Astronomy 121 Study Guide (4)

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Mercury, The Planet Venus, The Atmosphere and Surface of Venus, The Planet Mars, The Atmosphere and Surface of Mars

Why is Mercury rarely seen with the naked eye?

-- Mercury is always close to the Sun in the sky, which means that the Sun is usually visible when Mercury is.

Why did early astronomers think that Mercury was two separate planets?

-- Because Mercury can only be seen at sunrise or at sunset, early astronomers thought that one planet could only be seen at sunrise and the other planet could only be seen at sunset.

Employ the concept of escape speed to explain why Mercury has no atmosphere.

More massive objects have a better chance of retaining their atmospheres because the more massive an object, the greater the speed needed for atoms and molecules to escape. Because Mercury's escape speed is only 4.2 km/s, Mercury has a lot less pulling power, meaning that it cannot retain an atmosphere. The Earth's escape speed is 11.2 m/s.

Give two similarities and two differences between the surfaces of the Moon and Mercury.

-- Similarities: Are both covered in rock and heavily cratered. They are also have no permanent atmospheres.
-- Differences: Mercury is denser and has a strong magnetic field. It is also a planet and its orbit around its primary is not synchronous.

What does it mean to say that Mercury is in a 3:2 spin-orbit resonance? Why didn't Mercury settle into a 1:1 spin-orbit resonance with the Sun like the Moon did with Earth?

-- 3:2 spin-orbit resonance means that Mercury has exactly three rotations for every two revolutions. It did not develop a 1:1 spin-orbit resonance because its orbital speed is greatest at perihelion and least at aphelion (it has an elliptical orbit).

What is a scarp? How are scarps thought to have formed? What is the basis for believing that the scarps on Mercury formed after most meteoritic bombardment had ended?

-- A scarp is a cliff that was formed when the interior cooled and shrank long ago. The scarps are believed to have formed after most meteoric bombardment had ended because the scarps cut across multiple craters. Had the scarps been formed beforehand, you would expect them to be battered and weathered in the same way that the surface is by crater marks.

Unlike the Earth, the surface of Mercury undergoes extreme temperature changes. Why?

-- Because Mercury's escape speed is so low, it has no atmosphere, which means that no greenhouse effect and no convection can take place to stabilize its surface temperature.

How is Mercury's evolutionary history similar to that of the Moon? How is it different?

-- Mercury cooled more slowly than the Moon because it is bigger and more craters were erased which created intercrater plains. As its iron core cooled it shrunk, creating scarps along the planet's surface. Mercury does not have any evidence of maria.

Explain why Mercury is never seen overhead at midnight in Earth's sky.

-- The Sun is on the exact opposite side of where you are at midnight. Because Mercury is so close to the Sun, it will always be below the horizon at midnight. The angular distance between the Sun and Mercury is 28 degrees, and because the Earth rotates at 15 degrees an hour, you can only see Mercury for at most two hours on any given night.

Why does Venus appear so bright in the sky? On what does the brightness of a planet depend?

Venus is so bright in the sky because its high altitude clouds allow it to reflect a lot of the Sun's light. The brightness of a planet depends on both its ability to reflect light and on its distance from Earth.

Why is Venus always near the Sun in the sky?

-- Venus is always in the same region of the sky as the Sun because it's orbit lies within Earth's.

Why do astronomers think that the near resonance between Venus's rotation and orbit, as seen from Earth, is not really a resonance?

-- Because the resonance between Venus' rotation and orbital motion is nearly 5:1. If it were actually a resonance then it would be exactly 5:1, and there is no known interaction between Venus and Earth that can explain this phenomenon. The slight discrepancy (3 hours in 584 days) means that there is no actual resonance.

Describe one observational problem associated with Venus's near resonance of rotation and orbit.

-- Venus rotates almost exactly five time between one closest approach to Earth and the next which means that it presents nearly the same face to Earth at the closest approach. This means that we have much more information on one side of the planet than on the other side.

What is our current best explanation of Venus's slow, retrograde rotation?

-- Venus was struck by a large body early in its evolution, and its impact was sufficient enough to reduce the planet's spin to almost zero.

If you were standing on Venus, how would the Earth look?

-- Well you couldn't see it due to Venus' thick cloud cover, but if you could then you would see what would look like a very bright star in the night sky because the Earth is fairly close to Venus and its oceans are highly reflective. However, you would not see phases of Earth because Earth is outside Venus' orbit.

How did radio observations of Venus in the 1950s change our understanding of the planet?

