Mysteries abound in the Solar System. Though it can sometimes seem like we've learned a lot, you can pick any object in the Solar System and quickly come up with unanswered questions. That's certainly true of tiny Mercury.
Mercury's mystery lies in its core. Ground based radio observations during the 1960s and 1970s showed that it had a massive core.
The Mariner 10 mission in 1975, the first mission to Mercury, provided more accurate measurements, and the Messenger mission from 2010 – 2015 provided the most convincing evidence that the planet's core is massive.
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For some reason, the diminutive planet has a core that makes up about 70% of its mass. That's much greater than Earth's core (30%) and Mars' core (25%). This is sometimes called the "Mercury Problem".
The main working hypothesis for the Mercury problem says that the planet is the victim of a collision with a different-sized object. The cataclysmic collision stripped much of the planet's mantle and crust away, leaving only a thin crust and mantle overlying the massive core.
Unfortunately, simulations show that collisions between bodies with very different masses were very rare.
New research in Nature Astronomy says that while a collision between Mercury and another object is responsible for Mercury's unusual interior structure, the other object was not larger than Mercury.
It's titled Formation of Mercury by a grazing giant collision involving similar-mass bodies. The lead author is Patrick Franco from Institut de Physique du Globe de Paris, Université Paris Cité, CNRS, Paris, France.
"The origin of Mercury still remains poorly understood compared with the other rocky planets of the Solar System," Franco and his co-researchers write.
"To explain its internal structure, it is usually considered to be the product of a giant impact. However, most studies assume a binary collision between bodies of substantially different masses, which seems to be unlikely according to N-body simulations."
One of the reasons for their rarity is that the impactor had to be in an extremely eccentric orbit prior to impact, and that's rare.
The giant impact scenario proposes that an impact between a planetary embryo with 2.25 times the current mass of Mercury – a proto-Mercury – and an object six times smaller than that removed the embryo's mantle, and what's left resembles Mercury's internal structure.
But if those types of mismatched collisions were rare, what else could've happened?
Collisions between objects with similar masses were much more common in the young Solar System, according to detailed numerical simulations. The researchers say that contrary to the giant impact scenario, only a grazing impact with a similar mass object is needed to explain Mercury and its unusual interior structure.
"Through simulation, we show that the formation of Mercury doesn't require exceptional collisions. A grazing impact between two protoplanets of similar masses can explain its composition. This is a much more plausible scenario from a statistical and dynamic point of view," said lead author Franco in a press release.
"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, and the objective of the study was precisely to verify whether these collisions would be capable of producing a planet with the characteristics observed in Mercury."
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The early Solar System was much messier and chaotic than it is now. Rocky planetary embryos jockeyed for position in the inner Solar System and it wasn't clear which ones would eventually become planets. In that environment, collisions between similar mass objects were much more likely.
"They were evolving objects, within a nursery of planetary embryos, interacting gravitationally, disturbing each other's orbits, and even colliding, until only the well-defined and stable orbital configurations we know today remained," said Franco.
Franco and his co-researchers turned to smoothed particle hydrodynamics (SPH) simulations to test the idea. This widely used method simulates the behaviour of gases, liquids, and solids while they're in motion. SPH simulations are especially useful in the context of collisions like the ones between planets.
"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. The model's margin of error was less than 5%," Franco said.
Its unusual metal-to-silicate ratio refers to the fact that the core is metallic while the mantle and crust are silicate.
"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," Franco explains.
But if Mercury is the result of a mass-stripping collision, what happened to the material blasted into space? Modelling of the impact between different-sized objects results in Mercury re-accreting most of the lost mass, in which case Mercury wouldn't have the structure it does now.
"In these scenarios, the material torn away during the collision is reincorporated by the planet itself. If this were the case, Mercury wouldn't exhibit its current disproportion between core and mantle," Franco says.
"But in the model we're proposing, depending on the initial conditions, part of the material torn away may be ejected and never return, which preserves the disproportion between core and mantle," Franco argues.
Early in the Solar System, conditions could've prevented the mass from re-accreting.
"The scenario proposed in this work occurs during the initial tens of millions of years of planet formation, when several mechanisms could prevent substantial debris reaccretion," the authors write.
There would've been numerous planetesimals and planetary embryos that could've scattered the debris gravitationally.
Another possibility is that its neighbour Venus ended up a little bit more massive because of the impact.
"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," the researcher said.
Expanding on this understanding will require geochemical investigation of not only Mercury, but also of meteorites and possibly, hopefully, even a sample from Mercury itself.
There are concepts for a Mercury sample return mission, but they're restricted to conceptual status. In the mid-2000s, the ESA studied a solar-sail idea for a sample return mission to Mercury, but it was more of a thought experiment than a proposal.
Still, the solar sail idea won't go away.
The ESA/JAXA BepiColombo mission will reach Mercury in 2026 and features a pair of complementary orbiters that will perform a comprehensive study of the planet.
Together, they carry more than 20 science instruments. It will measure Mercury's solid and liquid cores and determine their sizes. It will also map the planet's magnetic and gravity fields.
The results may not confirm this new impact hypothesis, but more detailed data will undoubtedly advance the scientific understanding of Mercury.
"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," said Franco.
This article was originally published by Universe Today. Read the original article.