Mars
Mars is the fourth planet from the Sun, a rocky world with a thin carbon-dioxide atmosphere, ancient river-shaped terrain, and polar ice caps.
Martian atmosphere: The thin gaseous envelope surrounding Mars, composed mostly of carbon dioxide. Its low pressure and carbon-dioxide composition shape surface conditions and weather.
Olympus Mons: A shield volcano on Mars and the tallest known volcano in the Solar System. Its immense size records prolonged volcanic construction on a planet without moving tectonic plates.
Percival Lowell: An American astronomer who popularized the mistaken idea that Mars had artificial canals. His influential observations shaped early public beliefs about Martian life.
Earth: The third planet from the Sun and the only world known to support life. Earth’s active surface and thick atmosphere contrast with Mars’s cold, dry present.
Mars rover: A mobile robotic vehicle designed to investigate the surface of Mars. Rovers provide close-range geology and can travel between scientifically distinct sites.
Martian climate: The long-term patterns of temperature, winds, dust, and seasonal change on Mars. Axial tilt, dust, and atmospheric circulation drive its distinctive seasons.
Valles Marineris: A vast system of canyons near Mars’s equator, extending thousands of kilometers. Its scale and layered deposits expose the planet’s tectonic and erosional history.
Mariner 4: The first spacecraft to fly past Mars and return close-up images, in 1965. Its cratered images overturned expectations of a densely inhabited, Earth-like planet.
Venus: The second planet from the Sun, with a dense carbon-dioxide atmosphere and extreme surface heat. Venus and Mars show how rocky planets with carbon dioxide atmospheres can diverge.
Mars sample return: A planned effort to bring collected Martian rock and soil samples to Earth for analysis. Laboratories on Earth could test Perseverance’s samples with instruments too large for a rover.