Bad astronomy | Titan’s haze particles made in a lab and photographed in extreme detail

Titan is Saturn’s largest moon and the second largest moon in the solar system, roughly the same size as Mercury. Unique among moons, it has a thick atmosphere; Despite the lower gravity, the surface pressure is 1.5 times that of the Earth at sea level.

Its atmosphere is 95% nitrogen (Earth’s is 78%) and 5% methane. Normally it would be transparent, but the air on Titan is laden with haze – tiny particles about a micron wide (one millionth of a meter; a human hair is about 50-100 microns wide). These particles are suspended in the atmosphere, making it opaque.

Haze particles are formed when ultraviolet light from the Sun and / or subatomic particles circulating through space collide with nitrogen and methane, breaking them down into elements that are then rearranged into more complex molecules. Some of them are simple carbon rings, and others are much more complex molecules called PAH. Polycyclic aromatic hydrocarbons. It is not clear how the simple ones link together to form the larger ones, 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 is incredible. Those are individual molecules that you are seeing in those images. The scale bar is 0.5 nanometers, half a billionth one meter. However, they are not images like a photograph. It is literally impossible to do this in visible light; the wavelength of light is hundreds of nanometers, too much to see such small structures. Instead, they used what is 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 traces the grooves on a record. However, in this case, a molecule at the tip of a microscopic needle runs the length of a molecule and can detect the change in shape due to the atomic forces holding the molecule together. It is like running your fingers over an object to feel its shape.

The molecule samples 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 electrical discharge (a spark generator, essentially) to simulate ultraviolet and cosmic rays striking the gas. It’s not exactly like Titan – they did this 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 that of Earth, which, although very thin, is much higher than the upper part of Titan’s atmosphere where the reactions take place. However, the higher pressure allows the reaction rate to be much higher, so they don’t wait weeks for a decent sample.

They found about a hundred different molecules, a dozen of which they could examine under 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 produced 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 by Titan during that time, examining its surface and atmosphere. Simulations in the laboratory confirm this result.

In addition, the lab experiment created molecules made up of many connected rings, up to seven of them, which will help atmospheric scientists understand how more complex PAHs are formed from simpler molecules.

This job is important for many reasons. Titan’s atmosphere is charged with these things, collectively called tholins (Greek for “mud”, as they make molecules that color the environment yellow, orange, and reddish-brown), and are seen on other worlds as well; Pluto’s reddish landscape is due to the tholins.

Titan doesn’t have a water cycle like Earth, but it does have a methane cycle: liquid methane in vast lakes at its north pole evaporates into the atmosphere, rains down on nearby hills, and then flows back into lakes. Methane vapor can condense on suspended tholins, helping rain out, and then tholins can cover the moon’s surface. That’s very interesting, because nitrogen and carbon molecules are important in prebiotic chemistry, as they make up 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 that gave us the atmosphere, more or less, that we have today. Studying Titan is like studying ancient Earth. I don’t want to be too broad, but life evolved on Earth in that primitive 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 is certainly within the realm of science to investigate.

Titan is a strange world more than a billion kilometers from the Sun, and drier than any desert on our own 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 lander and a quadcopter. that will fly over the surface and examine the regions that are likely to have had or had conditions conducive to life.

What will you find there? These lab results are an important step in finding out.

*Writing those words makes me feel like a scientist from an old black and white science fiction movie.


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