PHYSICS 1040 - ELEMENTARY ASTRONOMY - HOMEWORK #4
1. The terrestrial planets are Mercury, Venus, Earth, and Mars. The terrestrial planets are small/large (circle one), more/less dense (circle one), and rocky/gaseous (circle one). They are composed mainly of heavy elements. The orbits of the terrestrial planets are nearly circular and are close to the Sun, so their surface temperatures are hot. The terrestrial planets all orbit the Sun in the same direction and move in the same plane, called the ecliptic plane.
2. The Jovian planets are Jupiter, Saturn, Uranus, and Neptune. The Jovian planets are small/large (circle one), more/less dense (circle one), and rocky/gaseous (circle one). They are composed mainly of light elements. The orbits of the Jovian planets are nearly circular and are far from the Sun, so their surface temperatures are cold. Together with the terrestrial planets, the Jovian planets all orbit the Sun in the same direction and move in the ecliptic plane.
3. The speed of a molecule in a gas depends on the temperature of the gas and the mass of the gas molecule. Higher gas temperatures mean higher/lower speeds (circle one), while more massive gas molecules mean higher/lower speeds (circle one). Because hydrogen is the least massive gas molecule, it will move the fastest. These molecules will thus be the most likely to exceed the escape velocity of a planet, and so escape from the planet's atmosphere. The terrestrial planets are small, hot, and have low escape velocities, so they have almost no hydrogen. Jupiter and Saturn are large, cold, and have high escape velocities, so they have retained their hydrogen.
4. Beyond Neptune= s orbit are numerous trans-Neptunian objects. The largest of these we know of is Eris, while the second-largest is Pluto. All of these objects orbit the Sun in the same direction as the planets, but their orbit are/are not (circle one) circular and are/are not (circle one) in the ecliptic plane. Astronomers estimate that there are 35,000 of these objects, which are composed mainly of rock and ice.
5. Impacts of rocky debris have formed craters on the planets and their moons. With time, these craters may be removed by the geologic activity caused by a planet= s or moon= s internal heat. This means that a surface with only a few craters is younger/older (circle one) than a surface with many craters. Also, smaller worlds cool off faster and therefore they have less/more (circle one) internal heat and less/more (circle one) geologic activity, so smaller worlds tend to have fewer/more (circle one) craters.
6. The magnetic field of a rotating planet or moon is caused by its molten interior. The molten material conducts electricity, and the rotating currents produce the magnetic field. This is called the dynamo mechanism. If a planet or moon has a magnetic field, we know that its interior is molten.
7. Astronomers use spectral line "fingerprints" to identify the atoms and molecules found in space. They find that the most common element is hydrogen, which makes up 71 percent of the mass of all the stars and galaxies in the universe. The next most common element is helium, which makes up 27 percent of the mass. All of the other elements combined make up only 2 percent of the mass of all the stars and galaxies in the universe. Astronomers believe that all of the hydrogen and helium were made 13.7 billion years ago in the Big Bang, the violent explosion of space at the beginning of time that gave birth to the universe. All other elements were made inside stars, and scattered throughout space by exploding massive stars called supernovae. The study of radioactive elements in meteorites shows that our solar system formed 4.56 billion years ago.
8. The law of conservation of angular momentum says that, if the mass of an isolated object doesn't change, then the value of its (size)
2 x (rotation speed) remains constant. This means that if the Sun were to become ten times smaller (with the same mass), then it would rotate (10)2 = 100 times faster. The law of conservation of angular momentum explains why a collapsing interstellar cloud of gas and dust flattens into a disk, called a protoplanetary disk. As the cloud collapses, its size becomes smaller so its rotation rate increases / decreases (circle one). The gas and dust near the cloud= s equator moves fastest and tends to flatten out, just like a ball of spinning pizza dough flattens out. Small clumps of dust then clump together to form planetesimals about 1 km across, and these then gravitationally attract each other to form protoplanets, about the size of our Moon. These then collided to form planets.9. The early solar nebula contained three types of materials: gases, ices, and rock-forming compounds.
a. gases have extremely low condensation temperatures (the temperature at which a gas condenses from a gas into a solid), and so are usually in a gaseous state (unless they are compressed under high pressure into a liquid).
b. ices have intermediate condensation temperatures between 100 K and 300 K.
c. rock-forming compounds have high condensation temperatures (between 1300 K and 1600 K), and so remain solid at high temperatures.
In the inner solar system, only substances with low/high (circle one) condensation temperatures could have remained solid, so the terrestrial planets are small, more dense, and rocky. The Jovian planets were formed where the temperature was cold enough for water to condense to form ice.
10. Today astronomers have discovered the presence of more than 270 planets (extrasolar planets) orbiting stars other than the Sun. The star and the planet each orbit their common center of mass, which is their natural balance point. This point is closer to the star than it is to the planet. Although astronomers cannot see the planet, they can see the star as it A wobbles@ in a small circle around the center of mass.