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Homework #3 Page 174 RQ 8 Explain why Venus and Mars both have carbon dioxide-rich atmospheres. How did Earth avoid such a fate? The presence or lack of water on the surface of a planet determines the amount of carbon dioxide (CO2) present in its atmosphere. Because CO2 dissolves in water, large oceans such as those on Earth can absorb CO2 out of the planet’s atmosphere and deposit it in carbonate rocks. The formation of oceans on a planet is dependent upon the planet’s surface temperature and atmosphere. Venus’s close orbit around the sun caused its climate to be slightly hotter than that of Earth, which prohibited the formation of oceans. Without these oceans, the atmosphere of Venus remained thick with CO2, giving the planet its signature runaway greenhouse effect. The heating due to the greenhouse effect completely removed any possibility that water could later condense on Venus. Mars has approximately the same percentage of CO2 in its much thinner atmosphere as does Venus, but its climate is radically different. While Mars does show surface features (channels, erosion) that suggest liquid water existed on its surface in the past, water today is frozen (polar caps, permafrost). Mars probably did not have enough liquid water on its surface to dissolve significant quantities of CO2 out of the atmosphere. Its surface temperature is much lower than that of Venus or Earth due to its larger orbit. Additionally, the small size of Mars meant that it has more difficulty holding onto a thicker atmosphere. It is likely that some water vapor in its atmosphere did escape. Page 204 RQ 11 What are the seasons on Uranus like? Uranus has the most extreme seasons of any planet, due to the 98 degree tilt of it rotational axis. Just like on Earth, the part of the planet pointing directly towards the sun is the region that has summer. But the orbit of Uranus is so odd that over one revolution the sun shines directly on the north pole, then the northern latitudes, followed by the equator, then the southern latitudes, then the south pole. The north pole and most of the northern hemisphere has summer and daylight, while the south pole and southern hemisphere is completely dark and frigid! Since Uranus takes 84 Earth years to orbit the sun, each season is about 21 years long. p. 204 RQ #15 What evidence do we have that catastrophic impacts have occurred in the solar system’s past? (Look at page 203) There are a number of clues: - fractures and craters on Jovian moons (not to mention craters on our own moon, craters on Mercury, Mars, Venus, … ) - planetary rings around the gas giants (these suggest many impacts that are still going on in which small particles are scattered, replenishing the rings) formation of Earth’s moon (remember the Giant Impact Theory?) density of Mercury (Mercury seems to have been larger at one point, but might have underwent a collision sometime after the planet differentiated. This would account for the size of Mercury’s iron core and high density.) - the peculiar rotation of Uranus (its rotation axis is tilted such that its equator is at an inclination of 98 degrees to its orbit) - backward rotation of Venus (something could have knocked it into this weird rotation pattern) - the high inclination of Pluto and the orbit of its moon, Charon, suggest that there was a past collision or close gravitational interaction with a passing body - peculiar orbits of Neptune’s moons Triton and Nereid The solar system isn’t such a safe place after all (we watched the Shoemaker-Levy comet hit into Jupiter (p. 181)—things are still happening). - p.86 RQ #4 Why do optical astronomers sometimes put their telescopes at the tops of mountains, while radio astronomers sometimes put their telescopes in deep valleys? For optical astronomers, visible light is easily scattered by the Earth’s atmosphere. Building a telescope on the top of a mountain keeps the telescope above a few miles of atmosphere, where the air is thinner (meaning the light won’t be scattered or blurred as much). In addition to this, the higher the mountain the higher above clouds and weather (rain and snow coming from the clouds) you will be. Water vapor (which makes up clouds) scatters light, building on top of a mountain minimizes this scatter. For a radio astronomer, the part of the spectrum he/she is looking at is at a much longer wavelength. This means that the intensity of the signals they look at is much weaker than those in the optical wavelengths. To get a bigger signal, it is necessary to increase the baseline, or distance across which the radio telescope can detect. This can be done either by using the interferometry technique (many telescopes separated over a large part of the Earth) or by building a really large telescope (like the 300-m one at Arecibo). You can’t build a really big telescope on a mountain (it just wouldn’t fit), so this is one logistical reason why a radio telescope would be built in a deep valley. One other concern a radio astronomer has is the interference caused by terrestrial radio sources (AM/FM radio, cell phones, etc.). Radio towers broadcasting these signals (for our cell phones and radio) are at high elevations so that more people can receive the signals. When building a radio telescope you want to minimize the effect of these sources, so you want to build your telescope in an isolated region away from large sources of radio signals. Building in a deep valley would ensure that these signals would be absorbed by surrounding features that were at higher elevations (mountains for instance), diminishing the contamination of astronomical radio sources. Problem: Voyager 2 was launched in 1977. It is now 6.5 billion miles from Earth. Calculate Voyager 2’s speed in miles per year. What is this speed in miles per hour? At this rate, how long (in years) would it take to reach a star 4.3 light years (26 trillion (1012) miles) away? Remember that: distance = rate x time In this case: We know the distance: 6.5 billion miles, or 6.5 x 109 miles We know the time: our year is 2003 and it was launched in 1977, so this is just our year minus the year launched. 2003 – 1977 = 26 years So, what’s the rate? This is the first question, the rate is the same as the speed. Solving the equation: Distance = rate x time Distance ÷ time = rate This means that the speed is just the distance (6.5 x 109 miles) divided by the time (26 years). The speed in miles per year is 2.5 x 108 miles per year. To find the speed in miles per hour we need to remember how to convert years into hours. We can set up a set of ratios based on the fact that there are 365 days in 1 year, and 24 hours in 1 day. So: 2.5 x 108 miles x 1 year x 1 day year 365 days 24 hours This gives us the speed in miles/hour: 2.85x104 miles/hour. To find out how long (time = ?? years) it would take to reach a star that is 4.3 light years away (distance = 2.6 x 1013 miles), we just use the same equation. We know the distance and we just solved for the speed. So, Distance ÷ rate (speed) = time We need to use the speed we calculated for miles per year in order to get the time in years. Time = (2.6 x 1013 miles)÷( 2.5 x 108 miles/year) = 104,000 years!