A new study might help explain how Mercury – the smallest and nearest planet to the sun – formed the way it did. Mercury has a surprisingly huge metallic core, comprising about 70% of the planet’s mass. In contrast, it is a little rocky planet with a thin mantle. Astronomers have been scratching their heads for years about Mercury’s outrageous composition and bizarre formation. An alternative model recently proposed by Patrick Franco could offer a much stronger explanation.
Through complex simulations, Franco phones in on the grazing collision between two protoplanets of comparable masses. He hypothesizes that this collision could account for the unusual characteristics of Mercury. This collision scenario could dramatically remove over 60% of Mercury’s primordial mantle, clearly explaining its increased metallicity. Scientists shared their results in the Nature Astronomy journal. These results have huge implications for our understanding of not only Mercury, but other rocky planets in our solar system.
The Mystery of Mercury’s Composition
Mercury’s core-mass ratio is exceptionally high (~0.3). This ratio is odd compared to other terrestrial planets, which typically have a less extreme core-to-mantle ratio. How such an imbalance evolved in the first place is one of planetology’s biggest open mysteries.
Franco’s model suggests that the Earth started out in much the same way as its rocky neighbors. The grazing impact he describes would have had an impact considerable enough to tilt this balance in the other direction dramatically.
“We assumed that Mercury would initially have a composition similar to that of the other terrestrial planets. The collision would have stripped away up to 60% of its original mantle, which would explain its heightened metallicity,” – Patrick Franco
The model uses the Lagrangian function invented by mathematician Joseph Louis Lagrange which allows for very accurate computations. As Franco emphasizes, the opportunity behind deep, highly-resolved simulations with smoothed particle hydrodynamics is in the detail. These simulations are able to reproduce, within remarkable margins of error (5% or less), both Mercury’s total mass and its peculiar metal-to-silicate ratio.
“Through detailed simulations in smoothed particle hydrodynamics, we found that it’s possible to reproduce both Mercury’s total mass and its unusual metal-to-silicate ratio with high precision,” – Patrick Franco
Simulating Celestial Collisions
The objective of this study was to use a simulation time span of 48 hours to examine the dynamics of the aforementioned collision. Franco and his colleagues looked at the incoming impact angle and found that it was 32.5 degrees. They will be remembering a relatively low impact velocity of 22.3 km/s. Such conditions are plausible within the early solar system’s chaotic environment, where protoplanets interacted gravitationally and frequently disturbed each other’s orbits.
Franco explained that the focus of their research was to investigate more common scenarios involving collisions between bodies of similar masses, rather than rare events involving significantly unequal bodies.
“Our work is based on the finding, made in previous simulations, that collisions between very unequal bodies are extremely rare events. Collisions between objects of similar masses are more common,” – Patrick Franco
According to the grazing impact scenario, a portion of this ejected material is unlikely to ever return to Mercury. This might account for the core-mantle disproportion that we see in today’s Earth.
Implications for Other Rocky Planets
Franco’s model provides food for thought that extends further than an understanding of Mercury. It raises exciting new questions about how other rocky planets underwent differentiation processes and material loss during their formative years. This research creates new opportunities to investigate how comparable impacts may have played a role in producing the conditions seen on Venus and Earth today.
Franco adds that if the collision occurred in closer, more adjacent orbits, it raises even more intriguing questions. These questions all center around how matter can move between still-forming planets.
“If the impact occurred in nearby orbits, one possibility is that this material was incorporated by another planet in formation, perhaps Venus. It’s a hypothesis that still needs to be investigated in greater depth,” – Patrick Franco
He stresses that Mercury is our solar system’s least explored planet. With some exciting new research and missions on the way, we’re in for some thrilling discoveries. Keep your eyes peeled!
“Mercury remains the least explored planet in our system. But that’s changing. There’s a new generation of research and missions underway, and many interesting things are yet to come,” – Patrick Franco