Dear Remmi,

Mars is about 140 million miles from our cozy planet. A human mission to that cold, rocky neighbor would mean being away from Earth for about 3 years. But the things those explorers would see and learn would be mind-blowing.

I asked my friend Katie Cooper about that. She’s a scientist at Washington State University. She studies how geology and physics work on Earth and other places in space.

She told me the weirdest thing on Mars is a ginormous volcano.

“Olympus Mons is the largest volcano in the solar system,” Cooper said. “It dwarfs Earth’s volcanoes! Its massive size helps us learn about how they form, what might make a giant volcano, and what controls the size of volcanoes on Earth. Plus, wouldn’t it be wild to watch an eruption on Olympus Mons—from a safe distance, of course?”

If we could drive across Olympus Mons, we’d cover 370 miles. That’s the same size as the whole state of Arizona. The volcano is 16 miles tall. That’s about triple the height of the tallest mountain on Earth’s surface. All in all, Olympus Mons is about 100 times bigger than the biggest volcano on Earth.

An overhead view of Olympus Mons, a large crater with a darker area in the center
This is Olympus Mons. The image came from a space probe called the Viking orbiter. Image: NASA

There are actually a bunch of massive volcanoes on Mars. The biggest ones—like Olympus Mons—are shield volcanoes. Sometimes we think of volcanoes as super steep, magma-filled mountains. But shield volcanoes slope up gently. The shape is like a warrior’s shield lying on the ground. The volcanoes that make the islands of Hawai’i are also shield volcanoes.

On Earth, shield volcanoes form when super-hot magma bursts up from deep inside the planet. The magma—called lava when it’s out of the Earth—oozes out an opening called a vent. Then it flows down the slopes of the volcano. The lava cools and hardens on the slopes. That makes the volcano bigger every time it erupts.

Scientists think it’s pretty much the same on Mars.

“The basic molten materials—or rocks that they’re melting—are similar to Earth’s because we all formed in the same kind of process,” Cooper said. “But Mars is a different size than Earth. Mars has different gravity.”

Less gravity means everything is lighter on Mars. A 5th grader who weighs 75 pounds on Earth would weigh 28 pounds on Mars. That’s about the same as a 2-year-old.

So, it might take longer for magma to bubble up to the surface of Olympus Mons. The underground chambers of magma could be larger and deeper. When a volcano erupts on Mars, the lava would probably flow more slowly and for longer. It would likely cool more quickly and throw ash farther. All that might help explain how volcanoes get so big there.

A photo of a meteorite that looks like a pair of rocks with grey and green areas
Meteorites are one way scientists study Martian volcanoes. These Nahklite meteorites formed from Martian magma about 1.6 billion years ago. They bashed off that planet when something banged into Mars. They traveled all the way to Earth in 1911. Image: NASA

Nobody has seen a Martian volcano erupt. Scientists aren’t sure if the volcanoes on Mars are still active. They think Olympus Mons’ last big eruption was 25 million years ago. The newest evidence suggests there could be molten rock still moving under the planet’s surface.

That’s important info for scientists who want to send rovers or people to Mars. It could even help us figure out what’s going on with the ice and liquid water inside Mars. Based on what we know about life on Earth, that’s the best place to look for extraterrestrial life.

It turns out the biggest mountain in the solar system could help us volca-know so much. And that really rocks.

Sincerely,

Dr. Universe