Mercury

Mercury is a small, dense, scorched world that orbits the Sun every 88 days. With virtually no atmosphere to redistribute heat, its surface swings by more than 600 °C between day and night — the steepest temperature range on any planet. It is the second densest body in the solar system after Earth, suggesting an oversized iron core that fills nearly 60% of its volume. Mercury rotates three times for every two orbits, locked into a 3:2 spin-orbit resonance unique among planets.

Effectively none. Mercury holds only a thin exosphere of atoms — sodium, oxygen, hydrogen, helium, potassium — sputtered off the surface by the solar wind and micrometeoroid impacts. Sodium atoms form a faint comet-like tail trailing the planet, photographed by ground-based telescopes.

Heavily cratered and geologically ancient, the surface resembles the Moon's but is etched with long lobate scarps — cliffs hundreds of kilometres long that formed as the planet's interior cooled and contracted. The Caloris Basin, 1,550 km wide, is one of the largest impact craters in the solar system. Permanently shadowed crater floors near the poles host confirmed deposits of water ice.

Only two spacecraft have visited. Mariner 10 made three flybys in 1974-75, mapping under half the surface. NASA's MESSENGER orbited from 2011 to 2015, finally completing global mapping and confirming polar water ice before being deliberately crashed into the surface. The joint ESA-JAXA BepiColombo, launched 2018, is using a long looping trajectory and is scheduled to enter Mercury orbit in November 2026.