Mars vs Earth — Side by Side

Mars and Earth are almost twins. Almost.

If you lined up the eight planets and asked which one looks most like Earth, the honest answer is Mars. Both are rocky. Both spin on a tilted axis. Both have polar ice caps, four seasons, dry land, weather, and clouds. From far away, they're recognizably the same kind of world.

Up close, the differences pile up fast. Mars is colder, smaller, drier, and has air you couldn't survive a single breath of. Here's exactly how the two planets stack up.

Size and Mass

Earth is the bigger of the two. Mars's diameter is about 6,791 km (4,219 miles), which is roughly half of Earth's 12,742 km (7,918 miles). If Earth were a tennis ball, Mars would be a golf ball.

Mars also weighs a lot less. It has only about 11% of Earth's mass, partly because it's smaller and partly because its core has less iron in it. That smaller mass is the reason Martian gravity is much weaker than ours.

Gravity

Stand on Mars and you'd weigh 38% of what you weigh on Earth. A child who weighs 80 pounds on Earth would weigh just 30 pounds on Mars. Adults could jump nearly three times higher and lift much heavier things. (Astronauts who've trained on simulators say it actually feels disorientingly bouncy.)

That low gravity sounds fun, but it's also one of the biggest problems for sending humans to Mars long-term. Astronauts who spend many months in low gravity lose bone density and muscle mass, even with daily exercise. We don't yet know what spending years on Mars would do to a human body.

Length of Day

This one is genuinely strange. A day on Mars is 24 hours and 37 minutes long. Just 37 minutes longer than a day on Earth.

Out of all the planets in the solar system, Mars is the closest match to Earth's day length. Mercury's day lasts 59 Earth days. Venus's day lasts 243. Jupiter spins so fast a day there is under 10 hours. Mars sits right next to Earth at roughly 24 hours, almost as if someone planned it.

Scientists who work with Mars rovers actually live on "Mars time" while they're operating them — going to bed 37 minutes later each night to stay synced with the rover's daylight hours.

Length of Year

Here the planets diverge. Mars takes 687 Earth days to circle the Sun, almost twice as long as Earth. So if you lived on Mars, your birthday would only roll around every two Earth years.

Both planets have four seasons, because both tilt on their axes at almost the same angle (Earth at 23.4°, Mars at 25°). But because the Mars year is twice as long, each Mars season lasts about twice as long as ours.

Atmosphere and Air Pressure

This is the big one.

Earth's atmosphere is a thick blanket of nitrogen (78%) and oxygen (21%), with a little bit of everything else. We can breathe it. Plants can grow in it. It traps heat, blocks dangerous radiation, and keeps liquid water on the surface.

Mars's atmosphere is almost the opposite. It's 95% carbon dioxide, with tiny amounts of nitrogen, argon, and barely any oxygen at all (around 0.16%). And it's about 100 times thinner than Earth's.

Even if Mars's air had oxygen in it, the pressure is so low you couldn't survive — your blood would actually start to bubble in your veins. That's why every drawing of an astronaut on Mars shows them in a sealed spacesuit. They need it to stay alive.

Temperature

Mars is freezing. The average temperature is about –63 °C (–81 °F). On a sunny summer day at the equator it can climb to a chilly 20 °C (70 °F), and on a winter night near the poles it drops to –153 °C (–225 °F).

Earth's average temperature is about 15 °C (59 °F). The two planets get different amounts of sunlight (Mars gets less than half as much, because it's farther from the Sun), but the bigger reason Mars is so cold is that thin atmosphere again. There just isn't enough air to trap any of the heat the surface picks up during the day.

Water

Earth is covered in liquid water. Oceans, rivers, lakes, rain.

Mars has water too — but almost all of it is frozen. There's water ice in the polar caps and just below the surface across most of the planet. In 2018, scientists found what looks like a lake of liquid water buried under the south polar ice cap, and there's some salty liquid water that occasionally seeps down crater walls in summer. But you wouldn't find anything like an ocean on Mars today.

Long ago, things were different. Around 3.5 billion years ago, Mars probably had rivers, lakes, and possibly even a shallow ocean covering big parts of the northern hemisphere. The dried-up riverbeds and ancient deltas are still there, mapped from orbit and visited by rovers.

Moons

Earth has one big moon. Mars has two tiny ones.

Phobos and Deimos are nothing like our Moon. They're small, lumpy, and shaped like potatoes — both probably captured asteroids. Phobos is only 22 km wide, Deimos just 12 km. Earth's moon is over 3,400 km across.

Quick Comparison Table

So Could Mars Become Like Earth?

Some scientists think Mars could one day be made more Earth-like. The idea is called terraforming — slowly thickening Mars's atmosphere, warming the surface, and melting the ice into water.

The catch is that it would take thousands of years even with technology we don't have yet. And without a magnetic field to protect the new atmosphere, the Sun would just strip it away again. So unless we find a way to give Mars a magnetic field, terraforming is more science fiction than science fact for now.

Still, the comparison shows just how lucky Earth is. Mars probably had everything it needed to be a second living world once. Then it lost its magnetic field, lost its air, and froze. Earth held on to all of it.

Frequently Asked Questions