Dec. 4th - Moons of the Solar System

Question 1

How are Jovian moons organized best?

3 categories, relating to geological activity

Answer 1

• Small moons less than about 300 kilometers in diameter
• Medium-size moons ranging from about 300 to 1500 kilometers in diameter
• Large moons more than 1500 kilometers in diameter

Question 2

How do Jovian moons resemble the terrestrial planets?

Answer 2

• Each is spherical with its own geology
• Some have atmospheres, hot interiors, even magnetic fields
• The two largest moons (Titan and Ganymede) are larger than Mercury, while four others (Io, Europa, Callisto, and Triton) are larger than the largest known dwarf planets (Pluto, Eris)

Question 3

How did most of the medium/small sized moons form?

Answer 3

• Accretion within the disk of gas surrounding young Jovian planets
• Explains their circular orbits on the plane, and why they rotate in the same direction as their parent planet

Question 4

Do the moons face in different directions?

Answer 4

• NO - same face always towards the planet due to tidal locking (like on earth)

Question 5

Where do most of the small moons end up originating from?

Answer 5

• Asteroids caught into an orbit, explaining their lack of uniform orbit (even rotating or orbiting backwards)

Question 6

Jupiter’s 4 Galilean Moons

Io: The Volcano World

Answer 6

• Most volcanically active world in our solar system
• Large volcanoes: eruptions so frequent that they continually repave surface (does not have a single impact crater anywhere)
• Io probably also has tectonic activity, because tectonics and volcanism generally go hand in hand
• However, debris from volcanic eruptions has probably buried most tectonic features.

Question 7

Jupiter’s 4 Galilean Moons

Io: Outgassing

Answer 7

• The primary gases released from Io’s volcanoes are sulfur dioxide (SO2), sulfur, and a hint of sodium
• Some escapes into space, where it supplies ionized gas (plasma) to the Io torus and Jupiter’s magnetosphere, and some of it gives Io a very thin atmosphere.
• Much of the gas condenses and falls back to the surface, where the sulfur gives Io its distinctive red and orange colors and the sulfur dioxide makes a white frost
• As hot lava flows across the surface, it can re-vaporize the sulfur dioxide surface ice in much the same way that lava flowing into the ocean vaporizes water on Earth.

Question 8

Jupiter’s 4 Galilean Moons

Io: internal heat

How can it have enough heat for volcanism, yet be similar to Mercury?

Answer 8

• ANS: TIDAL HEATING from Jupiter
• Jupiter’s mass makes the tidal friction much stronger than earth to the moon
• Moreover, Io’s orbit is slightly elliptical - therefore affecting its changing tidal bulges
• NET EFFECT: Io is constantly being pulled in different directions, which generates friction inside of it

Question 9

Jupiter’s 4 Galilean Moons

Why is Io’s orbit elliptical?

Answer 9

• EX: orbital resonance
• Io, Ganymede, and Europa share an orbital resonance with Jupiter (4:1:2 ratio); once all line up, they apply a strong gravitational tug on one another, causing ellipses

Question 10

Jupiter’s 4 Galilean Moons

Europa

Answer 10

• Europa is covered by water ice
• Nevertheless, its fractured, frozen surface must hide an interior made hot by the same type of tidal heating that powers Io’s volcanoes, though tidal heating is weaker on Europa because it lies farther from Jupiter.

Question 11

Jupiter’s 4 Galilean Moons

How does Europa’s hot interior affect its icy surface?

2 hypotheses

Answer 11

• Europa only has a few impact craters, which hints at recent resurfacing
• The answer is either:
  1. liquid water rising up from an ocean that lies beneath the icy crust or interior water ice that is just warm enough to undergo convection, so that some of it rises up and flows across the surface
  2. Theoretical models based on measurements of the strength of gravity over different parts of the surface indicate that Europa has a metallic core and a rocky mantle surrounded by enough water to make a layer of ice about 100 kilometers thick. The models suggest that the upper 5 to 25 kilometers of the water should be solidly frozen as an icy crust, but that tidal heating should provide enough warmth to turn the underlying ice into a layer of either liquid water or relatively warm, convecting ice.

Question 12

Jupiter’s 4 Galilean Moons

Europa: Magnetic Field

Answer 12

• Europa is one of only a few moons in the solar system to have a magnetic field, and its magnetic field changes as Jupiter rotates.
• The simplest way to explain this change is to hypothesize that Europa’s magnetic field is created (or induced) in response to the rotation of Jupiter’s strong magnetic field.
• This type of response is possible only if Europa has a liquid layer of electrically conducting material. A salty ocean would fit the bill, but convecting ice would not

Question 13

Jupiter’s 4 Galilean Moons

Europa: Evidence for a Deep Ocean

Answer 13

• Taken together, the evidence from surface photos, gravitational measurements, and magnetic fields makes a strong case for a deep ocean of liquid water on Europa
• Because Europa must have a hot interior from tidal heating, it seems reasonable to imagine that volcanic vents dot the seafloor and sometimes erupt, creating rising plumes of warm water
• These plumes may lead to the formation of subsurface lakes within the icy crust, which could cause cracking of the icy surface above. Surface features that look like jumbled icebergs may be explained by such cracking