-- Originally we thought that Venus' surface was only 10 to 20 degrees higher than Earth's and was hospitable towards life. However, the radio observations determined that Venus' surface was much hotter than anticipated, making it completely uninhabitable.

How do the continents of Venus differ from Earth's continents?

-- There are only two continents that occupy roughly 8% of its surface (as opposed to Earth's continents which occupy 25% of its space). There also seems to be no evidence of plate tectonics on Venus.

What is the evidence for active volcanoes on Venus?

-- The level of sulfur dioxide above Venus' clouds shows large and fairly frequent fluctuations.
-- Bursts of radio energy have been observed from regions of the planet's surface. These bursts are similar to those produced by lightning discharges that occur in the plumes of erupting volcanoes on Earth.

Given that Venus like Earth has a partially melted iron core, why doesn't it have a magnetic field like Earth's?

-- Venus' exceptionally slow rotation prevents it from creating a dynamo effect which is necessary to create a magnetic field in this case.

What did ultraviolet images returned by Pioneer Venus show about the planet's high-level clouds?

-- Fast moving upper layers of clouds with velocities of up to 400 km/hr.

Name three ways in which the atmosphere of Venus differs from that of Earth.

-- Venus' atmosphere is very dense while Earth's is less dense.
-- Venus' atmosphere is made up mostly of CO2 and Earth's is made up mostly of N2.
-- V's atmosphere is 50 km thick. E's is 10 km thick.

What are the main constituents of Venus's atmosphere? What are the clouds in the upper atmosphere made of?

-- 96.5% CO2 and 3.5% nitrogen. Clouds made of sulfuric acid.

What component of Venus's atmosphere causes the planet to be so hot? Explain why there is so much of this gas in the atmosphere of Venus compared with its presence in Earth's atmosphere. What happened to all the water that Venus must have had when the planet formed?

-- Venus' greenhouse effect is much more intense than the one found on Earth. Venus' atmosphere, being mostly made of C02, a greenhouse gas, trapped heat from the Sun which dramatically raised its temperature. There is so much C02 in the atmosphere because the surface temperature was hot enough that the outgassed water vapor and CO2 would have remained in the atmosphere creating a runaway greenhouse gas effect. The water was all evaporated.

Earth and Venus are nearly alike in size and density. What caused one planet to evolve as an oasis for life, while the other became a dry and inhospitable world?

-- Solar intensity at Venus' orbit is twice that at seen at Earth's orbit. The higher temperatures evaporated the water oceans and the UV from the Sun led to the destruction of water molecules. No water molecules, no life.

If Venus had formed at Earth's distance from the Sun, what do you imagine its climate would be like today? Why do you think so?

-- I think it would be a lot like Earth's because it would remain within the orbital limit of 0.7 and 1.0 AU that would keep the surface temperature at just the right level so that a runaway greenhouse effect would not occur.

How do the impact craters on Venus differ from those on other bodies in the solar system?

-- There are no craters that are smaller than 3 km across, which means that due to Venus' thick atmosphere, only large meteors actually land on the surface.

What is the evidence that volcanism of various types has changed the surface of Venus?

-- Larger craters, presence of coronae, lava domes, and shield volcanoes.

Do you think there might be life on Venus? Explain your answer.

-- No. Because of the runaway greenhouse effect present on Venus, its surface temperature is much too hot to support water molecules, and the lack of water molecules on the planet's surface means that no life can be present. Plus, its sulfuric acid clouds are not conducive to life either.

Do you think Earth is in any danger of a runaway greenhouse effect like that on Venus?

-- No, because Earth is sufficiently far away from the Sun that its surface temperature is low enough so that a runaway greenhouse gas effect will not take place. It would take a fairly substantial increase of surface temperature on Earth to provide conditions that are conducive to creating a runaway greenhouse effect.

Why is opposition the best time to see Mars from Earth?

-- Opposition is when the distance between Mars and Earth is at its minimum. Because Mars and Earth are as close as possible, Mars will appear brighter than it will when it is farther away.

Why are some Martian oppositions better than others for viewing Mars?

-- Because the planets do not move at constant speeds around their orbits. Oppositions occur at roughly 780 day intervals. Oppositions near Martian perihelion are less frequent, occurring once every 15 years.

How were the masses of Mars's moons measured, and what did these measurements tell us about their origin?

-- They were measured by their gravitational effect on the Viking orbiters. Their densities are far less than that of any world we have yet encountered, which suggests that they did not form along with Mars. They would then be captured asteroids that contain information about the very early solar system.

What are the Martian 'canals'?

-- An optical illusion created by poor-quality telescopes.

What is the evidence that Mars never melted as extensively as did Earth?

