Astronomy 201 Review 2 Answers
What is hydrostatic equilibrium? How does hydrostatic equilibrium maintain the Sun's stable size? Hydrostatic equilibrium, also known as gravitational equilibrium, describes a balance between gravity and pressure. Gravity works to contract while pressure works to expand. Hydrostatic equilibrium is the state where the force of gravity pulling inward is balanced by pressure pushing outward. In the core of the Sun, hydrogen is being fused into helium via nuclear fusion. This creates a large amount of energy flowing from the core which effectively creates an outward­pushing pressure. Gravity, on the other hand, is working to contract the Sun towards its center. The outward pressure of hot gas is balanced by the inward force of gravity, and not just in the core, but at every point within the Sun.
What is the Sun composed of? Explain how the Sun formed from a cloud of gas. Why wasn't the contracting cloud of gas in hydrostatic equilibrium until fusion began?
The Sun is primarily composed of hydrogen (70%) and helium (28%) with the remaining mass in the form of heavier elements (2%). The Sun was formed from a collapsing cloud of interstellar gas. Gravity contracted the cloud of gas and in doing so the interior temperature of the cloud increased because the contraction converted gravitational potential energy into thermal energy (contraction leads to heating). The cloud of gas was not in hydrostatic equilibrium because although the contraction produced heat, it did not produce enough heat (pressure) to counter the gravitational collapse and the cloud continued to collapse. Much of the thermal energy generated from contraction was radiated away at the cloud's surface despite the increase in interior temperature. As the cloud continued to collapse, the temperature and density at the center finally increased to the point where nuclear fusion was sparked. At this point, the energy being radiated away at the surface matched the energy generated at the core. Once fusion began, the Sun entered a state of hydrostatic equilibrium.
Describe the different layers of the Sun. What is interesting about the temperatures of the chromosphere and corona relative to the other layers of the Sun? If we cannot directly see the Sun's interior, how do we know about the Sun's internal structure?
From the center and proceeding outward, the layers of the Sun are the core, radiation zone, convection zone, photosphere, chromosphere, and the corona. The core is the hottest (about 15 million K) and most dense region of the Sun where nuclear fusion occurs. Energy created in the core will take on the order of a million years to finally reach the surface. This slow migration of photons from the core to the surface is known as radiative diffusion. The radiation zone (about 10 million K) is the region of the Sun where the primary means of energy transport is radiative diffusion. The convection zone describes the region where energy is transported by the rising of hot gas and the falling of cool gas (convection). The photosphere is above the convection zone. The photosphere is the visible portion of the Sun. The average temperature of the photosphere is about 5800 K which corresponds to a peak photon color of about green/yellow. One of the characteristic features of the photosphere is the granulation pattern due to the convection occurring below. The chromosphere basically corresponds to the lowest region of the Sun's atmosphere. The temperature of the chromosphere is about 10,000 K, which corresponds to ultraviolet radiation. Above the chromosphere is the corona, the uppermost region of the Sun's atmosphere. The temperature of the corona is about 1 million K and most of the X­rays emitted by the Sun are emitted from the corona. Beginning with the core and extending out to the photosphere, the temperature of each successive region decreases. Interestingly, the temperature then rises in the chromosphere and rises again by a factor of ten in the corona. It is thought that waves carry energy along magnetic field lines to heat the chromosphere and the corona and these waves are generated by turbulence in the convection zone. Models, Sun quakes, and solar neutrino observations allow us to study the Sun's interior. We can create models according to the laws of physics and check them against observations. Sun quakes describe pressure waves generated in the Sun that create vibrations of the solar surface which can be detected by Doppler shifts. Neutrinos are a byproduct of the fusion reactions occurring within the Sun. Models suggest we should detect a certain rate of neutrinos, which if detected would support the models and increase our confidence in our understanding of the nuclear physics going on in the interior of the Sun. While neutrinos can provide a probe of the interior, there is still much mystery surrounding them and they are very difficult to detect.
How does nuclear fusion work? More specifically, what is basically included in hydrogen burning?
Nuclear fusion describes the process where nuclei combine to form a nucleus with a greater number of protons and neutrons. Very high temperatures and densities are required for nuclear fusion. Nuclear fusion in the core of the Sun is fusing hydrogen into helium by way of the proton­proton chain. Basically, two hydrogen nuclei fuse into deuterium; deuterium fuses with a neutron to produce helium­
3; two helium­3 nuclei fuse to form helium­4. A lot of energy is produced along this chain of reactions. How is energy transported in the radiation and convection zones?
