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The first hints that the moon had an Earth-like interior came from NASA's Apollo missions. Data gathered by the lunar landers' instruments suggested that the celestial body was differentiated — or layered with denser material at the center and less dense material nearer the surface — as opposed to uniform rock all the way through. Apollo astronauts even left seismometers on the moon, which later revealed that it experiences moonquakes... However, scientists were only recently able to sort through the massive data sets from the Apollo missions and other lunar probes to get a clearer picture of the moon's insides....

For decades, astronomers assumed that Mars was geodynamically dead — a planet without rumbling earthquakes and erupting volcanoes. Though remnants of towering volcanoes exist on the surface of the Red Planet today, these colossal structures have been dormant for millions of years. With little to no heat firing the planet's engine, scientists reasoned, Mars became dormant long ago. However, over the last five years, this assumption has been proven wrong. NASA's InSight mission has detected quakes and even evidence of recent volcanism around one Martian region, known as Elysium Planitia. And now, they think they know why this activity is...