-- The presence of iron on Mars' surface suggests that Mars never melted as extensively as Earth did and that it therefore did not undergo complete differentiation. Also, a lack of current volcanism.

How would Earth look from Mars?

-- Because from Mars' perspective Earth would be an inferior planet, it would never stray too far from the Sun and would have phases like the Moon or Venus does. Due to Earth's oceans' reflectiveness Earth would appear to be ocean blue. Earth would be best visible during morning and evening twilight.

Why is Mars red?

-- Chemical reactions between Mars' iron-rich surface and the free oxygen in the atmosphere creates iron oxide (rust) that is red in color.

Why are Martian volcanoes so large?

-- Mars' low surface gravity creates much larger volcanoes than are present on Earth. As lava flows and spreads to form a shield volcano, its eventual height depends on its ability to support its own weight. Because Mars' gravity is so low, its weight is less which creates a higher mountain.

Describe the major large-scale features on the surface of Mars.

-- Olympus Mons -- Mars' largest volcano.
-- Tharsis Bulge -- Considered to be Mars' only continent. Is roughly the size of North America and has three very large volcanoes on it.
-- Hellas Basin -- Mars' lowest point. It is an impact feature that would have caused major redistribution of Martian crust.
-- Valles Marineris -- Mars' largest tectonic fracture.

Describe the atmospheric conditions in the Southern Hemisphere of Mars during the summer.

-- Martian summers in the Southern Hemisphere are significantly colder than those that you would find in the Northern Hemisphere. The temperature would be somewhere lower than 150K and giant dust storms would rage in the SH blowing northward.

Describe the two Martian polar caps - seasonal and permanent composition, and the differences between them.

-- Northern cap: residual of water ice, seasonal of carbon ice. Southern cap: mostly carbon ice (colder than northern cap)

Why couldn't you breath on Mars?

-- Mars' air is very thin and there is not enough oxygen in its atmosphere because Mars' atmosphere is made up of 95% CO2 while Earth's only contains .03% CO2.

What is the evidence that water once flowed on Mars?

-- We find many topographical features on Mars such as dry riverbeds and runoffs that suggest that water once flowed on Mars.

Is there liquid water on Mars today? Justify your answer.

-- No, there is no liquid water on Mars. Mars' atmospheric pressure is much too low to sustain liquid water. If it were theoretically present, it would rapidly evaporate.

Is there water on Mars today in any form? Again, justify your answer.

-- Yes, it appears that water certainly exists in the solid state (permafrost) at its poles, and possibly even underground as a solid. It is hypothesized that if all of Mars' water was released to the surface that it would cover Mars' surface by several meters.

What is the evidence that Mars once had an extensive ocean? Where was it?

-- By looking at Mars' surface, astronomers see evidence of terrace beaches that could be the remains of a shoreline from a large body of water. It would have been on the planet's northern plains.

If Mars were colonized, what resources could Mars provide and what would have to be brought from Earth?

-- Mars' vast stored of frozen water could theoretically be extracted, purified, and heated up to become water. Furthermore, its large iron deposits could be used in some elements of construction. However, essentials such as food, fuel, and electricity would have to be brought from Earth.

Since the atmosphere of Mars is mainly the greenhouse gas carbon dioxide, why isn't there a significant greenhouse effect warming its surface?

-- Mars does not have enough of an atmosphere to trap the necessary amount of CO2 to create a greenhouse effect. Mars' atmosphere disappeared either because of significant collisions with celestial bodies or because it escaped due to its small gravity. In any case, there is simply not enough atmosphere to create an insulating layer to hold in the trapped IR radiation.

Compare and contrast the evolution of the atmospheres of Mars, Venus, and Earth.

-- All three have secondary atmospheres created by volcanoes.
-- Venus, which is closest to the Sun, experienced temperatures which prevented the CO2 from settling out of the atmosphere and dissolve into the oceans. Venus became a victim of the runaway greenhouse effect and the H2O in the atmosphere was destroyed by UV sunlight.
-- Earth was far enough from the Sun that liquid water could form, eventually creating oceans, which dissolved much of the CO2 present in the atmosphere. The small amounts of H2O and CO2 in the Earth's atmosphere create an insulating layer that creates a greenhouse effect which is just enough to keep the Earth warm and liquid.
-- Mars once had liquid water and enough water to dissolve the CO2 in the atmosphere, but because it was less massive than Venus or Mars it lost its CO2. Once Mars lost its CO2, the atmosphere thinned to the point that most of the IR radiation escaped. Also, Mars's volcanism stopped very early in its formation which prevented the atmosphere from having its H2O and CO2 renewed.

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