Many impact craters on Mars were filled with ice in past climates. Sometimes this ice flows or slumps down the crater walls into the center and acquires concentric wrinkles as a result. This image shows an example of this.
There are other ways that scientists know the material in the crater is icy. Surface cracks that form polygonal shapes cover the material in the crater. They are easy to see in this spring-time image because seasonal frost hides inside the cracks, outlining them in bright white. These cracks form because ice within the ground expands and contracts a lot as it warms and cools. [More at link]