Solar activity and the inner planets

The terrestrial planets are small, dense and rocky. Mercury, the nearest planet to the Sun and the smallest (radius 2440 km), has almost no atmosphere, surface temperatures of ≈ +430 °C by day and ≈ –180 °C at night; it orbits the Sun in 88 days and rotates in 58.65 days (a 3 : 2 resonance), so its solar day equals 176 Earth days. Its surface is cratered and it has a large iron core. Mariner 10 and MESSENGER (2011–2015) explored it; the European–Japanese BepiColombo was launched in 2018 and is planned to enter orbit around Mercury in late 2026. Venus – radius 6052 km, similar in size to the Earth – has an atmosphere that is ≈ 96 % CO₂, a surface pressure of ≈ 92 atm and a temperature of ≈ 465 °C (a strong greenhouse effect), with clouds of sulfuric acid. It rotates backward (243 days – longer than its 225-day year). The first soft landing was Venera 7 (1970) and Magellan mapped it by radar (1990–1994); new missions are being planned.