Planet Mercury: Result of early hit-and-run collisions

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. 

Image courtesy NASA/JPL/Caltech.

Planet Mercury's unusual metal-rich composition has been a longstanding puzzle in planetary science.

According to a study published online in Nature Geoscience July 6, Mercury and other unusually metal-rich objects in the solar system may be relics left behind by collisions in the early solar system that built the other planets.

The origin of planet Mercury has been a difficult question in planetary science because its composition is very different from that of the other terrestrial planets and the moon.

This small, innermost planet has more than twice the fraction of metallic iron of any other terrestrial planet. Its iron core makes up about 65 percent of Mercury's total mass; Earth's core, by comparison, is just 32 percent of its mass.

How do we get Venus, Earth and Mars to be mostly "chondritic" (having a more-or-less Earth-like bulk composition) while Mercury is such an anomaly?

Erik Asphaug

For Arizona State University professor Erik Asphaug, understanding how such a planet accumulated from the dust, ice and gas in the early solar nebula is a key science question.

There have been a number of failed hypotheses for Mercury's formation.

None of them until now has been able to explain how Mercury lost its mantle while retaining significant levels of volatiles (easily vaporized elements or compounds, such as water, lead and sulphur).

Mercury has substantially more volatiles than the moon does, leading scientists to think its formation could have had nothing to do with a giant impact ripping off the mantle, which has been a common popular explanation.

To explain the mystery of Mercury's metal-rich composition, ASU's Asphaug and Andreas Reufer of the University of Bern have developed a new hypothesis involving hit-and-run collisions, where proto-Mercury loses half its mantle in a grazing blow into a larger planet (proto-Venus or proto-Earth).

One or more hit-and-run collisions could have potentially stripped away proto-Mercury's mantle without an intense shock, leaving behind a mostly-iron body and satisfying a number of the major puzzles of planetary formation, including the retention of volatiles, in a process that can also explain the absence of shock features in many of the mantle-stripped meteorites.

Asphaug and Reufer have developed a statistical scenario for how planets merge and grow based on the common notion that Mars and Mercury are the last two relics of an original population of maybe 20 bodies that mostly accreted to form Venus and Earth. These last two planets lucked out.

"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, who is a professor in ASU's School of Earth and Space Exploration (SESE).

"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," explains Asphaug.

"We propose one or two of these hit-and-run collisions can explain Mercury's massive metallic core and very thin rocky mantle."

According to Reufer, who performed the computer modeling for the study, "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."

"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.

More Information: Mercury and other iron-rich planetary bodies as relics of inefficient accretion: Authors: E. Asphaug & A. Reufer - Nature Geoscience (2014) doi:10.1038/ngeo2189:

Pluto satellites' orbital ballet may hint of long-ago collisions

A best-fit colour image/map of Pluto generated with the Hubble Space Telescope and advanced computers. Image: NASA

A large impact 4 billion years ago may account for the puzzling orbital configuration among Pluto's five known satellites, according to a new model developed by planetary scientists from Southwest Research Institute (SwRI).

Starting with Charon, Pluto's nearest and largest moon, each of the successively more distant—and much smaller—moons orbits Pluto according to a steadily increasing factor of Charon's own orbital period.

The small satellites, Styx, Nix, Kereberos and Hydra, have orbital periods that are almost exactly 3, 4, 5 and 6 times longer than Charon's.

Harold "Hal" Levison

"Their distance from Pluto and the orbital arrangement of the satellites has been a challenge for theories of the small satellites' formation," said lead investigator Dr. Harold "Hal" Levison, an Institute scientist in SwRI's Planetary Science Directorate at Boulder, Colo.

Models for the formation of Charon leave plenty of small satellites, but all of them are much closer to Pluto than the current system that we see today," said Levison.

A major problem has been understanding how to move these satellites outward, but not lose them from the Pluto-Charon system or have them crash into Charon.

He said, "This configuration suggests that we have been missing some important mechanism to transport material around in this system."

The SwRI study, funded by a grant from NASA's Outer Planetary Research program and Lunar Science Institute, considered the earliest and most dynamic epoch of the Pluto/Charon system.

It is thought that Charon was formed by a large impact during a period in solar system history when such collisions were dramatically more frequent.

Pluto's moon Charon

Any initially surviving satellites would likely be destroyed in collisions, but these shattered moons wouldn't be lost; rather, their remains would stay in the Pluto/Charon system and become the starting point for building new satellites.

Thus there would have been many generations of satellite systems over the history of Pluto and Charon.

In modeling the destruction of the satellites, the SwRI study found that there may be a method for moving them, or their building blocks, outward, due to the competing effects of Charon's gravitational kicks and collisions among the debris of the disrupted satellites.

Charon is the largest satellite of any planet or dwarf-planet, weighing in at 1/10 the mass of Pluto (the Moon is just 1/81 the mass of Earth), and so it could rapidly slingshot the small satellites outward if they were to approach too closely.

Kevin Walsh

Meanwhile, collisions among small satellites can change orbits to keep things away from Charon. When combined, this leads to a series of satellites colliding, breaking to pieces, moving outward and then rebuilding.

"The implications for this result are that the current small satellites are the last generation of many previous generations of satellites," said Dr. Kevin Walsh, another investigator and a research scientist in SwRI's Planetary Science Directorate at Boulder, Colo.

"They were probably first formed around 4 billion years ago, and after an eventful million years of breaking and rebuilding, have survived in their current configuration ever since."

Astronomers Modeling galaxy mergers

An generated optical image of stars in a pair of colliding galaxies after a time of 0.5 billion years, as simulated with a new computer code. 

Credit: P. Hopkins

Astronomers think that many galaxies, including our own Milky Way, have undergone similar collisions during their lifetimes.

Although galaxy collisions are important and common, what happens during these encounters is not very well understood.

For example, it seems likely that massive black hole(s) will form during the interactions, as the two galaxies' nuclei approach each other.

Galaxy-galaxy interactions also stimulate vigorous star formation as gravitational effects during the encounters induce interstellar gas to condense into stars.

The starbursts in turn light up the galaxies, especially at infrared wavelengths, making some systems hundreds or even thousands of times brighter than the Milky Way while the starbursts are underway.

Studying these luminous galaxies not only sheds light on how galaxies evolve and form stars, since they act as lanterns over cosmological distances it also helps scientists study the early universe.

All this impressive progress, however, hinges on an accurate understanding of mergers and how they work.

The general approach is to study many local examples to categorize their behaviors, and then model these cases with computational codes that simulate mergers.

The combination of precise observations and detailed modeling, iteratively applied, helps scientists improve both their understanding of the galaxies and the physical parameters and processes included in the modeling codes.

With these in hand, astronomers can start to probe the more distant universe where the objects are not as easy to measure.

Lars Hernquist

CfA astronomer Lars Hernquist and five of his Harvard colleagues (many of whom were his past students) have now shown that feedback processes from bursts of star formation play a key role in determining how merging galaxies develop, at least when two massive galaxies collide.

Prior models did not fully account for the role played by gas that is driven away by the radiation from a star burst, but which can sometimes fall back the galaxy.

The new paper is particularly effective in describing star formation in the tails and bridges of interacting systems, something had previously been lacking.

ESA announce 6th European Conference on Space Debris - Collision Video

Watch live streaming video from eurospaceagency at livestream.com

iPhone and Mobile Webstream for this video: http://iphone.livestream.com/eurospaceagency

Esa's Heiner Klinkrad of TU Braunschweig, just opened the 6th European conference on Space Debris in ESA's ESOC installation in Darmstadt, Germany,