Astronomy 201 Review 3 Answers

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1 Astronomy 201 Review 3 Answers Explain why the jovian planets are so much different from the terrestrial planets and why Jupiter has the most mass, followed by Saturn, and so forth. The jovian planets formed in a much colder region of the solar system where ice could condense. The terrestrial planets could only have cores of rocky material, whereas the jovian planets could have cores of rock and ices making them much larger. These larger planetesimals accreted surrounding hydrogen and helium, so much so that each jovian planetesimal was in effect a miniature solar nebulae where the planet formed at the center and moons formed within the surrounding swirling disk. This explains why the jovian planets have so many more moons than the terrestrial planets. The jovian planets have about the same mass in their cores, but the surrounding layers of hydrogen and helium decrease as you go outward from Jupiter because accretion rates went much faster for Jupiter and decreased outward toward the more spread out regions. By the time the solar wind blew the gas away, Jupiter had had more time to accrete so it grew the largest. Compare the atmospheres of the jovian planets including defining characteristics, weather patterns, and structure. The atmospheres of the jovian planets are very dynamic and while the atmospheric structure is very similar between each jovian planet, the primary difference among them is temperature, due to the decreasing temperature with increasing distance from the Sun. Jupiter's atmosphere is primarily hydrogen and helium, in addition to a small fraction of hydrogen compounds. The hydrogen compounds, although accounting for a very small fraction of the atmospheric mass, largely govern Jupiter's appearance. Jupiter has a thermosphere, stratosphere and troposphere and three distinct cloud layers, each at an altitude where the temperature is just right for a particular gas to condense (ie ammonia, ammonium hydrosulfide and water). Convection in the troposphere is responsible for the different cloud layers. The regions of rising air are called zones and the regions of falling air are called belts. The defining characteristic of Jupiter's atmosphere is the Great Red Spot which is a mysterious high pressure storm that has been brewing for centuries. Saturn, Uranus and Neptune all have distinct cloud layers as well, governed by the temperature at which different gases condense. Saturn has the same three cloud layers as Jupiter, however, due to Saturn's atmosphere being colder, the clouds are deeper in the atmosphere. Clouds on Uranus and Neptune are due to methane snowflakes. Saturn has stripes similar to Jupiter, but they are much less bold. Neptune is also banded and had a high pressure storm similar to the Great Red Spot on Jupiter called the Great Dark Spot. Uranus also has storms due probably to the extreme change in seasons due to being tipped on its side. Jupiter

2 has bold brown and red colors whereas Saturn is less distinctively tan and yellow. Uranus is blue green and Neptune is a deep blue color. Uranus and Neptune derive their colors from methane, whereas the hydrogen compounds on Jupiter and Saturn are responsible for their colors. Compare the interiors of the jovian planets as well as their sizes. Jupiter's composition is probably a nearly uniform combination of hydrogen and helium with each layer defined by the phase of the material, namely, gaseous hydrogen, liquid hydrogen and metallic hydrogen. The metallic hydrogen plays an important role in generating Jupiter's strong magnetic field. Jupiter's core is made of rock, metal and hydrogen compounds and is very very dense. The temperature, pressure and density are greatest at the core's center and decrease outward. Jupiter has a tremendous amount of internal heat probably due to contraction (gravitational potential energy converted to thermal energy) and it actually emits more heat than it receives. Saturn is made also of hydrogen, helium and hydrogen compounds and is about the same size as Jupiter, though one third Jupiter's mass. Saturn also has metallic hydrogen, but much deeper in comparison to Jupiter and therefore there is less overall metallic hydrogen on Saturn. Saturn also has a significant source of internal heat, however, the mechanism driving this heat is different from the contraction generating internal heat in Jupiter because Saturn is too small to generate all of its heat by contraction. The helium rain supposition attempts to explain Saturn's internal heat generation by contending that the relatively high density helium rain droplets that condense at high levels in Saturn due to its pressure and low interior temperature, slowly fall to the interior, thereby gradually providing a mechanism of differentiation (denser things fall to the center and less dense things above that). Uranus and Neptune have cores similar to Jupiter and Saturn, but the layers surrounding their cores are different in that the pressures are too low to form liquid or metallic hydrogen around their cores. That being said, their cores may be liquid making for oceans deep inside them. Uranus emits practically no more heat than it receives which is expected, but Neptune unexpectedly emits nearly twice the heat it receives. The source of Neptune's additional heat is a mystery and it's unusual to think that Neptune would still be contracting to produce additional heat due to its relatively small size. Describe the magnetospheres of the jovian planets. A magnetosphere traps charged particles causing them to spiral along magnetic field lines and in doing so, the more energetic particles collide with atoms and molecules in the upper atmosphere causing them to radiate and produce an aurora. Jupiter has the brightest auroras due to the large number of trapped charged particles. Each jovian planet is surrounded by a

