In January 2006, NASA launched the New Horizons space probe to explore the horizons of the Solar System and know what’s beyond all the planets. The space probe sailed about 5.9 billion kilometres for almost a decade, with state-of-the-art instruments on board, before it met Pluto, a now downgraded ‘dwarf planet’. Three and a half years later and still gliding, it encountered ‘2014 MU69’—the farthest object in the Solar System ever to be visited by a spacecraft. Today, this oddly named object has a fancy moniker—Arrokoth—and we now know a whole lot about this distant Kuiper Belt Object than ever before, thanks to three new studies.
The Kuiper Belt is a doughnut-shaped region of icy objects found beyond the orbit of Neptune, the farthest planet of the Solar System. Akin to the asteroid belt located between Mars and Jupiter, it has remnants, called Kuiper Belt Objects, from the period when the Solar System was formed. “The Kuiper Belt is the most preserved region of the Solar System for understanding how the Solar nebula came to form the building blocks of planets, called planetesimals,” says Dr S. Alan Stern. He is the Principal Investigator of New Horizons, from Southwest Research Institute, USA and was speaking at a press briefing during the AAAS 2020 Annual Meeting at Seattle, Washington, USA.
“Although the Solar System is about 4.56 billion years old, we know very little of what happened when it was about 10-50 million years old. During that period, massive planets were being formed, and material from the nebula was being flung inwards and outwards. The Kuiper Belt Objects date back to that period and serve as a treasure trove of historical information,” says Dr Ritesh Kumar Mishra, an Indian planetary scientist and Humbolt Fellow.
Pluto and Arrokoth, which attracted the attention of New Horizons, are also Kuiper Belt Objects that are icy and cold.
“Arrokoth is a frozen artefact of the past—a time machine into our Solar System’s formation,” says Dr Henry Throop, a planetary scientist at the Planetary Science Institute, Arizona, USA, who has been associated with the New Horizons mission since 2002.
In a series of new studies, published in the journal Science, scientists have now provided an accurate description of Arrokoth, its structure, composition and origins.
Arrokoth: A new name and a fresh perspective
Astronomers ringed in 2019 New Year’s Day with the exciting news of New Horizons capturing the first image of the most distant object ever explored in the Solar System. Then nicknamed ‘Ultima Thule’, the spacecraft captured its pictures from a distance of about 27,000 kilometres. Based on the initial fly-by data, it was known that ‘2014 MU69’ had two connected lobes—the bigger ‘Ultima’ and the smaller ‘Thule’. Scientists also had concluded that it had no rings, satellites or massive impact craters. Now, after more than a year, scientists have used over ten times more data to provide accurate details about the geology, colour, structure and origin of Arrokoth.
The enigma of Arrokoth starts with its name. Once called ‘2014 MU69’, the sobriquet ‘Ultima Thule’ was never official. “Ultima Thule was always intended to be an unofficial name,” says Dr Throop. In November 2018, the International Astronomical Union (IAU) decided to call it ‘Arrokoth’. “It means ‘sky’ in the local Powhatan language of the Native Americans in the US state of Maryland,” he explains. Maryland has a special regard for Arrokoth—the Applied Physics Laboratory, from where the operations of New Horizons and the Hubble Space Telescope are controlled, is in Maryland.
Our Solar System started with the Sun forming from a large disc of dust and gas, called the nebula. Most objects in the Solar System are thought to have been created by collisions and accretion of smaller materials into larger ones. However, the first study shows that is not the case with Arrokoth. Using simulations, the researchers theorise that the two lobes, which were once independent, came close to each other and merged gently without a bang. “The speeds were no more than 7 miles per hour,” says Dr William McKinnon, who headed the Geology and Geophysics division of New Horizons.
“You can tell from looking at the surface that its history has been gentle. It is not cratered by violent impacts as the speeds in the outer solar system are very low,” explains Dr Throop.
There are many such two-lobed ‘contact binaries’ found towards the outer end of the Kuiper Belt, and the researchers believe that they too were formed by gentle collisions like Arrokoth.
