Download File - Mr. Catt`s Class

Chapter 8
8-1 thru 8-2
Courtesy of NASA, J. Bell (Cornell U.), and M. Wolff (SSI)
The
Terrestrial
Planets
8-1 Mercury
Mercury as Seen from Earth
1. The least seen planet by people on Earth.
Because of its proximity to the Sun it can be seen
low on the horizon just before sunrise in the east
or just after sunset in the west.
2. Mercury exhibits phases like Venus.
3. Features on Mercury are hard to discern from
Earth because Mercury is small and, since it is
seen near the horizon, its light passes through
more of the Earth’s atmosphere than if it were
overhead.
Figure 8.03: Mercury from Mariner 10
Courtesy of NSSDC/NASA
Mercury via Mariner — Comparison with the Moon
1. Mariner 10 flew by Mercury three times (starting in 1974),
returning a total of 4,000 photographs.
2. Mercury appears similar to our Moon:
Both are covered with many impact craters
– Mercury’s craters are less prominent and its surface has
less extensive ray patterns
– The planet’s surface gravity is twice that of the Moon so
loose material doesn’t stack as steeply or travel as far.
3. Mercury lacks the larger maria seen on the Moon.
Because Mercury cooled more slowly, meteorites were able
to penetrate its crust over a longer period, which allowed
lava to flow out and obliterate older craters.
This resulted in the smooth plains seen between the evenly
spread craters.
4. More scarps—long cliffs in a line—are found on Mercury
than on the Moon. Most are believed to have formed from
the shrinking of Mercury’s crust after cooling.
Courtesy of NSSDC/NASA
5. A large “bulls-eye”
impact crater called
Caloris Basin (the size
of Texas) is visible in
Mariner’s images.
Courtesy of NSSDC/NASA
6. Mariner confirmed that
Mercury has negligible
atmosphere.
Figure 8.06: The right edge of
Caloris Basin crater, Mercury
Structural Characteristics
1. Mercury’s density is slightly
smaller than Earth’s while its
surface gravity is only 38% that
of Earth’s.
Because surface rocks on
Mercury look similar to Earth
rocks, Mercury must have a very
large iron core, perhaps 65%–
70% of its total mass.
2. Astronomers speculate that early
on a collision with a large
asteroid blasted away most of
Mercury’s rocky mantle.
Figure 8.09
3. Mariner 10 did detect a magnetic field on Mercury,
but it is 1% as strong as Earth’s.
– Mercury’s magnetic field suggests that part of
its metallic core must be molten in order for the
dynamo effect to operate.
– Recent measurements of heat loss indicate that
its core is not molten, but the question is open.
The dynamo effect explains the generation of magnetic
fields as a consequence of circulating electric charges, such
as in an electric generator or in molten magnetic material
within a planet’s core.
Mercury’s Motions
1. Compared to the other planets, Mercury
– circles the Sun in less time (88 days),
– moves faster in its orbit (48 km/s) and (except for Pluto)
– has the most eccentric orbit (varying between 46 and 70
million km from the Sun).
2. Radar observations show that Mercury rotates once very
58.65 Earth days (precisely 2/3 of its orbital period of 87.97
Earth days).
– This sidereal day is quite different from Mercury’s solar
day, which is 176 Earth days long.
– Thus a Mercurian day lasts two Mercurian years.
Figure 8.10: Rotation of Mercury during orbit
3. The coupling between Mercury’s
rotation revolution periods is
probably due to the unbalanced
nature of its mass.
The leading hypothesis is that the
impact object that created the Caloris
Basin was very dense, and its
presence under Mercury’s surface
has caused the planet to be lopsided.
4. High temperatures on Mercury can
reach 450°C (842°F), well above the
melting point of lead (330°C or 626°F).
On the night-side of Mercury,
temperatures can fall to –150°C (–
238°F).
5. Radar observations show a high albedo for
Mercury’s polar regions, suggesting the presence of
ice.
Yet ice should evaporate over the ages. The
presence of ice for so long remains a mystery.
Courtesy of NASA/JPL-Caltech
Figure 8.11
Question 1 (8-1 thru 8-2 PPT Questions)
What is the reason that Mercury’s day lasts longer than
Mercury’s year?
8-2 Venus
Structural Characteristics
1. Like Mercury, Venus is visible only in the evening
sky after sunset or in the morning sky before
sunrise.
2. Venus is Earth’s sister planet with a bit smaller
diameter (95% of Earth’s), smaller mass (82%) and
smaller density (95%).
3. The surface gravity of
Venus (91% of Earth’s)
and the similarities
between the two planets’
surface rocks suggest
than Venus has a dense
interior with probably a
metallic core.
4. Venus does not seem to
have an intrinsic magnetic
field. It is possible that its
field is now in the process
of reversing.
Figure DP03.03: Interior of Venus
Venus’s Motions
1. Its orbit is almost circular with a period of 225 days; orbital
speed is a nearly constant 35 km/s (78,300 mi/hr).
