Planet field guide · For families
Mercury: the small planet with a very long day
Mercury is the smallest planet and the closest planet to the Sun. It is rocky, heavily cratered and full of surprises. This guide gives young explorers the science, while offering adults useful questions and a simple activity to try together.
Mercury at a glance
- Place from the Sun
- First of the eight planets
- Year
- 88 Earth days to orbit the Sun once
- Solar day
- About 176 Earth days from one noon to the next
- Moons
- Zero
- Surface
- Rocky, with many impact craters
A “solar day” means the time from one local noon to the next. Mercury spins once in about 59 Earth days, but it also races around the Sun. Those two motions combine to make its noon-to-noon day last about 176 Earth days—two Mercury years.
Why isn't Mercury the hottest planet?
Mercury is closest to the Sun, so its sunlit ground gets extremely hot. But Mercury has almost no atmosphere to spread or hold heat. When a place on the planet turns away from sunlight, its surface loses heat and becomes bitterly cold.
Venus is farther from the Sun, yet it is the hottest planet. Its thick atmosphere traps heat very effectively. This is a useful reminder that distance from the Sun is only part of a planet's temperature story: an atmosphere matters too.
Ice in a world of extremes
Some deep craters near Mercury's poles never receive direct sunlight. Their shadowed floors can stay cold enough for water ice to remain there, even though nearby sunlit ground can become scorching hot. NASA's MESSENGER spacecraft gathered evidence for water ice in these permanently shadowed regions.
The ice is not spread over a sunny landscape like snow on Earth. It is hidden in very cold places inside polar craters. Scientists study it with spacecraft measurements and radar observations, because visiting and sampling those dark crater floors is difficult.
Mercury's almost-empty sky
Mercury does not have a thick atmosphere like Earth. It has a very thin exosphere: a sparse collection of atoms around the planet. Some atoms are knocked off the surface by sunlight, charged particles from the Sun and tiny space rocks. Many do not stay for long.
That thin exosphere cannot keep the surface warm or protect it from impacts in the way Earth's atmosphere does. The craters on Mercury preserve a long record of objects striking its rocky surface.
How spacecraft explored Mercury
Mercury is challenging to study from nearby because spacecraft travelling close to the Sun face intense heat and strong gravity from the Sun. NASA's MESSENGER mission entered orbit around Mercury in 2011 and mapped the planet, measuring its surface, magnetic field and thin exosphere. The mission ended in 2015 after the spacecraft used its remaining fuel and impacted the planet.
Before MESSENGER, Mariner 10 flew past Mercury three times in 1974 and 1975, sending back the first close-up views. Together, these missions turned Mercury from a blurry point in the sky into a world that scientists could map and investigate.
Try this together
Make a crater-shadow model
- Put a ball or orange on a table in a dim room. Ask an adult to hold a torch a safe distance away.
- Shine the torch on the ball to represent sunlight. Look for a dent, crease or small hollow that stays dark.
- Turn the ball slowly. Notice how some places move into the light while a deep hollow can remain shadowed.
- Talk about how a permanently shadowed crater on Mercury could stay much colder than sunlit ground.
For grown-ups: This model shows the idea of a shadow, not Mercury's scale, shape or actual temperature. Never point a torch or any other light into someone's eyes, and never look directly at the Sun.
How we check the science
Gavin, Denny and their missions are fictional. Planet facts are checked against primary science sources before they are used in a guide. We keep the language simple for children, and we link to the source material so parents and educators can explore further.
- NASA Science: Mercury facts — orbit, rotation and planet overview.
- NASA Science: MESSENGER mission — mission history and discoveries.
Science pages can change as missions return new evidence. We will update this guide if its sources or the scientific understanding change.