The primary method of energy transport in the radiation zone is radiative diffusion: the slow migration of photons from the core to the surface. Energy is transported in the convection zone by convection: the rising of hot gas and the falling of cool gas.
What are Sunspots and what causes them? Describe the Sunspot cycle. Sunspots are dark spots on the surface of the Sun. The plasma within Sunspots is about 4000 K, and therefore they appear dark in contrast to the even brighter surrounding material (5800 K). Sunspots occur in regions with strong magnetic fields and tend to occur in pairs connected by looped magnetic field lines. Since the Sun rotates differentially (rotates more quickly at the equator), the magnetic field lines are dragged into twisted contortions. Trapped gas within a strong magnetic field contortion cools because it is isolated from surrounding material via the magnetic field lines. The Sunspot cycle is the approximately eleven year period over which the variation in the number of Sunspots occurs. Sunspots first appear at mid­latitudes at solar minimum and then become more common near the equator at the next solar minimum. What is solar wind? What is it made of? How does it effect us?
Charged particles streaming out of the corona constitute the solar wind. The solar wind consists of roughly equal numbers of protons and electrons, in addition to a small fraction of heavier nuclei. The solar wind escapes primarily through coronal holes, which are regions of the corona where the magnetic field is “broken,” unlike the regions around Sunspots. The solar wind has a significant effect on the Earth's ionosphere and magnetic field, as well as the auroras and telecommunications systems.
What causes the granulation features we observe on the Sun? What is limb darkening? What are solar prominences? What are coronal mass ejections? What are spicules?
Up close, the photosphere resembles boiling water with a bubbly, almost cellular appearance. This granulation feature is due to the convection occurring below the photosphere in the convection zone. Limb darkening is the darker appearance near the edge of the Sun. It occurs because the observer's line of sight passes through shallower, cooler, and more absorbing layers of photosphere as it moves away from the center of the Sun's disk. Solar prominences are loops of hot gas that extend very high above the photosphere. The hot gas of solar prominences follows magnetic field lines. Coronal mass ejections are huge bubbles of gas threaded with magnetic field lines that are ejected from the Sun over the course of several hours. Coronal mass ejections are often associated with eruptive solar prominences in the chromosphere and occur most frequently at solar maximum. Spicules are narrow, usually vertical, short­lived jets of solar material that extend from the chromosphere into the corona. They appear spiky and have relatively short lifetimes of about five to fifteen minutes.
Describe the formation theories of our Moon. How is it thought that the moons around Mars came about?
Prior to the Apollo missions, the three original theories governing the Moon's origin were the capture hypothesis, the fission hypothesis, and the binary accretion hypothesis. The capture hypothesis states that the moon was formed elsewhere in the solar system and then captured by the Earth. It is very improbable because conditions would have to be extremely precise and just right requiring that a very unlikely set of orbital coincidences be satisfied. The fission hypothesis states that the moon was “spun off” of the Earth at a time when it was rapidly rotating and when it was still molten. The binary accretion hypothesis states that the Earth and Moon formed as a “double planet,” and remained rotating about one another. The binary accretion model suggests that the Earth and Moon should have the same composition, which is not the case. After the Apollo missions returned samples of the lunar surface, the above theories were discounted and in the 1970's the giant impactor theory was proposed. In this scenario, a Mars­sized planetesimal struck the side of the Earth and this glancing blow shot large amounts of Earth's mantle and large amounts of the impactor into orbit. This material rapidly accumulated by impact processes into the Moon. The giant impactor theory explains the unique composition of the Moon. The Moon is depleted in iron due to the Earth having already been partially differentiated (the iron was in the core) when the impactor struck Earth. The Moon's depletion in volatiles (like water and potassium) is explained by the extremely high temperatures associated with the giant impactor theory. The glancing blow described in this theory also explains the angular momentum of the Earth­Moon system. Finally, since this theory proposes that the Moon formed from large amounts of Earth's mantle, we would expect the Moon to be enriched in refractory elements (elements found in the Earth's core) and we find this to indeed be the case. The Martian moons, Phobos and Deimos, are thought to have been captured asteroids.
How do the surfaces of the terrestrial planets (also include the Moon) compare? What does cratering history tell us? How do the sizes of volcanoes (if any) compare on the terrestrial planets and the Moon? Which terrestrial planets have water, or have shown evidence of water?
