“That can’t be right.” These are terrible or wonderful words for a scientist. It’s that moment when they look at the results of an experiment and see something they truly did not expect, good or bad. It happened to Sarah Hörst, graduate student and lead researcher on a project for the University of Arizona (Tucson, USA) that was attempting to create, or simulate, the atmosphere of Saturn’s moon, Titan.
Titan is one of the most unique objects in our Solar System. It’s a large moon, fifty-percent bigger than Earth’s Moon. It’s the only planetary satellite that has a dense atmosphere. In fact, the atmosphere is denser than Earth’s. At the surface, the atmospheric pressure is roughly equivalent to 15 feet under water on Earth. Titan’s atmosphere is nothing like Earth’s. It’s composed mostly of nitrogen, which combines to make atmospheric methane and ethane. It rains methane on Titan. The atmosphere is so thick; it could best be described as organic smog.
It’s the organic part that excites astrobiologists. Titan’s atmosphere is thought to be full of organic molecules, the ones based on carbon, hydrogen, and oxygen, and it’s often speculated that some of those molecules could be prebiotic, precursors to the kind of amino acids that make up life on Earth. So far, however, the probes we have sent to Titan (Cassini and Huygens) have given scientists only a limited insight into the atmosphere of Titan. This has prompted attempts to simulate the atmosphere of Titan in the laboratory or in computer models.
The basic proposition of the University of Arizona project was to simulate the atmosphere of Titan in the lab. They did not expect to have an exact copy; after all, nobody knows the composition of Titan’s atmosphere. What they did hope to do is create conditions for the formation of particulate molecules (the smog or aerosols) that would be found in the upper atmosphere of Titan. That’s where the Sun’s energy, primarily ultraviolet rays (UV), can break apart some molecular chains and reform them into others. Lower in the atmosphere and at the surface, the temperature is too cold (-192 degrees Fahrenheit, -124 degrees Centigrade) for much molecular activity.
Hörst worked with colleagues in Paris who developed a unique synthesis chamber where raw gaseous materials could be bombarded with microwave energy. This simulates the energy of the Sun and produces aerosol molecules from the gases. Using electric fields, the aerosols are kept afloat within the chamber until they become so big that they drop to the bottom of the chamber. These are the molecule chains of most interest to biologists, which Hörst brought to a lab in Grenoble for analysis by mass spectrometer.
This analysis revealed about 5,000 molecular formulas, but spectrometry reveals nothing about the configuration of the molecules. It was suspected there could be as many as 20,000 different molecular structures – but which ones? Then the idea occurred to them that they might feed a computer a list of the molecular formulas for amino acids (the basis of proteins) and nucleotide bases (the stuff of DNA and RNA) and have the computer compare that against the atmospheric compounds. That’s when Hörst had her ‘That can’t be right moment.’ Looking at the results of the computer run, she quickly identified biotic molecules. In fact, all five nucleotide bases (cytosine, adenine, thymine, guanine, and uracil) were represented, as well as twelve amino acids.
The researchers weren’t trying to make these prebiotic molecules, but they arose spontaneously from the mixture of gasses and the presence of an electromagnetic catalyst (microwaves). This does not prove anything about the atmosphere of Titan. It suggests that in conditions that prevail in the upper atmosphere of Titan such chemistry might take place.
It’s also a very big leap from the prebiotic molecules to life. Scientists don’t know how that worked on Earth, much less how it might work in the radically different environment of Titan. There is plenty of speculation, and that speculation is part of how science makes advances to more accurate hypotheses and more definitive experiments.
What this study does show is that key organic compounds can be produced in the atmosphere. Whether life can form in the atmosphere, or requires some other kind of environment is still not known.
“There are a lot of reasons why life on Titan would probably be based on completely different chemistry than life on Earth,” Hörst added, “one of them being that there is liquid water on Earth. The interesting part for us is that we now know you can make pretty much anything you want in an atmosphere. Who knows this kind of chemistry isn’t happening on planets outside our solar system?”
That could be right.