Exploring Mercury

Mercury is the closest planet to the Sun. Its closeness greatly influences Mercury’s atmosphere, temperature, and surface.

Fastest Orbit

Mercury is about 1/3 the size of the Earth. It’s is only slightly larger than our moon and Mercury itself possesses no moons. Mercury is 3,032.3 miles (4,880 kilometers) wide. To put it in perspective, if the Earth were the size of a nickel then Mercury would be the size of a blueberry. Mercury’s closeness to the Sun makes it the fastest planet to orbit around it, just taking 88 Earth days, traveling at nearly 29 miles (47 kilometers) per second, faster than any other planet.

Mercury’s orbit is highly elliptical, almost egg-shaped. At its closest, Mercury can be just 29 million miles (47 million kilometers) and at its furthest Mercury can be 43 million miles (70 million kilometers) from the Sun. In contrast, Mercury spins very slowly on its axis making one day on Mercury the same duration as 59 Earth days. When Mercury is the closest to the Sun, it moves faster. During this time, depending on where you stand on Mercury, you could see the Sunrise briefly, stop and set again. On the other side of the planet, the same thing happens but in reverse.

Visitors from Earth

  • The first spacecraft that visited Mercury was Mariner 10. It launched on November 3, 1973, and flew by Venus and Mercury, imaging 45 percent of Mercury’s surface.
  • After Mariner 10, the MESSENGER spacecraft flew by Mercury three times and orbited the planet for four years before crashing on its surface. MESSENGER launched in August 2004 and orbited the planet from 2011 to 2015.
  • The European Space Agency (ESA) and the Japanese Aerospace Exploration Agency (JAXA) are in a joint mission to launch their first mission to explore Mercury in 2018. In May 2018, their spacecraft BepiColombo arrived at Europe’s Spaceport for their six-month countdown.


mosaic image of mercury by the Mariner 10 spacecraft in 1974.

This mosaic by the Mariner 10 spacecraft on the 29 March 1974, was taken while it retreated from the planet Mercury. Consisting of 18 images, these images show somewhat more of the illuminated surface.

Mercury’s Surface

Mercury is a terrestrial planet type with a central core, a rocky mantle, and solid crust. Very similar to Earth, it has a large metallic core with a radius of about 1,289 miles (2,074 kilometers), making up 85 percent of the planet’s radius. The planet’s crust, or outer shell, is only about 250 miles (400 kilometers). Its theorized that Mercury formed 4.5 billion years ago.

Temperatures on Mercury’s surface are extreme yet, despite being the planet closest tot he Sun, it is not the hottest -that title belongs to Venus. During the day Mercury’s surface can reach 800 degrees Fahrenheit  (430 Celsius). During the night, the surface temperature can drop down to minus 290 degrees Fahrenheit (minus 180 degrees Celsius) this is because Mercury has no atmosphere to retain the heat.

“Mercury possesses a thin exosphere made up of atoms blasted off the surface by the solar wind and striking meteoroids. Mercury’s exosphere is composed mostly of oxygen, sodium, hydrogen, helium, and potassium.”

Mercury’s surface resembles our moon’s with scattered craters from meteoroid and comet impacts. “Craters and features on Mercury are named after famous deceased artists, musicians or authors, including children’s author Dr. Seuss and dance pioneer Alvin Ailey.”

Two of the largest impacts, Caloris and Rachmaninoff were created early in the solar system’s history. Mercury’s distinctive “crater rays” are formed when an asteroid hits the surface. The impact blasts material away from the crater forming the distinctive lines. The crushed particles flown away from the impact are more reflective and thus more noticeable. With time and the exposure of the space environment, the rays darken.

Enhanced image of mercury to bring up its geological features

The MASCS instrument was designed to study both the exosphere and surface of Mercury. To learn more about the minerals and surface processes on Mercury, the Visual and Infrared Spectrometer (VIRS) portion of MASCS collected single tracks of spectral surface measurements since MESSENGER entered orbit.


Resources for Educators

  1. NASA’s Kid-Friendly Mercury
  2. NASA’s Solar System Exploration, Mercury
  3. YouTube channel: CrashCourse, Astronomy