transmitted back to Earth at the “snail speed” of 500 bits per second. The full data collected will be downlinked by this September.

"Arrokoth is the most distant, most primitive and most pristine object ever explored by spacecraft, so we knew it would have a unique story to tell," said New Horizons Principal Investigator Alan Stern, of the Southwest Research Institute in Boulder, Colorado. "It's teaching us how planetesimals formed, and we believe the result marks a significant advance in understanding overall planetesimal and planet formation."

The spacecraft is continuing its journey into the Kuiper Belt, the region that Pluto and Arrokoth inhabit. Speeding at 50,400 kilometers (31,300 miles) per hour, it is now over 7.1 billion kilometers (4.4 billion miles) from us.

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