If you’ve ever enjoyed a fizzy drink, you know how a gas can bubble out of a liquid. Pick up a bottle of soda, for example, and it looks like an ordinary liquid. But when you open the bottle, small bubbles start to form, rising to the top of the liquid. If you’re not careful, the bubbles can even cause the soda to froth out of the bottle. This frothing effect might also explain strange island-like structures on the surface of Saturn’s moon Titan, which seem to appear and disappear from time to time. 

The gas in a fizzy drink is carbon dioxide. Since carbon dioxide is absorbed fairly easily by water, water can be infused with CO₂ under pressure. When you open the bottle, the pressure drops and the carbon dioxide bubbles out. This is also what gives beer its foamy head. Although CO₂ has long been used to make drinks bubbly, in recent years some folks have started infusing drinks with nitrogen instead. Nitrogen isn’t absorbed as readily in water, and when released by a drop in pressure nitrogen bubbles tend to be smaller, which some folks feel adds a distinctive character to everything from beer to coffee. Nitrogen can be absorbed into other liquids as well, and this may be occurring naturally on Saturn’s moon Titan.

A false color image of Titan’s surface shows lakes and rivers similar to that of Earth. Credit: NASA/JPL/USGS.

Titan is an interesting world. Though it is much colder than Earth, it has interesting similarities. It has a thick nitrogen atmosphere, cloudy skies, and rain. It even has rivers and lakes. These are not composed of water, but of liquid methane (CH₄). Methane plays a role on Titan similar to water on Earth. Just as Earth has a water cycle of rain, rivers and evaporation, Titan has a methane cycle. Titan’s lakes can also contain ethane (C₂H₆) in smaller amounts

On Earth, carbon dioxide can be absorbed by lakes and oceans. In fact, as CO₂ levels have risen in our atmosphere, so to have CO₂ levels in the oceans, leading to ocean acidification. Since Titan’s atmosphere is 98% nitrogen, nitrogen can be absorbed into its methane lakes and seas. Recently a team simulated nitrogen absorption in Titan’s lakes, infusing a mix of liquid methane and ethane with nitrogen. They then cooled the mix to the point where the ethane began to freeze into solid ice. On Earth, water ice floats, but ethane sinks to the bottom of the methane. The team found that as the ethane freezes and sinks, nitrogen is released creating a bubbly froth on the liquid surface. This effect could explain a strange phenomenon seen on Titan’s surface, known as “magic islands.”

Because of Titan’s thick atmosphere, we can only observe its surface indirectly, by bouncing radar signals off the surface. This allows us to image both the solid terrain and liquid lakes, and has revealed a surface that looks strikingly similar to Earth. However a few features that look like islands are seen in some images but not others. It’s unlikely that large land masses would simply disappear and reappear, so they have been a bit of a mystery. But if these “islands” are regions of a lake where nitrogen is bubbling up to the surface, they would look similar to an island terrain on radar. Once the fizzing is over, the lake would settle down and the “island” would disappear.

So it seems that Titan’s lakes can at times resemble a fizzy drink, making the world a bit more strange than we had first supposed.

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We don’t generally think of Saturn’s moon Titan as an Earth-like world. It has no breathable atmosphere, and with a surface temperature of about 90 Kelvin life as we know it is out of the question. But there are many parallels between Titan and Earth, and so we can see the moon as a kind of colder, smaller cousin to our own planet.

Both Earth and Titan have thick nitrogen atmospheres. Earth’s also has about 20% oxygen, but the atmospheric dynamics are similar. On Titan, methane plays a similar role to water on Earth. Titan has clouds, rain and large lakes or seas. It has seasons following the changing tilt of its orbital plane relative to the Sun. As a result, the terrain of Titan is interestingly similar to Earths, with rivers, flood plains, and mountains.  It even has ice volcanoes, and so is geologically active.

Because of its lower temperature, and the way methane obscures visible light, we have to look in the infrared to see much of these details. A recent image by the Cassini mission does just that. Shown above, the false-color image gives infrared wavelengths more Earth-like hues. The result is Titan as an Earth-like world. It’s a great example of how sometimes a world so different from our own can also be hauntingly similar.

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Titan is the largest moon of Saturn, and the second largest moon in our solar system. It has a greater diameter than Mercury. It is also the only moon with a thick atmosphere. It has liquid methane rivers and lakes, and has a seasonal climate.

And like our moon, we have landed a probe on its surface. In 2005 the Huygens probe made a one-way journey to the surface of Titan. You can see a video of that landing above.

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Saturn’s largest moon Titan is in some ways very similar to Earth.  Sure, it is significantly colder, and has a much thicker atmosphere, but it has something no other world besides Earth. Lakes and seas. These are not water lakes, but methane.  With a surface temperature of about 94 K, and a thick atmosphere, Titan is perfect for a methane cycle similar to the water cycle on Earth.  On titan, methane gas exists as a vapor in the atmosphere, which can condense and fall as rain.  This collects as rivers and lakes on the surface of the moon.

With the Cassini mission we’ve been able to map some of the lakes and seas on Titan.  They resemble the shape of lakes we see here on Earth. But one thing they don’t seem to have is waves.  Some of Titan’s lakes are larger than the Great Lakes of Earth, which can experience sizable waves.  But radar measurements of Titan’s lakes have found no evidence of waves. Since Titan has a lower gravity than Earth, waves should actually be more likely. It’s been calculated that a breeze of only 2 – 3 km/hr would be enough to produce measurable waves. We know that such wind levels exist on Titan, so the lack of waves is a bit of a mystery.

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