PHYSICS 1040 - ELEMENTARY ASTRONOMY - HOMEWORK #6

1. Mercury data: Average distance from Sun = 0.387 A.U.

Orbital period = 88.0 days

Rotational period = 58.65 days

Diameter = 0.383 x Earth=s diameter

Average density = 5.4 x water=s density

Average temperature (day) = 350 oC

Average temperature (night) = -170 oC

 

Venus data: Average distance from Sun = 0.723 A.U.

Orbital period = 224.7 days

Rotational period = 243 days

Diameter = 0.949 x Earth=s diameter

Average density = 5.2 x water=s density

Average temperature = 460 oC (day and night)

 

Mars data: Average distance from Sun = 1.52 A.U.

Orbital period = 1.88 years

Rotational period = 24 hours 37 minutes

Diameter = 0.533 x Earth=s diameter

Average density = 3.9 x water=s density

Average temperature = -23 oC (day and night)

2. Mercury= s rotational period is exactly 2/3 of its orbital period, a phenomenon call spin-orbit coupling. As a result, the length of one solar day on Mercury (from noon to noon) is equal to 2 of Mercury= s years! Tidal forces are responsible for maintaining this relation between Mercury= s rotational and orbital periods.

3. Like the Moon, Mercury has many impact craters. Long ago a huge impact formed the Caloris Basin on Mercury. Seismic waves from this impact traveled through Mercury and were focused on a small region exactly on the opposite side of the planet where today we see a jumbled terrain. With so many craters, Mercury=s surface is very young/old (circle one), so astronomers do not believe that Mercury has much geologic activity, and should not have a molten core. We have found that Mercury has a huge iron core that reaches 75 percent of the way to the surface. This makes Mercury the most iron-rich planet in the solar system. Mercury has a weak magnetic field, only 1 percent of the strength of Earth= s magnetic field. A small part of Mercury= s iron core must therefore be molten to generate the observed magnetic field, but not molten enough to cause geologic activity.

4. Ignore this problem!

5. Venus= atmosphere is almost all carbon dioxide (CO2): 96.5 percent of its atmospheric molecules are CO2. Nitrogen (N2) makes up most of the remaining 3.5 percent of the molecules. Venus= upper atmosphere rotates around the planet in just 4 days in a retrograde direction, with winds of 350 km/hr. Near the ground, however, the winds are down to about 5 km/hr. The atmosphere is so massive that the atmospheric surface pressure on Venus is 90 times greater than it is on Earth. This is the same pressure found 1 km below the surface of Earth= s oceans.  Venus= clouds contain tiny droplets of sulfuric acid! The acid does not drop as rain, however, because it evaporates before it hits the ground. The sulfur comes from volcanoes on Venus, some of which may still be active. However, the surface of Venus shows no evidence of plate tectonics.

6. The early atmospheres of Venus and Earth started out with about the same amounts of carbon dioxide (CO2) and water (H2O). The CO2 came from volcanoes, and much of the water came from impacts with comets. But Venus has almost no water and a carbon dioxide atmosphere, while Earth has vast oceans and little carbon dioxide in its atmosphere.

a. What happened to Venus' water? Venus is close enough to the Sun for the Sun's heat to have eventually evaporated any water present on the surface of the young Venus. The sunlight broke up the water molecules in the atmosphere into hydrogen and oxygen atoms. The lighter hydrogen atoms moved fast enough to escape into space. The heavier oxygen atoms combined with other substances in Venus' atmosphere. Thus Venus now has almost no water. Because carbon dioxide is a greenhouse gas, Venus has a runaway greenhouse effect that heats its surface by an extra 400oC.

b. What happened to Earth's carbon dioxide? Because Earth is farther from the Sun than Venus, its water did not evaporate, but remained in Earth's oceans. Carbon dioxide dissolves in water, and so the carbon dioxide in the young Earth's atmosphere collected in the oceans. This carbon dioxide then became chemically bound into carbonate rocks, such as limestone and marble, that formed in the oceans. Thus Earth= s atmosphere now has little carbon dioxide, and a gentle greenhouse effect that keeps Earth= s average temperature above the freezing point of water.

7. The elevation of Mars= northern hemisphere is, on average, about 5 km lower than Mars= southern hemisphere. Also, Mars= northern lowlands show very many/few (circle one) craters, while Mars= southern highlands show many/few (circle one) craters. This means that the surface of the northern lowlands is relatively young/old (circle one), and that the surface of the southern highlands is relatively young/old (circle one). Mars does not have plate tectonics (the movement of a planet= s crustal plates), so the absence of craters in the north is not due to geologic activity.

8. Mars has some gigantic surface features. Olympus Mons is the largest volcano in the solar system, it is the size of the state of Utah and is 24 km high, three times as high as Mount Everest on Earth. Because plate tectonics is absent on Mars, Olympus Mons sat on a volcanic hot spot for millions of years. Valles Marineris is a canyon that is 4000 km long (the distance between Los Angeles and New York City), 400 km wide, and 8 km deep. This is a rift valley, where Mars= crust split along a fault line.

9. Mars changes its appearance over the course of a Martian year. This is caused by Mars= elongated orbit. Mars= seasons are caused by the 25o tilt of its rotation axis, similar to the cause of Earth=s seasons. However, Mars is much closer to the Sun when it is summer in the southern hemisphere. The Sun warms the soil around the southern ice caps while the ice caps remain very cold. This temperature difference powers strong winds that produce swirling dust storms. These storms can cover the entire surface of Mars, causing large areas of the planet to change color as the dust storms cover or reveal surface features.

10. Although Mars is now very dry, in the past some areas of Mars were covered by water. Robot rovers on Mars have found evidence of sedimentary rocks that can form only in standing water like a lake or shallow sea. Also, photos from spacecraft orbiting Mars have found channels and gullies carved by flowing water. Today, this water is frozen. Some frozen water is found beneath the surface of Mars. Frozen water is also found in Mars= polar ice caps, which also contain frozen carbon dioxide (CO2).

 

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