Mysteries of a Magnetic Mercury

On Mercury a solar day is about 176 Earth days long. During its first Mercury solar day in orbit the MESSENGER spacecraft imaged nearly the entire surface of Mercury to generate a global monochrome map at 250 meters per pixel resolution and a 1 kilometer per pixel resolution color map. Credit: NASA/JHU APL/CIW

On Mercury a solar day is about 176 Earth days long. During its first Mercury solar day in orbit the MESSENGER spacecraft imaged nearly the entire surface of Mercury to generate a global monochrome map at 250 meters per pixel resolution and a 1 kilometer per pixel resolution color map. Credit: NASA/JHU APL/CIW

Mercury, the smallest planet in our solar system, is a tiny ball of rock that flies around the Sun in just 88 Earth days. The planet has little atmosphere to moderate its surface temperature, which swings between baking hot and freezing cold due to its proximity to the Sun. In fact, Mercury has a wider variation in temperature than any other planet in the Solar System. On the nighttime side of the planet (facing away from the Sun) temperatures drop to 100 K (-280°F). During the day, temperatures can reach 700 K (800°F).

Beneath Earth's solid crust are the mantle, the outer core, and the inner core. Scientists learn about the inside of Earth by studying how waves from earthquakes travel through the planet. Image credit: World Book illustration by Raymond Perlman and Steven Brayfield, Artisan-Chicago

Beneath Earth’s solid crust are the mantle, the outer core, and the inner core. Scientists learn about the inside of Earth by studying how waves from earthquakes travel through the planet. Image credit: World Book illustration by Raymond Perlman and Steven Brayfield, Artisan-Chicago

Due to its size and extreme environment, Mercury has often received less attention than other planets like Mars, which astrobiologists are keen to study in order to gain clues about the potential for life beyond Earth. However, Mercury does have valuable lessons to teach us.

Although it’s small, Mercury is a rocky world and does have some similarities to Earth. This provides the opportunity for comparative planetology, where scientists compare features of the Earth and Mercury (or other celestial bodies) to learn more about what makes the Earth unique as a habitat for life.

One important similarity between the Earth and Mercury is that they both have iron cores. However, new research shows the similarity of the two planets’ interiors might only be skin deep.

The Earth’s core is divided into two parts: a solid inner core, and a liquid outer core. When the Earth rotates, the liquid outer core moves, and this generates our planet’s magnetic field. While it spins, parts of the liquid core are solidifying, causing the solid inner core to grow, and producing energy in the process.

The size of Earth’s liquid core and the speed at which the planet rotates determines the force of our magnetic field. This is important for life, because the magnetic field plays an vital role in shielding Earth from incoming radiation that would otherwise be harmful for living organisms.

Things are very different inside Mercury. Data from NASA’s MESSENGER (Mercury Surface, Space Environment, Geochemistry, and Ranging) spacecraft has shown that Mercury’s magnetic field is lop-sided, and is three times stronger in the northern hemisphere of the planet.


Mercury’s Surprising Core and Landscape Curiosities. Credit: Carnegie Institution (YouTube)

Scientists from the University of California at Los Angeles (UCLA) have proposed a theory for why this is the case.

“We had figured out how the Earth works, and Mercury is another terrestrial, rocky planet with an iron core, so we thought it would work the same way. But it’s not working the same way,” said Christopher T. Russell, co-author and professor in UCLA’s department of Earth, planetary and space sciences, in a recent press release.

Magnetic field lines differ at Mercury's north and south poles. As a result of the north-south asymmetry in Mercury's internal magnetic field, the geometry of magnetic field lines is different in Mercury's north and south polar regions. In particular, the magnetic "polar cap" where field lines are open to the interplanetary medium is much larger near the south pole. This geometry implies that the south polar region is much more exposed than in the north to charged particles heated and accelerated by solar wind–magnetosphere interactions. The impact of those charged particles onto Mercury's surface contributes both to the generation of the planet's tenuous atmosphere and to the "space weathering" of surface materials, both of which should have a north-south asymmetry given the different magnetic field configurations at the two poles. Image Credit: NASA/Johns Hopkins University Applied Physics Laboratory/Carnegie Institution of Washington

Magnetic field lines differ at Mercury’s north and south poles. As a result of the north-south asymmetry in Mercury’s internal magnetic field, the geometry of magnetic field lines is different in Mercury’s north and south polar regions. In particular, the magnetic “polar cap” where field lines are open to the interplanetary medium is much larger near the south pole. This geometry implies that the south polar region is much more exposed than in the north to charged particles heated and accelerated by solar wind–magnetosphere interactions. The impact of those charged particles onto Mercury’s surface contributes both to the generation of the planet’s tenuous atmosphere and to the “space weathering” of surface materials, both of which should have a north-south asymmetry given the different magnetic field configurations at the two poles. Image Credit: NASA/Johns Hopkins University Applied Physics Laboratory/Carnegie Institution of Washington

The team re-created Mercury’s core with mathematical models and tried to see if they could produce a similarly asymmetric magnetic field. The model included a lot of factors, such as the speed of Mercury’s rotation and the motion of fluid in the planet’s core. What they found is that Mercury’s core also has a region where iron is turning from liquid to solid. Unlike Earth, this region is at the outer boundary of the core instead of inside. Mercury’s bizarre magnetic field comes down to reactions between the core and the mantle of the planet. The study reveals that not all planets generate magnetic fields in the same way.

An artist's concept of MESSENGER orbiting Mercury. Credit: NASA

An artist’s concept of MESSENGER orbiting Mercury. Credit: NASA

Mercury is not a location in the Solar System where life is thought to be possible. However, studying Mercury can help astrobiologists understand how rocky planets form, and the processes that lead one rocky planet to become a haven for diverse forms of life, and another to become a tiny, lifeless ball of rock.

 

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