Earth punctured by tiny cosmic missiles
By Robert Matthews, Science Correspondent
FORGET dangers from giant meteors: Earth is facing another threat from outer space. Scientists have come to the conclusion that two mysterious explosions in the 1990s were caused by bizarre cosmic missiles.
The two objects were picked up by earthquake detectors as they tore through Earth at up to 900,000 mph. According to scientists, the most plausible explanation is that they were "strangelets", clumps of matter that have so far defied detection but whose existence was posited 20 years ago.
Formed in the Big Bang and inside extremely dense stars, strangelets are thought to be made from quarks - the subatomic particles found inside protons and neutrons. Unlike ordinary matter, however, they also contain "strange quarks", particles normally only seen in high-energy accelerators.
Strangelets - sometimes also called strange-quark nuggets - are predicted to have many unusual properties, including a density about ten million million times greater than lead. Just a single pollen-size fragment is believed to weigh several tons.
They are thought to be extremely stable, travelling through the galaxy at speeds of about a million miles per hour. Until now, all attempts to detect them have failed. A team of American scientists believes, however, that it may have found the first hard evidence for the existence of strangelets, after scouring earthquake records for signs of their impact with Earth.
The team, from the Southern Methodist University in Texas, analysed more than a million earthquake reports, looking for the tell-tale signal of strangelets hitting Earth.
While their very high speed gives strangelets a huge amount of energy their tiny size suggests that any effects might be extremely localised, and there is unlikely to be a blast big enough to have widespread effects on the surface.
The scientists looked for events producing two sharp signals, one as it entered Earth, the other as it emerged again. They found two such events, both in 1993. The first was on the morning of October 22. Seismometers in Turkey and Bolivia recorded a violent event in Antarctica that packed the punch of several thousand tons of TNT. The disturbance then ripped through Earth on a route that ended with it exiting through the floor of the Indian Ocean off Sri Lanka just 26 seconds later - implying a speed of 900,000 mph.
The second event took place on November 24, when sensors in Australia and Bolivia picked up an explosion starting in the Pacific south of the Pitcairn Islands and travelling through Earth to appear in Antarctica 19 seconds later.
According to the scientists, both events are consistent with an impact with strangelets at cosmic speeds. In a report about to be submitted to the Seismological Society of America, the team of geologists and physicists concludes: "The only explanation for such events of which we are aware is passage through the earth of ton-sized strange-quark nuggets."
Professor Eugene Herrin, a member of the team, said that two strangelets just one-tenth the breadth of a hair would account for the observations. "These things are extremely dense and travel at 40 times the speed of sound straight through the Earth - they'd hardly slow down as they went through."
The good news is that, despite their force, the impact of strangelets on an inhabited area would, probably, be less violent than that of a meteor. Prof Herrin said: "It's very hard to determine what the effect would be. There would probably be a tiny crater but it would be virtually impossible to find anything."
Scientists say that the discovery of strangelets would be a significant breakthrough, solving several long-standing mysteries. These include the nature of "dark matter", which, astronomers say, makes up more than 90 per cent of our galaxy. With their high density and stability, strangelets may account for much of this invisible matter.
Prof Frank Close, a particle physicist at Oxford University, said that confirmation of the events was crucial. "The first step is to see if one can find more examples and eliminate all other interpretations," he said. "If you're looking for very exotic and rare events, you need to be able to tell if it's the real thing or just an artefact."
According to Prof Herrin, the two events agree with predictions for strangelet impacts, which are expected to occur about once a year. He added, however, that finding more would be difficult, as seismic databases now automatically remove all signals not linked to earthquakes. He said: "To find more events we need to get at the data before that happens."
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