Beyond Earth, the general scientific consensus is that Mars is the best place to look for evidence of extraterrestrial life. However, it is by no means the only place. Aside from the many extrasolar planets that have been classified as “potentially habitable”, there are many other candidates here in our solar system. This includes the many icy satellites believed to have internal oceans that could harbor life.
Among them is Titan, Saturn’s largest moon, in whose atmosphere and surface all kinds of organic chemistry take place. For some time now, scientists have suspected that studying Titan’s atmosphere could provide important clues about the early stages of the evolution of life on Earth. Thanks to new research by the technology giant IBM, a team of researchers has succeeded in restoring the atmospheric conditions on Titan in a laboratory.
Her research is described in an article entitled “Imaging Titans Organic Haze at the Atomic Scale,” which recently appeared in the February 12 issue of The Astrophysical Journal Letters. The research team was led by Dr. Fabian Schulz and Dr. Julien Maillard and included many colleagues from IBM Research-Zurich, the University of Paris-Saclay, the University of Rouen in Mont-Saint-Aignan and the Fritz Haber Institute of the Max Planck Society.
This artist’s concept of a lake at the north pole of Saturn’s moon Titan shows raised edges and wall-like features as seen by NASA’s Cassini spacecraft around the Winnipeg Lacus of the moon. Photo credit: NASA / JPL-Caltech
Much of what we know about Titan today is thanks to the Cassini spacecraft, which orbited Saturn from 2004 to 2017 and completed its mission by immersing itself in Saturn’s atmosphere. During this time, Cassini took many direct measurements of Titan’s atmosphere and revealed a surprisingly Earth-like environment. Basically, titanium is the only other body in the solar system in which a dense nitrogen atmosphere and organic processes take place.
Particularly interesting is the fact that scientists believe that the Earth’s atmosphere may have been similar about 2.8 billion years ago. This coincides with the Mesoarchean Era, a time when photosynthetic cyanobacteria created the first reef systems and slowly converted Earth’s atmospheric carbon dioxide into oxygen gas (which eventually led to its current balance of nitrogen and oxygen).
While the surface of Titan is believed to contain clues that might improve our understanding of how life came about in our solar system, getting a clear view of this surface has been a problem. The reason for this lies in the titanium atmosphere, which is permeated by a dense photochemical haze that scatters light. As Leo Gross and Nathalie Carrasco (study co-authors) stated in a recent article published on the IBM Research Blog:
“Titan’s haze consists of nanoparticles made up of a large number of large and complex organic molecules that contain carbon, hydrogen and nitrogen. These molecules form in a cascade of chemical reactions when radiation (ultraviolet and cosmic) hits the mixture of methane, nitrogen, and other gases in atmospheres like titans. “
The PAMPRE experiment, which simulates the atmosphere of Titan. Photo credit: Nathalie Carrasco
As a result, there is still much that scientists do not know about the processes that power Titan’s atmosphere, including the exact chemical structure of the large molecules that make up this haze. For decades, astrochemists have conducted laboratory experiments on similar organic molecules known as tholines – a term derived from the Greek word for “muddy” (or “cloudy”).
Tholines refer to a wide variety of organic carbonaceous compounds that form when exposed to solar UV or cosmic rays. These molecules are common in the outer solar system and are typically found in icy bodies where the surface layer contains methane ice that is exposed to radiation. Their presence is indicated by a surface that is reddish in appearance or has sepia-colored spots.
For their study, the team led by Schulz and Maillard conducted an experiment in which they observed Tholine at different stages of education in a laboratory setting. As Gross and Carrasco explained:
“We flooded a stainless steel vessel with a mixture of methane and nitrogen and then triggered chemical reactions by means of an electrical discharge, which mimicked the conditions in the titanium atmosphere. We then analyzed over 100 resulting molecules that make up Titans Tholine in our laboratory in Zurich, and obtained images with a resolution of around a dozen with our self-made low-temperature atomic force microscope. “
NASA’s Cassini spaceship looks at the night side of Saturn’s largest moon and sees sunlight that is scattered through the periphery of Titan’s atmosphere and forms a colored ring. Photo credit: NASA / JPL-Caltech / Space Science Institute
By dissolving molecules of different sizes, the team gained insight into the various stages in which these vapor molecules grow, as well as their chemical composition. Essentially, they observed a key component in Titan’s atmosphere as it formed and accumulated to create Titan’s famous haze effect. Conor A. Nixon, a researcher at NASA’s Goddard Space Flight Center (who was not involved in the study) said:
“This paper shows groundbreaking new work on the use of atomic-scale microscopy to study the structures of complex organic molecules with multiple rings. A typical analysis of laboratory-generated compounds using techniques such as mass spectroscopy shows the relative proportions of the various elements, but not the chemical bond and structure.
“For the first time, we are seeing the molecular architecture of synthetic compounds that are similar to what is believed to be the cause of the orange haze of Titan’s atmosphere. This application now offers an exciting new tool for sample analysis of astrobiological materials, including meteorites and returned samples from planetary bodies. “
In addition, their results could shed light on the mysterious methane-based hydrological cycle of Titan. On earth, this cycle consists of water that changes between a gaseous state (water vapor) and a liquid state (rain and surface water). On Titan, the same cycle takes place with methane, which passes from the atmospheric methane gas and falls as methane rain into the famous hydrocarbon lakes of Titan.
A proposed eight-winged drone (also known as a “dragonfly”) could be ideally suited for exploring Saturn’s moon Titan in the decades to come. Photo credit: APL / Michael Carroll
If so, the research team’s results could reveal the role that chemical haze plays in Titan’s methane cycle, including whether or not these nanoparticles can float on its methane lakes. In addition, these results could reveal whether similar atmospheric aerosols contributed to the creation of life on Earth billions of years ago.
“The molecular structures that we have now shown are known to be good absorbers for ultraviolet light,” describe Gross and Carrasco. “That in turn means that the haze may have acted as a shield protecting DNA molecules on the earth’s early surface from harmful radiation.”
If this theory is correct, the team’s results would not only help scientists understand the conditions under which life arose here on Earth, but they could also point to the possible existence of life on Titan. The mysterious nature of this satellite first became apparent to scientists in the early 1980s when the spacecraft Voyager 1 and 2 both flew through the Saturn system. Since then, scientists have put together
By 2030, NASA plans to send a robotic rotary wing aircraft named Dragonfly to Titan to explore its surface and atmosphere and look for possible signs of life. As always, in the meantime, through theoretical work and laboratory experiments, the scientists will narrow the focus and increase the likelihood that the mission (once it arrives) will find what it is looking for.
Further reading: IBM, The Astrophysical Journal Letters