[ti:Scientists Have New Theory for What Killed the Dinosaurs]
[by:www.21voa.com]
[00:00.00]更多听力请访问21VOA.COM
[00:00.04]American researchers have a new theory
[00:03.88]about how an object struck the Earth
[00:07.56]and caused the dinosaurs to die off.
[00:12.24]Scientists mostly agree on where the impact happened
[00:17.28]about 65 million years ago.
[00:22.00]They say a huge object struck an area off the coast
[00:25.88]of what is now Mexico.
[00:30.44]Astronomers have said the most likely cause of the strike
[00:35.80]was either an asteroid or a comet.
[00:40.72]In recent years, researchers have presented evidence
[00:45.60]that the impact was caused by an asteroid.
[00:50.16]The theory suggests the asteroid came
[00:54.12]from an area between the orbits of Mars and Jupiter.
[01:00.28]But a study by two astronomers from Harvard University
[01:05.16]presents a new theory: that the crash was caused by a comet.
[01:11.32]The researchers say the comet came from an area
[01:16.56]containing icy debris on the edge of the solar system.
[01:22.24]The area is known as the Oort cloud.
[01:27.12]Their theory states that the comet
[01:29.92]was pulled into the solar system by Jupiter’s gravity.
[01:35.32]The comet then came very close to the sun,
[01:39.24]whose tidal force caused it to break up into pieces.
[01:44.60]The researchers believe one of the pieces crashed into the place
[01:49.92]that scientists have identified in Mexico.
[01:54.32]The team based its theory on a model
[01:58.00]created to predict the probability
[02:01.36]that a long-period comet from the Oort cloud would hit Earth.
[02:07.52]Long-period comets take more than 200 years to orbit the Sun.
[02:14.12]Because comets come from frozen areas of the outer solar system,
[02:20.36]they are icier than asteroids.
[02:23.76]They are known for leaving long trails of gas and dust as they melt.
[02:31.20]The new study was recently published in Scientific Reports.
[02:37.84]The lead author was Amir Siraj, an astrophysics student at Harvard.
[02:45.84]"Jupiter is so important because it's the most massive planet
[02:51.20]in our solar system," he told the French press agency AFP.
[02:57.48]Siraj said the findings showed that Jupiter’s large influence
[03:03.92]pushes “these incoming long-period comets into orbits
[03:09.60]that bring them very close to the sun.”
[03:13.60]The comets experience such a large tidal force from the sun
[03:18.96]“that the most massive of them would shatter
[03:22.64]into about a thousand fragments,” he said.
[03:27.20]Each of those fragments would be large enough
[03:30.92]to produce a crater the size of the Mexican site, he added.
[03:37.00]That massive impact is estimated to have been equal
[03:42.88]to the strength of about 10 billion nuclear bombs.
[03:48.52]The U.S. Space Agency NASA has estimated the strike
[03:53.64]created a huge crater about 180 kilometers wide and 900 meters deep.
[04:02.28]The event is believed to have caused widespread wildfires,
[04:08.16]earthquakes and ocean waves.
[04:12.32]It also released chemicals into the atmosphere, leading to severe cooling.
[04:18.84]Scientists blame the event for destroying
[04:23.16]more than 70 percent of plant and animal life.
[04:28.16]In addition, all dinosaurs that were not bird-like died out.
[04:34.20]The researchers say their theory
[04:37.40]can be tested by further studying the crater in Mexico,
[04:42.60]as well as possibly those on the moon.
[04:46.92]In addition, space explorers might also be sent
[04:51.40]to collect comet material for examination.
[04:56.80]The study also suggests
[04:59.20]that similar impacts can be expected to strike Earth
[05:03.76]about once every 250 to 730 million years.
[05:10.68]But the other lead researcher on the project,
[05:14.72]Harvard professor Avi Loeb, noted that that is just an estimate.
[05:21.48]“You never know when the next one will come,” he said.
[05:26.56]“The best way to find out is to search the sky.”
[05:31.24]I’m Bryan Lynn. 更多听力请访问21VOA.COM
END OF TRACK. "END OF TRACK." The two men bowed. "Whoever was that person you were talking to?" she enquired, as soon as they stood together. The took of triumph faded from her eyes, she had grown worn and weary. The roses were wilting on the walls, the lights were mostly down now. Hetty, looking in to see if anything was wanted, found herself driven away almost fiercely. I only saw Master Jervie once when he called at tea time, The year 1747 was opened by measures of restriction. The House of Lords, offended at the publication of the proceedings of the trial of Lord Lovat, summoned the parties to their bar, committed them to prison, and refused to liberate them till they had pledged themselves not to repeat the offence, and had paid very heavy fees. The consequence of this was that the transactions of the Peers were almost entirely suppressed for nearly thirty years from this time, and we draw our knowledge of them chiefly from notes taken by Horace Walpole and Lord Chancellor Hardwicke. What is still more remarkable, the reports of the House of Commons, being taken by stealth, and on the merest sufferance, are of the most meagre kind, sometimes altogether wanting, and the speeches are given uniformly under fictitious names; for to have attributed to Pitt or Pelham their[112] speeches by name would have brought down on the printers the summary vengeance of the House. Many of the members complained bitterly of this breach of the privileges of Parliament, and of "being put into print by low fellows"; but Pelham had the sense to tolerate them, saying, "Let them alone; they make better speeches for us than we can make for ourselves." Altogether, the House of Commons exhibited the most deplorable aspect that can be conceived. The Ministry had pursued Walpole's system of buying up opponents by place, or pension, or secret service money, till there was no life left in the House. Ministers passed their measures without troubling themselves to say much in their behalf; and the opposition dwindled to Sir John Hinde Cotton, now dismissed from office, and a feeble remnant of Jacobites raised but miserable resistance. In vain the Prince of Wales and the secret instigations of Bolingbroke and Doddington stimulated the spirit of discontent; both Houses had degenerated into most silent and insignificant arenas of very commonplace business. "It certainly will be. Miss Widgeon," answered Maria, with strictly "company manners." "One who has never had a brother exposed to the constant dangers of army life can hardly understand how glad we all feel to have Si snatched from the very jaws of death and brung back to us." "Just plug at 'em as you would at a crow, and then go on your way whistlin'?" persisted Harry. "Hurroo!" echoed Hennessey; "that's the ticket." "Come forward, keeper," continued the baron, "and state how these arrows came into your hands!" "Yes." HoMEJULIA京香2018下载
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