Janssen Crater is an impact crater in the Arabia quadrangle on Mars at 2.7° N and 322.5° W. and is 158.0 km in diameter. A picture below show these layers, as well as dark slope streaks. The darker the streak, the younger it is. (wikipedia entry)
A crater is a circular depression likely created by an impact event. On Mars large craters are named after deceased scientists, writers and others who have contributed to the lore of Mars. Small creaters (less than 60km) are named after small towns and villages.