What’s it Like Inside a Super-Earth?

We know a ton about the inside of Earth. We know it has both an inner core and an outer core and that the churning and rotation create a protective magnetosphere that shields life from the Sun’s radiative power. It has a mantle, primarily solid but also home to magma. We know it has a crust, where we live, and plate tectonics that moves the continents around like playthings.

But what about Super-Earths? We know they’re out there; we’ve found them. What do we know about their insides? Earth’s structure, and its ability to support life, are shaped by the extreme pressure and density in its interior. The pressure and temperature inside Super-Earths are even more powerful. How does it shape these planets and affect their habitability?

Super-Earths are more massive planets than Earth but not as massive as our Solar System’s ice giant planets, Uranus and Neptune. Uranus and Neptune are 14.5 and 17 times more massive than Earth. Generally, a Super-Earth is about two times more massive than Earth to ten times more massive than Earth. There are no firm definitions for Super-Earths, and different sources use different mass categories.

Of the almost 5,000 confirmed exoplanets, NASA calls 1539 of them Super-Earths. Super-Earths are defined by their masses, not by their composition or other characteristics. There are a lot of them, and some of our nearest stellar neighbours appear to host Super-Earths. There are different classifications for Super-Earths according to their densities and compositions. But many of them have similar densities to Earth and are likely compositionally similar.

This screenshot from the NASA exoplanets website shows the types of planets discovered beyond our Solar System. Image Credit: NASA

A team of researchers at Carnegie University and other institutions investigated the effect of extreme pressure and temperature on minerals inside Super-Earths. They performed lab experiments to simulate their interiors. They subjected minerals to extreme pressures and temperatures to see what would happen to them in the mantles of this common type of planet.

The paper is “Ultrahigh-pressure disordered eight-coordinated phase of Mg2GeO4: Analogue for super-Earth mantles.” The lead author is Rajkrishna Dutta, a post-doctoral fellow at Carnegie’s Earth and Planets Laboratory. The paper is published in the Proceedings of the National Academy of Sciences.

Earth’s interior dynamics support life in different ways. The magnetosphere generated by the core is one way. The magnetosphere directs harmful solar radiation away from the planet’s surface and prevents the solar wind from stripping away the atmosphere.

On Earth, plate tectonics and mantle convection have a thermostatic effect on the climate. Volcanoes release heated material and CO2 into the Earth’s atmosphere, stopping Earth from getting too cold. The same processes regulate the amount of CO2 by subducting carbonates back into rock with the help of rainfall. Plate tectonics also creates the complex chemistry necessary for life. So scientists think that plate tectonics and mantle convection play a critical role in life’s appearance and Earth’s ongoing habitability.

How would the extreme conditions inside a Super-Earth affect its habitability?

“The interior dynamics of our planet are crucial for maintaining a surface environment where life can thrive—driving the geodynamo that creates our magnetic field and shaping the composition of our atmosphere,” explained Carnegie’s Rajkrishna Dutta, the paper’s lead author. “The conditions found in the depths of giant, rocky exoplanets such as super-Earths would be even more extreme.

Silicate minerals make up most of Earth’s crust. The high temperature and pressure exerted on silicate minerals create key boundaries between the upper and lower mantle deep inside our planet. Studies of rocky exoplanets show they might also have silicate crusts. They exhibit the same density, roughly, as Earth.

Since Super-Earths can be so much more massive than Earth, the temperature and pressure inside them would be even more extreme than on Earth. The researchers wanted to probe those conditions and the effect they have on silicate minerals. They wanted to know if new types of silicates would emerge and if they would behave differently.

Silicate minerals make up most of the Earth’s mantle and are thought to be a significant component of the interiors of other rocky planets. On Earth, the structural changes induced in silicates under high pressure and temperature conditions define key boundaries in Earth’s deep interior. The research team was interested in probing the emergence and behaviour of new forms of silicate under conditions mimicking those found in distant worlds. The illustration is courtesy of Kalliopi Monoyios.

Under normal conditions, most silicates are organized into the same orientation called a tetrahedral structure. A tetrahedral structure has one central atom bonded with four other atoms.

This is a 3D ball model of a tetrahedral molecule. A central atom is located at the center, with four substituents situated at the corners of a tetrahedron. Image Credit: Public Domain, https://commons.wikimedia.org/w/index.php?curid=1454649

Mg2SiO4, also known as Forsterite, is one of the most abundant silicate minerals in the Earth’s mantle above about 400 km (250 miles.) It’s likely abundant in rocky Super-Earths, too. Modelling shows that new phases of silicates emerge in extreme temperatures and pressures inside Super-Earths, but there’s no way to observe them. Calculations show that it requires about 490 GPa of pressure for new silicate phases to emerge. But there’s no way to simulate that pressure.

Fortunately, scientists can use an analog for silicates that responds the same way but at less extreme temperature and pressure. That analog is germanium. Specifically, it’s magnesium germanate, or Mg2GeO4. Calculations show that magnesium germanate also changes to a new phase at high pressure, but the threshold is lower. New phases emerge at about 175 GPa, and that pressure can be created in a lab.

The research team used a diamond anvil to subject Mg2GeO4 samples to extreme pressure, then heated them with a laser. They exposed the magnesium germanate to two million times Earth’s normal atmospheric pressure and watched as a new crystalline structure emerged.

