Mercury

Mercury is the innermost terrestrial planet in our solar system.
It is hard to observe Mercury using optical telescopes.
Giovanni Schiaparelli’s studies revealed that Mercury has one side always facing the Sun.
Mercury has a rotation period of 59 days and orbits the Sun in 88 days, making a solar day equal to two Mercury years.

In 1965, radar observations disproved Schiaparelli’s perspective on Mercury's rotation.
Mercury completes one rotation on its axis every 59 days.
The same face of Mercury faces the Sun every other orbit.
A solar day on Mercury lasts for two Mercury years, with the Sun directly overhead every two years.
The axis of rotation is perpendicular to its orbit.

Mercury has no detectable atmosphere due to its size, proximity to the Sun, and high temperatures.
Similar to the Moon, it has an insignificant atmosphere, leading to high surface temperatures along with low gravity that causes gases to escape.
Daytime temperatures can reach up to 700K, while nighttime temperatures can drop to about 100K.
It boasts the widest temperature range in the solar system (600K) and polar temperatures of about 125K.
There is a possibility of water ice in permanently shadowed craters.

Data from the 2008 Messenger flyby indicates that Mercury's surface is similar to that of the Moon.
The craters on Mercury are not as deep or high, due to lower gravity.
Surface features include intercrater plains—flat areas between craters.
Older craters may be filled in with volcanic activity, resembling the Moon’s maria but with lighter coloration.

Scarps are long cliffs that cut across craters and are not due to volcanic or geological activity.
They likely formed from the cooling and contraction of the crust about 4 billion years ago.

The Caloris Basin, formed by the impact of a large asteroid, is the largest geological feature on Mercury.
It measures 1400 km across and features rings of mountains.
Volcanic activity has contributed to creating the intercrater plains within the basin.

The Mariner 10 mission discovered a weak magnetic field around Mercury, about 1/100th the strength of Earth’s magnetic field.
The prevailing theory suggests that magnetic fields are created from rapid rotation and a liquid metal core.
However, Mercury’s slow rotation raises questions about whether it possesses a liquid metal core.
The magnetic field can deflect solar wind and is stronger at the north pole compared to the south pole.
The exact mechanism for generating Mercury's magnetic field remains unknown.

Mercury likely possesses a large iron-rich core with a radius of about 1800 km, constituting 60% of its mass.
Its high density is estimated at 5400 kg/m3.
It has the highest core volume to planetary volume ratio among terrestrial planets.

Mercury has been geologically inactive for approximately 4 billion years, resembling the Moon's geologic inactivity.
It features a solid mantle, with no active volcanoes or moving tectonic plates.
In its first half billion years, Mercury underwent melting and differentiation, and experienced meteor bombardment.
The planet cooled slowly and developed a thin crust; it experienced more volcanic activity than the Moon as lava filled and covered many craters.