Simulations show Mercury may have been victim of hit-and-run collision

Jul 07, 2014 by Bob Yirka report
New simulations show that Mercury and other unusually metal-rich objects in the solar system may be relics left behind by hit-and-run collisions in the early solar system. Credit: NASA/JPL/Caltech

(Phys.org) —A pair of researchers with Arizona State University has created computer simulations that show that Mercury may have a relatively large metal core because it was the victim of a hit-and-run collision with another proto-planet—a collision that resulted in much of its non-metallic mantle being stripped away by the larger body. In their paper published in the journal Nature Geoscience, Erik Asphaug and Andreas Reufer describe how they ran many computer simulations designed to better understand planet formation in our solar system, and found that under just the right circumstances, a glancing blow collision between proto-Mercury and another proto-planet could have resulted in the current makeup of Mercury.

Mercury, as every school kid knows, is the planet in our solar system closest to the sun. It's also an anomaly because its core makes up approximately 60 percent of its mass (as compared to just 30 percent for Mars, Venus and Earth). Scientists have proposed theories to explain such an oddity, usually suggesting a of some sort must have taken place. A normal impact would have resulted in a loss of lighter elements, however, thus it's been difficult to make such theories work. To better understand what might have happened Asphaug and Reufer ran that showed a proto-Mercury colliding with various sized planets at various speeds and angles. They found that if a planet about the size of modern Earth collided with proto-Mercury at just the right angle and speed, it would have been possible for proto-Mercury to lose a large portion of its mantle due to it being stripped away. Furthermore, they noted that such an impact would not result in the proto-Mercury being caught in the larger planet's gravity, effectively turning it into a moon. Such a collision, they also note would have resulted in much of the material ejected into space falling back to the target body, resulting in it being more metal rich than before the impact.

Simulations can't prove what happened of course, especially when they are used to describe theoretical events that occurred so long ago, but because they are based on known factors such as the current state of the planets in our and data from spacecraft such as MESSENGER that entered Mercury's orbit back in 2012, they can offer scientists realistic options to consider when studying planetary evolution.

Explore further: Mercury passes in front of the Sun, as seen from Mars

More information: Mercury and other iron-rich planetary bodies as relics of inefficient accretion, Nature Geoscience (2014) DOI: 10.1038/ngeo2189

Abstract
Earth, Venus, Mars and asteroids such as Vesta and, perhaps, Lutetia have chondritic bulk compositions with massive silicate mantles surrounding iron cores. Anomalies include Mercury with its abundant metallic iron (about 70% by mass), the Moon with its small iron core, and metal-dominated asteroids. Although a giant impact with proto-Earth can explain the Moon's small core, a giant impact origin for Mercury is problematic. Such a scenario requires that proto-Mercury was blasted apart with far greater specific energy than required for lunar formation, yet retained substantial volatile elements and did not reaccrete its ejected mantle. Here we present numerical hydrocode simulations showing that proto-Mercury could have been stripped of its mantle in one or more high-speed collisions with a larger target planet that survived intact. A projectile that escapes the planet-colliding orbit in this hit-and-run scenario7 ultimately finds a permanent sink for its stripped mantle silicates. We show that if Mars and Mercury are derived from two planetary embryos that randomly avoided being accreted into a larger body, out of 20 original embryos (the rest having accreted into Venus and Earth), then it is statistically probable that one of those had its mantle stripped in one or two hit-and-run collisions. The same reasoning applies to pairwise accretion of planetesimals in the early Solar System, in which the relic bodies, which avoided becoming accreted, would be expected to have a wide diversity of compositions as a consequence of hit-and-run processes.

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JohanVDMeer
1 / 5 (12) Jul 07, 2014
The author of the article rightly points out that this simulation can never be considered to represent the actual events that transpired in the creation of Mercury.
What is most problematic of the whole simulation exercise is that it's based on the accretion theory of planetary formation which is flawed at best and a total denial of even basic physics / mechanics at its worst.
It's quite well known that there are no physically actual known circumstances in which rocks can smash into each other and accrete into bigger and bigger bodies.
The simulations which show that such events can occur must make use of the most special of parameters to achieve this.
In real life this accretion model cannot be substantiated - and therein lies the flaw of this whole simulation exercise.
dramamoose
5 / 5 (9) Jul 07, 2014

It's quite well known that there are no physically actual known circumstances in which rocks can smash into each other and accrete into bigger and bigger bodies.


Quite well known? Then why is it that the accretion model is the most popular amongst planetary scientists? I think you're making the mistake of applying simple logic to this scenario; you can't imagine it therefore it couldn't have happened. The problem is the human mind is really, really bad at figuring out what kinds of effects millions of years worth of gravity can have on some dust clumps
Torbjorn_Larsson_OM
5 / 5 (10) Jul 07, 2014
@JohanVDMeer:

It should also be rightly pointed out that while a stochastic model will never be _precisely_ the events, i.e. the "actual" events, it would if it is correct be the distribution of events, the "actual" event distribution.

They do these models precisely because they follow the basic physics and predict the observed distributions best. "The currently accepted method by which the planets formed is known as accretion," [ http://en.wikiped..._planets ]

See how predictive this accretion model is, it also predicts the seen diversity of compositions of accreted bodies. And note that all these scientists claim accretion, as opposed to what you speculate in. ("a total denial of even basic physics / mechanics".)

Tellus/Theia is a prominent example of a known impact event resulting in accretion. E.g. Tellus had ~ 90 % of the resulting Earth mass.
malapropism
5 / 5 (8) Jul 07, 2014
It's quite well known that there are no physically actual known circumstances in which rocks can smash into each other and accrete into bigger and bigger bodies.

Hmm, well, I suspect that many vulcanologists would disagree with you on that. And before you go protesting that "smashing rocks together" isn't the same as accreting rocks and lava, or lava and lava, I'd note that "smashing rocks together" is likely to generate, amongst other outcomes, heat. And at the sort of collision speeds likely to be seen in many such space encounters, probably quite a lot of it.
malapropism
5 / 5 (7) Jul 08, 2014
In real life this accretion model cannot be substantiated

I assume from this statement that you are refuting that a meteorite that falls to the surface of the earth does not accrete to the body of the planet but somehow then gets itself up and flies off again into space in order to maintain its separate existence?

As I suppose must also have happened to the remains of Shoemaker-Levy 9 in its close encounter with Jupiter. Or the Sept. 11, 2013, asteroid that hit the Moon and was captured on video doing so? (Oh, really, I can't be bothered; just Google it!)

So following on from my initial assumption above, you must have a mechanism for these bodies to fly away again. I await this with bated breath...
Whydening Gyre
not rated yet Jul 09, 2014
I'm curious as to where that second body repositioned itself after the "glancing blow". Venus, maybe? Or possibly Earth?

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