I’ve discovered over the years that one of the most popular space-related misconceptions is that Earth is the only planet, indeed the only place, in the solar system that has water.  I am constantly amazed at the frequent news headlines that this robot or that scientist has made the astounding discovery that there’s water on Mars.  This is, in fact, very old news.  We’ve known for at least 200 years that the northern Martian ice cap is mostly water ice (the southern cap is mostly dry ice or frozen carbon dioxide). 

Despite popular opinion to the contrary, water is extremely common in the solar system and Earth harbors only a small fraction of the compound that’s absolutely necessary for life-as-we-know-it.  But the important factor for life-as-WE-know-it is that virtually all solar system water occurs in water’s solid form; ice.

The water ice we’re familiar with is called Ice 1.  Because water is a polar compound, Earth’s water ice is chaotic in structure and less dense than the liquid form of water.  This sole fact has allowed life to form on our planet.  If Ice I was denser than water, the oceans would freeze from the bottom up and life-as-we-know-it could not exist.  Our kind of life REQUIRES liquid water.

But frozen water comes in at least 12 different forms depending on the temperature and pressure at which it freezes.  The water ices above Ice 3 have a crystalline form and are denser than liquid water.  They can never naturally occur on Earth, it isn’t cold enough.  These ices are not ice as we know it, but water rocks.  These ices can also take a molten or semi-solid form and we suspect that on some of the outer moons of the gas giants there are volcanoes that erupt molten water.

But that’s not all.  Jupiter has four planet-sized moons.  One of them, Io, is the most volcanically active body in the solar system and it’s made of silicate rocks.  The other three, Europa, Ganymede and Callisto, are all made of solidly frozen water in one of its crystalline forms.  But here’s the important part.  All of them wobble as they spin around Jupiter, just as a raw egg wobbles when it spins, and this implies that they are not solidly frozen.  Each of these enormous moons apparently contains a vast liquid water ocean.  Europa’s ocean may be 30 to 40 miles deep and the ocean inside Ganymede (the solar system’s largest Moon) may be 300 to 600 miles deep.  If these moons do contain liquid water, they harbor much more water than the seas of Earth.

Water erupts from Enceladus

We’re speculating about Europa, Ganymede and Callisto, but there is no more speculation about Enceladus, one of Saturn’s moons.  Enceladus is the brightest moon in the solar system reflecting over 90% of the sunlight that falls on it.  It’s that reflective because it’s a giant 300-mile wide frozen snowball.  But the Cassini mission that’s orbiting Saturn right now has taken pictures of Enceladus erupting liquid water.  There is no question that there’s liquid water inside this moon.

Artist’s view of an erupting water volcano on Enceladus

And just last month there is more news from the Cassini spacecraft in orbit around Saturn.  Enceladus apparently isn’t alone.  Two more of Saturn’s moons, Tethys and Dione, are flinging great streams of particles into space.  The discovery suggests the possibility of some sort of geological activity, perhaps even volcanic, on these icy worlds.

Until this discovery, only Enceladus was known to be an active world, with huge geysers spraying gases hundreds of miles above the moon’s surface.  Scientists are excited because activity like these particle streams suggests that Tethys and Dione have either not reached equilibrium or that energy is somehow being supplied in the system.  The activity on Enceladus was detected first by Cassini's Dual Technique Magnetometer (MAG).  This led the flight team to schedule a particularly close pass of Enceladus, which revealed a wealth of data about Enceladus’ alien geysers – and spectacular pictures, too.

In the case of Dione and Tethys, more fly-bys are scheduled in the future, which will allow the team and the other instruments a close-up look at the moons.  Before that happens, the Cassini team will mine the data already collected during the Tethys and Dione flybys of 2005 for further evidence of volcanic activity.  And remember, if there is volcanic activity on Tethys and Dione, the ‘lava’ that’s being erupted probably isn’t molten rocks; it’s molten water.  Tethys and Dione may also harbor liquid water oceans inside them.

So who cares if there’s water inside these moons?  Well, Tethys and Dione are small worlds, (Tethys and Dione are both about 600 miles in diameter), and Enceladus is half their size at around 300 miles in diameter.  But a 300 mile wide ball can still hold a LOT of water.  To give you some reference point, if all the Earth’s water was gathered into a sphere, it would be about 850 miles across.  So these three moons together probably hold more liquid water than Earth.

Then consider the three large Galilean satellites of Jupiter, Europa, Ganymede and Callisto.  Europa is about the same size of Luna, our moon, roughly 2000 miles in diameter.  Europa probably has more liquid water than Earth all by itself.

If you make a list of the ten largest objects in the solar system (smaller than the Sun) and rank them by size, the list looks like this: 1. Jupiter, 2. Saturn, 3. Uranus, 4. Neptune, 5. Earth, 6. Venus, 7. Mars, 8. Ganymede, 9. Mercury, and 10. Callisto.

Ganymede is bigger than Mercury and Callisto is only marginally smaller.  There’s apparently a LOT of liquid water in the solar system and Earth probably only has a small fraction of it.  And what could that mean?  Well, as one of the scientists who studies Europa said “Do you really think you can have a liquid water ocean for 6 billion years and NOT have life in it?”  What kind of life would develop inside a moon?  There could be aliens in our own solar system and they may be all wet!