Titan is Saturn’s largest moon and the second largest moon in the solar system, about the same size as Mercury. Unique among moons, it has a thick atmosphere – despite its lower gravity, its surface pressure is 1.5 times Earth’s at sea level.
The atmosphere is 95% nitrogen (Earth’s 78%) and 5% methane. Normally that would be transparent, but Titan’s air is full of haze – tiny particles about a micron across (one millionth of a meter; a human hair is about 50-100 microns wide). These particles float in the atmosphere, making it opaque.
The haze particles are formed when ultraviolet light from the sun and / or subatomic particles flying through space strike the nitrogen and methane and break it up into elements that then rearrange themselves into more complex molecules. Some are simple rings of carbon, and some are much more complex molecules called PAHs – polycyclic aromatic hydrocarbonsIt has not been clear how the simple ones come together to form the larger ones, but now this process has been simulated for the first time in a laboratory and the results examined using a powerful type of microscope that examines the basic atomic configurations of the molecules.
That is amazing. They are individual molecules you see in those images. The scale bar is 0.5 nanometer, half 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 such small structures. Instead, they used what is called atomic force microscopy
This uses a technique analogous to the way gramophones work, by using a needle on the end of an arm that follows the grooves in a record. In this case, a molecule at the end of a microscopic needle passes 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 its shape.
The samples of molecules were made in a laboratory to simulate Titan’s atmosphere. Scientists filled a stainless steel vessel with a gas mixture the same as Titan’s air and used an electric discharge (essentially a spark maker) to simulate the UV and cosmic rays that hit the gas. It’s not exactly like Titan: they did this 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 top of Titan’s atmosphere where the reactions take place. However, the higher pressure makes the reaction speed much faster, so they don’t wait weeks to get a decent sample.
They found more than one hundred different molecules, of which they were able to examine a dozen with their microscope. Many are simple carbon rings and more complex PAHs as expected. But they also discovered that a nitrogen atom was embedded in many of the PAHs, creating so-called N-PAHs. These molecules were detected in Titan’s atmosphere by the Cassini mission, which orbited Saturn for 13 years and made more than 100 passages of Titan during that time, examining its surface and atmosphere. The simulations in the lab confirm this result.
In addition, the lab experiment created molecules made of many connected rings, up to seven, that will help atmospheric scientists understand how the more complex PAHs are made from simpler molecules.
This work is important for many reasons. Titan’s atmosphere is loaded with this stuff, collectively called tholins (Greek for “mud,” as they make molecules that color the environment yellow, orange, and reddish brown), and they are seen on other worlds too; Pluto’s reddish-colored landscape is due to tholins.
Titan does not have a water cycle like Earth, but it does have a methane cycle: liquid methane in vast arctic lakes evaporates into the atmosphere, rains down on the nearby hills and then flows back into the lakes. Methane gas can condense on the floating tholins, causing it to rain, and then the tholins can cover the surface of the moon. This is very interesting, because nitrogen and carbon molecules are important in prebiotic chemistry because they form amino acids, which in turn are the building blocks of proteins.
Earth’s early atmosphere was probably very similar to Titan’s, before the Great Oxygenation about 3 billion years ago, which gave us the atmosphere we have more or less as we have it today. Studying Titan is like studying the old Earth. Not too broad, but life evolved on Earth in that early atmosphere, so it’s not too much of a surprise to wonder if anything similar is happening on Titan. We certainly don’t know if life is brewing or thriving there, but it is certainly within the realm of science to look at it.
Titan is an alien world more than a billion kilometers from the sun, and drier than any desert on our own planet. Yet there are painful similarities that we can study in the lab. NASA is already in the early stages of planning a mission to Titan called Dragonfly – a lander and quadcopter drone flying over the surface and research into regions likely to have or have had living conditions.
What will it find there? These lab results are an important step to find out.
When I type those words, I feel like a scientist in an old black and white science fiction movie.