Question 14

Jupiter’s 4 Galilean Moons

Ganymede’s surface appears to have a dual personality:

Answer 14

• Has an icy surface, like Europa
• Some regions are dark and densely cratered, suggesting that they look much the same today as they did billions of years ago.
• Argues for occasional upwelling of liquid water or icy slush to the surface.
  This material would cover craters before refreezing, explaining why there are so few craters in this terrain.
• The long grooves are probably made by the eruption of water or slush along a crack in the surface. As the water in the crack freezes, it expands and pushes outward, creating the groove.
• Other regions are light-colored with very few craters.
• In some cases, fairly sharp boundaries separate the two types of terrain.

Question 15

Ganymede: similarities to Europa

Answer 15

• Could the differences across Ganymede’s surface have the same explanations as Europa?
• Also has a magnetic field subject to Jupiter’s rotation, suggesting the presence of a salty ocean beneath the surface
• How does Ganymede stay so hot?
• Not necessarily due to tidal friction (like Io), but rather due to its large size - was able to retain more heat from radioactive decay

Question 16

Callisto: surface

Answer 16

• Heavily cratered iceball
• Circular patches on its surface are impact craters (bright because the large impacts revealed “cleaner” ice underneath)
• Craters make sense on an old surface like Callisto’s, but other features are more difficult to interpret.

For example, close-up photos show a dark, powdery substance concentrated in low-lying areas, leaving ridges and crests bright white

Question 17

Callisto: internal heat

Answer 17

• Despite its relatively large size (the third-largest moon in the solar system), Callisto lacks volcanic and tectonic features.** This tells us that it lacks any significant source of internal heat. **
• In fact, gravity measurements by the Galileo spacecraft showed that Callisto never underwent differentiation: Dense rock and lighter ice are mixed throughout most of its interior, which means that its interior never warmed significantly.
• The lack of heat is not surprising: Callisto has no tidal heating because it does not participate in the orbital resonances that affect the other three Galilean moons, and we do not expect its icy interior to contain enough radioactive material to supply much heat through radioactive decay.
• Nevertheless, it is possible that Callisto, too, may hide a subsurface ocean. As with Europa and Ganymede, Galileo found that Callisto has a varying magnetic field that suggests the presence of a salty interior ocean.

Question 18

What geological activity do we see on Titan and other distant moons?

Titan

Answer 18

• Saturn’s Titan is the second-largest in the solar system
• It is also unique in having a thick atmosphere—we cannot see through it with visible light.
• Titan’s color comes from chemicals in its atmosphere
• The atmosphere is about 95% molecular nitrogen (N2), not that different from the 77% nitrogen content of Earth’s atmosphere.
• Titan’s atmospheric composition can be understood in terms of our general understanding of atmospheric production and loss processes

Question 19

Titan: atmospheric gases

Answer 19

• Titan’s icy composition supplies methane and ammonia gas through vaporization of surface ices and liquids, and possibly also volcanic eruptions.
• Solar ultraviolet light breaks down some of those molecules, releasing hydrogen atoms and leaving highly reactive compounds containing carbon and nitrogen. The hydrogen atoms can leave Titan forever by thermal escape, while the remaining molecular fragments can react to make the other ingredients of Titan’s atmosphere.
• Methane and ethane are both greenhouse gases and therefore give Titan an appreciable greenhouse effect that makes it warmer than it would be otherwise.
• Still, because of its great distance from the Sun, its surface temperature is a frigid 93
• The surface pressure on Titan is about 50% higher than the sea-level pressure on Earth,

Question 20

We have at least two other reasons for special interest in Titan:

Answer 20

1. First, its complex atmospheric chemistry produces numerous carbon compounds—the chemicals that are the basis of life.
2. Second, although it is far too cold for liquid water to exist on Titan’s surface, conditions are right for methane or ethane rain, which creates rivers flowing into lakes and seas.

Question 21

Titan: Northern versus Southern polar regions

Answer 21

• Images reveal polar storm clouds and riverbeds leading into the lakes, suggesting that Titan has a methane/ethane cycle resembling the water cycle on Earth
• Northern summers are apparently wetter, based on the large fraction of lakes located there. This may be due to Saturn’s elliptical orbit, which brings it (and Titan) closer to the Sun during northern summer. The more intense northern summer may create stronger circulation and storms with enough rainfall to fill the northern lakes.
• Perhaps the most astonishing result from the Cassini/Huygens mission is how familiar the landscape looks in this alien environment with these unfamiliar materials.
• Evidently, the similarities between the physical processes that occur on Titan and Earth are far more important in shaping the landscapes than the fact that the two worlds have very different compositions and temperatures.