Space Rocks

Above, a meteor outburst during the Perseid meteor showers of 1995. Friction in the atmosphere turns rocks into fireballs.

Credit: S. Molau and P. Jenniskens, NASA Ames Research Center

Earth gets alien visitors all the time, in the form of space debris that makes it through our atmosphere. The terms can be confusing, so here's a glossery: Meteoroids are defined by the International Astronomical Union as "a solid object moving in interplanetary space, of a size considerably smaller than an asteroid and considerably larger than an atom." When a meteoroid enters a planet's atmosphere, its bright path across the sky is called a meteor. When a meteor makes it through the atmosphere without burning up entirely, the remnants found on earth are called meteorites. www.space.com

This 3-inch (8 centimeter)-wide meteorite fragment is part of the Allende meteorite, the most-studied meteorite ever. This car-sized chunk of rock flamed through Earth's atmosphere in February, 1969. It broke into thousands of smaller pieces, found strewn over the desert in the northern Mexico state of Chihuahua. 

The Allende meteorite is a carbonaceous chondrite, a rare type of meteorite that makes up only about 4 percent of known meteorites. The Allende meteorite contains components that are more than 4.5 billion years old, making the rock a snapshot of the conditions present in the earliest days of the solar system.

Credit: H. Raab

Until the 1980s, scientists believed that all meteorites came from the asteroid belt between Mars and Jupiter. This white-speckled rock changed all that. Researchers found it in Antarctica in 1981 and noticed its similarity to the moon rocks that Apollo astronauts brought back to Earth. Sure enough, tests showed that this rock came from the moon. In the next 15 years, 11 other moon rocks would be found on Earth.

Credit: NASA Johnson Space Center

Occasionally, meteorites bring something new to Earth. This 4.5-billion-year-old meteorite landed in northwest Africa. Inside, scientists discovered a mineral called krotite, which had never been found in nature before. Krotite forms at high temperatures and low pressure, and was likely one of the first minerals in the newly emerging solar system.

Credit: Chi Ma

Another controversial chunk of rock, this meteorite from Mars was found in 1984 in the Allan Hills ice field in Antarctica and dubbed ALH84001. In 1996, researchers announced in the journal Science that structures in the meteorite might be fossilized microbial life, setting off the kind of media storm that you'd expect from a story about potential life on Mars. But most scientists remain skeptical that the Allan Hills specimen is evidence of life, and studies since 1996 have been unable to conclusively prove that the original claims are true.

Credit: NASA/Johnson Space Center's Meteorite Processing Laboratory

Earth isn't the only planet with alien rocks. NASA's Mars Exploration Rover Opportunity found this iron meteorite on the Red Planet, the first meteorite ever found on another world. 

The basketball-sized rock, dubbed "heat shield rock" because it sits near the debris of Opportunity's heat shield, is mostly nickel and iron.

Credit: NASA

In 1492, this stone fell from the sky outside the walled city of Ensisheim, located in the Alsatian region France. The stone's descent was seen as a sign from God; the extraterrestrial origin of meteorites would not be accepted for another 300 years. The Ensisheim meteorite was brought into the city and chained up in church to keep it Earth-bound. 

Credit: Courtesy of Heritage Auctions

The Esquel meteorite, consisting of iron-nickel and olivine, was discovered in central Argentina. It is an example of a rare meteorite that may have been born from the collisions of magnetic asteroids in the early solar system, scientists say. 

Image released Nov. 15, 2012.

Credit: Arlene Schlazer

Rare Meteorites Born In Asteroid Crashes

by Charles Q. Choi, SPACE.com Contributor   |   November 15, 2012 02:01pm ET

www.space.com

Rare, gem-studded meteorites that resemble stained-glass windows when backlit may have come from magnetic asteroids that splintered apart in ancient collisions, scientists say.

The solar system once may have been full of swarms of these tiny magnetic asteroids, investigators add.

The space rocks known as pallasites, first discovered in 1794, are very rare, with only about 50 known. These meteorites are mixtures of iron-nickel metal and translucent, gem-quality crystals of the green mineral olivine.  

"How you get a mixture of metal and these gem-like crystals has been a longstanding mystery," lead study author John Tarduno, a geophysicist at the University of Rochester in New York, told SPACE.com. "Because of the density differences of these materials, you'd normally think they'd separate into two different groups."

Chemical analyses have suggested the pallasites came from at least three different asteroids.

The researchers speculated that any magnetized material within these meteorites might shed light on their formation, since asteroids would possess magnetic fields only under certain special circumstances.

Magnetic meteorite mystery

The researchers looked at metal specks encapsulated within olivine crystals in two pallasites. These crystals are far better at recording past magnetic conditions than the surrounding metal is.

The investigators used a laser to heat the metal grains past their individual Curie temperatures — the point at which a metal loses its magnetization. The grains were then cooled in the presence of a magnetic field in order to become re-magnetized. By monitoring the grains using a highly sensitive measuring instrument called a SQUID ("superconducting quantum interference device"),  the research team was able to calculate the strength of the magnetic field that these metal particles once possessed.

The scientists found these metal specks were once strongly magnetized. This suggests the meteorites came from asteroids that were themselves once strongly magnetic, perhaps 4.2 billion to 4.4 billion years ago.

Earth's magnetic field is created by its dynamo, the churning in its molten metal core. Since asteroids are relatively small, they would have cooled quickly and no longer possess molten cores or magnetic dynamos. However, recent analyses suggest that Vesta, the second-largest asteroid in the solar system, once possessed a magnetic dynamo.

Ancient asteroid crashes

Past research had suggested that pallasites originate in the boundary layer between an asteroid's metallic core and rocky mantle, arising from the mixing of material one might find there. However, this would not explain the magnetization — if the pallasites formed this way, they would not have cooled sufficiently to become permanently magnetized before any dynamo in the asteroid decayed.

Instead, the research team's computer models suggested these magnetic pallasites formed when asteroids collided with much larger asteroids, protoplanet-sized bodies about 250 miles (400 kilometers) wide. The impact would have injected a liquid mix of iron and nickel from the cores of the smaller asteroids into the larger ones, explaining the jumble of materials seen within the meteorites. The pallasites would have formed while the dynamos of these protoplanets was still active.

"If pallasites really are made of metal from one object and minerals from another, then there might be chemical 'fingerprints' we can look for to prove this hypothesis," study author Francis Nimmo, a planetary scientist at the University of California, Santa Cruz, told SPACE.com. "Another critical measurement to make is to get the ages of the minerals. Our models predict particular age ranges for these minerals, which can be tested against age measurements."

Tarduno noted the meteorites they analyzed represent only one of the parent asteroids of pallasites. "We'd like to sample some of the others," he said. "The techniques we've used here can be applied to meteorites of other small bodies as well."

Past research suggests thousands of protoplanets at least 60 miles (100 km) wide once dwelt within the solar system. The new findings suggest many of these might have been magnetic.

"The more small bodies we study, the more dynamos we find," Nimmo said. "The problem is that we don't understand what is driving those dynamos. Did they operate like the Earth's dynamo, or are they driven another way — for example, by their iron cores sloshing around after a giant impact?"