A1.  The two outer moons are larger and composed mostly of ice, while the two inner ones are somewhat smaller and composed mostly of rock. This pattern is similar to the pattern of terrestrial and giant planets in the solar system. The large moons around Jupiter formed from a gas disk orbiting the planet in much the same way that the planets formed from a gas disk orbiting the sun.

A2. 

Moon – maria, lava-filled basins

Io - active volcanoes

Europa – ice-covered oceans

Ganymede – intersecting grooves, dark spots

Callisto – crater covered, possible ocean

Titan – dense atmosphere

Triton – geysers, rigid ice surface, frost covered

A3.  Callisto’s surface is completely covered by craters and shows no signs of other geologic activity. While Ganymede also has some craters, it’s surface also shows a complicated system of parallel ridges and grooves and large areas of different colors and materials. These observations suggest that Ganymede has undergone some plate tectonic activity. The interiors of Callisto and Ganymede also differ: Callisto is undifferentiated and has no core, while Ganymede has a dense core surrounded by a mantle.

A4.  Io’s surface is pock-marked by many dark volcanic features. It’s surface is yellowish orange with no impact craters because of the intense volcanic activity. Europa’s surface is very smooth and cris-crossed by many lines. It’s icy surface bears a strong resemblance to pack ice on Earth – solid ice floating on a liquid ocean. Callisto’s surface is impact-scarred and fairly dark. Ganymede’s surface also shows many impact craters, but has a more mottled appearance with large dark spots scattered across the surface. Systems of parallel grooves also cris-cross the surface. These features are probably the result of the shrinking of the surface as the moon cooled long ago.

A5.  Moons in the solar system can be classified as tiny (less than 100 km in diameter), intermediate ( between 100 km and 1000 km in diameter), large rocky (larger than 1000 km in diameter and composed mostly of rocky material), and large icy (larger than 1000 km in diameter and composed mostly of icy material).

A1.  The most compelling evidence for a liquid ocean on Europa is the appearance of the surface. It is extremely smooth, as if it is unable to support the weight of tall features. It has many criss-crossing ridges and grooves from the constant shifting of the surface. Pieces that look like icebergs are also found at several locations on the surface.

A1.  Io’s orbit around Jupiter is locked in a 2:1 resonance with Europa’s orbit. Every other orbit they line up at the same place in Io’s orbit. This causes Io’s orbit to become slightly elongated at that point. The changing tides on Io caused by its changing distance from Jupiter cause the moon to slightly expand and contract internally. The friction created by this internal motion heats the interior, which is released by volcanic activity.

A2.  Io’s orbit is in a two-to-one resonance with Europa’s orbit around Jupiter. This resonance has pulled Io’s orbit into a slightly elongated shape. When Io is closer to Jupiter it’s shape is distorted by the tidal forces from Jupiter; when it is further away it’s shape relaxes somewhat. This internal flexing of Io causes friction, which leads to the build up of heat, which in turn is released by numerous volcanoes.

A3.  Its orbit has a 2 to 1 resonance with Europa’s orbit just outside its orbit. The repeated tugs of Europa at a fixed point in Io’s orbit gradually have pulled its orbit into an elongated shape. As the distance between Jupiter and Io changes during Io’s orbit, the pull of gravity it feels from Jupiter also changes, causing the shape of the moon to be alternately squeezed and expanded. The internal friction created by this changing shape generates internal heat, which powers the volcanic activity on the moon.