We have a lot of knowledge about the inside of Earth. It has both an inner core and an outer core, and that the rotation creates a protective magnetosphere that shields life from the Sun's radiative power. The mantle is mostly solid but also home to magma. We know that it has a crust and plate tectonics that move the continents around.
What about Super-Earths? We know they are out there and we have found them. What do we know about their insides? Earth's structure is shaped by the extreme pressure and density in its interior. The temperature and pressure inside Super-Earths are even more powerful. How does it affect the habitability of these planets?
Our Solar System's ice giant planets, Uranus and Neptune, are more massive than Super-Earths. Neptune is 17 times larger than Earth. Earth is about ten times more massive than a Super-Earth. Different sources use different mass categories for Super-Earths.
NASA calls 1539 of the almost 5,000 confirmed exoplanets Super-Earths. Super-Earths are defined by their mass. 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. Many of them have similar densities to Earth.
Researchers at Carnegie University and other institutions studied the effects of extreme pressure and temperature on minerals inside Super-Earths. They did lab experiments to recreate their interiors. The minerals were subjected to extreme pressures and temperatures to see what would happen to them in the mantles of a common type of planet.
The lead author of the paper is a post-doctoral fellow at Carnegie. The paper was published in the National Academy of Sciences.
Life is supported in different ways by Earth's interior dynamics. One way is the magnetosphere generated by the core. The magnetosphere prevents the solar wind from stripping away the atmosphere and directs harmful solar radiation away from the planet's surface.
The climate on Earth is affected by plate tectonics and mantle convection. The Earth's atmosphere is stopped from getting too cold by the release of heated material and CO2 from the volcanos. The same processes regulate the amount of CO2 by subducting carbonates back into rock. The chemistry needed for life is created by plate tectonics. Scientists think that plate tectonics and mantle convection play a critical role in life.
The extreme conditions inside a Super-Earth would affect its habitability.
The interior dynamics of our planet are crucial for maintaining a surface environment where life can thrive.
Most of Earth's crust is made of sibel minerals. Key boundaries between the upper and lower mantle are created by the high temperature and pressure of silicate minerals. Studies show rocky exoplanets may have silicate crusts. The density is roughly the same as Earth.
The temperature and pressure inside Super-Earths would be more extreme than on Earth, since they are so massive. The researchers wanted to know how the conditions affect silicate minerals. They wanted to know if new types of silicates would behave differently.
Most silicates have the same orientation called a tetrahedral structure. There is one central atom and four other atoms.
One of the most abundant silicate minerals in the Earth is Mg 2 SiO 4. It is abundant in rocky Super-Earths. There is no way to observe the new phases of silicates that emerge inside Super-Earths. It requires 490 GPa of pressure for the new phases to emerge. There is no way to mimic that pressure.
Scientists can use an analogue for silicates that responds the same way but at less extreme temperature and pressure. That is germanium. It is german magnesiumate, or Mg2GeO4. At high pressure, magnesium germanate changes to a new phase, but the threshold is lower. Pressure can be created in a lab when new phases emerge at 175 GPa.
The research team heated the samples with a laser after using a diamond anvil. The magnesium germanate was exposed to two million times Earth's normal atmospheric pressure.
The central germanium atom had eight oxygen atoms instead of four. The new mineral could affect the internal temperature and dynamics of Super- Earths.
The understanding of deep Earth dynamics was changed by the discovery that silicates could take on a structure oriented around six bonds. The discovery of silicate-perovskite and post-perovskite is referred to by Tracy. The structure of perovskite is formed under high pressure. It's not stable at Earth's surface and only exists in the lower part of Earth's mantle. Scientists discovered natural silicate perovskite in a meteorite.
Tracy said that the discovery of an eightfold orientation could have revolutionary implications for how we think about exoplanet interiors.
The boundaries between Earth's core, mantle, and crust are created by earthquakes. There is a sudden jump in seismic velocity. The different structures of minerals help create discontinuities. The heat flow from the planet's interior to the surface is regulated by the structure of the minerals. Habitability is determined by the structure of the minerals.
In an email exchange with Universe Today, the study lead author explained the big picture.
The germanate is an analogue of the silicate. We expect to see a silicate eight-coordinated phase in the deep mantle of large super-earth planets. A tighter, denser crystal structure is suggested by having an eight-coordinated phase.
How does a denser crystal structure affect a planet?
A transition from the six-coordinated post-perovskite leads to a significant difference in volume. There is a chance of a discontinuity in the mantle of those giant planets. This can create a boundary layer for subducting plates if the transition is negative.
How will this affect Super-Earths? Earth's interior structure is important in maintaining habitability. It must be the same on Super-Earths.
Not much is known about the geology of large exoplanets. The study is still preliminary and more work needs to be done to understand the effects of the structure on the rheological properties of the structure. The two very different-sized cations occupy the same site because of the structure being disorderly. It suggests that at extreme conditions, materials may behave differently and undergo more chemical mixing.
Different results have been produced by previous studies on Super-Earths. Super-earths have more geological activity than Earth. The plate tectonics would be more vigorous if the plates were thinner and under more stress. The research shows that Super-Earths have stronger crusts. There is no consensus among scientists that plate tectonics is necessary for life.
We don't understand the interior structure or dynamics of distant exoplanets. One more data set will be given to us by the InSight lander, which is gathering data on the interior of Mars. We are not near a comprehensive understanding of interiors.
That doesn't mean there isn't progress to be made.
Understanding the interiors of these planets is mostly based on laboratory experiments and theoretical computations.
As we discover more and more exoplanets, that work will increase.
Just as the discovery of 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 a new world.
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