Deep Magma Oceans Could Make Super-Earths Habitable: A New Study Reveals
The Earth's habitability is a delicate balance between the Sun's solar wind and cosmic radiation. The Sun's solar wind would strip away the planet's atmosphere, but Earth's magnetosphere acts as a protective shield. Similarly, cosmic rays, which are high-energy particles that can damage living tissue, are deflected by the magnetosphere. Scientists are now exploring whether exoplanets, particularly super-Earths, can be habitable if they have a similar protective shield.
A recent study published in Nature Astronomy suggests that a magma ocean could create a protective shield for super-Earths. Super-Earths are the most common type of exoplanet, and if magma oceans can boost their habitability, the chances of life existing elsewhere are greater than previously thought. The research, titled 'Electrical conductivities of (Mg,Fe)O at extreme pressures and implications for planetary magma oceans', was led by Miki Nakajima, an associate professor in the Department of Earth and Environmental Sciences at the University of Rochester.
During planet formation, planets undergo many impacts that can generate magma oceans. When these crystallize, part of the magma densifies via iron enrichment and migrates to the core-mantle boundary, forming an iron-rich basal magma ocean (BMO). These BMOs could provide the same type of magnetic protection that Earth enjoys, but only if their iron content is high enough. The BMO could generate a dynamo in early Earth and super-Earths if the electrical conductivity of the BMO, which is thought to be sensitive to its Fe content, is sufficiently high.
The study also addresses the question of what created Earth's early magnetic shield. The researchers point out that several existing models try to explain Earth's early geodynamo without a solid inner core, and each of them has its problems. An alternative, which is the main focus of this work, is dynamo generation in a basal magma ocean (BMO).
The lead author, Nakajima, emphasized the importance of a strong magnetic field for life on a planet. However, most terrestrial planets in the solar system, such as Venus and Mars, do not have magnetic fields because their cores don't have the right physical conditions to generate a magnetic field. Super-earths, on the other hand, can produce dynamos in their core and/or magma, which can increase their planetary habitability.
To test their idea, the research team performed experiments on an Fe-rich BMO analogue. They conducted laser-driven shock experiments on ferropericlase, which mimic the extreme pressures and temperatures present inside super-Earths. They also ran simulations that calculate the long-term evolution of super-Earths.
The results show that the extremely intense pressures inside super-Earths, which can have as many as 10 Earth masses, forces the molten rock in their deep mantles to become electrically conductive. This conductivity can last a long time and can be much stronger than Earth's, boosting the potential habitability of this common type of planet. It also helps explain Earth's ancient magnetosphere.
The experiments and model show that dynamos created by basal magma oceans are most likely stronger than those created by core-driven dynamos, at least for super-Earths with masses greater than about 3 to 5 Earth masses. The BMO-driven dynamo can last for a few billion years, which may be detectable with future observations of super-Earths.
The closer a planetary dynamo is to a planet's surface, the stronger the magnetic shield is. Taken together, the results not only explain super-Earth magnetic shields but also the early Earth's magnetic shield that provided shelter while life on early Earth got started. A BMO-driven dynamo can be the dominant source of the surface magnetic field for billions of years, which crucially includes the formative stages of planetary evolution.
Exoplanet scientists know that protective magnetospheres are critical to habitability. Figuring out how to detect and measure them is a critical and very challenging issue in exoplanet science. In 2021, the Hubble may have observed the very first exoplanet magnetosphere around Kepler-3b, an exo-Neptune about 122 light-years away. However, the detection is still somewhat ambiguous, and illustrates the difficulty in detecting them. From such a great distance, these fields are extremely weak.
Powerful radio-telescopes are likely the solution to detecting these fields. If we ever put a radio-telescope on the Moon, like FarView or FARSIDE, it should be able to study distant magnetic fields. New ground-based radio observatories could also do it, as could a mission like the Lunar Surface Electromagnetics Experiment (LuSEE-Night).
Like many things in astronomy, only better future observations can shed more light on the issue. Nakajima expressed excitement and gratitude for the support from collaborators across various research fields, and looked forward to future magnetic field observations of exoplanets to test their hypothesis.
