Magnetism, not cataclysm may be the cause of Mercury's giant iron core
Mercury’s disproportionately massive core may be the result of the Sun’s powerful magnetic influence rather than the consequence of a cataclysmic collision with another body in the ancient past, according to the results of a new study.
Since the advent of spaceflight, humanity has sent just three spacecraft to unravel the secrets held by Mercury, the innermost planet of our solar system. In the 1970s, NASA’S Mariner 10 spacecraft made three separate flybys of the planet, capturing high-resolution imagery and capturing data on Mercury's magnetic field in the process.
Decades later, on the 17th of March 2011, the agency’s MESSENGER spacecraft became the first probe to enter orbit around Mercury. It then spent four years characterizing the alien world in unprecedented detail. Meanwhile, the joint European/Japanese mission BepiColombo is still en route to the planet, and is set to arrive late in the year 2025.
As a result of these endeavors astronomers have learned a great deal about Mercury, yet it still harbors a plethora of mysteries that the scientific community has yet to resolve.
One such mystery is related to the planet’s internal structure. An analysis of data collected by orbiting spacecraft taking detailed readings of Mercury’s gravitational signature had in the past revealed that the planet has a disproportionally massive iron core relative to the size of its mantle.
To be more specific, it is estimated that the core makes up roughly three quarters of Mercury’s mass, and has a radius of roughly 1,289 miles (2,074 km), while the rocky outer shell of the planet measures a mere 250 miles (400 km) in depth. This makes it the second-densest planet in the solar system.
Until now, the leading theory as to why Mercury has such an unusually large core for such a tiny planet centered around the idea that it was in fact once a much larger planet that had fallen afoul of a planetary collision some time in the distant past. According to that theory, the cataclysmic force of the interaction would have been sufficient to strip away much of Mercury’s outer shell, leaving behind a shallow mantle to cover the once larger planet’s core.
However, according to the new study, Mercury’s unusual structure may actually be the result of the natural influence of the Sun’s magnetic field.
The authors created a new computer model of the primordial cloud of dust and gas from which the planets of the solar system would eventually form, and simulated the effect of a young Sun’s magnetic field on the swirling mass. It was discovered that the magnetic influence of our parent star drew the iron grains embedded throughout the cloud closer. This resulted in the planets that formed closest to the Sun having a significantly larger iron core than those that would one day orbit in the farther reaches of the solar system.
The researchers combined their model with earlier research on planetary formation, in order to calculate the rate at which the material would be pulled towards the Sun. They found that their model’s predicted planetary compositions correlated well with the real-life planets that make up our solar system in the present day.
Alongside shedding light on how our home solar system came to coalesce and subsequently mature, the new research could also have significant implications for astronomers hoping to gain a greater understanding of distant exoplanets spread throughout the galaxy.
"You can no longer just say, 'Oh, the composition of a star looks like this, so the planets around it must look like this,'" explained William McDonough, a professor of geology at the University of Maryland and one of the authors of the new study. "Now you have to say, 'Each planet could have more or less iron based on the magnetic properties of the star in the early growth of the solar system.'"
The team is now looking for an alien star system in which rocky planets are known to orbit, with which they can further test their theory.
The paper has been published in the journal Progress in Earth and Planetary Science.
Source: University of Maryland