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Seismic data reveal the boundary from the crust to the core



Structure inside Mars

An artist̵

7;s impression of Mars’ internal structure. The top layer is the shell, and below it is the mantle, which sits on a solid inner core. Credit: Photos provided by NASA / JPL-Caltech)

Using data from NASAInSight Lander above MarsRice University’s seismologists have made the first direct measurements of the three subsurface boundaries from the crust to the core of the red planet.

InSight deploys its wind and heat shields

This February 2, 2019 photo shows the robotic arm aboard NASA’s InSight lander deploying a dome-shaped shield that shields the lander’s seismograph from wind, dust, and extreme temperatures. Credit: Image courtesy of NASA / JPL-Caltech

“Ultimately it could help us understand planet formation,” said Alan Levander, co-author of a study published this week. Letters of geophysical research. While the Martian crust’s thickness and the depths of its core have been calculated with some models, Levander said InSight data allows for the first direct measurements, which can be used to test the models and finally improve them.

“In the absence of plaque tectonics on Mars, its early history is largely preserved relative to Earth,” said study co-author Sizhuang Deng, a Rice graduate student. “Estimates of the depths of the seismic boundary on Mars could provide guidance for a better understanding of its past and the formation and evolution of terrestrial planets in general.”

Finding clues about Mars’ interior and its forming processes are the main targets of InSight, a robotic lander that landed in November 2018. The probe’s dome-shaped seismoscope allows scientists to listen to the noises inside the planet, in many ways doctors can listen to the patient’s heartbeat with a stethoscope.

Seismometers measure vibrations from seismic waves. Like ripples marking the location where a pebble disturbs the surface of a pond, seismic waves flow through the planets, marking the location and size of the disturbances such as meteor impact or earthquakes, cleverly known as Mars on the red planet. InSight’s seismometer recorded more than 170 of these between February and September 2019.

Mars Mosaic Viking Orbiter

A mosaic of Mars is made up of 102 Viking Orbiter images. Credit: Image courtesy of NASA / JPL-Caltech

The seismic waves are also subtly altered as they pass through different rocks. Seismologists have been studying patterns in seismic records on Earth for over a century and can use them to map the locations of oil and gas fields and much deeper strata.

“The traditional way to investigate structures below Earth is to analyze earthquake signals using dense networks of seismic stations,” Deng said. “Mars is much less tectologically active, which means it will have fewer Martian events than Earth. Moreover, with just one seismic station on Mars, we cannot use methods based on seismic networks.

Levander, Rice’s Professor of Earth, Environment and Planetary Science, and Deng analyzed InSight’s 2019 seismic data using a technique called ambient noise autocorrelation. “It uses continuous noise data captured by a Mars seismic station to extract clearly reflected signals from seismic boundaries,” Deng said.

The first boundary Deng and Levander measured was the split between the Mars crust and crust nearly 22 miles (35 km) below the lander.

The second zone is the mantle transition zone where iron magnesium silicate undergoes a geochemical change. Above the region, the elements form a mineral called olivine, and below it, heat and pressure compress them into a new mineral called wadsleyite. Known as olivin-wadsleyite transition, this site has been found 690-727 miles (1,110-1,170 km) below InSight.

“The temperature in the olivine-wadsleyite transition is the key to building Mars’ thermal models,” Deng said. “From the depth of the transition we can easily calculate the pressure, and from there, we can calculate the temperature.”

The third boundary he and Levander measured was the boundary between the Martian crust and its rich iron core, which they found about 945-994 miles (1,520-1,600 km) below the lander. A better understanding of this boundary, says Deng, “can provide information about the development of the planet from both a chemical and a thermal point of view.

Ref: “Mars’ Autocorrelation” by Sizhuang Deng and Alan Levander, August 4, 2020, Letters of geophysical research.
DOI: 10.1029 / 2020GL089630

The research was supported by Rice’s Department of Earth, Environmental and Planetary Sciences.




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