快猫短视频

IN THE basement of London鈥檚 Natural History Museum sit some rocks so ancient they make the demise of the dinosaurs upstairs seem like it was only yesterday. One is from a meteorite that in 1857 made villagers jump out of their skins when it fell to Earth in India, heralded by a fanfare of sonic booms.

Weighing over 70 kilograms and peppered throughout with small round pieces of silicate, the Parnallee meteorite is now giving scientists cause for concern. An analysis suggests that this 4.6 billion-year-old space rock and others like it may be testimony to a dramatic new view of how planets and asteroids formed. Forget tranquil evolution, the Earth may owe its very existence to furious winds that blew red-hot rock out from the Sun at hundreds of kilometres per second.

It鈥檚 a controversial idea. Until recently all scientists believed that the formation of the Solar System was a fairly sedate affair. For the past two centuries researchers thought asteroids and planets grew from dust and gas that quietly clumped together in a disc of matter orbiting the young Sun. But astrophysicist Frank Shu of the National Tsing Hua University in Hsinchu, Taiwan, now has serious doubts. Analysis of the oldest meteorites has convinced him that the Solar System鈥檚 youth was much more violent than anyone ever imagined.

Shu hadn鈥檛 been planning to upset the status quo. He was trying to explain how some embryonic stars blast out energetic streams of hydrogen from their poles. First spotted in the early 1980s, these 鈥渂ipolar outflows鈥� mystified theorists. Stars are born when a giant cloud of dust and gas collapses to form a spinning star surrounded by a swirling disc of matter. To understand how the torrents of hydrogen form in these young stars, Shu, who was then at the University of California in Berkeley, calculated how the material in the disc interacts with the young star鈥檚 magnetic field.

According to his model, the burning star heats dust and gas at the inner edge of the disc so fiercely, that electrons are ripped out of the atoms to leave charged ions. Under the influence of the star鈥檚 magnetic field and gravity, this ionised material falls onto the star. Shu also worked out that all the dust and gas has huge angular momentum, much greater than that of the star itself. According to a key law of physics, the total angular momentum of a closed system must remain the same. Because of this, as the ionised material is sucked inwards, some of it is flung back out at high speed.

Eureka moment

But Shu鈥檚 calculations showed something else. The powerful stream of charged particles has a magnetic field of its own, strong enough to change the Sun鈥檚 field around the dusty disc (see Diagram). Instead of threading through the disc, the magnetic field lines at this point are pinched into an X shape at its inner edge. It was a crucial difference. As gas and dust gravitated in from the cool outer reaches of the disc towards the fiery inner edge, the intense magnetic field here blew them far and wide. Shu called this the X-wind.

He and his colleagues continued to work on the model and found that it could explain the bipolar outflows from embryonic stars that astronomers were seeing with telescopes. But as time passed, he discovered that the X-wind has other talents.

The eureka moment came in the mid-1990s when Shu was in Wellesley, Massachusetts, at a conference on star formation. Al Cameron of the University of Arizona gave a talk on meteorites that have journeyed to Earth from the asteroid belt. He speculated that some kind of wind might explain why most meteorites contain puzzling round blobs of silicates called chondrules, all about a millimetre across. Rich in magnesium and iron, chondrules are thought to have clumped together to produce boulders that went on to attract and merge with other boulders, eventually forming rocky planets in the inner Solar System, including Earth.

The trouble is no one knows how chondrules formed in the first place. They must have started out as loose balls of dust in the cloud of matter surrounding the Sun. But their round shape suggests the dust grains then melted briefly to make a single blob of rock. 快猫短视频s suspect that chondrules must have been molten for at least ten minutes. Anything less would have prevented the silicate crystals seen inside chondrules from growing. And if they had been molten for more than an hour, the potassium and sodium they contain would have fizzled out of the globules.

