Our moon, while celebrated in song and literature for its beauty, does not harbor life. With no atmosphere and little known water, conditions on our moon are not adequate to support life as we understand it. There are, however, a total of 61 moons orbiting the 9 planets in our solar system and some of them have atmospheres, organic molecules, water, or heat energy – the conditions necessary for life to exist.

The larger moons of Jupiter, for instance, have some of the qualities that would make life possible. These moons – Europa, Io, Callisto, and Ganymede – are called the Galilean moons because they were first spotted by Galileo Galilei in 1610. The moons were individually named by the German astronomer Simon Marius, who discovered them the same year as Galileo.

Other materials could possibly combine to create life, but not as readily as the combination of liquid water and organic materials like carbon. Carbon is plentiful throughout the universe and, more importantly, it can form an amazing variety of intricate compounds. Liquid water helps this process along by acting as a stable medium for the organic molecules to dissolve in, while still being fluid enough so the molecules can interact. No other combination of elements that we know of can beat this happy partnership between carbon and liquid water in the creation of life.

This blend of elements may have occurred on Europa, the Jovian moon with the strongest possibility for the existence of life. When the Galileo spacecraft sent back images of Europa, we saw that the surface ice layer was covered with various cracks and gashes. These fissures seem to indicate tectonic activity much like the tectonic activity on Earth, where the continents are always shifting.

Underneath this surface layer of pure ice, Europa is believed to have liquid water melted by radioactive decay and tidal heating. Scientists differ on the thickness of this ice layer: Estimates range from fewer than 20 kilometers (12 miles) to more than 150 kilometers (90 miles).

There is no definitive proof that liquid water exists on Europa. Recently, however, the Galileo spacecraft found evidence that Europa has a periodically varying magnetic field. This finding is best explained by the presence of a salt-water ocean beneath the moon’s icy crust.

Another indication of possible life is the presence of organic molecules, which the Galileo space probe has detected on Europa. The Galileo spacecraft’s near-infrared mapping spectrometer sent back data indicating the presence of combinations of oxygen, carbon, sulfur, hydrogen and nitrogen on Europa. The data also included a suggestion of the presence of tholins, complex organic compounds.

“This doesn’t mean there is life on Europa. The exciting thing now is the evidence that Europa may have all three of the ingredients [for life: heat, water, and organics],” said University of Hawaii geophysicist Thomas McCord.

The Galileo probe also detected organic molecules on the Jovian moons Callisto and Ganymede. Like Europa, Callisto and Ganymede are composed of a rocky core and an outer layer of ice. Ganymede shows some indication of tectonic activity, while Callisto does not. Consequently, parts of Ganymede show cracks similar to the cracks on Europa. Callisto, whose surface remains unchanged by any internal activity, is covered with ancient impact craters from meteors.

Ganymede, like Europa, has an oxygen atmosphere, while Callisto appears to have its oxygen locked up in ice and rocks. Callisto’s atmosphere is composed mostly of carbon dioxide. Recent evidence from the magnetometer aboard the Galileo spacecraft indicate that both Ganymede and Callisto may also have liquid oceans deep beneath their icy surfaces.

Jupiter’s moon Io hosts an environment very different from that of the planet’s three other giant moons. Io’s surface is liberally dotted with volcanoes, giving it – in combination with the sulfur-rich surface – a uniquely colorful appearance. Io is a greenish-yellow moon speckled with red, orange, white, and black markings.

Analysis conducted by Galileo determined that the lava of Io’s volcanoes reaches temperatures of 1430 to 1730 degrees Celsius (2,600 to 3,140 degrees Farenheit), exceeding the temperatures of earthen lava, which only reaches about 1090 degrees Celsius (2,000 degrees Farenheit). Io seems to have enough heat and energy to sustain life, and it does have a thin sulfur dioxide atmosphere, but it lacks water. Io’s volcanoes are constantly resurfacing the planet, and the intense heat generated by this activity probably caused any water present to evaporate billions of years ago.
Io is one of the few bodies in the solar system known to be volcanically active (along with Neptune’s moon Triton, the planet Venus, and of course, Earth). Data from the Galileo probe suggest that Ganymede may also have experienced volcanic activity in the past, although it is not believed to be volcanically active today.

Europa could possibly also have volcanic activity. Because Io’s volcanic processes are driven by internal tidal friction, it is thought that the weaker tidal friction on Europa could generate a less intense form of volcanic activity. This tidal friction is caused by gravitational effects that pull and stretch the moon. The closer a moon is to Jupiter, the greater its tidal friction.

The occurrence of underwater volcanic activity on other worlds is an exciting prospect in the search for extraterrestrial microbial life. Recent findings have shown that Earth’s submarine volcanic systems are linked to an abundance of carbon-based life forms. These microbes, called thermophiles – organisms that like extreme heat—live and thrive around hydrothermal vents in the seafloor. Whether life can actually originate in such environments is still highly controversial, however.

“The Galilean satellites are located in the temperature region of the solar system where water ice and other volatiles become stable over the age of the solar system . The composition and chemistry of the surfaces of these objects is of interest because they provide clues to the origin of our solar system and because they contain water ice and may contain organic molecules that are essential for the initiation of life,” notes McCord.

Jupiter itself seems to be a long shot in the search for extraterrestrial microbial life. True, the planet is warm and has plenty of organic materials. Jupiter has wet and dry areas like the desert and tropical regions of Earth. Telescopes on Earth and on the Galileo space probe even have detected areas on Jupiter with clouds of water that could indicate rainfall. But other qualities of Jupiter seem too extreme to sustain life.

For instance, a great deal of Jupiter is composed of liquid metallic hydrogen, an element only possible at pressures exceeding 4 million bars. (The atmospheric pressure on Earth at sea level is a little over one bar, or 14.7 pounds per square inch.) Any life that could withstand such conditions would still have to face Jupiter’s torrential winds, which clock in at an astounding rate of 400 miles per hour, twice as fast as tornadoes on Earth.

While Europa seems to be the only part of the Jovian system that may have all of the ingredients necessary to cook up the dish called “life“—liquid water, organic molecules, and heat energy—the other Jovian moons and the planet Jupiter can not yet be completely ruled out. There is so much about that region of space, and about the existence of life itself, that we still don’t know.

What Next?

The Galileo probe continues its investigations of the Jovian system. The Galileo Europa Mission (GEM) is a highly focused follow-on to Galileo’s Jupiter system exploration and a precursor for future missions to Europa and Io. GEM will conduct a detailed study of Europa over 14 months, then plunge repeatedly through the Io Plasma Torus to reach volcanic Io.