* Because of its slow rotation, Mercury's solar day is twice as long as its year. Although daytime temperatures are very high, temperature plummets during the long nights. The range of surface temperatures is greater for Mercury than for any other planet or satellite.
* The dominant features of Mercury's surface are impact craters.
Because of Mercury's relatively large gravity, debris from impacts
was not thrown as far as for lunar craters. Thus young craters
have not been effective at erasing old craters.
* A pattern of great scarps shows that Mercury's crust has been
altered since it formed. This probably happened when Mercury shrank,
fracturing the crust. All internal activity in Mercury probably
ended about 3.8 billion years ago.
* Mercury's atmosphere is extremely tenuous. It consists of gases
trapped from the solar wind or released when meteoroids struck
Mercury.
* The average density of Mercury is almost as large as that of
the Earth. This shows that Mercury contains a large proportion
of heavy elements, the most abundant of which is probably iron.
* Mercury's high iron content may have resulted from the loss
of most of its rocky exterior. This may have happened when Mercury
collided with another body almost as large as Mercury.
* Venus is an especially important planet to understand, because
it is similar to the Earth in many ways. By comparing the Earth
and Venus, we can see how small initial differences in the two
planets have led to significant differences today.
* Unlike the other terrestrial planets, Venus has a retrograde
rotation. The rotation of Venus is very slow, resulting in a solar
day that is 117 Earth days long.
* The lower atmosphere of Venus is as hot as an oven. The large
amount of carbon dioxide in Venus's atmosphere results in a surface
pressure 92 times as large as that of the Earth.
* The thick cloud layer of Venus is made of concentrated sulfuric
acid droplets. The main cloud layer is 25 km thick. Nevertheless,
enough sunlight penetrates the clouds to make Venus's surface
as bright as the Earth's on a cloudy day.
* The thick atmosphere of Venus produces a strong greenhouse effect
that is responsible for Venus's high atmospheric temperature.
* Venus's surface has many impact craters. However, there are
no small impact craters because small meteoroids are destroyed
in the dense atmosphere before they reach the ground. Venus appears
to have undergone planetwide lava flooding about a half billion
years ago.
* The surface of Venus shows abundant evidence of volcanic activity
and crustal distortion. However, surface features seem to have
been formed through vertical motion in Venus's crust rather than
plate tectonics. A possible reason that plate tectonics is inhibited
on Venus is that Venus's high crustal temperature may make the
crust too buoyant to be easily forced into the interior.
* Similar amounts of nitrogen and carbon dioxide have been released
from the interiors of the Earth and Venus. In the case of Venus,
almost all of the outgassed nitrogen and carbon dioxide remains
in the atmosphere. Whatever water that was released from Venus's
interior, however, has been removed from the atmosphere.
* The climates of Venus and the Earth were probably quite similar
at first. However, the warmer temperature of Venus's atmosphere
caused it to be more humid than that of the Earth. The additional
water vapor partially blocked the escape of infrared radiation
and produced a greenhouse effect, further warming Venus. The atmosphere
quickly became much hotter, evaporating surface water. Without
oceans, carbon dioxide could not be deposited as carbonaceous
rocks and therefore entered the atmosphere, resulting in the thick,
hot atmosphere of Venus today.
arachnoid , corona, intercrater, plain, retrograde, rotation, scarp, shield, volcano, smooth plains
1. In which of the following circumstances can Mercury be observed? high in the sky at noon.
2. Wich of the following is true about Mercury's rotation? planet revolves 3 times for every 2 revolutions because of tidal resonance
3. The scarps which meander across the surface of Mercury probably
were formed when the crust buckled as it cooled
4. Which of the following objects has a surface which most nearly
resembles that of Mercury? the moon
5. What does the density of Mercury imply about the nature of
its interior? it has a large iron-nickel core
6. Which of the following best describes the rotation of Venus?
slow and backward (retrograde)
7. Why is the surface of Venus so hot? infrared radiation emitted
by the surface can't escape into space
8. Venus atmosphere contains much more carbon dioxide than does
the atmosphere of the Earth. What has happened to the carbon dioxide
that formerly was in the atmosphere of the Earth? it is incorporated
in surface rocks
9. The principal constituent of the atmosphere of Venus is: carbon
dioxide
10. What evidence is there that Venus has plate tectonics? there
is no evidence of plate tectonics