A hit within a hit
On Phobos, a moon of mars, we have a seriously huge impact with another deep on inside it.
Enlargement taken from:
By NASA/JPL-Caltech/University of Arizona - English Wikipedia - Original image and description., Public Domain, Link
The grooves are much older than these craters and probably date back to mars's formation. Get a good look at them. Phobos is not likely to be around much longer. Only another 50 million year or so before it gets so close to mars that it is torn apart by its gravity and becomes a ring, like saturn's rings.
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