At 2 million atmospheres, the central germanium atom bonded with eight oxygen atoms instead of four. The new mineral is called an “eight-coordinated, intrinsically discorded mineral,” and it could substantially affect the internal temperature and dynamics of Super-Earths.

Working with magnesium germanate, Mg2GeO4, analogous to one of the mantle’s most abundant silicate minerals, the team gleaned information about the potential mineralogy of super-Earths and other giant, rocky exoplanets. Under about 2 million times normal atmospheric pressure, a new phase emerged with a distinct crystalline structure that involves one germanium bonded with eight oxygens. Scientists expect the new eight-coordinated, intrinsically discorded mineral to strongly affect these planets’ internal temperature and dynamics. Image courtesy Rajkrishna Dutta.

“The discovery that under extreme pressures, silicates could take on a structure oriented around six bonds, rather than four, was a total game-changer in terms of scientists’ understanding of deep Earth dynamics,” explained study co-author Sally June Tracy. Tracy refers to the discovery of silicate-perovskite (now called bridgmanite) and post-perovskite. Perovskite is a mineral structure that only forms under high pressure. It’s not stable at Earth’s surface and mainly exists in the lower part of Earth’s mantle. Scientists first discovered natural silicate perovskite in a heavily shocked meteorite.

“The discovery of an eightfold orientation could have similarly revolutionary implications for how we think about the dynamics of exoplanet interiors,” said Tracy.

Seismic discontinuities create the boundaries between Earth’s core, mantle, and crust. A seismic discontinuity is a sudden jump in seismic velocity across a boundary. The different structures of minerals under high pressure and temperature help create these discontinuities. So in effect, the structure of the minerals contributes to regulating the heat flow from the planet’s interior to the surface and also to plate tectonics. As a result, the structure of the minerals is a big part of what determines habitability.

Minerals determine the boundaries in Earth’s core under different pressures and temperatures inside the planet. This figure shows the pressure as it increases with depth in green. It also shows the speed changes in S (red) and P (blue) seismic waves. The seismic discontinuities coincide with mineral changes. Image Credit: University of Maryland

In an email exchange with Universe Today, study lead author Rajkrishna Dutta explained the big picture.

“The germanate is an analogue for the silicate. So, we expect to see a silicate eight-coordinated phase in the deep mantle of large super-earth planets. Having an eight-coordinated phase suggests a tighter, denser crystal structure.”

How does a tighter, denser crystal structure affect a planet’s interior?

“A transition from the six-coordinated post-perovskite leads to significant (~2.5%) difference in volume. This suggests the possibility of a seismic discontinuity in the mantle of those giant planets. Depending on the slope of the transition (expected to be negative), this can create a boundary layer for subducting plates and mantle convection.”

The question is, how exactly will this affect Super-Earths’ potential habitability? Earth’s interior structure plays a considerable role in maintaining habitability. The same must be true on Super-Earths.

This artist’s impression shows the planet K2-18b, its host star and an accompanying planet in this system. As of 2019, K2-18b is the only super-Earth exoplanet known to host both water and temperatures that could support life. What role does the planet’s interior structure play in the potential habitability? Image Credit: ESA/Hubble, M. Kornmesser

“Not much is known about the geology of the large exoplanets. Our study is still preliminary, and further work needs to be done to understand the effect of the structure on the thermodynamic and rheological properties of this structure. But, interestingly, this structure is disordered, so the two very different-sized cations occupy the same site. This suggests, at such extreme conditions, materials may behave very differently and undergo more chemical mixing,” Dutta explained.

Previous studies on Super-Earths have produced different results. Some research shows that Super-Earths have much more powerful geological activity than Earth. The tectonic plates would be thinner and under more stress so that that plate tectonics would be more vigorous. Other research shows that Super-Earths would have much stronger crusts that inhibit plate tectonics. And there’s no widespread agreement among scientists that plate tectonics is necessary for life.

Unfortunately, we’re nowhere near understanding the interior structure and dynamics of distant exoplanets. The InSight lander is gathering data on the interior of Mars, which will give us one more data set on planetary interiors. But we’re nowhere near a comprehensive understanding of exoplanet interiors.

The InSight lander is gathering data on Mars’ interior. Though Mars is far from being a Super-Earth, the data InSight is gathering will help scientists understand planetary interiors better in general. The SEIS (Seismic Experiment for Interior Structure) instrument is under the white protective in this image. Image Credit: NASA/JPL

However, that doesn’t mean there’s no progress to be made.

“But, understanding of the interiors of these planets are mostly based on laboratory experiments and theoretical computations,” Dutta said. “This is a domain where a lot of collaborative work between geoscientists, astrophysicists, chemists is required. With more experiments and modelling, we believe we can get a clearer picture of these planetary interiors.”  

That work will only increase as we discover more and more exoplanets.

“Just as the discovery of widespread, six-coordinated germanates/silicates profoundly altered our understanding of silicate crystal chemistry and its role in the Earth’s deep interior, the discovery of an eightfold-coordinated, intrinsically disordered germanate opens the possibility of previously unexplored crystal–chemical behavior in the silicate minerals of large, rocky exoplanets,” the authors write in their conclusion.

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Evan Gough

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