July 8, 2014
New Evidence Suggests Mercury Was Formed From A Stellar Collision
Gerard LeBlond for redOrbit.com - Your Universe Online
Planet Mercury has an unusual metal-rich composition which has baffled the scientific community. However, new evidence may have solved this mystery.
According to a new study published online in Nature Geoscience on July 6, Mercury along with other space objects in the solar system with an unusual metal-rich composition, may be left over from stellar hit-and-run collisions in the early formation of the solar system.
The composition of Mercury is quite different from other planets and the moon, which makes it difficult to determine its origin. It has more than twice the metallic iron core than Earth. Mercury’s core is 65 percent iron compared to Earth, which has a 32-percent iron core.
Earth, Mars and Venus all have similar compositions while Mercury is an anomaly, according to Arizona State University professor Erik Asphaug and Andreas Reufer of the University of Bern. The key question is how the planet was formed from the dust, ice and gas of the early solar nebula?
Until now, many theories of how Mercury was formed have been disproved. The new study explains how Mercury lost its mantle while retaining high levels of volatiles -- easily vaporized elements such as water, lead and sulfur. A previously common explanation was a giant impact had nothing to do with the formation.
The new theory explains that one or more hit-and-run collisions could have stripped away proto-Mercury’s mantle leaving behind a mostly-iron formation while retaining the volatiles. This could explain the absence of shock features normally left in mantle-stripped formations.
Asphang and Reufer have put together a scenario on how planets merge and grow based on the concept that Mars and Mercury are the last two of around 20 interstellar bodies that formed Venus and Earth via accretion.
“How did they luck out? Mars, by missing out on most of the action – not colliding into any larger body since its formation – and Mercury, by hitting the larger planets in a glancing blow each time, failing to accrete,” explains Asphaug, “It’s like landing heads two or three times in a row – lucky, but not crazy lucky. In fact, about one in 10 lucky.”
“The surprising result we have shown is that hit-and-run relics not only can exist in rare cases, but that survivors of repeated hit-and-run incidents can dominate the surviving population. That is, the average unaccreted body will have been subject to more than one hit-and-run collision. We propose one or two of these hit-and-run collisions can explain Mercury’s massive metallic core and very thin rocky mantle,” he added.
“Giant collisions put the final touches on our planets. Only recently have we started to understand how profound and deep those final touches can be,” stated Reufer, who performed the computer models.
“The implication of the dynamical scenario explains, at long last, where the ‘missing mantle’ of Mercury is – it’s on Venus or the Earth, the hit-and-run targets that won the sweep-up,” says Asphaug.
The study also revealed a problem with modern theories of planet formation, being that protoplanets merge into larger bodies when they collide.
“Protoplanets do merge and grow, overall, because otherwise there would not be planets, but planet formation is actually a very messy, very lossy process, and when you take that into account, it’s not at all surprising that the ‘scraps,’ like Mercury and Mars, and the asteroids are so diverse,” Asphaug explains.
Questions that arose from the new study like where’s the stripped mantle rock and meteorites from these collisions? “It’s not missing – it's inside the mantles of the planets, ultimately. It got gobbled up by the larger growing planetary bodies in every hit-and-run series of encounters,” explains Asphaug.
To show this, scenarios were run on computer simulations demonstrating proto-Mercury colliding with various sized planets at different speeds and angles. A planet around the size of Earth could have collided with proto-mercury at just the correct angle and stripped away a large portion of its mantle, resulting in the material falling back into the planet and forming a more metal-rich body.
The simulations don’t prove what actually happened, but offer new opinions on planetary evolution.