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24 Feb 2020 12:49
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  •   Home > News > International

    New data from New Horizons' fly-by of Arrokoth could settle the controversy over how our solar system first formed

    New data from New Horizons' fly-by of Arrokoth — the farthest, most primitive object we've ever visited — could settle the controversy over how our solar system first formed.

    It looks like a deformed red snowman, orbiting the Sun from the icy reaches well beyond Pluto.

    Arrokoth — named after a Native American word meaning "sky" in the Powhatan/Algonquian language — is the farthest object in the solar system we've ever visited.

    A year ago, NASA's space probe New Horizons zipped past the tiny space rock in the Kuiper Belt, a ring of primitive objects about 50 times as far away as the Earth is from the Sun.

    Now new data from that mission, published in three papers in the journal Science, provides a more detailed picture of Arrokoth's geology, colour, and how it formed.

    Not only does this give us clues about the origins of Arrokoth, but it has implications for the entire solar system, said New Horizons principal investigator Alan Stern of the Southwest Research Institute in Boulder, Colorado.

    "The most important single discovery that's reported here is a decisive breakthrough in our understanding of the early stages of planet formation," Dr Stern said.

    Primitive objects, known as planetesimals, like Arrokoth are thought to be the building blocks of the planets in our solar system.

    Getting to know Arrokoth

    The first results of the mission, published in May 2019, gave us a glimpse of the size, shape and colour of Arrokoth (then commonly known by its nickname Ultima Thule which was later dropped to avoid Nazi links).

    The latest papers on the 36-kilometre-long rock are based on 10 times as much data.

    One of its defining features is its double lobes, which appear to be much flatter than they initially thought.

    Simulations of how the two lobes got stuck together suggests the process was gentle rather than a crash-and-smash event.

    The new data also reveals the surface is ultrared, composed of "the reddest material in the solar system" which is likely due to organic molecules.

    Indeed, methanol ice and other unidentified complex organic molecules were detected on the surface, although water was not.

    And unlike other small bodies such as comets and asteroids, Arrokoth has a "lightly-cratered smooth surface".

    "As a big object, it's the most pristine object we have ever seen in the solar system," said astronomer Jonti Horner of the University of Southern Queensland, who wasn't involved in the research.

    Professor Horner said the lack of craters suggests the surface has been unaltered for 4 billion years when planet formation in the solar system finished.

    Together the evidence from all three papers could help us resolve a long-standing controversy of how the building blocks of our solar system first formed.

    Crash and smash vs gentle kisses

    For decades there have been two competing theories about how our planets were born.

    One theory, called hierarchical accretion, was that planetesimals formed when quite disparate parts of the gas and dust cloud from which Sun and planets arose crashed and smashed together.

    The other, the pebble accretion model, suggests local clouds of pebbles form binary objects, which are gravitationally bound to each other and spin in the same plane.

    They gradually spin together very slowly, until they just gently nestle and eventually stick.

    "So you're not looking at a chance collision between two things that orbit the Sun but rather, two things that formed together, that then spiral in and just gradually nestle together in a very gentle, almost kissing collision," Professor Horner said.

    Until now we didn't know which theory was right, Dr Stern said.

    "[That's because] we'd never been to an object like Arrokoth, that really preserved the right evidence."

    But, he said, all of the evidence from the photographs, spectra and other data New Horizons collected on Arrokoth pointed to one thing: planets were formed in local pebble clouds.

    "The hierarchical accretion model doesn't wash."

    "It's rare in my field that we have a case that's so clear cut, but this really is a gigantic advance for understanding the formation of our solar system."

    Professor Horner agrees: "These things are quite small and they're quite fragile, so if you try and make that from a splatter collision, it's really hard to do because if you collide with any energy at all, you smash the thing apart."

    "The collision that formed [Arrokoth] had to be very slow and very gentle, which means that the objects had to be gravitationally bound to each other."

    Andrew Prentice, a planetary scientist and mathematician at Monash University, who has developed an alternative model of how Arrokoth could have formed from what was originally a single body, said the new data "captured everything that so far has been learnt about Arrokoth".

    "[But] one worry I have is no carbon dioxide ice has been detected, and yet I had predicted it's a major component [in my model]."

    The value of visiting objects like Arrokoth

    Professor Horner said the papers showed the real value of missions like New Horizons.

    "It shows how valuable these objects are to visit, but it also signposts the fact that the next time we go out there we'll probably do things very differently," he said.

    He'd prefer a next generation mission to spend time orbiting objects like Arrokoth instead of flying past like a speeding bullet.

    "You'd get a lot more science for not much more investment."

    Dr Stern also hopes to go deeper into the Kuiper Belt and explore many more of these objects in more detail.

    "Just as anthropologists want to understand the origins of civilisation," he said, "we planetary scientists want to understand how planets came to be, because they're the basis for which life exists across the galaxy."

    © 2020 ABC Australian Broadcasting Corporation. All rights reserved

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