Atomic force microscopy helps clear the haze surrounding Saturn’s moon Titan

In the cold outskirts of the solar system, Saturn’s largest moon, Titan, may hold clues on the early stages of the evolution of life on Earth. But those clues are literally hiding in haze. Titan’s atmosphere is shrouded with a brownish-orange fog made of organic aerosols, the origin and nature of which still puzzles astronomers.

We have now unveiled in the laboratory new details on how the famous Titan haze may have formed and what its chemical make-up looks like. We resolved molecules of different sizes, giving snapshots of the different stages through which molecules grow to build up the haze. We report our findings in the latest issue of the Astrophysical Journal.

Titan’s haze consists of nanoparticles made of a wide variety of large and complex organic molecules containing carbon, hydrogen and nitrogen. These molecules form in a cascade of chemical reactions when (ultraviolet and cosmic) radiation hits the mix of methane, nitrogen and other gases in atmospheres like Titan’s.

Planetary scientists believe that, some 2.8 billion years ago, the Earth may have been surrounded by a similar haze to that of Titan. So, studying Titan’s haze today gives scientists a sort of time machine to travel back to the origins of life on our own planet.

Thanks to the Cassini spacecraft which orbited Saturn from 2004 to 2017 astronomers learned a great deal about Titan’s haze from direct measurements in the moon’s atmosphere. But they still don’t understand all the details of the reaction cascade by which the nanoparticles are made from raw materials as simple as methane and nitrogen. Nor do they know the exact chemical structure of the large molecules that make up the haze.

But, luckily, we can study Titan here on Earth as well. For decades, astrochemists have been recreating Titan’s haze in the lab in an attempt to understand the haze components better. The components of this lab analogue of Titan’s haze are known as tholins, a term derived from a Greek word meaning mud and coined in a 1979 Nature paper co-authored by late astronomer and author Carl Sagan. That’s exactly what we did. We flooded a stainless-steel vessel with a mixture of methane and nitrogen and then triggered chemical reactions through an electric discharge, thereby mimicking the conditions in Titan’s atmosphere.

We then analyzed over 100 resulting molecules composing Titan’s tholins in our lab at Zurich, obtaining atomic resolution images of around a dozen of them with our home-built low-temperature atomic force microscope.

Over decades, scientists have pieced together a general picture of the chemical composition of Titan’s haze. In it, the main part of the haze is made of nitrogen-containing polycyclic aromatic hydrocarbons. We now found confirmation for that scenario and obtained real space images of those molecules in our lab-made duplicate of Titan’s haze. Adding to that picture, we found that pentagonal aromatic rings are common. The structure of the larger molecules that we observed tells us about the way they grow.

The chemical structures revealed in our study could be related to the haze’s wettability, which impacts the methane-based hydrologic cycle on Titan and determines whether or not the nanoparticles float on Titan’s hydrocarbon lakes. Finding these new details on the chemical structure of tholins adds to our understanding not only of Titan’s haze but also of the likelihood that aerosols might have favored life on the early Earth in the past. The molecular structures we have now imaged are known to be good absorbers of ultraviolet light. That in turn means that the haze may have acted as a shield protecting DNA molecules on the early Earth’s surface from damaging radiation.

“This paper shows ground-breaking new work in the use of atomic-scale microscopy to investigate the structures of complex, multi-ringed organic molecules. Typical analysis of laboratory-generated compounds using techniques such as mass spectroscopy reveals the relative proportions of the various elements, but not the chemical bonding and structure. For this first time here we see the molecular architecture of synthetic compounds similar to those thought to cause the orange haze of Titan’s atmosphere. This application now provides an exciting new tool for sample analysis of astrobiological materials, including meteorites and returned samples from planetary bodies,” said Conor A. Nixon, Research Space Scientist, NASA Goddard Space Flight Center, who is not involved in the paper.