Are tides a peculiarity of our planet? Often, other bodies in our solar system are thought of as static, somewhat boring large rocks that sit around doing nothing. (Ok – they orbit. But what else?)
And yet, when we look at what causes tides, there is no real reason to believe that other planets or satellites should not experience the same phenomenon. Tides on Earth are caused by the Moon’s gravitational pull. Because the Moon is a decent fraction of the Earth’s mass, it is capable of varying the gravitational field we “feel” from the Sun enough to actually make a difference. Water feels the Moon's field differently than the rest of the crust beneath it, and therefore it is either pulled closer to, or pushed farther away from the Moon, thus creating the effect called tides. Therefore, all that is needed is something that will slightly but significantly alter the main gravitational field on a planet, as well as some sort of fluid that can move around more easily than the surface beneath it.
So let’s take a look around us. Mars has two satellites, but they are very small and can't do much of anything. Furthermore, Mars doesn't really have fluids floating around, there is no atmosphere, and, at least on the surface, no liquid water. One is then inclined to look at the giant gas planets like Saturn and Jupiter—they practically are balls of fluid, and have very large moons. The problem here is that these planets are so large that the moons are minuscule in comparison, and the gravitational field's variation is minuscule.
Now it’s time to think out of the box: what if a moon, and not a planet, experienced tidal effects? Out of all moons, the one where conditions are perfect for tides is Titan, one of Saturn's more famous satellites. In fact, when compared to Earth, it's an even better place to find tides. Titan's atmosphere is extremely rich and even heavier than Earth's. Furthermore, even though it does not have a moon of its own, Titan has something better—Saturn. Think of it as the Moon having tides because of effects from the Earth, rather than vice-versa.
The one tricky thing is that Titan, just like our own Moon, is in a locked orbit with Saturn—it is always showing its host planet the same face. What this means is that variations in its gravitational field are not caused by rotation around its own axis (as is the case for Earth) but the revolution around Saturn, which reaches points of minimum (perigee) and maximum (apogee) distance from the planet. This difference in distance is what causes temporal variations in the gravitational field which leads to tidal force 400 times stronger than those on Earth!
What these tidal forces do is create pressure differences in the very rich atmosphere surrounding the moon, which allow winds to form. These winds are quite mild, and yet, extremely regular. Now, one would ask, how do we know about these winds on a moon that isn’t exactly close to us? Is it just the result of some computer model which makes a bunch of unrealistic assumptions to simplify calculations?
Nope. Here comes the cool part—there is evidence, really neat evidence. When the ESA Huygens probe landed on Titan it was able to take quite a few aerial shots of the surface, and one of the things it noticed were these very long filaments, which are in fact dunes—dunes practically identical to those present in our very own deserts. This leads scientists to believe that the effect forming them must be the same—very regular winds, caused themselves by these tidal effects.
And yet Titan is not alone, as long as we don't limit ourselves to actual tides. One of its neighbors, called “Io,” a moon of Jupiter, has active volcanoes because of tides. Being the closest moon to Jupiter it feels very strong tidal forces, which cause the rocks in its interior to push against each other, creating heat that is sufficient to drive volcanic activity.
So, we are not alone...at least when it comes to tides.
- blog authored by David Caratelli