PV Gap

Corey Said:

In what sense are Jupiter and its moons like a miniature solar system?

We Answered:

The moons of Jupiter orbit that planet the way planets would orbit a star (the same could be said for Saturn, Uranus, and Neptune). The moons of Jupiter are very small compared to the planet, just as planets are very small compared to the parent star.

The galilean moons decrease in density with distance from Jupiter (Io = 3.5 g/cm^3, Europa = 3.01, Ganymede = 1.94, and Callisto = 1.83). This is a trend (though it doesn't hold completely for all Jupiter's moons).
A possible explanation is the farther from Jupiter the moon is, the more volatiles (ices) a moon is able to collect during formation (ices will decrease the density of an object).

Esther Said:

Which four moons of our solar system have an atmosphere?

We Answered:

"Of the known moons in the solar system, four have been found to have atmospheres: Jupiter's volcanically active moon Io (tenuous sulfur dioxide), Jupiter's ice-coated moon Europa (tenuous oxygen), Saturn's largest moon Titan (dense nitrogen and methane), and Neptune's largest moon Triton (tenuous nitrogen and methane). The ability of a moon to retain an atmosphere is related to its mass and its distance from the Sun, although tenuous atmospheres may be regenerated by a variety of processes."

Christian Said:

Why does Titan have a dense atmosphere, whereas other large moons in the solar system don’t?

We Answered:

Because most of the moons have no gases present therefore no atmosphere. Our moon for instance has no gas so it can't have an atmosphere. Titan is also bigger than most moons so that probably plays a role, much of the atmosphere isnt lost into space as easily.

Mario Said:

Why don’t other planets and moons in our solar system show signs of plate tectonics?

We Answered:

Mercury, as well as all the moons in our solar system with the exception of the four large moons of Jupiter and Titan, are too small to maintain a liquid core for any significant amount of time. If the core becomes solid, there can no longer be plate tectonics.

Mars does show hugely obvious signs of plate tectonics - the Valles Marineris is the most well known one, a split between two plates deeper than the Grand Canyon and longer than the distance between New York and LA. However, since Mars is also small, the core has also become solid somewhere between 2 and 3.5 billion years ago.

Venus does show signs of plate tectonics. There are two continental plates known, and at least four "oceanic" plates (although there is no liquid water, the oceanic plates can be recognized by being denser and deeper than the continental plates).

Io, the first moon of Jupiter, does show signs of plate tectonics. It is the single most active volcanic place in the solar system, with barely an hour going by without a volcanic eruption. This is due to tidal forces of Jupiter constantly grinding the insides of Io to melting point.

Europa, the second moon of Jupiter, might possibly have plate tectonics, but to a much lesser extent than Io. However, Europa is covered by an ice cap over a kilometer thick - we are unable to see beneath it to check.

Callisto and Ganymede, the third and fourth moons of Jupiter, and Titan, a moon of Saturn, are similar in size to the Earth, but do not show signs of ever having been active volcanically. This is probably because they formed more gradually from rings around Jupiter, and were therefore much cooler and never had the chance to be very active.

The outer planets' solid cores are all covered by at least 10,000 kilometers of gaseous and liquid hydrogen. Whether or not there is plate tectonics in their cores is unknown, although due to the pressure (and in case of Jupiter, deuterium fusion), they are probably either comparable to the Earth's core (solid, uniform, strongly magnetic) or they're so hot that they turned to plasma.

Earth has not cooled down due to it's size, and because of the tidal forces exerted by the moon.