Among the thousands of visitors to Mt. Etna this year, one group came not just to look at one of most famous volcanoes on Earth. Dozens of scientists trekked up Etna together this fall to observe what Etna has in common with Mars. Researchers interested in what makes the red planet tick can’t study the planet in person-at least not yet. To help them interpret what they see in Mars images and other remote sensing data–and to test their instruments and procedures–they turn to Earth.