Spectroscopic images of those rocks taken with the Compact Reconnaissance Imaging Spectrometer for Mars (CRISM) showed that they were composed of minerals that only form in the presence of water.

FORBES: NASA Finds Remains Of Ancient Martian Lake

The image from the High Resolution Imaging Science Experiment camera on NASA's Mars Reconnaissance Orbiter represents the first time a Mars landing has been captured by another spacecraft.

NPR: Phoenix Sends Color Photos of Mars to NASA