Red, cold and undisturbed
The initial images from New Horizons’ fly-by had indicated that Arrokoth measures about 30 kilometres in length and has a flat surface. However, the new results show that the two lobes are lightly cratered, unlike previously studied objects in the Solar System. This observation adds to the evidence that Arrokoth is well-preserved since the end of the planet formation era, dating back to 4 billion years ago. Since most objects in this region have low velocities and temperatures, there are no erosions or craters like those seen on asteroids and comets.
The surface of both the lobes are red in colour, and the colour distribution is mostly uniform, found the third study.
“It’s not red paint, but just that the surface reflects light of wavelengths in the red region,” says Dr Will Grundy, who heads the Composition Science Theme Team of New Horizons.
The smaller lobe appears to be redder than the larger one.
The researchers used multiple images captured from various angles to reconstruct a three-dimensional view of Arrokoth. The two lobes are flat and aligned with each other. Interestingly, the smaller one is more elongated, and the direction of elongation indicates that it was orbiting the larger one before their fusion. There is also a massive depression, named Maryland, on the smaller lobe with a diameter of about 6 kilometres. The larger lobe is much smoother, although both have some fissures and small dark hills.
Arrokoth, the Kuiper Belt Object explored by New Horizons [Image Credit: NASA/Johns Hopkins University Applied Physics Laboratory/Southwest Research Institute/Roman Tkachenko]
The researchers trace the origin of this red colour to the presence of torins—polymer-like organic solids that are presumably formed by simpler molecules due to high-energy radiations. Spectral analysis of radiation reflected by Arrokoth found the presence of methanol ice. Although there was no water detected, the researchers speculate that it could be present, but somehow masked or hidden from view.
Arrokoth is very cold—the poles have a temperature of about 50K (-223o C), and it drops towards the equator. However, the summer and winter temperatures vary significantly, with winter temperatures dropping to 1/6th of peak summer temperatures. It has a rotational period of 15.92 hours.
A glimpse of the past
Kuiper Belt Objects, like Arrokoth, provide valuable information on how planets are formed in the Solar System because they preserve their structure for billions of years. “In that sense, Arrokoth is a wonderful scientific present,” remarks Dr Stern, as it answers the debate of how planets are formed.
Astronomers have long grappled with two different paradigms for how the Solar System came into being. One idea is that planets formed by small objects coming together to form larger objects, which in turn came together to form even larger objects. This model is known as “hierarchical accretion theory”. An alternative model is the so-called “cloud collapse theory” according to which clouds of gas and dust around the Sun collapsed into a thin disc under the effect of the Sun’s gravity. The disc later radiated away its heat giving rise to the planets we see. Hitherto, astronomers lacked the data to make a clear case for one model or the other.
But now, New Horizons has discovered that Arrokoth came together slowly with ‘locally-sourced’ materials from a small part of a collapsing cloud of particles.
“The structure of Arrokoth serves as a decisive test on the theory of planet formation,” says Dr Stern. Now, scientists know that the cloud collapse theory is the more plausible one. Dr Mishra agrees, saying that “Kuiper Belt Objects help us validate our theoretical models on how planetary systems were formed and understand the distribution of materials in the Solar System.”
A three-dimensional view of Arrokoth. Scientists have used all available New Horizons images of Arrokoth, taken from many angles, to determine its 3D shape, as shown in this animation. The shape provides additional insight into Arrokoth’s origins. The flattened shapes of each of Arrokoth’s lobes, as well as the remarkably close alignment of their poles and equators, point to an orderly, gentle merger of two objects formed from the same collapsing cloud of particles. Arrokoth has the physical features of a body that came together slowly, with ‘locally-sourced’ materials from a small part of the solar nebula. An object like Arrokoth wouldn’t have formed, or look the way it does, in a more chaotic accretion environment.
[Credit: NASA/Johns Hopkins University Applied Physics Laboratory/Southwest Research Institute/Roman Tkachenko]
The fact that Arrokoth is so archaic can help us piece together the role of planetesimals in the early solar system. “We believe Arrokoth formed just slightly after the Sun formed, from the gravitational collapse of a cloud of dust and ice,” explains Dr Throop, talking about how significant the artefact we have now touched upon is.