This tantalizing close-up detail of a network of martian valleys was recorded from orbit this April by Mars Global Surveyor's camera. Water may once have flowed here but now sand dunes stripe the windswept valley floor.

What formed Ariel's valleys? This question presented itself when Voyager 2 passed this satellite of Uranus in January 1986. Speculation includes that heating caused by the ancient tides of Uranus caused moonquakes and massive shifting of the moon's surface.