3 magnetosphere. The size of a magnetosphere depends on both the magnetic field strength and the pressure of the solar wind against it. Just as the further away from the sun a planet is the lower the temperature, larger distances correspond to weaker solar wind pressure on a distant planet, so Uranus and Neptune, despite intrinsically weak magnetic fields, have moderately sized magnetospheres. Jupiter's magnetic field is about 20,000 times stronger than Earth's due to the thick layer of metallic hydrogen. Jupiter also has two sources of charged particles interacting with its magnetosphere: the solar wind and gas coming from Io. Io loses atmospheric gas very fast due to the constant bombardment of charged particles within Jupiter's magnetosphere. The magnetic axis of Jupiter is roughly aligned with its rotation axis as would be expected. Saturn has a weaker magnetic field than Jupiter because it has less metallic hydrogen. Saturn's magnetic axis is also aligned with its axis of rotation. Neptune and Uranus have no metallic hydrogen and therefore have decidedly weak magnetic fields probably caused by liquid materials in their cores. Unexpectedly, the magnetic axes of Neptune and Uranus are not aligned with their respective rotation axes. Where did the jovian moons come from? The larger moons probably formed around the swirling disk of material surrounding each jovian planetesimal during formation and this is supported by the fact that the large majority of the moons orbit the same direction that their planet rotates and lie close to the equatorial plane orbiting in nearly circular orbits. Most of the moons also rotate synchronously due to the strong tidal forces exerted by the jovian planets. Most of the small moons are probably captured asteroids with eccentric orbits and often large tilts relative to the equatorial plane. Why are jovian moons generally more geologically active than terrestrial planets? In some cases (Io and Europa), the moons have a continual source of internal heat due to tidal heating (from Jupiter) whereby any deviation from a circular (synchronous) orbit means a change in the strength and direction of the tidal force, altering the sizes and orientations of tidal bulges on the moons and generating internal friction. Additionally, the composition of the jovian moons includes a higher proportion of ice (directly related to where they formed in the solar system), relative to the terrestrial planets, which can undergo geological change at lower temperatures than the rocky materials that make up the terrestrial planets. Accordingly, the moons of the more distant planets are composed of a higher fraction of ice. What are the Galilean moons? Briefly describe them. The Galilean moons include Io, Europa, Ganymede, and Callisto. Io is the most volcanically

4 active celestial body in the solar system and therefore has an extremely young surface. Io has a thin atmosphere of sulfur dioxide produced through outgassing, which is also accountable for the additional ionized gas particles within Jupiter's magnetosphere. Sulfur is also the reason for the red and orange colors of Io. Io is also thought to have tectonic processes. Europa's surface and crust are made primarily of water ice and its surface is devoid of impacts. Europa might have an ocean of liquid water deep under its surface due to tidal heating which would be nice and would explain the lack of impacts. Europa has a magnetic field that is probably induced by Jupiter's strong magnetic field sweeping past and it changes as Jupiter rotates. A salty liquid ocean could drive such a magnetic field. Ganymede has a surface of water ice and regions of very young surface, as well as regions of very old surface. The regions of young surface are thought to be replenished by upwellings of liquid water that fill the craters prior to freezing. Ganymede may also have an ocean of liquid water supported by magnetic field measurements. Callisto is also covered with ice, but is heavily cratered. Callisto lacks volcanic and tectonic activity suggesting very weak internal heating. Callisto is not in an orbital resonance and thus has no tidal heating. Callisto's interior never underwent differentiation due to its extremely cool interior. Callisto has a magnetic field suggestive of a submerged salty liquid ocean. Ganymede, Europa, and Io are in a 1:2:4 orbital resonance and all are subject to tidal heating due to their slightly elliptical orbits, though the degree of tidal heating depends on the moon's distance from Jupiter. Describe Saturn's largest moon Titan. Titan's atmosphere is primarily nitrogen with the rest being argon, methane, and ethane. The latter two are greenhouse gases that significantly warm Titan. Titan is composed mostly of ices including methane and ammonia ice that, when sublimation occurs, adds to the atmosphere. Ultraviolet light may have substantially reduced the amount of methane by converting it to ethane, which may in turn form clouds and rain liquid ethane. The surface temperature and pressure on Titan are just about right for ethane to exist at its triple point, where it can exist as a gas, liquid and solid simultaneously. This, in addition to surface brightness variations, lend evidence to support oceans of liquid ethane. What is interesting about the moons of Uranus? Uranus has five medium sized moons. Specifically, there are two pairs of similar sized ( twin ) moons that vary significantly in the context of their geological activity and this continues to be a mystery. What are the two major moons of Neptune and why is Triton unique?

