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 toffeelike 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 solid-but-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. This work, conducted with one of the most technologically advanced seismometers ever built, represents “a major leap in planetary seismology,” said Paula Koelemeijer, a seismologist at Royal Holloway, University of London who was not involved in the research but co-wrote a perspective article in Science.
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. 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 is going on inside Mars.
It has not 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’ 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,” said Brigitte Knapmeyer-Endrun, a planetary seismologist at the University of Cologne and lead author of the paper on the Martian crust. Despite the challenges, the team managed to make a detailed X-ray of Mars’ guts.
InSight also found that Mars’ mantle as a whole is about half as thick as Earth’s — a lack of insulation that would have exacerbated Mars’ heat loss as it erupted heavily in its youth. (Mars’ small size also allowed plenty of its primordial heat to radiate into space.)
This thin mantle may also partly explain why Mars lost its protective magnetic field in the first 700 million years of its history. Earth’s magnetic field is powered by the circulation of iron-nickel currents within its liquid outer core. Presumably, Mars had a similar circulation, but the speedy cooling of its innards caused those currents to seize up, shutting off its magnetic dynamo.
Without a magnetic bubble to shield Mars from the sun’s radiation, its atmosphere was blown away like confetti. Water that once frequented its surface — if it was not soaked up by the rocks below — escaped into space, turning it into a frigid, irradiated desert.