Titan is the largest moon of Saturn, and the second largest moon in the solar system, about the same size as Mercury. Unique under moons, it has a thick atmosphere – despite its lower gravity, the surface pressure is 1.5 times that of Earth at sea level.
Its atmosphere is 95% nitrogen (the earth 78%) and 5% methane. Normally it would be transparent, but Titan’s air is full of haze – small particles about a micron (one millionth of a meter; human hair is about 50-100 microns wide). These particles hang in the atmosphere and make it opaque.
The haze particles are formed when ultraviolet light from the sun and / or subatomic particles that zip into space, dump into the nitrogen and methane, and then break it down into elements which then rearrange themselves 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 combine to form the larger ones, but the process has now been simulated for the first time in a laboratory and the results have been examined using a powerful microscope that shows the basic atomic configurations of the molecules. .
It’s amazing. That is individual molecules you see in those images. The scale bar is 0.5 nanometers, half of a billionth of a meter. However, they are not images like a photo. It is literally impossible to do this with visible light; the wavelength of light is hundreds of nanometers, too long to see structures so small. Instead, they used the so-called atomic force microscopy*.
It uses a technique analogous to the way phonographs work, by using a needle at the end of an arm that locates the grooves in a plate. In this case, however, a molecule runs at the tip of a microscopic needle next to a molecule and can detect the change in shape due to atomic forces holding the molecule together. It’s like running your fingers over an object to feel the shape.
The samples of molecules were created in a laboratory to simulate the atmosphere of Titan. Scientists have filled a stainless steel container with a gaseous mixture that is the same as the air of Titan and use an electric discharge (a spark manufacturer) to mimic 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 Earth, which, although very thin, is much higher than the top of Titan’s atmosphere where the reactions take place. However, the higher pressure causes the reaction rate to be much higher, so they do not wait weeks to get a decent sample.
They have about a hundred different molecules of which a dozen or so could be examined using their microscope. Many are, as expected, simple carbon rings and more complex PAHs. But they also found that a nitrogen atom is embedded in many of the PAHs that make N-PAHs. These molecules were detected in the atmosphere of Titan by the Cassini mission, which orbited Saturn for 13 years and during that time made more than 100 passages of Titan, and examined the surface and atmosphere. The simulations in the laboratory confirm this result.
Furthermore, the laboratory experiment made molecules consisting of many linked rings, up to seven of them, that will help atmospheric scientists understand how the more complex PAHs are made of simpler molecules.
This work is important for many reasons. Titan’s atmosphere is loaded with these things, collectively called tholins (Greek for “mud”, because they make molecules that color the environment yellow, orange and reddish brown), and it is also seen in other worlds; Pluto’s reddish landscape is due to tholins.
Titan does not have a water cycle like Earth, but does have a methane cycle: liquid methane in vast lakes at the North Pole evaporates into the atmosphere, rains on the hills in the area and then flows back into the lakes. Methane vapor can condense on the suspended tholines, and it helps rain, and then the tholins can cover the moon’s surface. This is very interesting because nitrogen and carbon molecules are important in prebiotic chemistry and form amino acids, which in turn are the building blocks of proteins.
The early 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 the ancient earth. Not to be too broad, but life evolved on Earth in that early atmosphere, so it’s not too stupid to wonder if something similar is happening on Titan. We certainly do not know if life there is brooding or flourishing, but it is definitely within the realm of science to look at it.
Titan is an alien world that is more than a billion kilometers from the sun, and drier than any desert on our own planet. Yet there are painful similarities we can study in the laboratory. NASA is already in the early stages of planning a mission to Titan called Dragonfly – a lander and a quadcopter drone which will fly over the surface and examine regions that are likely to have or have had living conditions.
What will it find there? These lab results are an important step in finding out.
*By typing the words, I feel like a scientist in an old black-and-white sci-fi movie.