Moon basin gouged by pieces of proto-planet

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Imbrium Basin, the circular feature in the top half of this image, is the largest basin on the near side of the moon.

Ever looked at the full moon to see a face staring back? If you zoom into the right eye – a vast basalt plain called Mare Imbrium – you’ll see a series of unusual parallel grooves surrounding a 1,250-kilometre-wide crater. How did those furrows come to be?

Experiments conducted by Brown University planetary scientist Peter Schultz and Sandia National Laboratories physicists David Crawford showed they may have formed as a proto-planet, around 250 kilometres in diameter, broke upon impact with the moon around four billion years ago.

Those fragments ploughed out lunar material into the distinct grooves we see today.

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Comparison of furrows produced by an impactor in experiments (above) and those produced in a computational model of an asteroid (red pattern, below). 
Peter Schultz

The work, published in Nature, gives planetary scientists insights into the population of objects in the asteroid belt during what’s known as the Late Heavy Bombardment, when the solar system was awash with asteroids, comets and other objects.

The pattern of rimmed furrows around Mare Imbrium – dubbed the Imbrium Sculpture by American geologist G K Gilbert in 1893 – were thought to have been formed by a large object that battered the moon, glancing off at an angle of around 30 º.

But some elements didn’t quite add up.

Rather than radiating neatly, like spokes on a wheel, a few furrows were parallel.

Did they form when dust and rock, ejected from the impact, settled into that particular pattern? Or was there something special about the object that crashed into the moon that produced the Imbrium Sculpture?

Schultz and Crawford tested the hypothesis by pummeling samples in the lab and modelling bigger impacts with computer simulations.

They found a proto-planet, about half the size of asteroid belt object Vesta and much bigger than previous estimates, could have produced the Imbrium Sculpture – but only if parts snapped off after it collided.

Those fragments could have produced parallel furrows as they bounced and gouged the surface.

Assuming a proto-planet 250 kilometres in diameter, the pair also calculated its likely speed when it barrelled into the moon.

Around 25 kilometres a second was fast enough to produce a ring of debris that closely matches the Montes Alpes, a mountain range to the basin’s east.

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