Mercury (Planet) — Facts, Moons, Atmosphere, and Exploration
Mercury is the smallest planet and the closest one to the Sun. It has a heavily cratered surface, almost no true atmosphere, and very large temperature changes between daytime and nighttime.
Quick Facts
| Planet type | Terrestrial (rocky) |
|---|---|
| Mean radius | 2,440 km |
| Mass | 0.33 × 10^24 kg |
| Gravity | 3.7 m/s² |
| Average temperature | 167°C |
| Length of day | 59 Earth days |
| Length of year | 88 Earth days |
| Number of moons | 0 |
| Rings | No |
| Distance from Sun (AU) | 0.39 |
What Makes Mercury Unique
Mercury is unusually dense for its size, which suggests it has a large metal core relative to its rocky mantle. Its orbit is also quite eccentric, so the distance from the Sun changes noticeably during one year.
The planet rotates slowly but orbits quickly. That combination creates a special 3:2 spin-orbit resonance: Mercury rotates three times for every two trips around the Sun.
Atmosphere and Weather
Mercury does not have a thick atmosphere like Earth or Venus. Instead, it has a very thin exosphere made of atoms knocked off the surface by sunlight and the solar wind. Because there is little air to trap heat, daytime can be extremely hot while nighttime can be extremely cold.
Weather in the usual Earth sense does not exist there. There are no clouds, no rain, and no winds moving through a dense atmosphere.
Moons and Rings
No known moons or rings orbit Mercury. Being so close to the Sun and relatively small makes long-term moon capture less likely.
Exploration Highlights
Mariner 10 delivered the first close-up views in the 1970s, showing a cratered world. Decades later, MESSENGER orbited Mercury and mapped nearly the whole planet, finding evidence of water ice in permanently shadowed polar craters. The BepiColombo mission is currently improving our understanding of Mercury's interior, magnetic field, and surface chemistry.
If You Like Mercury
If Mercury's extremes interest you, continue with Venus for greenhouse heating, Mars for a colder rocky world, and Earth for a life-supporting atmosphere.
Is Mercury an Inner or Outer Planet?
Mercury is an inner planet. It belongs to the four rocky planets closest to the Sun: Mercury, Venus, Earth, and Mars. You can compare the full groups in this guide to inner and outer planets.
Mercury Radius and Size
Mercury has an approximate mean radius of 2,440 kilometers. That makes it much smaller than Earth, with a radius a little over one third of Earth's 6,371 kilometers.
For a quick side-by-side view of every major world, see How Big Are the Planets Compared to Earth?. Mercury is the clearest example of how compact a planet can be while still remaining a major world.
Does Mercury Have an Atmosphere?
Mercury does not have a thick atmosphere like Earth or Venus. Instead, it has a very thin exosphere made of atoms such as oxygen, sodium, hydrogen, and helium that are constantly added and lost to space.
FAQ
Why is Mercury so cratered?
Its old rocky surface has been exposed to impacts for billions of years, and there is little atmosphere or active geology to erase craters quickly.
Can humans stand on Mercury?
Not with current technology. Extreme heat, cold, and intense solar radiation make human missions very challenging.
Does Mercury have ice even though it is near the Sun?
Yes. Some deep polar craters never receive direct sunlight, allowing water ice to remain in permanent shadow.
Is Mercury an inner planet?
Yes. Mercury is one of the four inner planets and the one closest to the Sun.
What is the radius of Mercury?
Mercury's mean radius is about 2,440 kilometers, so it is much smaller than Earth.
Does Mercury have an atmosphere?
Mercury does not have a thick atmosphere. It has a very thin exosphere instead.