Ultima Thule's mystery mounds STUNS NASA scientists: ‘That caught us by surprise’

Ultima Thule is the small, frozen object lying one billion miles beyond Pluto’s orbit. Photos captured by NASA’s New Horizons spacecraft captured during the historic encounter revealed Ultima Thule’s unique shape. But scientists have been left baffled by another aspect of Ultima Thule’s appearance – its oddly lumpy exterior.

Images beamed back of the distant object shocked the world when they revealed the bilobed Ultima Thule resembled a reddish “flattened snowman”.

Dr Alan Stern, New Horizons principal investigator said: ”These are not spherical lobes at all.

"That caught us by surprise – I think it caught everybody by surprise."

New Horizons imagery also revealed a number of mound-like features on the larger of the two lobes, the part dubbed Ultima.

Dr Stern added: ”They seem to be raised, but exactly what causes them we're not sure. It is still early days."

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The current hypothesis is the mounds are caused by convection of low-temperature ice.

And this was driven by the heat generated by the radioactive decay of aluminum-26.

But further work suggests this is an implausible theory.

NASA now believes the mounds may be the retained outlines of minute planets that merged to form Ultima’s lobe long ago.

But Dr Stern warned: ”There could be other processes as well. So this is an active topic of debate."

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Ultima Thule formed from a cloud of rocky, icy material far from the sun.

These smaller chunks first formed two larger objects, which then apparently orbited a common centre of mass as a binary pair.

These two bodies then slowly merged to form Ultima Thule.

The mission team has been able to put some "speed limits" on that merger, thanks to computer simulations.

Had the two lobes merged at only 22mph (35kmh), the material would not have merged at all.

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That collision would have been a glancing one, and Ultima and Thule would have gone their separate ways.

A collision at 11mph (18kmh) would lead to a merger, but one that would not have left Ultima Thule’s distinctive lobes intact.

So the NASA New Horizons team now believes the icy collision occurred at even slower speeds — perhaps as slow as 5.5mph (9kmh).

Indeed, the result of simulations with a 5.5mph merging speed "is strikingly like what we actually observe," Dr Stern said.

Mission scientists have seen no evidence of any type of atmosphere on Ultima Thule, nor have they spotted any signs of satellites or ring systems.

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But NASA’s New Horizons will continue examining the distant object for another 16 months, during which the spacecraft will finish beaming all of its data home to Earth.

The Ultima Thule flyby was the second such encounter for New Horizons, which famously cruised past Pluto in July 2015.

And scientists may get an opportunity for further close-ups from New Horizons in the future.

New Horizons is in surprisingly good condition and has enough fuel to zoom past a third object, if NASA approves another mission extension.

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