The Jovian planets - Jupiter, Saturn, Uranus and Neptune - probably have cores (suspected to be liquid metal for Jupiter and Saturn, but primarily ice for Uranus and Neptune), but are “mostly atmospheres,” comprised of dense gas. Below are photographs of the planets by robotic probes, and an artist’s diagram of their interiors. The Jovian interiors are composed primarily of H_2 and and He, with important amounts of methane ( CH_4 ), water ( H_2O ), ammonia ( NH_3 ) and related molecules. The Jovian planets thus reflect better the original material than the terrestrial planets where most of the gases have escaped.

Given the extremely high pressures at the interface with any surface, it seems unlikely that the Jovian planets harbor living organisms. We will therefore review them briefly and concentrate instead on their moons which seem more likely sites for life.

Jupiter has more mass than all of the other planets put together, and is only about 1⁄10 the mass it would need to initiate deuterium fusion reactions in its core (the “easiest” type of fusion for a star). Thanks to its mass and 10-year orbit, Jupiter would likely be the first planet discovered by an alien civilization searching for planets around our Sun using techniques like those we use today (as we will discuss in the final unit of the course). Jupiter shines more brightly in the night sky than any star, and is outshined only by the Moon and Venus. Its strong magnetic field causes occasional bright radio flares brighter than any other radio phenomenon in the Solar System apart from the Sun. Its atmosphere exhibits bands caused by high-altitude winds and gargantuan hurricane-like stoms, including the famous “Red Spot” storm, which is roughly the size of ten Earths and has persisted in its atmosphere for more than three centuries. Jupiter has four large moons, discovered by Galileo, that rival Mercury in size and some of which may offer suitable habitats for life, as we discuss below and will explore further in the weeks ahead.

Saturn is a less massive, more distant version of Jupiter exhibiting similar visual colors, including banded high-velocity clouds and storms, and having a similar atmospheric composition. Its appearance in telescopes is the most dramatic of all the planets, thanks to its giant ring system, likely the disrupted remains of a moon that wandered too close to the planet in the relatively recent past (less than 1 Ga ago) and was torn apart by Saturn’s tidal forces. Saturn has a single massive moon, Titan, roughly the size of Mercury, the only planetary satellite in the Solar System to have a substantial atmosphere and weather. Titan is another possible habitat for life which we will explore in more detail in the weeks ahead. Saturn, easily observed with the naked eye, is the most distant (slowest moving) of the planets known to ancient astronomers.

Uranus and Neptune have been called the “ice giants” because they exist in the cold outer reaches of the Solar System and exhibit atmospheres with significant ice content and a different mix of gases than Jupiter and Saturn, giving them their blue-green (or if you prefer, cyan) appearance in visual-light images. Both are too faint to be seen with the naked eye (with ocassional exceptions for Uranus), and were not discovered until after telescopes were in widespread astronomical use. Uranus is noted for the unusual 97 degree tilt of its rotational axis, which means it effectively rolls around the Solar System on its side instead of spinning like a top as the other planets do (Venus can be considered to be upside down, and spins slowly, which is also unusual). Neptune has a single massive moon, Triton, which orbits in the “wrong” (retrograde) direction and so is thought to be a captured Kuiper Belt object. Triton’s properties, including its very tenuous nitrogen atmosphere, are very similar to those of Pluto and other massive Kuiper Belt objects, and with current understanding it is too cold and too far from the Sun to possibly support life.

Our current picture of the interiors of these gas giants is shown in the figure below.

Moons of Jupiter

Jupiter has four large inner moons discovered by Galileo: Io, Europa, Ganymede, and Callisto. The pictures here are from the Galileo robot probe launched by NASA that orbited Jupiter and photographed its moons from 1995-2003.

Io is closest to Jupiter and, subjected to enormous tidal forces of the giant planet, should have its spin locked to its orbital period like our Moon’s spin is locked to its orbit, with the same side always facing its planet. But the pull of the next moon out, Europa, disturbs Io’s orbit, with the result that Io is constantly being “massaged” by Jupiter’s tidal forces, which heat and melt its icy and rocky interior. As a result, Io is the most volcanically active body in the Solar System (below). With many constantly erupting volcanos, its surface covered with recent lava flows, and its complete lack of any atmosphere, Io’s surface and interior are probably too harsh and unstable for life.

Europa (below, left) has an icy surface with long dark cracks and no craters. These surfaces show evidence of having been repeatedly melted and refrozen. Models of this phenomenon suggest there is a large ocean beneath a thick icy crust that may contain more liquid water than exists in all the oceans of Earth. As with Io, tidal forces of Jupiter likely provide sufficient heat to keep this water from freezing. The evidence for existence of these oceans, their likely properties, and their potential for harboring life will be a focus for us in the weeks ahead.

Ganymede (below, right) and Callisto exhibit relatively young surfaces that, again, suggest the presence of large quantities of liquid water - or, at the very least, “slush” - below the surface of both moons, again because of Jupiter’s tidal influence, combined with the tidal forces of the other inner moons. We will discuss prospects for life on these moons as well.

Moons of Saturn

Saturn’s moons, discovered by Christian Huygens in the 1600s, are now being investigated by the Cassini probe which arrived there in 2005. There are dozens of moons, most with ancient icy surfaces covered in craters. Some have large cracks suggesting heating and expansion from large collisions. Saturn’s largest moon is Titan, the only moon with a substantial atmosphere and observed weather patterns. The surface features are invisible due to clouds (below, left), and models suggest that Titan may have liquid N_2 (nitrogen), CH_4 (methane), or C_2H_6 (ethane) oceans.

Cassini dropped a research probe named Huygens (built by the European Space Agency) onto Titan in 2005. It took some remarkable photos during its descent (below, center) and after landing on the surface (below, right) - the only visible light photos we have of the surface of this alien world. The last photo shows stream-beds kilometers in length and surface material with a mix of light and dark colors. It is much too cold for liquid water; the liquid seen flowing on Titan’s surface (center) is likely to be methane or ethane.

Saturn’s moon Enceladus is another example of an icy moon like the Galilean satellites of Jupiter. The existence of liquid water under its surface has been confirmed by the Cassini mission’s observations of jets of water vapor erupting from its surface. We will discuss Enceladus along with Europa, Ganymede, and Callisto in the context of habitability.

Moons of Uranus and Neptune

Uranus and Neptune have a number of prominent moons, most with ancient, heavily cratered icy surfaces. Uranus’ moon Miranda ** (below, left) is remarkable: it looks like different terrains have been pieced together. Giant cliff separates two regions. Perhaps Miranda was shattered into pieces by collision with an asteroid/planetisimal, and reformed itself. Triton (below, right) is the largest of Neptune’s moons and has very strange charateristics: it orbits in the wrong direction, and its surface has two different terrains (smooth melted material, and rough colored material). The moon is extremely cold and the surface is mainly nitrogen ice ( N_2 , bluish) on one side and methane ice ( CH_4 , pinkish) on the other. It appears to have (active?) volcanos (erupting nitrogen? water? soot?). Triton is the only large moon with a retrograde orbit, so it may have melted during its capture by Neptune long ago. But the surface has very few craters, so it must have been altered many times since the original capture.