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Rosetta's comet sculpted by stress
by Staff Writers
Paris (ESA) Feb 19, 2019

First impressions of the Kuiper Belt object Ultima Thule (left) revealed a surprisingly familiar appearance to the comet that ESA's Rosetta spacecraft explored for more than two years (right). NASA's New Horizons flew by Ultima Thule on 1 January 2019, with subsequent images and data suggesting that its two lobes are rather more 'squashed' like a pancake, with respect to Comet 67P/Churyumov-Gerasimenko. Ultima Thule, which sits beyond the orbit of Neptune in the outskirts of the Solar System, is about 34 km long, with the two lobes measuring about 19.5 and 14.2 km across. By comparison, Comet 67P/C-G's two lobes measure 4.1 x 3.3 x 1.8 km and 2.6 x 2.3 x 1.8 km. The comet likely originated from the Kuiper Belt and now orbits around the Sun on a 6.5 year journey that takes it from just beyond the orbit of Jupiter at its most distant, to between the orbits of Earth and Mars at its closest. Additional images, charts and detailed captions can be found here and here

Feeling stressed? You're not alone. ESA's Rosetta mission has revealed that geological stress arising from the shape of Comet 67P/Churyumov-Gerasimenko has been a key process in sculpting the comet's surface and interior following its formation.

Small, icy comets with two distinct lobes seem to be commonplace in the Solar System, with one possible mode of formation a slow collision of two primordial objects in the early stages of formation some 4.5 billion years ago. A new study using data collected by Rosetta during its two years at Comet 67P/C-G has illuminated the mechanisms that contributed to shaping the comet over the following billions of years.

The researchers used stress modelling and three-dimensional analyses of images taken by Rosetta's high resolution OSIRIS camera to probe the comet's surface and interior.

"We found networks of faults and fractures penetrating 500 metres underground, and stretching out for hundreds of metres," says lead author Christophe Matonti of Aix-Marseille University, France.

"These geological features were created by shear stress, a mechanical force often seen at play in earthquakes or glaciers on Earth and other terrestrial planets, when two bodies or blocks push and move along one another in different directions. This is hugely exciting: it reveals much about the comet's shape, internal structure, and how it has changed and evolved over time."

The model developed by the researchers found shear stress to peak at the centre of the comet's 'neck', the thinnest part of the comet connecting the two lobes.

"It's as if the material in each hemisphere is pulling and moving apart, contorting the middle part - the neck - and thinning it via the resulting mechanical erosion," explains co-author Olivier Groussin, also of Aix-Marseille University, France.

"We think this effect originally came about because of the comet's rotation combined with its initial asymmetric shape. A torque formed where the neck and 'head' meet as these protruding elements twist around the comet's centre of gravity."

The observations suggest that the shear stress acted globally over the comet and, crucially, around its neck. The fact that fractures could propagate so deeply into 67P/C-G also confirms that the material making up the interior of the comet is brittle, something that was previously unclear.

"None of our observations can be explained by thermal processes," adds co-author Nick Attree of the University of Stirling, UK. "They only make sense when we consider a shear stress acting over the entire comet and especially around its neck, deforming and damaging and fracturing it over billions of years."

Sublimation, the process of ices turning to vapour and resulting in comet dust being dragged out into space, is another well-known process that can influence a comet's appearance over time. In particular, when a comet passes closer to the Sun, it warms up and loses its ices more rapidly - perhaps best visualised in some of the dramatic outbursts captured by Rosetta during its time at Comet 67P/C-G.

The new results shed light on how dual-lobe comets have evolved over time. Comets are thought to have formed in the earliest days of the Solar System, and are stored in vast clouds at its outer edges before beginning their journey inwards. It would have been during this initial 'building' phase of the Solar System that 67P/C-G got its initial shape.

The new study indicates that, even at large distances from the Sun, shear stress would then act over a timescale of billions of years following formation, while sublimation erosion takes over on shorter million-year timescales to continue shaping the comet's structure - especially in the neck region that was already weakened by shear stress.

Excitingly, NASA's New Horizons probe recently returned images from its flyby of Ultima Thule, a trans-Neptunian object located in the Kuiper belt, a reservoir of comets and other minor bodies at the outskirts of the Solar System.

The data revealed that this object also has a dual-lobed shape, even though somewhat flattened with respect to Rosetta's comet.

"The similarities in shape are promising, but the same stress structures don't seem to be apparent in Ultima Thule," comments Christophe.

As more detailed images are returned and analysed, time will tell if it has experienced a similar history to 67P/C-G or not.

"Comets are crucial tools for learning more about the formation and evolution of the Solar System," says Matt Taylor, ESA's Rosetta Project Scientist.

"We've only explored a handful of comets with spacecraft, and 67P is by far the one we've seen in most detail. Rosetta is revealing so much about these mysterious icy visitors and with the latest result we can study the outer edges and earliest days of the Solar System in a way we've never been able to do before."

"Bilobate comet morphology and internal structure controlled by shear deformation" by C. Matonti et al. is published in Nature Geoscience.


Related Links
Rosetta at ESA
Asteroid and Comet Mission News, Science and Technology


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IRON AND ICE
NASA telescopes take a close look at the brightest comet of 2018
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As the brilliant comet 46P/Wirtanen streaked across the sky, NASA telescopes caught it on camera from multiple angles. NASA's Hubble Space Telescope photographed comet 46P/Wirtanen on Dec. 13, when the comet was 7.4 million miles (12 million kilometers) from Earth. In this visible-light image, the comet's nucleus is hidden in the center of a fuzzy glow from the comet's coma. The coma is a cloud of gas and dust that the comet has ejected during its pass through the inner solar system due to h ... read more

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