Ever since the Cassini spacecraft was placed into orbit around Saturn in 2004, it has been mapping the surface of Titan and basically blowing everyone's minds by revealing how much this moon resembles a planet. It has a dense nitrogen-rich atmosphere, geologic activity on its surface, water ice, massive deserts, and methane oceans—planetary qualities that have led many space scientists to speculate that Titan may provide us with a snapshot of a young Earth, with NASA going so far as to label Saturn's largest moon as perhaps "one of the most Earth-like worlds we have found to date."
It is Titan's vast equatorial deserts and the massive hydrocarbon sand dunes found there that are of particular note, however. These awe-inspiring structures have recently led a team of scientists from France's Université de Bordeaux to believe that Titan has experienced significant climate change since its formation.
One of the largest mysteries literally surrounding the solar system's second largest moon is its atmosphere. Comprised of nitrogen and methane, it should have disappeared within 100 million years of its formation because methane is broken down by sunlight. The fact that it hasn't suggests that Titan's climate and atmosphere have changed over time, influenced by as yet undetermined features of the moon (although volcanic activity is a strong candidate). Understanding how and why these changes occurred can potentially provide valuable insight into the early development of Earth's own climate and how we might expect it to change in the future.
Studying Titan has been slow going largely because researchers do not have direct access to the moon's surface. With the exception of the Huygens probe, which touched down on the moon in 2005 and transmitted data for 90 minutes before succumbing to the harsh environment, much of what we know about Titan is thanks to Cassini's RADAR system. At once a synthetic aperture radar (SAR), a
scatterometer, an altimeter, and a radiometer, Cassini's RADAR instrument allows researchers to "see" through the thick swathes of methane clouds which obfuscate Titan's surface.
What the researchers found is that nearly one-fifth of the moon's surface is covered in linear sand dunes that are up to 2 kilometers wide, 100 kilometers long, and 150 kilometers high. Moreover, Cassini's RADAR also showed the presence of mega-yardangs on the moon's surface, massive rocky ridges formed by wind erosion that, like the dunes, run in an east-west direction parallel to Titan's equatorial winds.
Both of these landforms can provide valuable insight into the evolution of Titan's climate. Unfortunately, the images provided by Cassini's RADAR system are too low in resolution to allow serious investigation into their morphology.
Thus, in order to better study the history of Saturn's moon, the Université de Bordeaux researchers, led by planetary scientist Philippe Paillou, have turned their focus to Earth, a method known as comparative planetology. Using images from the Earth-orbiting TerraSAR-X satellite, Paillou and his team examined linear sand dunes in Egypt's Great Sand Sea and Namibia's Namib Desert, and mega-yardangs in Iran's Lut Desert and Chad's Borkou Desert to compare how radar interacts with geological structures on a planet's surface.
What Paillou and his colleagues were observing was how the terrestrial dunes and mega-yardangs backscattered the TerraSar-X's radar signal, allowing them to produce a surface scattering model for each type of formation: bare dunes (Egypt), sand-covered dunes (Namibia), young mega-yardangs (Iran), and old mega-yardangs (Chad). Once Paillou and his team had scattering models for each landform, they applied their findings to Titan.
Using images from a Cassini flyby in 2008, the team found that the equatorial dunes on Titan were not homogenous, but rather a hodgepodge of both bare and sand covered dunes.
"Being able to distinguish between different types of dune is significant," said Paillou. "It's important to know that there's more than one form of linear dune on Titan, to avoid false interpretations of radar images in the future."
The team then turned to other regions of Titan imaged in 2009 and 2012, the team discovered the presence of mega-yardangs in the area, structures that are thought to form on Earth after the sediment deposited by ancient lakebeds is eroded by the wind. This is significant because now only Titan's polar regions are able to keep liquid methane stable, which makes the presence of mega-yardangs at the moon's equator something of an mystery.
"Finding yardangs at mid-latitudes on Titan means that lakes might have existed there at some point in the moon's past," said Nicolas Altobelli, the European Space Agency's Cassini-Huygens Project Scientist. "As they're certainly not there now, it's a strong sign for natural climate change. This just adds to the moon's intrigue—there's much more for us to explore."