But why did they melt? Most meteorites come from the asteroid belt, which lies between the orbits of Mars and Jupiter. It is a freezing cold place today and it probably wasn鈥檛 much warmer billions of years ago. 鈥淲ithin the asteroid belt, the temperature was maybe 200 kelvin 鈥� that鈥檚 hardly enough to melt rock,鈥� says Shu.

Baffled, physicists had suggested that shock waves in the disc of gas and dust surrounding the Sun could have heated pockets to the temperatures needed to melt the chondrules. Or maybe primeval lightning bolts 鈥� electrical discharges in the swirling disc 鈥� could have done the trick. 鈥淚t鈥檚 always been a mystery how materials in the Solar System were originally heated 鈥� after all these years it鈥檚 frustrating that we still don鈥檛 know,鈥� says Sara Russell, a meteorite expert at the Natural History Museum in London.

Cameron also questioned why chondrules are all about a millimetre across. Very few are twice that size, or half that size. 鈥淚t鈥檚 completely unlike what you鈥檇 find in a truckload of gravel, for example, where you鈥檇 typically see all different sizes from small particles to big particles,鈥� says Shu. 鈥淭he chondrules seem to be size-sorted, just like a can of peas.鈥�

At the conference, Cameron argued that if some kind of wind had blown chondrules across the Solar System, the lightest ones would have drifted farthest while the heavy ones would have been left behind. So the chondrules carried to the narrow asteroid belt 鈥� where they could gang together into asteroids and remain in stable orbits 鈥� would all, naturally, be roughly the same size.

Shu suddenly realised he had just the kind of wind Cameron was describing 鈥� the X-wind. It all fitted together like a jigsaw. 鈥淣ear the Sun, it鈥檚 natural to get the temperatures and timescales to melt the chondrules,鈥� says Shu. 鈥淲e know that flares go up in young stars, and they have timescales of an hour. It鈥檚 natural to think these energetic events can melt rock.鈥� And when he went back to his office to do some calculations, he found that the numbers tallied: the X-wind should carry millimetre-sized chondrules from near the Sun to roughly the distance of the asteroid belt.

Although geological changes on the restless Earth have gradually obliterated all traces of the chondrules that originally made up the bulk of our planet, meteorites may still hold plenty of evidence for the X-wind. And when Shu learned that chondrules behave like little bar magnets, it was a further boost for his theory. In fact their magnetisation is so strong, it suggests that the molten droplets of rock solidified in powerful magnetic fields, around 0.1 to 1 millitesla. That鈥檚 much stronger than the magnetic field on the surface of the Earth (around 0.045 millitesla), let alone way out in the asteroid belt. What these meteorites have recorded, says Shu, is the ancient magnetic field of the Sun near the region where the X-wind launched the chondrules into space.

Things seemed to be going well for the X-wind theory. It could explain a host of strange properties of chondrules and star formation. But it didn鈥檛 work when Shu and his colleagues tried to use it to explain why certain meteorites contain the remains of a puzzling mix of radioactive elements (see 鈥淩adioactive tracers鈥�). So most scientists held on to the traditional view that the radioisotopes were made when a nearby star exploded and got mixed into large globules called calcium-aluminium inclusions (CAIs), which also melted and solidified a long time ago.

It was a tough time for the X-wind theory, but in the past couple of years new data has revived its fortunes. In 2001, Alexander Krot of the University of Hawaii together with Russell at the Natural History Museum and their colleagues reported their conclusions after analysing chondrules in two metal-rich meteorites from Libya and Antarctica. Their analysis suggested the chondrules formed at a temperature of 1500 kelvin or more, so that moderately volatile elements like sulphur fizzled out of them. But then something must have whipped the chondrules out of the resulting cloud of sulphur vapour, because little sulphur condensed onto them when they cooled (Science, vol 291, p 1776). 鈥淥ne way you can do that is if they鈥檙e suddenly flung outwards in the X-wind,鈥� says Russell.

Meanwhile, geochemist Kevin McKeegan of the University of California at Los Angeles and his team found that the famous Allende meteorite that fell onto Mexico in 1969 had once contained radioactive beryllium-10. This was dramatic news because stars never manufacture beryllium-10 in their interiors. Had it instead come from intense radiation processes in the early Solar System, namely the X-wind?