Mercury's surface is characterized by craters, bright ray systems, smooth lava plains, ridges, and valleys. Characteristic features include long cliffs and steep scarps, many of which are younger than the cratering. The surface of Venus resembles a stony desert landscape, with outcrops and patches of dark material. Estimates suggest that roughly 70% of the surface of Venus is covered by rolling plains, while about 20% of the surface is covered by depressional regions and the remaining 10% of the surface characterized by highlands. There are large craters that suggest impacts occurred before the atmosphere reached its current density. Earth's surface is mostly liquid water and most of the impacts have been dissolved due to the rapidly changing surface on Earth. The Martian surface seems to be mostly desert with some dark regions. Mars has numerous very large volcanoes, including Olympus Mons. Mars shows evidence for liquid water, but no evidence for recent volcanic activity has been found. If we see a lot of cratering on a planet, then we know that the planet is geologically inactive and the surface is older. Also, if we see a large amount of cratering on a planet with a substantial atmosphere, it suggests the impacts occurred before the atmosphere reached the density we see today. The size of volcanoes is determined by surface gravity, lava viscosity, the location of hot spots and plate tectonics, and geologic activity (erosion). For instance, the Earth has plate tectonics which means that regions where volcanoes can form move, whereas on Mars, there is no evidence of plate tectonics so a given hot spot will remain in the same place and over time a volcano at that point would have continued to build up. Furthermore, the lower surface gravity on Mars allows volcanoes to more easily grow taller. Finally, the more viscous the lava, the less it will flow away from the volcano, leading to a higher volcano altogether. Which terrestrial planets have atmospheres? How do the atmospheres compare? Why don't Mercury and the Moon have (substantial) atmospheres?
Venus and Earth have the most substantial atmospheres, Mars has more of an atmosphere than Mercury or the Moon, but quite less than Venus or Earth, and Mercury and the Moon have very little atmosphere present. During initial formation, the primary atmosphere for every terrestrial planet was composed basically of light gases like hydrogen and helium. This primary atmosphere was lost on the terrestrial planets due to things like relatively high surface temperature, low atomic mass and low escape velocity. For the jovian planets, the primary atmosphere is held, but for the terrestrial planets most of the original hydrogen and helium has been lost. The terrestrial planets will form a secondary atmosphere from outgassing due to tectonic activity. The composition of the outgassing material is basically the same on Venus, Earth and Mars consisting of water vapor, carbon dioxide, sulfur dioxide, nitrogen and traces of noble gases like neon and argon. The evolution of these materials after outgassing is what causes the atmospheres to exist as we see today. On Earth, the temperature was just right for liquid water to form and carbon dioxide released by outgassing was dissolved in the liquid water to produce carbonate rocks. This meant that nitrogen became the primary component of our atmosphere. On Venus there was (and is) no liquid water because of the high surface temperature, so carbon dioxide was unable to dissolve out of the atmosphere and therefore remains as the primary constituent of the atmosphere. The atmosphere on Venus is also characterized by clouds of sulfuric acid. Mars shows evidence for liquid water early on, but the temperature was insufficient to retain liquid water so carbon dioxide was the primary component in the atmosphere. However, Mars has had most of its atmosphere frozen out in the form of water ice or carbon dioxide ice leaving little carbon dioxide and noble gases to make up the very thin atmosphere. The atmospheric composition on a terrestrial planet is determined by a number of factors: (1) The planet's distance from the Sun determines its surface temperature and the higher the temperature, the higher the mean energy of the atmospheric components. (2) The size of the planet (mass and radius) determines the planet's surface gravity. They higher the surface gravity, the more easily the planet can retain an atmosphere because the escape velocity is higher. (3) The geologic activity of a planet is directly related to the amount of outgassing in the sense that the more geologically active a planet is, the higher the amount of outgassing, which leads to a thicker atmosphere. (4) Chemical reactions also influence the atmospheric composition because under various environmental conditions, different molecules are created and destroyed. Also, higher temperatures translate to faster reactions.
Describe the Greenhouse effect. What Greenhouse gases are present on the terrestrial planets? Why is Venus so hot?
The greenhouse effect is the phenomenon whereby a planet's surface is heated due to the trapping of infrared radiation by the atmosphere. The greenhouse effect is important in the atmospheres of Venus and Earth (and also on Titan). Water vapor and carbon dioxide are the most important greenhouse gases on Earth. Carbon dioxide is believed to have played a very significant role in early atmospheres of all the terrestrial planets and is currently the reason for the extremely high surface temperatures of Venus.