5 Nereid is a medium sized moon orbiting Neptune and Triton is the larger and much colder moon in retrograde rotation. Triton also orbits highly inclined relative to the equatorial plane of Neptune which suggests it was captured. Triton is icy, spherical, and large and it is thought that Triton orbited the Sun at one point instead of Neptune. Triton has a large number of impacts, but also shows evidence of past icy volcanism. Triton also shows evidence for past geologic activity in the cantaloupe terrain which comprises a region of wrinkly ridges. The source of internal heat powering that geological activity may have involved tidal heating, but it is not known for sure. Triton also undergoes extreme seasonal changes. Since Triton orbits in retrograde motion, it is being slowed down by tidal bulges and is slowly spiraling in. Eventually, it will reach the Roche zone and an extraordinary broad ring system will surround Neptune. Which of the jovian planets have rings? What accounts for gaps in ring systems? What is the origin of rings? All four jovian planets have rings. Saturn's rings are the most well known and they are made of individual particles consisting primarily of reflective water ice. Each particle orbits Saturn independently according to Kepler's laws. Gaps within rings are primarily due to orbital resonances which means that every time a given particle enters a specific point in its orbit, it feels a consistent gravitational push that clears it out and creates a gap. The Mimas 2:1 resonance is responsible for the Cassini division (which we know is not completely devoid of particles). Jupiter, Uranus, and Neptune also have rings, but they are much fainter and smaller than those surrounding Saturn. The rings around Uranus were actually discovered during observations of stellar occultation. The rings around the jovian planets lie close to their planets where tidal forces are strong. When the tidal forces exerted on an object become comparable to the gravitational forces holding it together, the object has reached the Roche zone and will be torn apart. Another explanation is that the constant bombardment of micrometeorites on small moons within rings provides a continual source of ring particles. What are the three categories of small bodies in the solar system? Describe asteroids and comets. Describe the origin of the asteroid belt. Asteroids, Kuiper belt comets, and Oort cloud comets make up the smaller bodies of the solar system. Asteroids are rocky chunks of material thought to be leftover from the formation of the solar system. A comet is an icy leftover orbiting the Sun usually with a very long period compared to asteroids. Meteors are pieces of material that streak across the sky and a meteorite is a piece of material that actually reaches the surface of Earth. The majority of asteroids are

6 located in the asteroid belt between Mars and Jupiter and this is probably due to orbital resonances between the asteroids and Jupiter. Orbital resonances with Jupiter also probably prevented a planet from forming where the asteroid belt is because the planetesimals that would have accreted into a planet there were not allowed to accrete due to the orbital resonances throwing material out of the region. Gaps in the asteroid belt are known as Kirkwood gaps. Comets consist of a nucleus, a coma, and a tail which consists of gas and dust; actually there are two tails, the plasma tail is made of ionized gas and the dust tail is made of small particles. Will Pluto and Neptune ever collide? Why? Neptune and Pluto are in a 3:2 orbit resonance which translates to Neptune always being a safe distance from Pluto. Also, Pluto is inclined roughly seventeen degrees relative to the equatorial plane of Neptune. What is the explanation for Pluto's higher than expected density? What evidence suggests Pluto is in fact a Kuiper belt object? It is thought that a giant impact with Pluto may have blasted away the low density outer regions of Pluto which ultimately led to the formation of its moon Charon. Pluto is about the size of a large Kuiper belt object and its inclined orbit is different from the relatively planar orbits of the planets.