Ever since the Cassini orbiter entered the Saturn system in July of 2004, scientists and the general public have been treated to a steady stream of data about this ringed giant and its many fascinating moons. In particular, a great deal of attention was focused on Saturn’s largest moon Titan, which has many surprising Earth-like characteristics.
These include its nitrogen-rich atmosphere, the presence of liquid bodies on its surface, a dynamic climate, organic molecules, and active prebiotic chemistry. And in the latest revelation to come from the Cassini orbiter, it appears that Titan also experiences periodic dust storms. This puts it in a class that has so far been reserved for only Earth and Mars.
The discovery was made by an international team scientists from the Institut de Physique du Globe de Paris (IPGP), the Laboratoire de Planétologie et Géodynamique (LPGNantes), the Southwest Research Institute (SwRI), the John Hopkins University Applied Physics Laboratory (JHUAPL), the European Space Agency, NASA’s Jet Propulsion Laboratory and multiple universities and research institutes.
The study was based on data acquired by the Cassini probe’s Visual and Infrared Mapping Spectrometer (VIMS), which acquired images of Titan during its many flybys between May 2009 and September 2000. As you can see from the animation above (click to animate), the atmosphere underwent a lot of changes in that time, revealing bright patches that were interpreted as dust storms.
As Sebastien Rodriguez, an astronomer with the IPGP at the Université Paris Diderot and the paper’s lead author, explained in a recent NASA press release:
“Titan is a very active moon. We already know that about its geology and exotic hydrocarbon cycle. Now we can add another analogy with Earth and Mars: the active dust cycle, in which organic dust can be raised from large dune fields around Titan’s equator.”
Already, scientists were aware of the ways in which Titan is similar to Earth. Aside from being the only other celestial object in the Solar System with a substantial nitrogen atmosphere, it is also the only other Solar body where stable bodies of liquid still exist on its surface. And like on Earth, this liquid cycles between the atmosphere and the surface.
But whereas on Earth, this cycle consists of water evaporating, forming clouds, and raining back down onto the surface (i.e. the hydrological cycle); on Titan, the entire process involves methane and ethane. Titan also experiences seasonal variations in weather, much like Earth. At the time when the Sun crosses Titan’s equator (the equinox) massive clouds can form in the tropical regions and create methane storms – one of which Cassini witnessed.
Rodriguez and his team first spotted three unusual spots in Titan’s equatorial region while analyzing infrared images taken by Cassini in 2009, around the time of the moon’s northern equinox. These bright patches were originally mistaken for methane clouds, but further investigation revealed that they had to be something completely different. As Rodriguez explained it:
“From what we know about cloud formation on Titan, we can say that such methane clouds in this area and in this time of the year are not physically possible. The convective methane clouds that can develop in this area and during this period of time would contain huge droplets and must be at a very high altitude — much higher than the 6 miles (10 kilometers) that modeling tells us the new features are located.”
Another possibility was that these spots corresponded to features on the surface, such as frozen methane rain or icy lava flows. However, these features would have had a different chemical signature and would have been visible for long periods of time, whereas these spots were only visible for periods ranging from 11 hours to five weeks.
To address this, the team conducted a series of models that revealed that these features were both atmospheric but still close to the surface. They also noticed that the bright spots were located directly above the dune fields around Titan’s equator – which are made up of tiny solid organic particles. This left only one explanation, which was that the spots were clouds of dust raised from the dunes.
Granted, the discovery of organic dust clouds is not entirely surprising. For years, scientists have known that this kind of dust forms when organic molecules are caused by the interaction of sunlight with methane in Titan’s atmosphere. When they grow large enough, they fall to the surface. In addition, the presence of dunes in the region is indicative of strong winds and the facts that the organic sands can be put in motion.
However, this research is intriguing in that it is the first time that dust storms have ever been observed on Titan. What’s more, the knowledge that winds could be transporting organic dust across large distances could also have implications for our understanding of the dynamics on Titan. Basically, Titan may be Earth-like in that it too has a global cycle of organic dust, with similar effects to what it has on Earth.
“We believe that the Huygens Probe, which landed on the surface of Titan in January 2005, raised a small amount of organic dust upon arrival due to its powerful aerodynamic wake,” said Rodriguez. “But what we spotted here with Cassini is at a much larger scale. The near-surface wind speeds required to raise such an amount of dust as we see in these dust storms would have to be very strong – about five times as strong as the average wind speeds estimated by the Huygens measurements near the surface and with climate models.”
Even though the Cassini mission ended on September 15th, 2017, when it was intentionally crashed into Saturn’s atmosphere, scientists are still sifting through the volumes of data it acquired during the 13 years it spent orbiting the ringed gas giant. The discovery that Titan experiences dust storms like Earth and Mars is merely the latest revelation to come from this mission.
In the future, several robotic missions are being planned for Titan, ranging from balloons and landers, aerial platforms, and helicopters, to paddleboats, and a submarine. Regardless of what form the next mission will take, its objectives will be clear: to learn more about Titan’s dynamic environment and search for evidence of possible life there.
Further Reading: NASA, Nature