It’s summertime here in the Northern Hemisphere, when beachgoers beat the heat by splashing around the cooling waters of Earth’s lakes and oceans.
Liquid water is ubiquitous on our planet, so there’s no shortage of places with soothing waves and a refreshing coastal breeze for fun in the sun—but Earth is not the only world orbiting our star that can promise such a wet getaway.
Next time you’re planning a beach vacation, you might consider Saturn’s moon Titan. It’s huge, indeed larger than the planet Mercury, but it’s still smaller and lower-mass than our own world, so its gravity is only about a seventh of Earth’s. That’s convenient for when you inevitably overeat at some tourist-trap greasy spoon. It also has an atmosphere similar to our own, with pressure only about half again higher.
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As a destination, Titan does have some disadvantages, however. Its air, like Earth’s, is mostly nitrogen. But instead of oxygen making up the rest, that remainder is composed of tough-to-breathe methane and molecular hydrogen. It’s a long trek to get there, too, with a one-way trip of more than a billion kilometers. But on the other hand, the sun is only about 1 percent as bright on Titan as it is on Earth, so you won’t need sunglasses or sunscreen!
But you will need mittens. The surface temperature of Titan is about –180 degrees Celsius (–300 degrees Fahrenheit). You’ll certainly forget all about sweltering summers!
Still, there is something Titan has that no other world in the solar system besides Earth can boast: persistent liquid on its surface.
In 2007 the Cassini spacecraft took images and radar observations that confirmed the existence of huge lakes on Titan. Most are near the moon’s north pole. But globally, they are several times the surface area of all the U.S. Great Lakes combined. That’s a lot of beachfront property.
There is one major difference between Titan’s lakes and our own, and to be honest, it could be a deal-breaker: Water is frozen as hard as rock there. So those lakes are instead filled with liquid methane.
This could make swimming somewhat problematic. Even a wet suit won’t help; neoprene shatters at such low temperatures. But let’s say we can find a material that is both flexible and so well insulated that you’re protected against freezing into a human ice cube. What would it be like to take a dip in Titan’s lakes?
No travel agent would dare capsize a vacation by putting this inconvenient fact on a promotional brochure: it would be nearly impossible. The problem is buoyancy.
This is the property of liquid that imparts an upward force to an object against gravity’s downward pull. An object displaces some of that surrounding liquid, which pushes back on it. As it happens, humans and water have about the same density, so the weight of the water you displace when you jump into a lake on Earth is roughly equivalent to your own weight. The forces balance, and so you float.
Interestingly, and perhaps counterintuitively, the change in gravity from Earth to Titan makes no difference. You weigh less as a result of the weaker gravity, but so does the liquid. This exactly cancels out, so if you had liquid water on some other, lower-surface-gravity world, you would still float in it the same way you do on Earth.
But Titan’s lakes are different; liquid methane is not water! At Titan’s temperature, methane has a density of very roughly half that of liquid water. This means the displaced liquid has less weight. So if you jumped into a Titanic lake, there’d be insufficient force to hold you up, and you’d sink. You could try to tread methane, but you’d have to flap around so hard that it would be incredibly tiring. Snorkeling is definitely out—the deadly air notwithstanding—but perhaps walking along the bottom of a methane lake would be a fun diversion.
Still you do have another option: surfing.
Body surfing would be extremely difficult in light of that whole “sinking” thing. (I think tethering yourself to lighter-than-Titan-air hydrogen balloons might be fun, if awkward, though I wouldn’t call it body surfing.) But could you actually grab a surfboard and hang ten?
Yes! But it would be different.
Titan’s lower gravity, as well as liquid methane’s lower density, does change how waves behave there. To see what those effects might be, a team of scientists modeled how wind and surface liquids interact to create waves and applied them specifically to other worlds, including Titan. They described their results in a paper published in the April issue of the Journal of Geophysical Research: Planets.
What they found is that waves start to pick up at lower wind speeds on Titan and could grow to be much larger than waves on Earth driven by similar winds. For example, they found that a wind speed of 10 meters per second generates waves that are 15 meters high—as tall as a five-story building!
The lower gravity might make that somewhat less intimidating, but the waves will also move more slowly as well, feeling downright eerie compared with Earth’s.
Assuming you can handle the terrifying prospect of uncannily languid, looming waves that could smother and snap freeze you, what would it be like to surf them?
The first issue is the surfboard. The lower gravity doesn’t affect buoyancy, but the lower density of the liquid methane means youll need a surfboard with about twice the area as you would back on Earth. You can make it twice as long or twice as wide, or a combination of both. If it’s too wide, though, paddling with your hands will be difficult, so I suggest making it longer. Paddling will be difficult anyway; the lower density liquid means that you’ll have to push twice as hard to move as fast as you would on Earth. The slower speed of waves on Titan, though, will make catching up to one easier. You’ll likely need a larger fin under the board to better control your movements, too.
Balancing on the board might be easier, given your lower weight, but it will take getting used to. Once you catch a wave, you’ll have to be careful standing up so that you don’t launch yourself into the air!
It’s unclear if waves on Titan will barrel—curve at the top and roll over, creating a liquid tube to ride through—because that’s a complicated effect that arises from waves’ speed and height and the slope of the surface underneath them. Riding a wave is also a complex combination of balancing your weight on the board, stepping back or forward to make the board nose rise or lower. This will be harder on Titan because your lower weight means your movements will have to be exaggerated compared with what they’d be on Earth.
To be totally fair, we don’t really know if there are waves on the Titanic lakes in the first place. Cassini didn’t detect any, but, for example, it may have been looking at the wrong time of year for windy conditions. Cassini only measured wind speeds of about 0.3 meter per second as well, which may be too low to raise a wave even in the lower gravity.
So perhaps it’s hard to imagine Titan becoming a surfing mecca. On the other hand, there are surfers who sacrifice most everything else to travel all over Earth looking for new experiences, new waves to ride. Shredding a 40-meter methane swell on a Saturnian moon may very well scratch that itch, too.