Mercury

The smallest planet and the closest to the Sun, Mercury is mostly iron core with a thin rocky shell. How it was built, why it has almost no atmosphere, and why there is ice near its poles.

Diameter: 4,879 km (38% of Earth)
Mass: 0.055 × Earth
Surface gravity: 0.38 × Earth
Distance from Sun: 0.39 AU (about 58 million km)
Year: 88 Earth days
Solar day: about 176 Earth days
Moons: 0
Surface temperature: about −180 °C at night to 430 °C by day

The build: a core with a thin coat

Mercury is a planet built almost entirely of foundations. Its iron core takes up about 85% of its radius — far more than any other rocky planet. Earth's core, by comparison, reaches only about halfway to the surface. That is why Mercury is the second-densest planet in the solar system even though it is the smallest.

Nobody is sure why. Ideas include:

A giant impact early in its history stripped away much of its rocky mantle.
The intense heat near the young Sun vaporised lighter rock before it could be added.
Iron-rich material was naturally concentrated in the innermost part of the protoplanetary disk.

ESA and JAXA's BepiColombo mission, launched in 2018, is designed to help answer this question from orbit.

Why there is almost no air

Mercury breaks most of the rules from step 4: it is small, so its gravity is weak, and it is close to the Sun, so any gas gets very hot and is blasted by the solar wind. What it has is an extremely thin exosphere of atoms knocked off the surface — so sparse that the atoms rarely collide with each other.

Without an atmosphere to spread heat, the temperature swings are extreme: hot enough to melt lead in the day and colder than Antarctica's coldest nights after dark.

A strange day

Mercury spins exactly three times for every two trips around the Sun. Because its rotation and its orbit work against each other, one full day–night cycle at any spot lasts about 176 Earth days — two of Mercury's years.

Ice on the hottest planet's neighbour

Mercury's axis is barely tilted, so the floors of some craters near its poles never see sunlight. NASA's MESSENGER spacecraft, which orbited Mercury from 2011 to 2015, found strong evidence of water ice hidden in these permanently shadowed craters.

A magnetic surprise

For such a small planet, Mercury was expected to have cooled completely. Instead, Mariner 10 discovered in 1974 that it has a global magnetic field — weak, about 1% of Earth's, but real. Part of its huge core must still be liquid.

Lessons for planet builders

A big iron core does not guarantee an atmosphere: size and distance matter more.
Orbits close to a star are hot, airless and battered by stellar wind.
Even hostile worlds can hide ice in the shadows.