(This passage was excerpted from material
published in 1996.)
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.
Q36:
The passage suggests that if the scientists mentioned in line 19 are correct in their speculations, the large multi-ring impact basins on the Moon with the most significant negative gravity anomalies probably
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