首页
外语
计算机
考研
公务员
职业资格
财经
工程
司法
医学
专升本
自考
实用职业技能
登录
外语
According to the dynamo theory, what two processes contribute to the creation of Earth’s magnetic field? Click on 2 answers.
According to the dynamo theory, what two processes contribute to the creation of Earth’s magnetic field? Click on 2 answers.
admin
2018-08-16
71
问题
According to the dynamo theory, what two processes contribute to the creation of Earth’s magnetic field? Click on 2 answers.
Listen to part of a lecture in an astronomy class.
Professor: Ok, the collection of Moon rocks that astronauts brought back to Earth in the early 1970s has provided us with a lot of information about the nature of the Moon, but there’s one thing about the rocks that has especially puzzled scientists ever since these missions: why did so many of them have a magnetic signature, evidence of having been magnetized in the past? After all, we know the Moon, unlike Earth, has no magnetic field today. Well, let’s back up a bit here and discuss Earth’s magnetic field.
It’s explained by the dynamo theory. The dynamo theory explains how Earth generates a long-lived global magnetic field. Now, we know Earth has a molten, metallic core. That is, a core made up largely of hot, liquid iron. Well, as the warmer molecules of this iron liquid rise, the cooler portions sink in a circulatory motion. This circulating metallic liquid inside Earth along with the rotation of the Earth creates electric currents, and these electric currents, in turn, create Earth’s magnetic field.
A molten core that generates a magnetic field is a dynamo, and the action of this dynamo magnetizes rocks on Earth, but the Moon doesn’t have a magnetic field now, and scientists have long believed that it has never had one. Why? Well, we know that the Moon is cold and it was also thought that the Moon was too small to have had a molten core, so it came as a big surprise when astronauts returned with rock samples that had been magnetized. Well, ever since those early missions, scientists have been debating the answer to this question. Some scientists believe that the shock of the impact of meteorites and asteroids left a magnetic signature on the Moon rocks. As we know, 3.9 billion years ago, relatively soon after the Moon formed, it underwent a period of heavy meteorite and asteroid bombardment. The scientists thought it was the impacts during this period of bombardment that shocked or altered many of the Moon rocks and left a magnetic signature on them.
Other scientists, however, have theorized that at one time, soon after the Moon formed, it could in fact have had a molten core and a magnetic field like Earth does, and that it was this, not meteor strikes, that was the source of the magnetic signatures on the Moon rocks. So who’s right? Well, the very detailed recent analysis of a particular Moon rock known as "the Trocto" may have finally helped to shed some light on this debate.
The Trocto is made of troctolite, a combination of minerals that crystallized very early in the Moon’s history, and I mean very early. This rock is about 4.3 billion years old and the magnetic signature found on Trocto was very different than the signature on other rocks. It suggests that the rock spent a very long period in a magnetic environment, millions of years.
You see, magnetization from a meteorite generates a short-lived magnetic field, which results in a certain magnetic signature, and the Trocto just doesn’t show that. In laboratory tests, scientists found it had been exposed to two stable and intense magnetic events, each followed by a long cooling period.
The first event, the one we’re most interested in, occurred about 4.2 billion years ago, about 300 million years after the Moon formed, the time when the Moon would most likely have a liquid metallic core. And because of the prolonged nature of the magnetic event, scientists speculate that it was caused by a magnetic field generated by a molten metallic core and not a meteor strike, so even if Trocto had been subjected to that later bombardment that occurred 3.9 billion years ago, it’s not the source of the magnetization that scientists found.
Well, this lunar dynamo theory certainly is very appealing to many of us. For one thing, we’ve been trying to explain the origin of the Moon for a long time, and this theory fits very nicely with the most widely accepted hypothesis of the Moon’s origin, the Giant Impact Hypothesis. The Giant Impact Hypothesis also allows for the possibility that the Moon had a small molten core. We’ll explore that theory and other ways the two theories give in a few minutes. Now, admittedly, having only one lunar rock as evidence isn’t enough for a lot of scientists, but it’s a good start.
选项
A、Circulating molten iron in Earth’s core
B、The Earth’s rotation
C、An intense magnetic event at the time Earth formed
D、The impact of meteorites on Earth’s surface
答案
A,B
解析
细节题。线索词为教授所说:This circulating metallic liquid inside Earth along with the rotation of the Earth creates electric currents,and these electric currents,in turn,create Earth’s magnetic field.如原文所述,地球内部循环的液态金属和地球自转会促成地球磁场的产生。C和D选项错误原因一致。强烈的磁场活动和陨石的冲击均在解释月球上许多岩石为何具有磁性特征。
转载请注明原文地址:https://kaotiyun.com/show/vAfO777K
0
托福(TOEFL)
相关试题推荐
Whatproblemsdothespeakersidentifyforthisproject?ChooseSEVENanswersfromtheboxandwritetheletters,A-H,nexttoq
CompletethenotesbelowWriteONEWORDONLYforeachanswer.TheGherkinBuildingCommissionedby:【L31】______firmcalledFoste
Completethenotesbelow.WriteNOMORETHANTWOWORDSforeachanswer.AdvertisingEffectTheimportantfactortoconsider
Whatisthemainopinionofeachofthefollowingpeople?ChooseFIVEanswersfromtheboxandwritethecorrectletter,A-G,ne
Whichattitudeisassociatedwiththefollowingpeopleduringtheconversation?ChooseSIXanswersfromtheboxandwritetheco
Completethenotesbelow.WriteONEWORDONLYforeachanswer.EffectsofurbanenvironmentsonanimalsIntroductionRecenturba
Choosethecorrectletter,A,BorC.PacificTapaClothPacifictapaclothisdifferentfromothertypesoftapaclothbecause
conversation本题询问该项目的工作内容。录音原文中的assistingwith是题目helpingwith的同义替换。英语教学项目的主要任务是帮助提高当地人说英语(Englishconversation)的能力,故空格处填入conversa
Whichofthefollowingquestionsdoesthepassageprimarilyanswer?Itcanbeinferredfromthepassagethatonemaindifferen
Whatdoesthepassagemainlydiscuss?Itcanbeinferredfromparagraph3thatchipmunksdonotaidinthedispersalofoaktr
随机试题
A.润肺化痰止咳B.润肺止咳化痰C.润肺化痰,止咳平喘D.敛肺定喘,止带,缩尿E.清肺化痰止咳,降逆止呕
宜在医师指导下选用非处方药补虚药的有
沥青混合料目标配合比设计阶段,经马歇尔试验确定OAC后,还应进行()检验。
对于集团股份支付,接受服务企业没有结算义务,应当将该股份支付交易作为权益结算的股份支付进行会计处理。()
本题涉及营业税、城建税(及教育费附加)、车船税、房产税。某市甲贸易公司2011年经营状况如下:(1)代理进口20批货物,共收取代理报酬55000元,代理成本15000元;(2)该公司拥有车辆情况:微型客车4辆;自重2吨货车3辆;(3)该公司拥有办公用房2处
祭月赏月活动始于周代,()始定为中秋节,南宋成为普遍的活动,明清以来盛行不衰。
可行性研究概括起来主要包括3个方面:________、经济和社会可行性。
Theairsurroundingusisimportanttoeveryone.Withoutair,wecouldnotexist.Everyoneunderstandsthat.Butairisnecessar
PresidentBush’svisitwasplannedto______30thanniversaryofPresidentNixon’svisittoChina.
A、Water’sspecificheatishigherthanthatofsand.B、Water’sspecificheatislowerthanthatofsand.C、Water’stemperaturec
最新回复
(
0
)