The Layers of Mars

The fate of almost everything on Earth’s surface is determined by infernal engines deep below. Mars is no different. Now, thanks to an intrepid robot parked on the Martian surface by NASA in November 2018, scientists have a map of our neighboring world’s geologic abysses, the first ever made of another planet. NASA’s InSight lander has been listening to marsquakes and tracking their seismic waves as they journey through the planet. A trio of papers published Thursday in the journal Science, using data InSight has collected, reveals the red planet to be something like a colossal candy treat imagined by a ravenous deity. Its crust is split into two or three layers of volcanic chocolate. The mantle below has a surprisingly sizable and rigid toffee-like filling. And the planet’s core is surprisingly light — less nougaty center, more syrupy heart. Paired with recent activities at the surface by new NASA and Chinese robotic rovers, these missions highlight stark differences between our blue world and the red one next door. This survey of the Martian insides has been a long time coming. Earth’s solidbut-squishy mantle was first glimpsed in 1889, when seismic waves from a quake in Japan dove in and out of the layer before emerging in Germany. Earth’s liquid outer core was discovered in 1914, and the solid inner core was revealed in 1936. Similar measurements of the moon were made when the Apollo astronauts left seismometers on its surface. Now the same basic and foundational measurements have been made on Mars.

But a handful emanate from deeper locales, ricocheting through the planet before reaching InSight. Seismic waves change speed and direction as they traverse different materials, so scientists could use these deep-seated quakes to see what’s going on inside Mars. It hasn’t been easy going. Working with a solitary seismometer means scientists get a decent look at just one region on Mars rather than the entire planet. And, to construct a detailed picture of the subsurface, plentiful powerful quakes that pass through much of the planet’s depths would be ideal. Unfortunately, Mars’s seemingly infrequent quakes are never more potent than a magnitude 4.0. “We just had to push forward and see what we could do with this data,”

Earlier missions to Mars have provided rough estimates of the dimensions and properties of its innards. But InSight’s seismological surveys provide precision. Models used to simulate the evolution of Mars can now be built on the foundations of these ground truths. Revelations from the InSight mission will also be useful for studying other worlds by providing scientists with an example that differs from Earth. “If you’re a doctor, and you only practice on one patient, you’re not going to be a very good doctor,” said Mark Panning, a planetary seismologist at NASA’s Jet Propulsion Laboratory in Pasadena, Calif., and a co-author on all three papers. Mars is more like a cousin of our planet than a sibling. Six times less voluminous, it is strangely small — and geochemical evidence suggests that “it’s this really ancient relic of the early solar system,” said Christine Houser, a seismologist at the Earth-Life Science Institute in Tokyo who was not involved with the research. Why is diminutive Mars so physically different from Earth and Venus, a planet thought of as Earth’s geologic twin? InSight’s forensic examination improves scientists’ chances at finding an answer — and, in the process, better understanding our planet’s place in the solar system. Over the past two years, the InSight lander has studied the red planet’s magnetism, its wobble as it orbits the Sun and the seismic waves created by its marsquakes. Most marsquakes occur at shallow depths. 16