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:

Cosmic turbulences result in Star and Black Hole formation

This image shows an artist's rendering of a protoplanetary disc. Credit: Pat Rawlings / NASA

Just how stars and black holes in the Universe are able to form from rotating matter is one of the big questions of astrophysics.

What we do know is that magnetic fields figure prominently into the picture.

However, our current understanding is that they only work if matter is electrically well conductive—but in rotating discs this isn't always the case.

Now, a new publication by Helmholtz-Zentrum Dresden-Rossendorf physicists in the scientific journal Physical Review Letters shows how magnetic fields can also cause turbulences within "dead zones," thus making an important contribution to our current understanding of just how compact objects form in the cosmos.

When Johannes Kepler first proposed his laws of planetary motion in the early days of the 17th century, he could not have foreseen the central role cosmic magnetic fields would play in planetary system formation.

Today, we know that in the absence of magnetic fields, mass would not be able to concentrate in compact bodies like stars and black holes.

One prominent example is our solar system, which formed 4.6 billion years ago through the collapse of a gigantic cloud of gas, whose gravitational pull concentrated particles in its center, culminating in the formation of a large disc.

Dr. Frank Stefani

"These accretion discs are extremely stable from a hydrodynamic perspective as according to Kepler's laws of planetary motion angular momentum increases from the center towards the periphery," explains HZDR's own Dr. Frank Stefani.

"To explain the growth rates of stars and black holes, there has to exist a mechanism, which acts to destabilize the rotating disc and which at the same time ensures mass is transported towards the center and angular momentum towards the periphery."

As early as 1959, Evgenij Velikhov conjectured that magnetic fields are capable of prompting turbulences within stable rotating flows.

Although it wasn't until 1991 that astrophysicists Steven Balbus and John Hawley fully grasped the fundamental significance of this magneto rotational instability (MRI) in cosmic structure formation.

Balbus and Hawley will be this year's recipients of the one million Dollar Shaw Prize for astronomy, which will be given in September 2013.

However, to ensure the MRI actually works, the discs have to exhibit a minimum degree of electrical conductivity.

In areas of low conductivity like the "dead zones" of protoplanetary discs or the far-off regions of accretion discs that surround supermassive black holes, the MRI's effect is numerically difficult to comprehend and is thus a matter of dispute.

HZDR scientists, who to date have been mostly concerned with an experimental study of the MRI, have now offered a new theoretical explanation for this phenomenon.

Read the full article here

More information: O.N. Kirillov, F. Stefani: Extending the range of the inductionless magnetorotational instability, in Physical Review Letters 111 (2013), S. 061103, DOI-Link: link.aps.org/doi/10.1103/PhysRevLett.111.061103

H1N1: UKs BMJ Report Extreme Doubts over Tamiflu results

UK medical experts from the British Medical Journal (BMJ) are questioning the effectiveness of antiviral drug Tamiflu commonly used against the swine flu virus spreading across the globe, according to a study reported in Britain Tuesday.

An investigation by the British Medical Journal acknowledges that the drug oseltamivir, which trades as Tamiflu, has "a very modest effect in reducing flu symptoms and infectivity in otherwise healthy adults."

But "researchers say there is insufficient published data to know if oseltamivir reduces complications in otherwise healthy adults," the media groups said in a joint statement.

The use of flu drugs like oseltamivir has increased dramatically since the A(H1N1), or swine flu, pandemic began in April 2009, with government rushing to stockpile treatments while persuading people to have vaccinations.

The global death toll since the virus was uncovered in April approached 8,770 in early December, with confirmed infections in 207 countries, according to World Health Organisation figures.

Claims about the effectiveness of drugs like Tamiflu against flu complications have been a key factor in governments' choosing to spend millions of dollars to hoard them, the British Medical Journal said.

The British government has spent about 500 million pounds (813.9 million dollars) on such drugs, they said.

But research on the drugs by scientists from Australia's Bond University was hampered by a "paucity of good data" available from Swiss pharmaceutical giant Roche that produces oseltamivir.

"As a result, they conclude that they have no confidence in claims that oseltamivir reduces the risk of complications of influenza in otherwise healthy adults, and believe it should not be used in routine control of seasonal influenza."

The researchers called on governments to set up studies to monitor the safety of drugs like Tamiflu, which are called neuraminidase inhibitors.

A team from the University of Birmingham concluded meanwhile that oseltamivir may reduce the risk of pneumonia in otherwise healthy people who contract flu.

"However, the absolute benefit is small, and side effects and safety should also be considered," the statement said.

Professor Nick Freemantle from the University of Birmingham said he saw "very little evidence to support the widespread use of oseltamivir in the otherwise healthy population who are developing signs of influenza-like illness."

"We have remarkably few resources in this country to spend on pharmaceuticals on health and it's surprising to see such widespread use of oseltamivir," he said.

British Medical Journal editor-in-chief Fiona Godlee warned that the review left unresolved important questions about effectiveness of the drugs.

"Governments around the world have spent billions of pounds on a drug that the scientific community now finds itself unable to judge," she said.

Roche has estimated sales of 1.6 billion pounds this year alone from the drug, the statement said.

NASA: Centaur Moon Impact produces disappointing result

In the final minutes of its plunge toward the moon, NASA's LCROSS spacecraft spotted the brief infrared flash of a rocket booster hitting the lunar surface just ahead of it – and it even saw heat from the crater formed by the impact.

But scientists remain puzzled and disappointed about why the event did not seem to generate a visible plume of debris as expected.

As hundreds of telescopes and observers watched, the highly publicised NASA mission to search for water on the moon reached its grand finale at 0431 PDT (1131 GMT) with a pair of high-speed crashes into a lunar crater named Cabeus.

During the crucial moments at NASA's Ames Research Center in Moffett Field, California, scientists and engineers with LCROSS (Lunar Crater Observation and Sensing Satellite) peered in silent concentration as successive images of the crater grew larger on their screens.

Spectroscopic hints
Nearby, some 500 bleary-eyed visitors that had gathered overnight outside mission control were watching the same pictures on a giant outdoor screen.

Yet, immediately after the scheduled impact time, there was no obvious sign of the spectacular explosion that many were expecting. "Impacting into the moon is an unpredictable business at best," Anthony Colaprete, principal investigator for LCROSS, said in a post-impact briefing.

Colaprete did not offer definitive word as to why the visual camera apparently did not detect the event but added there were interesting changes in spectroscopic data taken by the spacecraft that might have been produced by a debris cloud. "I'm not convinced that the ejecta is not in the data yet," he said.