Describe the different internal regions of terrestrial planets. Describe plate tectonics.
The three basic internal regions of the terrestrial planets are the core, mantle, and crust. The core is the dense central region which forms when heavier components sink to the planet's center during differentiation. Earth's core is composed primarily of iron alloyed with nickel and lighter elements like sulfur. The core is mostly liquid, with a small solid inner core. Mercury, Venus and Mars have iron cores as deduced from their densities and moments of inertia. Mercury's core is far larger, proportionately speaking, than those of the other terrestrial planets. The Moon is depleted in iron, but may have a small core. The mantle is the intermediate region of a planetary body that has undergone differentiation. The terrestrial planets and the Moon have silicate mantles. The crust is the outermost region of the terrestrial planets composed of the lightest silicate materials that floated to the top during differentiation. Plate tectonics describes the shifting of large sections of the lithosphere (crust together with uppermost region of mantle) due to convection in the mantle. Head­on collisions of plates cause compression and create mountain ranges. Stretching of plates produces faults and linear depressions.
Which of the terrestrial planets have strong magnetic fields? What do we think causes these magnetic fields?
Earth is the only terrestrial planet with a strong magnetic field. Mercury has a weak magnetic field that is approximately a dipole aligned along the axis of rotation and is probably due to dynamo action in a supposed liquid core. However, if Mercury's core is solid, then the magnetic field may uniquely be a remnant of magnetization of the crust that was acquired during formation. Venus, despite a large nickel­iron core, does not have a magnetic field, due most likely to the planet's very slow rotation unable to generate a field by dynamo action. However, a very weak magnetic field is induced in the ionosphere through interaction with the solar wind. Mars also does not have a strong magnetic field (it has a very very weak magnetic field), though it has an iron­rich core. Earth, of course, has a magnetic field attributed to the rotation of the liquid core using the dynamo effect to generate a field. In general, a planetary magnetic field requires a layer of electrically conducting material, convection of that material, and rapid rotation.
The Moon rotates synchronously. What is synchronous rotation? Mercury is in 3:2 spin­orbit coupling. What does this mean?
Synchronous rotation describes the coincidence of the rotational period of a celestial body with its orbital period. In other words, if something is in synchronous rotation, it rotates once about its axis of rotation for every revolution (1:1 spin­orbit coupling). If something is in 3:2 spin­orbit coupling, it rotates three times for every two revolutions. In the case of Mercury, it rotates three times for every two orbits around the Sun. Describe the nebular theory of solar system formation. What important characteristics of the solar system are explained by this theory? How do we explain “exceptions,” such as backwards rotation?
The nebular theory starts with the gravitational collapse of a small region of a cold, low density, interstellar cloud known as the solar nebula. The collapse was probably triggered by a nearby cataclysmic event. As the cloud shrank, the gravitational contraction became stronger increasing the temperature and density and changing the shape of the collapsing cloud. This lead to heating, spinning, and flattening, which explain the layout of the solar system in terms of a relatively coplanar and orderly spinning system. The planets were formed in the spinning disk surrounding the forming Sun. When the temperature is low enough, some atoms or molecules may bond and solidify, and may therefore “condense” out. Temperature differences led to different types of condensed materials. Within the frost line, hydrogen compounds stay gaseous, and beyond the frost line, hydrogen compounds may condense out. As time went on, the seeds grew larger by way of accretion ultimately forming planetesimals, where the sizes and compositions of the planetesimals reflected the seeds from which they were accreted. The outer planetesimals (that would become the jovian planets) grew many times the size of the Earth and their gravity was strong enough (and the temperatures that far out were low enough) that they began accreting the abundant hydrogen and helium gas. Although the outer planets accreted a lot of gas, most of it was blown out of the solar nebula by solar wind. The leftover planetesimals became the asteroids and comets. Most of the unorthodox orbital features are explained by impacts. A theory of solar system formation must account for the two classes of planets (terrestrial and jovian); it must be able to explain the large numbers of asteroids and comets; it must be able to account for the orderly patterns of motion we observe in the solar system; it must also allow for the exceptions we see. These “exceptions” include the sideways rotation of Uranus and Pluto relative to their orbits; the fact that Venus rotates “backward;” the existence of Earth's large moon; it must also allow for the backward orbit of Triton around Neptune.