Mars

Mars is the fourth planet from the Sun, orbiting at 1.524 AU. With a diameter of 6,792 km (about half of Earth's), it is the second-smallest planet. Mars is famous for its distinctive red-orange color, caused by iron oxide (rust) in its surface dust. The planet has a thin atmosphere (about 1% of Earth's pressure) composed mainly of carbon dioxide, with dramatic features including Olympus Mons — the largest volcano in the solar system at 21.9 km tall — and Valles Marineris, a canyon system stretching 4,000 km across the surface. Mars has two small, irregularly shaped moons: Phobos and Deimos.

Mars varies enormously in apparent size and brightness depending on its position relative to Earth. At opposition (occurring roughly every 26 months), it can reach magnitude -2.9 and show a disk of 25 arcseconds — bright enough to outshine all stars. At its farthest, it shrinks to a tiny 3.5-arcsecond dot. During favorable oppositions, a 6-inch telescope at 200-300x can reveal dark surface markings (Syrtis Major is the most prominent), the white polar ice caps, occasional dust storms, and limb clouds. Color filters help: a red/orange filter (#21 or #23A) enhances dark surface features, while a blue filter (#80A) shows clouds and hazes. Mars is best observed in the weeks surrounding opposition.

Known since prehistory, Mars's reddish color led the ancient Romans to name it after their god of war. Tycho Brahe's precise observations of Mars enabled Johannes Kepler to derive his laws of planetary motion in the early 1600s. Giovanni Schiaparelli mapped 'canali' (channels) on its surface in 1877, which was mistranslated as 'canals,' sparking decades of speculation about Martian civilization. Mariner 4 provided the first close-up images in 1965, revealing a cratered, barren surface. Mars has been explored by numerous rovers and landers, including the Perseverance rover which has been collecting samples for eventual return to Earth.