NASA's Mars Reconnaissance Orbiter left Earth on August 12, 2005 — almost exactly 15 years ago. The satellite began circling the red planet on March 10, 2006, and since then it has studied Martian temperatures, detected minerals on the planet's surface, and taken nearly 7 million images of our closest planetary neighbor.The orbiter's main goal is to help scientists understand the role water has played on Mars throughout the planet's history, and the degree to which water still exists on the planet as liquid, vapor, or ice. In honor of the orbiter's 15th anniversary, here are some highlights from its massive portfolio of photos, and what each image reveals about Mars.