Voici pourquoi:

* Mare Nubium is a large, dark, smooth plain of solidified lava, a lunar mare. These mares were formed billions of years ago, during the early bombardment period of the Moon's history.

* Crater Tycho , on the other hand, is a relatively young impact crater, clearly visible with its bright rays radiating outward. Its age is estimated to be around 108 million years old.

Since Tycho was formed long after the mare, we know it's younger.