Imagine that you had a circular wire brush

Imagine that it didn't just spin on its axis, but its axis also described a circle

Imagine that the brushes are tilted to brush out from the middle to the edge

That's the brush on the RAT...

Now, suppose that your not-so imaginary brush is used on a rock with a weak dusty crust

In the outer regions, where the brush moves rapidly and presses hard, the dust is scoured - indeed the dusty crust flakes right off

In the central portion, the crust DOESN't break off, and just gets some spiral scours and scratches

The central bit is irregular, because the real world is imperfect, but it looks like a semicircular remnant of the dusty crust, with some spiral scratches..