Image: This colorful view of Mercury was produced using imagery from the color imagery campaign during the first Messenger mission. These colors are not those of Mercury, they have been enhanced to differentiate the chemical and mineralogical aspect of the rocks that make up the surface of Mercury.
Image credit: NASA / Johns Hopkins University Applied Physics Laboratory / Carnegie Institution of Washington.
Mass: Mercury is the smallest terrestrial planet in the solar system, with a diameter of about 4,880 kilometers, or about one-third the size of Earth.
Mercury's mass is about 3.3 x 10^23 kg, which is equivalent to about 5.5% of Earth's mass. Although smaller than Earth, Mercury is the most massive planet of all the telluric (or rocky) planets in the solar system, including Venus, Earth, and Mars. Due to its mass, Mercury has a stronger gravity than one would expect given its small size. Mercury's gravity is about 38% of Earth's, which means that if you weigh 100 kg on Earth, you would weigh about 38 kg on Mercury.
Orbit: Mercury's orbit is the most eccentric of all the planets in the solar system, which means that the distance between Mercury and the Sun varies greatly during its orbit. At its furthest point from the Sun, called aphelion, Mercury is about 70 million kilometers from the Sun, while at its closest point, called perihelion, Mercury is about 46 million kilometers from the Sun. In addition, Mercury's orbit precedes, that is, the orientation of its orbit changes slowly over time. This precession was one of the first proofs of Albert Einstein's theory of relativity.
Day: Mercury has one sidereal day (time it takes to complete one full rotation on its axis) which is approximately 176 Earth days. It is the longest day of all the planets in the solar system.
Atmosphere: Mercury has a very thin and tenuous atmosphere. Due to its low gravity and lack of a strong magnetic field, Mercury's atmosphere is constantly being eroded by solar wind and charged particles from cosmic radiation.
Mercury's atmosphere is mostly made up of scattered gas particles, mostly helium and the rare gas called "argon", along with traces of sodium, potassium, oxygen and hydrogen.
The density of Mercury's atmosphere is very low, about 10^14 times lower than that of Earth, which means that it can only hold a negligible amount of air or water vapour.
Surface: Mercury's surface is marked by immense plains, craters, mountains and steep cliffs.
The plains are mostly made up of basalt, a volcanic rock that has sunk and filled impact craters. Craters are ubiquitous and some of them are very large, such as the Caloris Basin, which is around 1,550 km in diameter. The steep mountains and cliffs, called "ridges", are the result of the contraction of the planet's crust.
Mercury's surface is also characterized by variations in color and texture, reflecting the chemical and mineralogical composition of the rock. Measurements made by the MESSENGER mission revealed that Mercury's surface contains a large amount of ferrous silicates and heavy metals, such as iron, aluminum and titanium. These elements likely accumulated as a result of the planet's accretion, which occurred about 4.5 billion years ago.
Due to its close orbit to the Sun, Mercury's surface is also subject to extreme temperatures, which can reach over 400 degrees Celsius (750 degrees Fahrenheit) during the day, while nighttime temperatures can drop below -170 degrees. Celsius.
Temperature: Due to its eccentric orbit, Mercury has the most extreme temperature of any planet in the solar system. The surface temperature can reach up to 427°C on the day side and drop to -183°C on the night side.
Density: Mercury is a dense rocky planet. Its average density is about 5.4 g/cm³, which is almost 30% higher than the density of Earth. This high density suggests that the planet has an iron-rich core, which occupies about 60% of its radius. Simulations show that Mercury's core is made up of a mixture of iron and nickel, while the crust and mantle are mainly composed of silicates.
Mercury's high density is likely due to the effect of impact and gravitational compression on the planet during its formation, as well as its proximity to the Sun, which resulted in the loss of most of its primitive gas envelope. Mercury's high density also makes it more resistant to erosion, which is why the planet has retained many impact craters from the time the solar system was formed.
Craters: Mercury's surface is covered in impact craters, which is typical of rocky planets in the solar system. Mercury's craters vary in size, shape, and depth, ranging from small craters about 1 km in diameter to large impact basins hundreds of kilometers in diameter.
The largest impact crater on Mercury is Caloris Basin, which is about 1,550 km in diameter and surrounded by mountain ridges. Other notable craters include Kuiper Crater, Rachmaninoff Crater, and Debussy Crater.
Impact craters on Mercury bear witness to the violent history of the formation of the solar system. They were created when large rocky bodies, such as asteroids or comets, hit the planet at extremely high speeds, creating gigantic shocks and throwing material great distances.
Impact craters are also used to study the geological history of Mercury, analyzing the size, number, and distribution of craters to determine the relative age of the planet's surface.
1973: The Mariner 10 mission was launched in 1973, it made three flybys of Mercury in 1974 and 1975. The mission mapped around 45% of Mercury's surface and revealed information about the planet's internal structure.
2004: The MESSENGER mission (MErcury Surface, Space ENvironment, GEOchemistry, and Ranging) was launched in 2004, it orbited Mercury for four years between 2011 and 2015. The mission made it possible to map the planet 100%, to discover water ice in the polar craters and to obtain precise data on the chemical and geological composition of the planet.
2018: The joint mission of the European Space Agency and the Japan Aerospace Exploration Agency (JAXA), called BepiColombo, was launched in October 2018. The mission includes two probes, one to study the surface of the planet and the other to study its magnetic field. It is scheduled to arrive on Mercury in 2025.
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