Titan is Saturn’s largest moon and the second largest moon in the solar system, about the same size as Mercury. Unique on Mondays, it has a thick atmosphere – despite its lower gravity, the surface pressure is 1.5 times higher than that of the Earth at sea level.
Its atmosphere is 95% nitrogen (Earth is 78%) and 5% methane. Normally, this would be transparent, but Titan’s air is filled with fog – tiny particles of about one micron (one millionth of a meter; human hair is about 50–100 microns wide). These particles are suspended in the atmosphere, making it opaque.
Fog particles are formed when ultraviolet light from the Sun and / or subatomic particles that attach around space crash into nitrogen and methane, breaking it down into elements that then rearrange into more complex molecules. Some of them are simple carbon rings, and others are much more complex molecules called PAHs – polycyclic aromatic hydrocarbons. It was not clear how the simple ones bind 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 molecules.
That’s amazing. These are individual molecules see in those pictures. The scale bar is 0.5 nanometers, half billion of one meter. However, there are no 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*.
It uses a technique analogous to the way phonographs work, using a needle at the end of an arm that tracks the grooves in a recording. In this case, however, a molecule from the tip of a microscopic needle runs along a molecule and can detect changes 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.
Molecule samples were created in a laboratory to simulate Titan’s atmosphere. The scientists filled a stainless steel vessel with a gas mixture that is the same as Titan’s air and used an electric shock (essentially a spark maker) 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 are not very sensitive to temperature. They also used a gas pressure of about 0.001 on Earth, which, although very thin, is much higher than the peak of Titan’s atmosphere where the reactions take place. However, the higher pressure allows the reaction speed to be much higher, so don’t wait weeks to get a decent sample.
They found over one hundred different molecules, which they could examine with a microscope. Many are simple carbon rings and more complex PAHs, as expected. But they also found that many of the PAHs incorporated a nitrogen atom into 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 over 100 Titan passes during that time, examining its surface and atmosphere. Laboratory simulations confirm this result.
Moreover, the laboratory experiment created molecules made up of many connected rings, up to seven of them, which will help scientists in the atmosphere understand how more complex PAHs are made from simpler molecules.
This work is important for several reasons. Titan’s atmosphere is loaded with these things, collectively called tholins (In Greek it means “mud”, because they make molecules that color the environment yellow, orange and reddish-brown) and are seen in other worlds; Pluto’s reddish-colored landscape is due to the tolin.
Titanium does not have a water cycle like Earth, but it does have a methane cycle: liquid methane from the vast lakes at its north pole evaporates into the atmosphere, rains on nearby hills, then flows into lakes. Methane vapors can condense on suspended tolines, helping them to rain, and then tolines can cover the surface of the moon. This is very interesting because nitrogen and carbon molecules are important in prebiotic chemistry, forming amino acids, which in turn are the building blocks of proteins.
The early atmosphere of 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 too wide, but life evolved on Earth in that early atmosphere, so it’s not too bad to wonder if something similar is happening on Titan. We certainly don’t know if life develops or thrives there, but it’s certainly in the realm of science to look at it.
Titan is an extraterrestrial world more than a billion kilometers from the Sun and drier than any desert on our planet. However, there are painful similarities that we can study in the laboratory. NASA is already in the early stages of planning a mission to Titan called Dragonfly – a landing drone and quadcopter which will fly above the surface and examines regions likely to have or have had favorable conditions for life.
What will he find there? These laboratory results are an important step in finding out.
*Just writing these words makes me feel like a scientist in an old black and white SF movie.