How is this possible? The difference between the two New Horizons images is the solar phase angle, the direction from which the Sun is shining. The lava flow probably has a rough surface. When the Sun is high overhead, as in image C, New Horizons sees mostly the dust lying on the tops of the rough surfaces. When the Sun is coming from the side, as in image D, the rough surface of the older lava flow results in lots of shadowed areas, making the old lava flow visible under the frost.