A venusian volcanic valley

Based upon analysis of the number and distribution of impact craters across the whole of Venus, the entire surface appears to be the same age: about 500 million years old. This is an astonishing result since 500 million years is young for a planet in our solar system. Like the Earth, Venus is thought to be about 4.5 billion years old. This suggests that 500 million years ago (1/9th Venus' current age) Venus completely resurfaced itself--turned itself inside out. What caused such a cataclysmic event, or if indeed this is what really happened, remains a mystery.