For the first time, the levels of liquid in Titan’s lakes have been found to fall and rise with the seasons, just like on Earth.

Titan, Saturn’s large, planet-like moon, is the only location in the Solar System – aside from the Earth of course – that has a hydrological cycle. While Earth’s hydrological cycle runs on water, Titan’s distance from the Sun makes for much chillier conditions, and a hydrological cycle that is based on liquid methane, ethane and propane.

Now, after analysing Cassini spacecraft data collected over a period of four years, scientists have found that the depth of liquid in Titan’s southern hemisphere lakes have dropped at a rate of about one metre per year. For the case of Ontario Lacus (named for its similar size to Lake Ontario on Earth), the scientists found that between June 2005 and July 2009, a period of time that covers the transition between mid-summer and autumn on Titan, the shoreline receded by around ten kilometres.

The results are based on radar altimetry and Synthetic Aperture Radar (SAR) data, which measures the roughness of a surface such that a flat, smooth surface such as a lake appears dark, while rough features (such as mountains) appear bright. By combining the SAR and radar altimetry data, the scientists could build up a picture of the absorptive properties of the liquid. “It argues that the liquids are relatively pure hydrocarbons made up of methane and ethane and not a gunky tar," says Oded Aharonson, associate professor of planetary science at Caltech.

"The liquid is not highly attenuating," explains Alexander G. Hayes, also from Caltech, "which means it is fairly clear to radar energy – that is, transparent, like liquid natural gas. Because of this, radar can see through the liquid in Titan's lakes to a depth of several metres. Then the radar hits the floor, and bounces back. Or, if the lake is deeper than a few metres, the radar is completely absorbed, producing a 'black' signature."

Taking into account the liquid’s optical properties, the researchers could then focus on the depth of the lake. "We were able to determine the bathymetry of the lake out to a depth of about eight metres," says Hayes, who adds that the lake is shallowest and most gently sloped along its southern edge, in areas where sediment is accumulating. Along its eastern and northern shore, where it hits a mountain range, the slope of the lake is somewhat steeper, which Hayes calls the ‘beachhead’.

Comparing images of Ontario Lacus separated by four years revealed that the lake had shrunk. "The extent to which the lake has receded is related to the slope – i.e., where the lake is shallow, the liquid will have receded more," says Hayes. "This allows us to deduce the vertical height by which the lake depth has dropped, which is about one metre per year."

The rate of evaporation of methane from nearby lakes was also determined by comparing the radar signatures from images taken in December 2007 and May 2009, in terms of how ‘dark’ the signature appeared. In all the lakes the radar darkness decreased or disappeared entirely, translating as a reduction of liquid. "We got the same result: one metre per year of liquid loss," says Aharonson.

As yet, no analogous changes have been noted in the northern hemisphere lakes, which is now entering spring. "We would expect it will happen, but we don't know how it would manifest in the data if the lakes in the north are significantly deeper," says Aharonson "We'll continue to look for this effect with future radar images, to disentangle the seasonal variations from longer-term climate variations we previously have proposed."