Mercury vs Earth: How Different Are They?

Earth is the planet you know best. So the easiest way to understand Mercury is to put it next to Earth and see how different it really is.

Spoiler: very different. Mercury is much smaller, much hotter (and colder), much closer to the Sun, and almost entirely without air. Mercury also has no moon, no seasons, no weather, and no oceans. About the only thing Mercury and Earth share is that they're both rocky planets — and even there, Mercury is mostly metal.

Quick Comparison: Mercury vs Earth

Size: Mercury Is Much Smaller

Mercury's diameter is about 3,032 miles (4,879 km). Earth's diameter is about 7,918 miles (12,742 km). Mercury is only 38% the width of Earth.

If Earth were the size of a basketball, Mercury would be about the size of a tennis ball. Mercury is so small that it's actually only a tiny bit bigger than Earth's Moon. Two of Jupiter's and Saturn's moons (Ganymede and Titan) are larger than the planet Mercury.

But Mercury is heavy for its size. Mercury's density is 5.43 g/cm³, just slightly less than Earth's 5.51 g/cm³, which makes Mercury the second densest planet after Earth. That's because most of Mercury is solid iron. The iron core takes up about 85% of Mercury's radius. Earth's iron core is only about 17% of its radius. Mercury is essentially a giant metal ball with a thin rocky shell.

How Far Is Mercury From Earth?

This one's tricky, because the distance changes all the time. Both planets are moving, and on different orbits.

• When Earth and Mercury are on the same side of the Sun, they can come as close as 48 million miles (77 million km) apart.
• When they're on opposite sides of the Sun, they can be as far as 138 million miles (222 million km) apart.

Surprisingly, scientists have shown that Mercury is actually Earth's closest neighbor on average. That's because Venus, which gets a lot closer to Earth at its closest, also gets much farther away when it's on the far side of the Sun.

Gravity: How Much Would You Weigh on Mercury?

Mercury's gravity is 38% as strong as Earth's. To find out what you'd weigh on Mercury, multiply your Earth weight by 0.38.

• If you weigh 60 lbs on Earth, you'd weigh about 23 lbs on Mercury.
• If you weigh 100 lbs on Earth, you'd weigh about 38 lbs on Mercury.
• If you weigh 150 lbs on Earth, you'd weigh about 57 lbs on Mercury.

Your jumps would also be much higher. A jump that takes you 1 foot off the ground on Earth would launch you almost 3 feet up on Mercury, and you'd hang in the air longer too. The same is true for any ball you threw, kicked, or hit. A baseball home run on Earth would clear the entire stadium on Mercury.

Day, Year, and Seasons: Totally Different

This is where Mercury gets really weird.

A year on Mercury is just 88 Earth days — way shorter than an Earth year. But a day on Mercury (from sunrise to sunrise) is 176 Earth days — about half a year on Earth, and twice as long as a Mercury year.

So if you lived on Mercury, you'd celebrate a new birthday every three Earth months, but you'd only see the Sun rise twice in a Mercury year.

Mercury also doesn't have seasons. Earth has seasons because it tilts 23.5 degrees on its axis, so different parts of the planet point toward the Sun at different times of year. Mercury barely tilts at all — only 2 degrees — so its seasons are basically nonexistent. Read more about a day on Mercury.

Atmosphere: Earth Has One, Mercury Barely Does

Earth's atmosphere is what makes our planet livable. It's mostly nitrogen and oxygen, with traces of other gases. It traps heat at night, blocks dangerous radiation, and burns up most meteors before they hit the ground. Without it, Earth would be a much harsher place — a lot like Mercury, in fact.

Mercury has what scientists call an exosphere, which is so thin it's basically a vacuum. It's made of small amounts of oxygen, sodium, hydrogen, helium, potassium, and calcium — atoms that get knocked off the surface by sunlight and tiny meteoroids.

Because Mercury's atmosphere is so thin:

• The sky stays black at noon.
• Day-to-night temperature swings 1,100°F.
• Meteoroids hit the surface directly, so the planet is covered in craters.
• Sound can't travel — it would be silent on Mercury.

Read more about Mercury's almost-non-existent atmosphere.

Could You Survive on Mercury?

No. Mercury is one of the most dangerous places in the solar system for a human.

• The day side would cook you — 800°F is hot enough to melt metal, never mind a person.
• The night side would freeze you — -290°F is colder than anywhere on Earth.
• You couldn't breathe — the exosphere has no breathable air at all.
• Solar radiation is intense — with no atmosphere or strong magnetic field to block it, the Sun's radiation would damage your cells.

Astronauts who ever visit Mercury will need a fully sealed, heavily shielded habitat — probably one buried under regolith (Mercury's rocky surface dust) or built inside one of those permanently shadowed polar craters where the temperature is at least stable. More on whether humans could ever live there.

The One Thing They Share

For all their differences, Mercury and Earth are both terrestrial planets. That means they're both small, rocky, and have a metal core, in contrast to the gas giants like Jupiter and Saturn. Mercury, Venus, Earth, and Mars all formed from the same dusty inner part of the solar system about 4.5 billion years ago, made of similar materials.

The differences you see today — Earth's oceans, Mercury's craters, Venus's clouds, Mars's red dust — come down to a few key things: how big each planet is, how far it sits from the Sun, and what kind of atmosphere it managed to keep.

Mercury, the smallest and the closest, kept the least.

Frequently Asked Questions