Seismic action on Mars is revealing new details about the inner structure of the Red Planet.

Why it matters: Mars' interior holds the key to understanding how the planet and its atmosphere formed — and provides clues about how other rocky planets, like Earth, become habitable.

• "Mars is a second laboratory in planetary formation," says Bruce Banerdt, principal investigator of NASA's InSight mission to study the interior of Mars.
• "We're just starting to understand basic processes. If theories don’t explain the situation we find on Mars, we'll have to tweak them."

Driving the news: A trio of new studies published today in the journal Science reports the first direct measurements of the interior of Mars and, for that matter, another planet.

• The InSight mission measures seismic waves from fractures in Mars' surface thought to be caused by heat escaping the planet's still-cooling core.
• The low-frequency waves created by these Marsquakes are picked up directly by the seismometer or after they travel through the planet, bouncing off the core and surface.

What they found: The teams of researchers measured the strength and speed of the reflected waves from 8–11 Marsquakes to determine key aspects of the size and composition of Mars' interior.

Core: They estimateMars' core has a radius of about 1,140 miles, which is 100 miles larger than theories predicted and about half the radius of the planet.

• That means the density of the mostly iron and nickel core is less than previously thought and the center of Mars likely includes other elements, like sulfur, carbon, hydrogen and oxygen.
• The scientists also confirmed the core is still liquid, likely because of the other elements dissolved in it.
• The large size of Mars' core creates a "shadow zone" so that some waves don't reach the seismometer. Banerdt says he hopes future missions can capture more data by going to a different region of Mars and placing another seismometer on the planet.
• The intrigue: The estimated size of the core implies it is 15–18% sulfur, about twice what can be accounted for in existing models, according to Banerdt. "We don't know how to get that much sulfur," he says, adding it may have to do with the density of the mantle.

Mantle: Mars' middle layer, which insulates the core, is relatively thin, which may explain why the planet cooled more quickly than Earth and lost its protective magnetic field.

• On top of the mantle, there is a thick lithosphere — the rigid outer shell that makes up the crust and upper mantle — about 310 miles below the surface, the researchers report. The thickness of the lithosphere might explain why plate tectonics — or volcanic activity — isn't seen on the Red Planet today.
• The mantle, which the data suggest consists of just one rocky layer compared to Earth's two, makes up the bulk of the planet. "We have to understand the mantle to understand the planet," Banerdt says.

Crust: The outermost layer of the planet is estimated to be between 15 and 45 miles thick — not as thick and dense as researchers earlier predicted from satellite data — and made up of two or three layers.

• The researchers also found the crust is 13–21 times more enriched in radioactive elements that produce heat, which hints at its composition and how it formed through early volcanic activity.

Background: NASA's InSight lander touched down on the Elysium Planitia region of Mars in November of 2018 and the seismometer began taking measurements in February 2019.

• A key instrument designed to drill into the martian soil and measure how much heat is flowing from the planet's interior to its surface failed multiple times before NASA called it quits on the probe earlier this year.
• How a planet loses heat — from radioactive elements decaying or from the cooling of its core — influences its volcanic and tectonic activity. Those measurements would have helped to independently validate the seismometer data, but Banerdt says they're able to self-verify the seismometer system in other ways.

What's next: InSight received a mission extension until at least the end of 2022.

• In that time, Banerdt says they'll focus on detecting more quakes, collecting data to understand seasonal and annual variations in weather, and looking at whether some types of seismic events vary between seasons on the planet.