2. Venus’s surface is shrouded by heavy clouds. Since 1961
we have been bouncing radar signals off its surface to
learn about its rotation rate and surface features.
3. Venus’s sidereal rotation period is 243 days,
its revolution period is 225 days, and
these combine to produce a solar day that is 117 Earth
days long.
4. Venus’s axis is tilted 177°; because the angle is
greater than 90°, Venus’ direction of rotation is
backward compared to most other directions of
rotation and revolution in the solar system.
5. The definition of a planet’s North Pole is based on
the right-hand rule: grab the planet with your
right hand so that your fingers point in the
direction of the planet’s rotation; your thumb
then points to its north pole.
Figure 8.12: Rotation axis of Venus relative to Earth's
The Surface of Venus
1. Since 1962, many spacecraft from the U.S. and Russia have
visited Venus.
The former Soviet Union landed 11 spacecraft, some of
which produced close-up photos of the surface.
Figure 8.13: Surface of Venus from Venera 13
Courtesy of Soviet Planetary Exploration Program/ NSSDC/NASA
2. Photos of sharp-edged rocks confirm that winds at the
surface are fairly calm.
3. Orbiting probes Pioneer Venus 1 (1978) and Magellan
(1990) have produced detailed radar maps of Venus’s
surface.
– About two-thirds of Venus’s surface is covered with
rolling hills.
– Highlands occupy <10% of the surface, with lower-lying
areas making up the rest.
Figure 8.15: Venusian feature map
Courtesy of NASA/JPL-Caltech
Figure 8.16: Crater Howe seen by Magellan
Courtesy of NASA/JPL-Caltech
4. Venus has about 1,000 craters that are larger than a few
kilometers in diameter.
Venus has no craters older than ~800 million years; average
surface age is estimated at 500 million years, about twice as
old as Earth’s.
5. Venus shows past evidence of volcanic and tectonic activity:
mountains, large lava flows, volcanoes.
There is no evidence of current volcanic activity.
6. The presence of volcanism suggests that Venus has a molten
interior.
However, there is no evidence to suggest the presence of
plate tectonics.
Question 2 (8-1 thru 8-2 PPT Questions)
What is some evidence that Venus is a dead planet?
(dead planet refers to a planet who no longer has a
molten interior)
Advancing the Model: Our Changing View of Venus
1. Venus is most brilliant when its elongation is 39, which
occurs about 36 days before or after its new phase.
2. It was during Venus’ solar transits in 1761 and 1769 that
parallax measurements allowed us to measure the actual
distance between Earth and Venus and thus the actual value
of the A.U.
Figure 8.B01: Changing
view of Venus
The Atmosphere of Venus
1. Venus’s atmosphere is
composed of 96% carbon
dioxide (CO2), 3.5% nitrogen
(N2), and small amounts of
water (H2O), sulfuric (H2SO4)
and hydrochloric acid (HCl).
Venus is inhospitable.
2. The upper atmosphere is very
windy; wind speeds reach 350
km/hr (218 mi/hr). The wind
speed decreases to almost zero
as one descends toward
Venus’s surface.
3. The atmospheric pressure
on Venus’s surface is
about 90 times that found
at the Earth’s surface.
4. Surface temperature of
Venus has been measured
at about 464°C (867°F).
5. Venus’s clouds form a
layer between altitudes of
50 and 70 km. A haze
layer extends from the
cloud layer down to 30
km. From there to the
ground the atmosphere is
quite clear.
Figure 8.18: Atmosphere of
Venus
A Hypothesis Explaining Venus / Earth Differences
1. Venus’s high surface temperature (being closer to the Sun)
didn’t allow water to condense out of its atmosphere into
oceans that could absorb CO2 (as happened on Earth).
High in Venus’s atmosphere, the Sun’s UV light broke down
water molecules into hydrogen (which escaped the planet)
and oxygen (which combined with other elements).
2. CO2 remained in the atmosphere where it trapped the
outgoing infrared radiation from the planet’s surface, leading
to higher temperatures.
As Venus became hotter, more CO2 was baked out of the
surface rocks. This created a runaway greenhouse effect.
Figure 8.19: Greenhouse effect on Venus
3. Finally, equilibrium was reached between the
amount of radiant energy leaving the planet and
that striking it.
4. Earth’s temperature is elevated about 35C
(63F) by the natural greenhouse effect.
5. So much solar radiation is reflected from
Venus’s clouds that if Venus had no greenhouse
effect, its atmosphere would be cooler than
Earth’s.
6. For the past 150 years humans have been
injecting CO2 and other greenhouse-causing
chemicals into the atmosphere at an ever
increasing rate. Some studies indicate that
doubling the CO2 in the atmosphere could
increase Earth’s average temperature between
2.8°C (5.0°F) and 5.2°C (9.4°F).
7. Plant growth naturally decreases atmospheric
CO2. Unfortunately, we are destroying tropical
forests at the rate of 1 acre per second.
Question 3 (8-1 thru 8-2 PPT Questions)
Explain why Venus is the hottest planet when Mercury
is much closer to the Sun.