Mercury - 10 Facts for Kids

Mercury - 10 Facts for Kids
Enhanced-color image of Mercury. Credit: NASA/JHUAPL/Carnegie Institution of Washington

Mercury is close enough to the Sun for lead to melt during the day. Yet there is ice at the poles. Before we had space probes, Mercury was a mystery hidden in the Sun's glare, but that's changed.

1. Mercury is the smallest planet in the Solar System.

Now that Pluto is a dwarf planet, Mercury is the baby of the planets. Even Jupiter's moon Ganymede and Saturn's moon Titan are bigger than Mercury.

2. Mercury probably used to be bigger than it is now — and it's still getting smaller.

Mercury is like a Moon-sized metal ball with a rocky crust. The core is made of iron, and it's most of the planet. This suggests that Mercury used to be bigger, and that a collision made it lose an outer layer. It's also shrinking, though extremely slowly. As the hot core cools and contracts, Mercury's outer layer shrinks.

3. Mercury is so close to the Sun that it's hard to observe.

We used to know almost nothing about Mercury. Astronomers have to observe Mercury at twilight when it's low in the sky. But its features aren't clear then. Also half the planet had never been seen until 2008 when NASA's Messenger probe took pictures. Interestingly, 13 times a century we can see the disk of Mercury cross in front of the Sun. This is called a transit, and the most recent one was on November 11, 2019.

4. Astronomers used to think that Mercury didn't turn on its axis.

Since Mercury is close to the Sun, we could expect it to keep the same side facing the Sun, the way the Moon does as it orbits Earth. However, in 1965, astronomers studied the planet with radar, and discovered that it rotates three times every two years.

5. Mercury has a very elliptical orbit. An ellipse is a squashed circle.

The average distance between Mercury and the Sun is 58 million km (36 million miles). But it gets as far away as 70 million km (40 million mi) and as near as 46 million km (29 million miles). If you could stand on Mercury when it's closest to the Sun, the Sun would look three times as big as it does from Earth.

6. Mercury is the fastest-moving Solar System planet.

Mercury's average speed is 107,000 km per hour (67,000 miles per hour). It's so close to the Sun that it has to move fast to keep from being pulled in by the Sun's gravity. If you could move that fast, you could get all the way around the Earth in less than 25 seconds. No wonder the planet is named after Mercury, the swift Roman messenger god.

7. A day on Mercury is longer than a year on Mercury.

The time it takes a planet to go around the Sun is a year. A year on Mercury is very short, only 88 Earth days long. One way of measuring a day is the time from one noon to the next noon. On Earth that's 24 hours. However, on Mercury it's 176 Earth days. That means that a Mercury day is two Mercury years long.

8. Mercury has enormous temperature changes during a day.

The day side can heat up to 430°C (800°F). That's hot enough to melt lead. On the night side, the temperature can drop to -179°C (-290°F). This is as cold as some of the moons of Saturn, even though they are more than twelve times farther from the Sun than Mercury is.

9. There is ice on Mercury.

NASA's Messenger probe detected ice at the poles. There are places on Mercury that never get direct sunlight, so they always stay extremely cold.

10. One of the biggest impact craters in the Solar System is on Mercury.

Mercury is cratered like the Moon. Something pretty big hit Mercury in its early days to make a hole the size of the Caloris Basin. This impact crater is about 1550 km (960 miles) across. That's more than the distance from New York City to Chicago or from London to Rome.

You Should Also Read:
Transit of Mercury
Absolute Beginners - Seeing Mercury and Venus
Jupiter's Moons - Facts for Kids

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