The Earth, Moon, Mars and the giant planets

The Earth is the only known planet with life: radius ≈ 6371 km, mass 5.97 · 10²⁴ kg, an atmosphere of ≈ 78 % nitrogen and 21 % oxygen, ≈ 71 % of the surface covered by ocean; it has a magnetic field and plate tectonics. The Moon is the Earth’s natural satellite: radius 1737 km, mass ≈ 1/81 of the Earth’s, surface gravity ≈ 1.6 m/s² (≈ 1/6 of the Earth’s); it has no atmosphere, and its surface is made of dark “seas” (maria) and bright highlands with many craters. By the current hypothesis the Moon formed ≈ 4.5 billion years ago when a Mars-sized body (Theia) struck the Earth. The first craft to reach the Moon was Luna 2 (1959); the Apollo 11 astronauts landed on 20 July 1969 and in 1969–1972 twelve people walked on the Moon; in 2019 Chang’e 4 landed on the far side and in 2023 Chandrayaan-3 near the south pole. Mars has a radius of 3396 km, a very thin CO₂ atmosphere (pressure ≈ 0.6 % of the Earth’s) and a mean temperature of ≈ –60 °C; it has the tallest mountain in the Solar System, Olympus Mons (≈ 22 km), and there is evidence of ancient water. The rovers Curiosity (2012) and Perseverance (2021) work on Mars; with Perseverance came the Ingenuity helicopter, which made the first powered, controlled flight on another planet in April 2021.