Mercury is the smallest planet in our Solar System. It is also the closest planet to the Sun. Mercury takes about 88 Earth days to complete one trip around the Sun.

You can sometimes see Mercury from Earth. It looks bright, but it's often hard to spot because it's so close to the Sun. You can usually only see it during twilight (morning or evening) or during a solar eclipse.

We don't know as much about Mercury as we do about other planets. This is because it's hard to study with telescopes from Earth. Also, sending a spacecraft to orbit it is tricky. So far, two main spacecraft have visited Mercury: Mariner 10 and MESSENGER.

Mercury looks a lot like Earth's Moon. It has many craters and flat areas called plains. It doesn't have any moons orbiting it. Mercury also has a very thin atmosphere, which scientists call an exosphere.

Unlike our Moon, Mercury has a huge iron core. This core creates a magnetic field, which is about 1% as strong as Earth's. Because of its large core, Mercury is a very dense planet. The temperature on Mercury's surface can change a lot. It can be super hot, up to 427 °C (801 °F), in areas facing the Sun. But in deep craters near the poles, it can be extremely cold, as low as −183 °C (−297 °F).

People have known about Mercury for thousands of years. Ancient Greek astronomers once thought it was two different objects. They called the morning star "Apollo" and the evening star "Hermes." The name "Mercury" comes from the Romans. They named it after their swift god, Mercury, who was like the Greek god Hermes. The symbol for Mercury is even based on Hermes' staff.

Even though Mercury is the closest planet to the Sun, it's not the hottest. This is because it has almost no atmosphere to trap heat. So, any heat it gets from the Sun quickly escapes into space. The hottest planet in our Solar System is Venus.

Inside Mercury: A Dense, Rocky Planet
Mercury's Surface: Craters and Plains
How Mercury Moves: Orbit and Rotation
Space Missions to Mercury
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Inside Mercury: A Dense, Rocky Planet

Mercury is one of the four inner planets in our Solar System. Like Earth, it has a rocky body. It's the smallest planet, with a radius of about 2,440 kilometers (1,516 miles). Mercury is even smaller than some large moons, like Ganymede and Titan. However, it has more mass than these moons.

Mercury is made of about 70% metal and 30% silicate material (which is like rock). It's the second densest planet in the Solar System, only a little less dense than Earth.

Mercury's Surface: Craters and Plains

Animation of Mercury and the Earth orbiting the Sun

Mercury's surface looks a lot like the Moon's. It has flat areas, similar to the Moon's "mares," and many craters. Mercury was hit by many comets and asteroids billions of years ago. It was also hit during a time called the Late Heavy Bombardment. Mercury has so many craters because its thin atmosphere can't slow down incoming objects.

Pictures from the MESSENGER spacecraft show that Mercury might even have shield volcanoes. The surface temperature on Mercury can swing wildly, from 100 K (−173 °C) to 700 K (427 °C). Even with these extreme temperatures, scientists think there might be frozen water on Mercury, especially in shadowed craters.

Mercury is too small and hot for its gravity to hold onto a thick atmosphere for long. It has a very thin exosphere that includes gases like hydrogen, helium, oxygen, sodium, calcium, and potassium. This exosphere is always changing. Gases are lost to space and then replaced from different sources. For example, hydrogen and helium might come from the solar wind. Other gases, like helium, sodium, and potassium, can come from radioactive elements in Mercury's crust.

How Mercury Moves: Orbit and Rotation

Mercury has the most unusual orbit of all the planets. Its path around the Sun is not a perfect circle; it's more of an oval shape. Its distance from the Sun changes from about 46 million to 70 million kilometers (29 million to 43 million miles). As mentioned, it takes Mercury about 88 Earth days to orbit the Sun. Mercury's axial tilt (how much it leans) is very small, almost zero degrees.

Space Missions to Mercury

Many spacecraft have been sent to Mercury to learn more about it.

Mariner 10: The First Visitor

Mariner 10

The first spacecraft to visit Mercury was NASA's Mariner 10. It flew past Mercury three times between 1974 and 1975. Mariner 10 sent back the first close-up pictures of Mercury's surface. These pictures showed many geological features, including lots of craters.

Unfortunately, each time Mariner 10 flew by, the same side of Mercury was lit by the Sun. This meant it could only map about 45% of the planet's surface. During its first flyby, Mariner 10 found that Mercury has a magnetic field. This was a surprise because Mercury rotates very slowly, and scientists didn't expect it to have a strong magnetic field. The later flybys helped scientists learn more about this magnetic field, which is similar to Earth's.

Mariner 10 ran out of fuel on March 24, 1975, and its mission ended. Scientists believe it is still orbiting the Sun today.

MESSENGER: Mapping the Whole Planet

MESSENGER

The second spacecraft to reach Mercury was NASA's MESSENGER. Its full name stands for MErcury Surface, Space ENvironment, GEochemistry, and Ranging. It launched on August 3, 2004. MESSENGER flew past Earth in 2005 and Venus in 2006. It then made three flybys of Mercury in 2008 and 2009. These flybys helped map the parts of Mercury that Mariner 10 couldn't see.

On March 18, 2011, MESSENGER successfully entered an elliptical orbit around Mercury. It was the first spacecraft to orbit the planet. MESSENGER's main goals were to study Mercury's high density, its geology (how it formed), its magnetic field, its core, and whether there was ice at its poles. It also studied where Mercury's thin atmosphere comes from.

MESSENGER completed its mission and crashed into Mercury's surface on April 30, 2015, as planned.

BepiColombo: Europe and Japan's Mission

Bepicolombo

The European Space Agency (ESA) and the Japanese Space Agency (JAXA) worked together to create a mission called BepiColombo. This mission launched on October 20, 2018.

BepiColombo is special because it carries two probes (smaller spacecraft) that will orbit Mercury. One probe will map the planet's surface, and the other will study its magnetosphere (the area around the planet controlled by its magnetic field). BepiColombo is expected to arrive at Mercury in 2025. Once it arrives, it will release both probes into their orbits to begin their studies.

List of planets

Images for kids

Mercury (first from the left) to scale along with the other largest and rounded bodies of the Inner Solar System, the four terrestrial planets, as well as the Moon and Ceres (first from the right).
MASCS spectrum scan of Mercury's surface by MESSENGER
Enhanced-color image of Munch, Sander and Poe craters amid volcanic plains (orange) near Caloris Basin
Tolstoj basin is along the bottom of this image of Mercury's limb
Picasso crater — the large arc-shaped pit located on the eastern side of its floor are postulated to have formed when subsurface magma subsided or drained, causing the surface to collapse into the resulting void.
Composite of the north pole of Mercury, where NASA confirmed the discovery of a large volume of water ice, in permanently dark craters that are found there.
Graph showing relative strength of Mercury's magnetic field
Image mosaic by Mariner 10, 1974
False-color map showing the maximum temperatures of the north polar region
False-color image of Carnegie Rupes, a tectonic landform—high terrain (red); low (blue).
Mercury, from Liber astronomiae, 1550
Ibn al-Shatir's model for the appearances of Mercury, showing the multiplication of epicycles using the Tusi couple, thus eliminating the Ptolemaic eccentrics and equant.
Transit of Mercury. Mercury is visible as a black dot below and to the left of center. The dark area above the center of the solar disk is a sunspot.
Elongation is the angle between the Sun and the planet, with Earth as the reference point. Mercury appears close to the Sun.
Water ice (yellow) at Mercury's north polar region
MESSENGER being prepared for launch
Mercury transiting the Sun as viewed by the Mars rover Curiosity (June 3, 2014).
Estimated details of the impact of MESSENGER on April 30, 2015