Our solar system is a fascinating place: a collection of unique and magnificent worlds orbiting a brightly burning star. These worlds are, for most of us, quite familiar and recognizable. Going out from our Sun, you have the inner planets, all relatively small and rocky compared to the planets all farther out. There’s Mercury, named after the Roman messenger god, which lives up to its name as it zips around the sun in just 88 days. There’s Venus, a world similar in shape and size to our own but with a runaway greenhouse effect that makes the surface hot enough to melt lead. There’s our home, the Earth, the only habitable planet that we’re aware of; a world of green and blue, with a breathable, temperate atmosphere and a vast diversity of life. Then there’s Mars, an ancient desert world once thought to have a considerable atmosphere and, perhaps, water oceans. The outer planets, lying at increasingly greater distances from the Sun, are also recognizable. Jupiter has its massive red spot, a twirling hurricane that’s lived for hundreds of years; Saturn has it’s magnificent rings. Uranus is tilted, and rotates, completely on its side; Neptune has an orbit that brings it outside that of Pluto’s every 200 years or so.

These great planets are familiar to us, and each are incredibly beautiful and awe-inspiring in their own ways. They have been a source of constant study, and scientific revelation, to astronomers and scientists curious about the nature of our solar system and planets other than our own. However, perhaps the greatest beauty and the most magnificent possibilities are found on worlds which aren’t as familiar to the average person. These worlds have been recently reveled to us, and they still hold many tantalizing secrets and answers, but they have opened and broadened our understanding of our universe, and could possibly become the most important places, other than the Earth, in the solar system. They might have liquid oceans. One has a considerable atmosphere and surface features familiar to us on Earth. One shoots off massive geysers of water and organic molecules. All three might sustain, and possess, life. These worlds I refer to are moons, and of them I refer most specifically to Jupiter’s moon Europa, and Saturn’s moons Enceladus and Titan.

Europa

Europa is Jupiter’s sixth closest moon. It’s still one of the larger worlds in the solar system, being slightly smaller than our own Moon. Its surface is a thick ice, but it is what is below the surface that makes Europa such a fascinating world. Most planetary scientists believe that vast liquid water oceans exist beneath the ice, warmed by the flexing pull of Jupiter’s gravity.

The effect of Jupiter’s gravity is impressive. Io, a moon slightly larger than our own and closer to Jupiter than Europa, is covered in volcanoes. The inside of the moon is churning magma, tugged by the force of gravity. These gravitational effects operate on Europa as well, though weakened because of distance. They warm the inside of the planet, heating the oceans beneath as the got closer to the core.

Europa has emerged as one of the most promising candidate for extraterrestrial life. The heat of Europa’s core, which is tugged and pulled on by Jupiter, could create volcanic activity on the surface of Europa’s ocean floor, and this would provide a habitat similar to Earth’s deep-oceans (assuming Europa indeed does possess these oceans). Life has been found to exist on Earth around deep-sea vents, and thus could exist deep in Europa. The possibility of life there has left astrobiologists and astronomers wanting to study Europa in further detail.

Beyond Europa and Jupiter is the next gas planet, Saturn. It has around it two incredible moons. Closer of the two is a moon very similar in situation to Europa.

Enceladus

The sixth-largest of Saturn’s moons, Enceladus is a a tiny world. It is only once-seventh the diameter of our own Moon, and its diameter is small enough to fit within the length of the Island of Great Britain. It experiences heating from Saturn’s gravitational pull, so the insides of this tiny world are warmed.

Enceladus seems to have a liquid ocean underneath its icy surface, like Europa. Volcanoes at the southern pole of Enceladus shoot large geysers of water ice into space, some of which falls back onto the moon and some of which go into Saturn’s rings. These geysers have indicated that there could be a close, sub-surface body of liquid water.

In May 2011 NASA scientists at the Enceladus Focus Group Conference stated that Enceladus “is emerging as the most habitable spot beyond Earth in the Solar System for life as we know it”.(1) Enceladus and Europa, two icy worlds with potential undersea oceans, might be the best chance we have to discovering if life exists outside the Earth.

Another moon of Saturn’s is also worthy of notice. It is unlike Europa or Enceladus, however. This world is unique in its nature, and worthy of study. It is, in some ways, resemblance of our own planet.

Titan

Titan is the largest moon of Saturn, with a diameter almost 2/3rds the size of Mars. Titan is fascinating because it the only natural moon in the solar system known to have a dense atmosphere, and the only world other than the Earth where bodies of surface liquid have been found to exist.

Titan’s atmosphere is thick, almost twice as thick as Earths, and is rich in nitrogen. A thick haze hangs over the planet. The surface of Titan is geographically young, indicating erosion and volcanic activity. There exist bodies of liquid on Titan, composed primarily of methane. At the temperatures and pressures which Titan’s surface is subject to, Methane operates in a fashion similar to how Water operates on Earth, as a liquid, solid, and gas.

Titan has also been presented as a possible habitat for extraterrestrial life. It has been suggested that life could exist in the lakes of methane on Titan, which would use hydrogen in place of oxygen and to metabolize and exhale methane instead of carbon dioxide. However, these methanogenic life-forms are, to date, only hypothetical.

These moons are interesting and unique, and they present large possibilities for study. Europa and Enceladus are icy worlds, and might have liquid water oceans underneath them. These oceans could provide habitats for life, living near hydrothermal vents on the ocean floor. Perhaps one day probes will be sent to drill into the icy surfaces of these planets and explore the oceans, sensing for biological traces and signatures

Titan presents an example of a world similar to our own, but also uniquely different. Pictures of its surface display a world similar to our own geographically, with rivers and oceans and bodies of land. Yet this world is much different than our own in composition, and demonstrates that natural processes we are familiar can operate with different chemicals. Perhaps life exists on Titan as well, using different chemicals than we do in the processes required to live.

Future study of our solar system will likely focus on these worlds. They present fantastic opportunities for discovery and to learn more about our own world.

Cited

(1) “Enceladus named sweetest spot for alien life” http://www.nature.com/news/2011/110531/full/news.2011.337.html