Mars
Mars is the fourth planet from the Sun and the most explored planet beyond Earth — the only world on which robotic rovers have driven across the surface. Its landscape is defined by rust-colored dust, extinct volcanoes, vast canyons, polar ice, and evidence that rivers and lakes once flowed there. Two small moons, Phobos and Deimos, orbit close to the planet, and an armada of spacecraft has studied Mars since the first flyby in 1965.