Icy moons cracked by short sharp tidal tugs: study

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The New Horizons spacecraft snapped this image of Charon, Pluto’s largest moon, as it flew by in July 2015.

Cracks around icy moons in the far reaches of the Solar System may have developed when another similarly sized object whizzed by four billion years ago.

Alice Quillen from the University of Rochester in New York and colleagues put the hypothesis to the test using computer simulations.

They saw that if another body briefly caught an icy object in its gravitational pull, even only for a few hours, it could tear its crust.

When New Horizons flew past Pluto’s largest moon Charon last year, it exposed what appeared to be an enormous crack around its equatorial belt.

In February, scientists suggested a frozen ocean below busted its skin.

But there are other moons in the Solar System with cracks, such as Enceladus’ tiger stripes.

Quillen hypothesised the cracks could have appeared 3.8 to 4.1 billion years ago or so.

The Solar System was a very different place during this period, called the Late Heavy Bombardment. Planets and moon were barraged by comets and asteroids before they (mostly) settled into orbit around the Sun.

Charon, at that point, may not have been in Pluto’s orbit. And a similarly sized object, giving it a short sharp tug as it flew by, could have torn its crust into the crack we see today. Conversely, a constant tidal force, as Charon experiences with Pluto, is not energetic enough to rent its surface. This is known as brittle elastic behaviour.

“If you take silly putty and throw it on the floor it bounces – that’s the elastic part,” Quillen explains. “But if you pull on it rapidly and hard enough, it breaks apart.”

She and her colleagues modelled icy moons as N-bodies, where their surface was supported by a series of “springs”.

Computer simulations showed when a comparable object flew by, a spherical icy moon elongated, thanks to the gravitational tug, then flicked back to oscillate a few times.

The initial few vibrations were enough to get cracks going. Subsequent deformations increased their length. Some looked like the long crack on Charon.

They admit a few limits to their simulations. Their “moons” had crusts of constant thickness and elasticity. But icy moon crusts aren’t like that. Pre-existing fractures or spin weren’t included, nor were effects of multiple tidal tugs over long periods.

The work will be published in the journal Icarus. A preprint is available on ArXiv.

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