The white circle marks the location where the Deepwater Horizon rig exploded April 20. The dark black swirls in the center of the image are oil.
Taking photos of the oil spill in the Gulf of Mexico from space isn't as easy as you might think.
Though NASA has satellites that take images of the Gulf daily, a whole slew of variables — cloud cover, water smoothness and sunglint, the reflection of sunlight off the surface — make getting high-quality and scientifically meaningful images of surface oil in the Gulf an inconsistent process.
The NASA Earth Observatory explains that because ocean waters are never perfectly smooth, the sun's reflection gets scattered off the surface in many directions. This yields a broad stripe of sunlight across the ocean in most satellite photographs.
But things change when you add oil to the water. As NASA says:
Oil smooths the surface of the water, dampening small waves and ripples. This smoothing changes the way the ocean reflects sunlight; in a sense, it makes the water a better mirror.
And depending on where the oil is in relation to the sun's reflection, the oil slick either appears brighter than clear water or much darker.
You can find photos and a full explanation of how NASA is imaging the oil spill online at the NASA Earth Observatory.