GMAT真题阅读―重力异常

GMAT真题阅读一重力异常Whenalargebodystrikesaplanetormoon,materialisejected,therebycreatingaholeintheplanetandalocaldeficitofmass.Thisdeficitshowsupasagravityanomaly:theremovalofthematerialthathasbeenejectedtomaketheholeresultsinanareaofslightlylowergravitythansurroundingareas.Onewouldthereforeexpectthatallofthelargemulti-ringimpactbasinsonthesurfaceofEarth'sMoonwouldshowsuchnegativegravityanomalies,sincetheyare,essentially,largeholesinthelunarsurface.Yetdatacollectedin1994bytheClementinespacecraftshowthatmanyoftheselunarbasinshavenoanomalouslylowgravityandsomeevenhaveanomalouslyhighgravity.Scientistsspeculatethatearlyinlunarhistory,whenlargeimpactorsstrucktheMoon'ssurface,causingmillionsofcubickilometersofcrustaldebristobeejected,densermaterialfromtheMoon'smantleroseupbeneaththeimpactorsalmostimmediately,compensatingfortheejectedmaterialandthusleavingnolowgravityanomalyintheresultingbasin.Later,however,astheMoongrewcoolerandlesselastic,reboundfromlargeimpactorswouldhavebeenonlypartialandincomplete.Thustodaysuchgravitationalcompensationprobablywouldnotoccur:theouterlayeroftheMoonistoocoldandstiff.当一个大型物体撞击一颗行星或者卫星时，会出现物质喷射，所以行星上会出现坑和质量的亏损。这种亏损表现为重力的异常：被喷射出去从而形成坑的物质的移除导致了一个区域的重力略低于其周边区域。所以有人会预期所有月球表面的'多重环形冲击盆地都会显示负的重力异常，因为从本质上来说，他们都是月球表面的大坑。但是1994年由Clementine宇宙飞船手机的数据显示许多这种月球盆地没有异常的低重力并且一些甚至有异常的高重力。科学家们推测在月球历史的早期，当大型冲击体撞击月球表面时，导致了成千上万立方千米的地壳物质喷射，来自月球月幔的高密度物质迅速上涌覆盖了冲击体，补偿了喷射出去的物质，所以在形成的盆地中没有留下异常的低重力。然而在此之后，随着月球变得越来越冷，越来越没有弹性，来自大型冲击体的回弹变得部分而不完整。所以在今天，这样的重力不常将不会再出现：因为月球的表面太过寒冷和坚硬。01Question1Accordingtothepassage,thegravitationalcompensationinthehighlightedtextiscausedbyofthefollowing?AdeficitofmassresultingfromthecreationofaholeinthelunarsurfaceThepresenceofmaterialfromtheimpactorinthedebriscreatedbyitsimpactThegradualcoolingandstiffeningoftheMoon'souterlayerTheejectionofmassiveamountsofdebrisfromtheMoon'scrustTherapidupwellingofmaterialfromthelunarmantleQuestion2Thepassagesuggeststhatifthescientistsinthehighlightedtextarecorrectintheirspeculations,thelargemulti-ringimpactbasinsontheMoonwiththemostsignificantnegativegravityanomaliesprobablywerenotformedearlyintheMoon'shistorywerenotformedbythemassiveejectionofcrustaldebrisarecloselysurroundedbyotherimpactbasinswithanomalouslylowgravitywerecreatedbytheimpactofmultiplelargeimpactorswereformedwhentheMoonwasrelativelyelasticQuestion3Whichofthefollowingbestdescribestheorganizationofthepassage?Ananomalousfindingiscited,thedatausedtosupportthatfindingareanalyzed,andthefindingismodified.Atheoryaboutaphenomenonisintroduced,dataseemingtodisprovethattheoryareanalyzed,andthetheoryisrejected.Aphenomenonisdescribed,afindingrelatingtothephenomenonisdiscussed,andapossibleexplanationforthatfindingisoffered.Adebateamongscientistsregardingtheexplanationforaparticularphenome