NASA-funded researchers said Thursday they had mapped the interior of Mars, using seismic data collected by the agency’s Mars InSight lander to reveal a planet with a molten core whose size and composition came as major surprises.

The interior map—the first ever created of another planet—shows that the internal structure of Mars differs dramatically from Earth’s. Mars has a thicker crust and a thinner underlying mantle layer as well as a core that is bigger, less dense and more liquid than the researchers had expected.

The scientists said their findings, which were described in three papers published Thursday in the journal Science, suggest that Mars formed millions of years before Earth, when the sun was still condensing from a cloud of glowing gas.

“It gives us our first sample of the inside of another rocky planet like Earth, built out of the same materials but very, very different,” Sanne Cottaar, a seismologist at the U.K.’s University of Cambridge, said of the new research. “It is impressive.”

Dr. Cottaar, who wasn’t involved in the new research, called the findings “a major leap forward in planetary seismology.”