PASADENA — Our view of the solar system has changed drastically in the last few decades. Take moons, for instance. The word moon implies something small and insignificant, at least compared to a planet. But so many of the moons, especially those around the outer planets, are fascinating and complex worlds unto themselves. Many of the most dramatic environments we know of, as well as some of the best chances for life outside the Earth, exist on the moons of our planetary family.
One of the most tantalizing moons is Titan, the largest moon of Saturn. Even among all the fascinating moons in the solar system, Titan is unique: it is the only moon that possesses a significant atmosphere. Not only does it have an atmosphere, but also layers and layers of orange, hazy clouds (the clouds were so thick that when the Voyager Spacecraft flew by in 1980, the surface was entirely concealed.) Titan's atmosphere is actually quite a bit denser than the Earth's, which is really saying something when you consider the low surface gravity on Titan. Most people don't think about the Earth's gravity holding down and compressing the gases of our atmosphere, but that's exactly what happens. The gravity on Titan's surface in only about one tenth of what you feel on Earth, but the air pressure is actually 60 percent greater than what you feel at sea level here. That's a lot of atmosphere!
Not that you could breathe Titan's air, though. The composition of Titan's atmosphere is not totally unlike Earth's; the air on both our worlds is mainly composed of nitrogen (78 percent of our air is nitrogen, compared to 90 percent on Titan). But unfortunately, Earth seems to be unique among all the planets and moons in having a significant amount of oxygen; Titan has practically none. There's also the problem of air temperature: even on the balmiest days, the temperature rarely gets much above 300 degrees below zero Fahrenheit.
Yet it's this extremely low temperature that may make Titan such an interesting place to visit. At these cryo-freezing levels, both methane (what we think of as natural gas) and ethane exist in liquid form. There may be puddles, lakes, or even oceans of liquid ethane and dissolved methane sloshing around on the surface. No one has directly observed any lakes or oceans yet, but the conditions on Titan heavily imply that such things could exist. You'll have to keep posted on that one.
How amazing it would be to stand on a frozen shore and watch waves of liquid ethane crash onto the beach. And yes, scientists are pretty sure that if such oceans exist, there would be waves and tides. Titan orbits fairly close to its parent planet, and Saturn is much more massive than the Earth. These two combined effects mean that tidal forces on Titan are over four hundred times stronger than what we feel on Earth.
Fortunately, that doesn't matter much because like our own moon, Titan has been tidally locked in its orbit around its planet. Even so, scientists believe waves exist. The elliptical shape of Titan's orbit changes its distance to Saturn over sixteen days, one "month" on Titan. These small changes are enough to generate a small, but noticeable tide of about ten feet. Saturn's gravity also generates tides in Titan's atmosphere (tidal winds! very cool), which keeps a constant wind-speed of a few miles per hour chopping up those liquid ethane oceans.
But frigid oceans and tidal weather patterns are not the only reasons Titan has gotten everyone so excited. For a long time now, scientists have been searching for environments outside the Earth that may support life, and Titan has loomed large on their list.
For starters, Titan's atmosphere is packed full of complex organic molecules. When sunlight (even as faint as the sun is all the way out around Saturn) travels through Titan's atmosphere, it creates all kinds of interesting chemicals from the methane and ethane already there. We know that there's acetylene, ethylene, and hydrogen cyanide in Titan's air, the last of which was a very important molecule (we think) for getting life started on Earth. And there's plenty of other gook floating around Titan's atmosphere, including brownish, smoggy hydrocarbons that may actually precipitate and rain down on the surface into viscous, oily puddles. In short, the chemistry on Titan is a lot like what we think the early Earth was like.
One of the key requirements for creating life, it seems, is the presence of a liquid. In order for life to begin, large, complex molecules have to be given the chance to mix around and interact with each other. If everything is frozen solid in ice, there's not much mixing to be had. If there's nothing but gas around, the molecules don't have much time to hang on to each other as they blow past each other in the air.
But liquid is just right for those kinds of interactions. In a liquid, molecules can mix, interact with each other, maybe even form longer, more complicated chains. With enough time and the right circumstances, the molecules may even learn to replicate themselves from smaller bits and pieces floating around in the muck. Let sit for a few million years, and lo and behold, you get DNA, or something similar. Life has begun.
Now, Titan is way, way too cold for liquid water to exist (at those temperatures, water is about as solid and dense as granite). But here's where the super-cold temperature may actually prove to be an advantage.
Given very low temperatures and high air-pressure, methane and ethane can exist in liquid form. Liquid methane, especially, has a lot of the same chemical properties as water, and may provide a similar environment for life to take hold. Recent observations suggest that conditions may not be quite right for liquid methane to exist on Titan's surface, but liquid ethane with dissolved methane in it is still a strong possibility. And that may just do the trick.
Still, scientists have found out that defining exactly where life begins is far more tricky than they first thought. We know of lots of complicated molecules, some of which can even replicate themselves, but we wouldn't call them alive. But somehow, life arose from them. Where is the cut-off, and would we even recognize the very first beginnings of life if we saw them on Titan, or another world?
It's an interesting question, and one that as led NASA to give the exploration of Titan a high priority. In 1997, NASA launched the Cassini spacecraft to explore Saturn, as well as its mysterious rings and moons. It's taken quite a while to get out there, but 2004, the probe will arrive at its destination and begin a many-year survey of the Saturn system. One of the most exciting aspects of the mission will doubtlessly be the release of the Huygens probe, a miniature laboratory that will be dropped from Cassini and sent down to plunge through Titan's hazy clouds. On its descent, for a mere three hours, the probe is scheduled to analyze Titan's chemistry, sniffing and tasting the various cloud layers before it touches down (on a cold dead surface? Sinking into liquid ethane oceans?) and ends its mission.
We're expecting a view not only of a distant, exotic world, but a glimpse back in time to what we may have looked like billions of years ago.