Massive rocky planets known as super-Earths may be much more solid deep inside than scientists previously understood, according to new research into how minerals behave under extreme pressure and heat.
Super-Earths are rocky planets with masses between about one and 10 times that of Earth.
Thousands of planets have been discovered beyond our solar system, and understanding what happens inside these larger rocky worlds could help scientists explain how they form, cool and change over billions of years.
The challenge is that conditions deep inside a super-Earth are far more extreme than anything found inside our own planet.
Enormous pressures can force common minerals to rearrange their atoms into completely different crystal structures.
One important example is magnesium orthosilicate, or Mg2SiO4, a mineral that is a major building block of rocky planets.
As this mineral moves deeper into a planet and experiences greater pressure, its structure changes. Under conditions found in Earth’s mantle, it can form a structure known as the spinel phase.
At still higher pressures, it breaks apart into two other minerals, bridgmanite and ferropericlase. Bridgmanite is believed to be the most abundant mineral inside Earth.
But something surprising happens under the much greater pressures expected inside super-Earths. Mg2SiO4 becomes stable again, this time taking on an unusual crystal structure known as the post-post-spinel phase.
Scientists believe this extremely high-pressure mineral could make up a significant portion of the deep mantles of some super-Earths. Knowing when it melts is therefore important for understanding whether the interiors of these planets are solid, partially melted or liquid.
Because reproducing super-Earth conditions in a laboratory is extremely difficult, researchers used advanced computer calculations instead.
In the study, published in AGU Advances, they simulated the behavior of post-post-spinel Mg2SiO4 at pressures reaching 1,300 gigapascals—many times greater than the pressure at Earth’s center.
The calculations revealed that the mineral is remarkably resistant to melting.
Depending on pressure, it remained solid until temperatures reached between about 9,780 and 14,897 Kelvin. Those temperatures are considerably higher than the melting points of several related minerals expected inside rocky planets.
The researchers also investigated what happens when iron is added, since real planetary mantles are unlikely to contain pure magnesium silicate. Even with moderate amounts of iron, the mineral’s melting temperature remained higher than the temperatures scientists expect inside the deep mantles of most rocky planets.
The results suggest that many super-Earths could have largely solid deep mantles rather than enormous regions of molten rock.
That could have important consequences. Whether a planet’s mantle is solid or partially molten affects how heat moves through its interior and how materials circulate. These processes could also influence the behavior of the planet’s metallic core and, ultimately, its ability to generate a magnetic field.
By learning how minerals survive under extraordinary conditions, scientists are getting a clearer picture of worlds that may be far larger—and internally very different—from Earth.
Source: KSR.
