A meteor is a tiny piece of cosmic dust usually no larger than a garden pea and often smaller than this. When they encounter the Earth's atmosphere, friction causes them to burn up to be seen as what are commonly called 'shooting stars'.
Large meteors, known as bolides or fireballs, are particularly bright and may survive passage through the atmosphere and land on the ground. These are then termed 'meteorites'. Meteorites have lost their cosmic velocity by the time they reach the ground. One that fell in Barwell in Leicestershire still had sufficient energy to break a house window, but insufficient to break a vase on the windowsill where it remained undetected for several days.
Although meteors are often seen sporadically in the sky, others form into meteor streams (or showers). The showers can be predicted and are associated with comets.
Text courtesy of Peter Brown
March 16, 2000 NASA Press Release
A meteorite that exploded over a remote area of northwest Canada in January may offer "a new window into the universe before the solar system was created," said a NASA scientist who has begun analyzing some of the meteorite fragments.
The very primitive composition and pristine condition of the 4.5-billion-year-old meteorite "offers us a snapshot of the original composition of the entire solar system before the planets formed," said Dr. Michael Zolensky, a cosmic mineralogist at NASA's Johnson Space Center (JSC) in Houston. "It tells us what the initial materials were like that went into making up the Earth, the Moon and the Sun." The age of the solar system is about 4.5 billion years.
"These meteorite fragments are of immense scientific value and interest," said Dr. Richard Herd, Curator of National Collections for the Geological Survey of Canada. "This rare find potentially will contribute to a better understanding of the nature of the universe." He added that finding previously undetected compounds in the fragments will have implications for both planetary and biological sciences worldwide.
The scientists described the fragments -- lumps of crumbly rock with scorched, pitted surfaces -- as resembling partly used charcoal briquettes: black, porous, fairly light and still smelling of sulfur.
Several factors combined to make this meteorite a cosmic bonanza for scientists. First, it is a carbonaceous chondrite, a rare type of meteorite that contains many forms of carbon and organics, basic building blocks of life. Carbonaceous chondrites, which comprise only about 2 percent of meteorites known to have fallen to Earth, are typically difficult to recover because they easily break down during entry into Earth's atmosphere and during weathering on the ground.
Zolensky said the last time a carbonaceous chondrite like this fell to Earth and was recovered was 31 years ago. "This is probably the only time in my career this will happen," he said.
The location and timing of the fireball also contributed to the scientific value of the samples. The fragments are part of a meteor that blew apart over a remote area of the Yukon Territory the morning of Jan. 18, 2000. The resulting sonic booms startled residents as far away as British Columbia and Alaska. The frozen, snow-covered ground of the remote Yukon provided near-ideal conditions for preservation, Herd said.
The finder, a local resident who has requested anonymity, collected the fragments in clean plastic bags and kept them continuously frozen. These are the only freshly fallen meteorite fragments recovered and transferred to a laboratory without thawing. Keeping the fragments continuously frozen minimized the potential loss of organics and other volatile compounds in the fragments.
About 2 pounds of meteorite fragments have been recovered so far. Of those, Zolensky has about a pound of fragments provided by the Canadian government and the University of Calgary. The finder loaned them to the university and to the National Meteorite Collection of the Geological Survey of Canada, Natural Resources Canada (NRCan) in Ottawa, which provided the still-frozen samples to JSC for study and analysis. NASA is working closely with NRCan scientists and is providing results of the analysis to them. "We are very sensitive to the fact that these are Canadian meteorites," Zolensky said. Any future studies will be done in cooperation with scientists worldwide.
Scientific analysis of the fragments has just begun. Tests have been limited to two non-destructive activities: making a thin section to analyze the mineralogy of the fragments, and measuring induced radioactivity. Tests for induced radioactivity, which are being carried out by Dr. David Lindstrom of JSC, measure the object's exposure to space radiation. This can be used to determine the size of the original meteoroid in space, estimates of which range up to 50 feet in diameter, with a mass of more than 55 tons.
The next step in the study of the fragments will be baseline analyses of the organics in the meteorite. This would require the destruction of some samples, and negotiations are under way with the finder for permission to do such tests.
