Titan is the largest moon on Saturn and the second largest moon in the solar system, about the same size as Mercury. Unique among the moons, it has a thick atmosphere; despite its lower gravity, the surface pressure is 1.5 times that of the Earth at sea level.
Its atmosphere is 95% nitrogen (78% Earth) and 5% methane. Normally, this would be transparent, but Titan’s air is laden with fog: small particles one micron in diameter (one millionth of a meter; a human hair is about 50–100 microns wide). These particles are suspended in the atmosphere, which makes it opaque.
Fog particles form when ultraviolet light from the Sun and / or subatomic particles that cover space sink into nitrogen and methane and divide into elements that reorganize into more complex molecules. Some of them are simple carbon rings and others are much more complex molecules called PAHs – polycyclic aromatic hydrocarbons. It is not clear how the simple ones relate to form the largest, but now, for the first time, this process has been simulated in a laboratory and the results have been examined using a powerful type of microscope that reveals the basic atomic configurations of the molecules.
This is awesome. These are individual molecules you are seeing in these pictures. The scale bar is 0.5 nanometers, half one billionth of a meter. But they are not images like a photograph. It is literally impossible to do this with visible light; the wavelength of light is hundreds of nanometers, too long to see such small structures. Instead, they used what is called atomic force microscopy*.
A technique analogous to the way phonographs work is used, by using a needle at the end of an arm that traces the slots in a register. In this case, however, a molecule at the tip of a microscopic needle traverses a molecule and can detect the change in shape due to the atomic forces that hold the molecule together. It’s like running your fingers over an object to feel its shape.
Samples of molecules were created in a laboratory to simulate Titan’s atmosphere. The scientists filled a stainless steel container with a gaseous mixture that is the same as Titan’s air and used an electric shock (essentially a spark) to simulate the UV and cosmic rays hitting the gas. It’s not exactly like Titan: they did it at room temperature, which is much warmer than Titan, but the reactions aren’t very sensitive to temperature. They also used a gas pressure of about 0.001 from Earth, which, while very thin, is much higher than the upper atmosphere of Titan where the reactions take place. However, the higher pressure allows the reaction rate to be much higher, so don’t wait weeks to get a decent sample.
They found more than one hundreds different molecules, a dozen or so of which could be examined with their microscope. Many are simple carbon rings and more complex PAHs, as expected. But they also found that many of the PAHs had a nitrogen atom embedded in them, which makes what is called N-PAH. These molecules were detected in Titan’s atmosphere by the Cassini mission, which orbited Saturn for 13 years and made more than 100 passes of Titan during that time, examining its surface and atmosphere. Laboratory simulations confirm this result.
In addition, the lab experiment created molecules made up of many connected rings, up to seven of them, that will help atmospheric scientists understand how more complex PAHs are made from simpler molecules.
This work is important for many reasons. Titan’s atmosphere is laden with these things, collectively called tholins (Greek for “mud,” as they make molecules that color the environment yellow, orange, and reddish brown), and are also seen in other worlds; The reddish landscape of Pluto is due to the tholins.
Titan does not have a water cycle like Earth, but it does have a methane cycle: liquid methane in vast lakes from its North Pole evaporates into the atmosphere, rains on nearby hills, and then returns to the lakes. Methane vapor can condense into suspended tholins, helping it to rain, and then tholins can cover the surface of the moon. This is very interesting, because nitrogen and carbon molecules are important in prebiotic chemistry, as they form amino acids, which in turn are the basic components of proteins.
The primitive atmosphere of the Earth was probably very similar to that of Titan, before the Great Oxygenation Event, about 3 billion years ago, which gave us the atmosphere, more or less, that we have today. Studying Titan is like studying ancient Earth. Not to be too broad, but life evolved on Earth in that early atmosphere, so it’s not too silly to wonder if something similar is happening on Titan. We certainly don’t know if life is brewing or thriving there, but it’s true that within the realm of science we look at it.
Titan is a world alien more than a billion kilometers from the Sun and drier than any desert on our own planet. However, there are painful similarities, which we can study in the laboratory. NASA is already in the early stages of planning a mission to Titan called Dragonfly: a drone lander and quadcopter which will fly over the surface and examine regions likely to have or have had conditions conducive to life.
What will you find there? These lab results are an important step in finding out.
*Just writing these words makes me feel like a scientist in an old black and white science fiction movie.