The case in favour of the X-wind strengthened when McKeegan鈥檚 team found that Allende also once contained beryllium-7. It too is radioactive, but its activity is short-lived with a half-life of only 53 days. So any formed during the birth of the Solar System will have long since vanished, leaving only a characteristic trail of decay products behind.

In Shu鈥檚 view, the only way Allende鈥檚 beryllium-7 could come from a supernova 鈥� the alternative theory for the formation of radioisotopes in meteorites 鈥� would be if the Solar System formed in less than 53 days. Much longer and all the element would have decayed before it could become incorporated into molten globs of rock. 鈥淣ow no one believes that,鈥� he laughs. 鈥淭he bible talks about seven days, but we think that鈥檚 metaphorical.鈥� For him, the beryllium-7 data is the best clue yet that his X-wind blew out of the young Sun.

Not everyone is convinced. In March, at the Lunar and Planetary Science Conference in Texas, Steven Desch of the Carnegie Institution of Washington argued that galactic cosmic rays could have included heavy beryllium-10 ions that became trapped in CAIs. And Russell says the beryllium-7 data is controversial, and very difficult to interpret.

Grossman is sceptical that a wind could melt chondrules and CAIs near the Sun, then hurl them all the way out to the asteroid belt to make rocks that 鈥渕agically鈥� still have the same relative abundances of non-volatile elements as the Sun. More likely, he says, some ingredients would separate out in the wind. He does not worry about the precise size distribution of chondrules. And to him, the X-wind theory is like a chameleon that changes colour every time new data appears. 鈥淵ou鈥檝e got to watch these theoreticians,鈥� says Grossman. 鈥淚t seems that each time I read a paper about the X-wind, it changes a little.鈥�

Other critics are adamant that you simply don鈥檛 need an X-wind to explain the radioisotopes in CAIs. But most are keeping an open mind. 鈥淧eople have very strong opinions, but I really don鈥檛 think there鈥檚 any smoking-gun evidence either way,鈥� Russell concludes. She is convinced that clinching evidence will come in the next couple of years, however, when meteorites at the Natural History Museum are probed in greater detail. With Matthieu Gounelle at the University of Paris, she is pioneering measurements of a rare isotope, vanadium-50, in CAIs. The X-wind model predicts they should contain an excess of the isotope vanadium-50, which is stable and so leaves an indelible signature. 鈥淭hat would certainly convince me,鈥� says Russell.

More clues about the X-wind will come when spacecraft return samples from asteroids and comets in the next few years (see 鈥淪pace dust collection鈥�). Shu predicts that comets will contain mini chondrules 鈥� the ones that the X-wind blew out way beyond Neptune. 鈥淛ust like a garden sprinkler, the X-wind doesn鈥檛 just sprinkle chondrules to the asteroid belt, it sprinkles them all over the Solar System,鈥� he says.

If he is right, planetary scientists will have to rethink their ideas about how Earth, Mars, Venus and Mercury formed. And it could be grim news for astronomers looking for cosy Earth-like planets around other stars. So far researchers have found more than 100 extrasolar planets, all of them gas giants like Jupiter or Saturn, which planet-hunting techniques find most easily. New projects in the pipeline hope to nail small, rocky, Earth-like planets orbiting their stars in warm zones where liquid water could flow and life could evolve. But if the X-wind theory is right, rocky planets as we know them might be rare.

Chondrules were the seeds for the growth of the rocky planets in our own Solar System, so the Earth may owe its existence to an X-wind that happened to blow just right. Too gentle a breeze wouldn鈥檛 have returned enough chondrules to the disc of the solar nebula for rocky planets to have formed. Had it blown too hard, the chondrules might have flown far into the frigid realms of the comets. Near other stars, the bricks and mortar for building Earth-like planets may, quite literally, have gone with the wind.