GWD17-Q35

horizonlily

GWD17-Q35

      When a large body strikes a planet

            or moon, material is ejected, thereby

            creating a hole in the planet and a local

Line     deficit of mass.  This deficit shows up

  (5)      as a gravity anomaly:  the removal of

the material that has been ejected to

make the hole results in an area of

slightly lower gravity than surrounding

areas.  One would therefore expect that

 (10)     all of the large multi-ring impact basins

on the surface of Earth’s Moon would

show such negative gravity anomalies,

since they are, essentially, large holes

in the lunar surface.  Yet data collected

 (15)     in 1994 by the Clementine spacecraft

show that many of these lunar basins

have no anomalously low gravity and

some even have anomalously high

gravity.  Scientists speculate that early

 (20)     in lunar history, when large impactors

            struck the Moon’s surface, causing

            millions of cubic kilometers of crustal

debris to be ejected, denser material

from the Moon’s mantle rose up

 (25)     beneath the impactors almost imme-

diately, compensating for the ejected

material and thus leaving no low gravity

anomaly in the resulting basin.  Later,

however, as the Moon grew cooler

 (30)     and less elastic, rebound from large

impactors would have been only partial

and incomplete.  Thus today such

            gravitational compensation probably

would not occur:  the outer layer of

the Moon is too cold and stiff.

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35. Q35:

According to the passage, the gravitational compensation referred to in line 33 is caused by which of the following?

                       

1. A deficit of mass resulting from the creation of a hole in the lunar surface
2. The presence of material from the impactor in the debris created by its impact
3. The gradual cooling and stiffening of the Moon’s outer layer
4. The ejection of massive amounts of debris from the Moon’s crust
5. The rapid upwelling of material from the lunar mantle

答案是E，但是我觉得E说的月球地底物质上升不是本质原因，而是A因为有这个deficit mass的形成(或者说这个撞击）才会造成密度高的物质往上挤，从而造成gravitational compensation。请教怎么理解？

盆子要革命

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