We have already explored our Sun, now Mercury is the next stop on our journey through our solar system. Mercury is the closest planet to the Sun and is the smallest planet in our solar system, coming in at only slightly larger than our Moon. On average, Mercury is only 36 million miles (58 million km) from the Sun. Sunlight takes 3.2 minutes to reach Mercury, in comparison to the 8 minutes it takes to reach Earth.
If you were to stand on Mercury, the Sun would be three times larger in the sky than it appears in Earth’s sky. Even though Mercury is the closest planet to the Sun it is not the hottest planet as one might expect. Venus is the hottest planet in our solar system. If Mercury is closer to the Sun than Venus, then why is it not hotter?
This is because Mercury does not have an atmosphere to retain any heat from the Sun. So even though its days can reach temperatures of nearly 800oF (430oC), without an atmosphere to hold in heat, its nights drop down to a whopping -290oF (-180oC) and its days do not get as hot as Venus (872oF/467oC).
Orbit and Rotation
The closer a planet is to the Sun, the faster it orbits. This makes Mercury the fastest planet in our solar system. It is zipping around the Sun at 29 miles (47 km) per second. Being closest to the Sun also gives Mercury the shortest year because it has the smallest orbit. A year on Mercury is 88 Earth days.
Mercury’s orbit is highly eccentric and is egg-shaped. At closest approach it is 29 million miles (47 million km) from the Sun and is 43 million miles (70 million km) at its furthest.
Image Credit: E. Otwell
While Mercury orbits around the Sun very fast, it spins slowly about its axis. A day on Mercury, one full rotation on its axis, is 59 Earth days long. Mercury’s axis is almost perfectly upright, tilted at only 2o. Earth’s axis is tilted 23.4o, this tilt is what gives us distinct but not extreme seasons. With barely any tilt, Mercury does not experience seasons like other planets.
Mercury was formed approximately 4.5 billion years ago. Mercury is a terrestrial planet meaning it is made up of a solid crustal surface, a rocky mantel, and a central core. Mercury’s core is large and metallic, making up about 85% of Mercury’s overall radius. Evidence suggest that Mercury’s core is partly molten. Mercury is the second densest planet in our solar system, after Earth.
Mercury looks a lot like our Moon. Covered in impact craters from collisions with meteoroids and comets. Craters on Mercury are named after famous deceased musicians, artists, and authors. To the human eye Mercury mainly looks greyish brown. The white streaks across the surface are called “crater rays” and are formed when there is an impact on the surface.
As the impact crater is formed dust and other crushed material is thrown from the sight of impact and falls to the surface, creating the rays. Over time, the rays will darken due to other impacts, solar-winds, and other effects of the space environment.
Mercury does not have an atmosphere; it only has a thin exosphere. This exosphere is mainly made up of oxygen, sodium, hydrogen, helium, and potassium. This exosphere is made up of atoms that are thrown off the surface of Mercury during impacts or from solar winds.
Mercury’s magnetic field has just 1% the strength of Earth’s at the surface. Despite this, it causes intense magnetic tornadoes when it interacts with the magnetic fields of a solar wind. These magnetic tornadoes funnel hot solar wind plasma to the surface of Mercury.
Missions to Mercury
NASA’s Mariner 10 was the first space craft to visit Mercury. Mariner 10 took images of about 45% of Mercury’s surface. NASA’s MESSENGER space craft performed three flybys, orbited Mercury for 4 years, and then crashed into its surface to end its mission.
The ESA and JAXA launched BepiColombo in 2018. This joint mission is made up of two space crafts the Mercury Planetary Orbiter, the main craft built by the ESA, and the Mercury Magnetospheric Orbiter, built by JAXA. BepiColombo had its first flyby on October 1st, 2021 and will have 8 more flybys to help direct it into orbit by late 2025. Its primary science mission will begin 2026.
Mercury is named after the fastest of the ancient Roman gods. Mercury was the Roman god of shopkeepers and merchants, thieves and tricksters, and travelers and transporters of goods. He is often identified with Hermes from Greek myth, the fast messenger of the gods.
- 1631 – Mercury was observed by Galileo Galilei and Thomas Harriott with the newly invented telescope. Pierre Gassendi watches Mercury transit over the face of the Sun.
- 1965 – Astronomers discover that mercury rotates three times for every two orbits around the Sun.
- 1974-1975 – Mariner 10 performs 3 flybys and photographs almost half of Mercury’s surface.
- 1991 – Ice is found in permanently shadowed areas of craters in Mercury’s polar regions.
- 2008-2009 – MESSENGER performs three flybys.
- 2011 – MESSENGER begins the orbital part of its mission.
- 2015 – MESSENGER crashes into Mercury to end its mission.
- 2018 – ESA and JAXA’s BepiColombo is launched.
- 2021 – BepiColombo’s first flyby.