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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).
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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!