"The nice thing about having a sample like this is that you don't really know what you're going to find or where it's going to lead," Zolensky said. "You can tuck samples away for the future when new questions come along that people can't even think up now."
American Association for the Advancement of Science Press Release 8 June 2000
Brine-pocketed salt crystals within the "Zag" meteorite may be among the oldest materials found in the solar system, a U.K. research team has found. This surprisingly old age could spur scientists to speed up the prevailing scenario of the solar system's evolution, and opens the possibility that hospitable conditions for life might have arisen earlier than previously thought. The researchers report their findings in the 9 June issue of Science.
Using radioisotope dating, scientists at the University of Manchester and the Natural History Museum in London determined that the salt crystals probably formed within about two million years of the solar system's birth. If this age is correct, it means that the dust, gas, and ice swirling around the newborn sun clumped together into rocky fragments far more quickly than researchers have assumed. These fragments were the parent bodies for primitive meteorites like Zag and the essential building blocks for asteroids and planets.
In the scenario proposed by first author James Whitby and his colleagues, Zag's parent body accreted rapidly into a rocky mass containing water and radioactive isotopes. The isotopes' decay generated enough heat to melt any ice within the rock matrix, and soon caused the liquid to evaporate altogether. The salt crystals, (mainly sodium chloride, or "halite,") precipitated during the evaporation process, similar to the way halite forms when sea water evaporates on Earth.
The Zag meteorite, which fell in Morocco in 1998, was the second meteorite found carrying halite crystals. Like those in the Monahans meteorite (see Science, 27 August, 1999, p. 1364 and 1377), these crystals contained microscopic inclusions of water, the key ingredient for life. Both finds have raised hopes of learning more about the possibility that life might have evolved elsewhere in the solar system. To do so, researchers would first have to determine when water existed in these parent bodies and for how long.
Previous studies of other meteorite minerals could only place the time of liquid water within 100 million years after the solar system's formation, explains Ulrich Ott, of the Max-Planck-Institut für Chemie in Mainz, Germany, in a Perspective article that accompanies the research paper. Rubidium dating of the halite in Monahans had suggested that the crystals might be some of the oldest materials in the solar system, but this dating method is less precise than the one used by Whitby and his colleagues.
"Those results showed that this was interesting halite. Immediately, our next question was, 'how old is it?'" Whitby said.
A second crucial task was to make sure that the halite hadn't precipitated from water on Earth that contaminated the meteorite after it landed.
To tackle both these questions, Whitby and co-authors Ray Burgess, Grenville Turner and Jamie Gilmour, of the University of Manchester, and John Bridges, of the Natural History Museum, analyzed xenon, iodine, and argon isotopes extracted from a minute sliver of a Zag halite crystal. They found a surprisingly large amount of xenon-129, which forms when iodine-129 decays. Iodine-129 was present in the early solar system but is not found on Earth.
"I popped the halite in the machine and got this amazing peak showing an abundance of xenon-129. That told us immediately it wasn't terrestrial material. I hadn't really expected that, so it was quite stunning, really," said Whitby.
Because scientists know the rate at which iodine-129 decays into xenon-129, Whitby and his co-authors could then calculate the age of the halite. They estimate that the crystals formed about two million years after the birth of the solar system 4.57 billion years ago, suggesting that liquid water departed from its parent body soon after it had appeared.
"The results from the I-Xe dating by Whitby et al. are particularly astonishing," writes Ott in the Perspective article.
Until the discovery of halite in Monahans and Zag, the oldest materials in the solar system were thought to be chondrules, glassy spheres that make up much of the mass in primitive meteorites. Although the origins of these particles are still under debate, many scientists believe that chrondrule formation continued for about 5 million years after the solar system's birth.
From their analysis of the crystals and the rocky matrix around them, the research team proposes that the halite grew very quickly on a newly formed asteroid that had just formed by the collision of smaller particles. About 300 million years later, another large impact smashed the loose fragments together into a more solid conglomerate, a piece of which became the Zag meteorite.
The presence of liquid water also has important implications for understanding the geology of moons and planets with large amounts of heat in their interiors. Volcanic activity is closely tied with the availability of water, which plays a major role in the formation of magma.
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