NASA always knows how to explain things so we can understand them.

The Moderate Resolution Imaging Spectroradiometer (MODIS) instrument on NASA’s Terra satellite captured a natural-color image. The oil slick appeared as a tangle of dull gray on the ocean surface, made visible to the satellite sensor by the sun’s reflection on the ocean surface. On May 1, most of the oil slick was southeast of the Mississippi Delta.

via NASA