Planetary scientists had believed that the entire surface of Titan was engulfed in an ocean of liquid ethane one kilometre deep. That is until the European Space Agency’s Huygens lunar probe landed on Titan in 2005 and found a surface covered in a sand-like material – not liquid ethane. Recent calculations by Donald Hunten from the University of Arizona, USA suggest that instead of accumulating in liquid form after being produced in Titan’s upper atmosphere, the ethane condenses onto thick smog that envelops the moon's surface (Nature 443 669).

This process forms a smog-and-dust-like material that Hunten calls "smust". The idea came to Hunten while he was analysing the vertical distribution of ethane in Jupiter's atmosphere. "Jupiter has smog particles too, and it occurred to me that they are very spongy and have lots of sites that would be good for ethane to adhere to. The same process should work on Titan." Indeed, Hunten has calculated that Titan could be covered in a layer of smust 2.6 km thick that could support dune-like structures. Such dunes have been observed on Titan by the ESA’s Cassini planetary probe, which launched Huygens and continues to monitor Titan.

The atmosphere of Titan is expected to contain vast quantities of ethane that has built up over time from photochemical reactions in the upper atmosphere. Similar reactions produce heavier hydrocarbons, which are responsible for Titan's dense, orange-brown smog.

According to Hunten’s calculations, condensation onto the smog is the only explanation for the observed mixing ratio (a measure of the relative abundance) of liquid ethane at the bottom of Titan’s stratosphere. "There is a deep minimum of the mixing ratio that coincides with the coldest region [of the stratosphere] near the tropopause. This suggests condensation, but it is not cold enough for the ethane to condense by itself."

Unfortunately it is unlikely that Hunten's proposal will be verified in the near future. "One never knows what Cassini might find," said Hunten, "but I don't expect anything that will point directly to the presence of smust particles."