NOTICE: This summary has been updated to correct misspellings of the last names of both Perspective authors. The Perspective authors are Sanne Cottaar and Paula Koelemeijer. We apologize for our error.
The first direct seismic observations from NASA's InSight lander, presented in three studies in this issue, provide clues to the composition of Mars. Researchers across these studies report preliminary findings from the Insight mission and begin to map - for the first time - the interior of a planet apart from Earth. "These three studies provide important constraints on the present-day structure of Mars and are also key for improving our understanding of how the planet formed billions of years ago and evolved through time," write Sanne Cottaar and Paula Koelemeijer in a related Perspective. Studying a planet's interior layers - its crust, mantle and core - can reveal key insights into its formation and evolution, as well as uncovering any geomagnetic and tectonic activity it hosts. Such deep interior regions can be probed by measuring the waves that travel through the planet's body following seismic events like a quake. Such methods have been instrumental in surveying the internal characteristics of Earth.
In early 2019, NASA's Martian lander InSight (Interior Exploration using Seismic Investigations, Geodesy and Heat Transport) began to detect and record marsquakes from its position on the surface of Mars, including several subcrustal quakes that resemble tectonic events on Earth. Brigitte Knapmeyer-Endrun and colleagues used marsquakes and ambient seismic noise to image the structure of the Martian crust below the InSight landing site, finding evidence for a multi-layered crust with either two or three interfaces. Extrapolating this data to the entire planet, Knapmeyer-Endrun et al. show how the average thickness of Mars' crust lies between 24 and 72 kilometers (km). Amir Khan et al. used direct and surface reflected seismic waves from eight low-frequency marsquakes to probe deeper and reveal the structure of Mars' mantle to a depth of nearly 800 km. Their findings suggest that a thick lithosphere lies close to 500 km below the surface and, like the Earth, likely has a low-velocity layer beneath it. According to Khan et al., Mars' crustal layer is likely highly enriched in heat-producing radioactive elements, which heats this region at the expense of the planet's interior. Deeper still, Simon Stähler and colleagues used the faint seismic signals reflected off the Martian core-mantle boundary to investigate the Martian core. They found that Mars' relatively large liquid metal core has a radius of nearly 1,830 kilometers and begins roughly halfway between the surface and the center of the planet, suggesting that the planet's mantle consists of only one rocky layer, rather than two, like in Earth. According to Stähler et al., the findings indicate that the iron-nickel core is less dense than previously thought and enriched in lighter elements. "Direct seismic observations on Mars represent a major leap forward in planetary seismology," write Cottaar and Koelemeijer. "Over the coming years, as more marsquakes are measured, scientists will refine these models of the red planet and reveal more of Mars' enigmatic mysteries."