What you are looking at researchers are fairly certain is a layer of ice excavated by the blast of Phoenix's retrorockets.
The possibility still exists that it's just rock, however, I doubt geologists would make that mistake, even with a black and white photograph. I was really hoping for a full color photograph letting us know once and for all if this is what we came to find, or if it's just a layer of rock. Based on the smoothness and the behavior of the soil around it, NASA's putting their money on ice.
The way we look at Mars just changed. Forever.