Only two bodies in the Solar System are known to have persistent surface liquid: Earth and Saturn's moon Titan. Unlike Earth, Titan's liquid is methane rather than water, and most of the methane is in the moon's thick atmosphere instead of on the surface. Therefore, instead of large oceans, Titan possesses smaller, shallower seas in the polar regions. The equatorial region is marked by sand dunes, similar to fields found on Earth. Studies of Titan's atmosphere show the tropics to be free of precipitation, making it drier than the driest desert on Earth.

However, analysis of Cassini space probe data from 2004 through 2008 may have found as many as five tropical lakes on Titan. Spectral analysis by Caitlin A. Griffith et al. revealed an oval region that absorbs infrared light, an effect consistent with a small lake of liquid methane at least 2 meters deep. They also identified four other candidate lakes, but the data was far less clear. In the absence of rain, the authors suggest the most likely source for these lakes is subsurface—making them desert oases.

Mapping Titan's surface is complicated by the moon's thick atmosphere, which is opaque to many wavelengths of light. While Earth's atmosphere has many broad "windows"—ranges of wavelengths that can penetrate to the surface, including visible and radio light—Titan has much narrower windows in the infrared and radio portions of the spectrum. The Cassini probe and the associated Huygens lander performed detailed chemical analysis of the atmosphere; they found the opacity is due to methane and other hydrocarbons, compounds of hydrogen and carbon responsible for smog on Earth.

Using Cassini radar data, scientists previously determined the presence of large methane seas above 50° of latitude, the first such liquid bodies found on any world other than Earth. (Titan is far too cold for liquid water: its average surface temperature is about -179° C.) On the other hand, the tropical regions—latitudes between 20° north and 20° south—appear to be mostly arid. This proposition is supported by atmospheric studies, which show the methane clouds responsible for rain are driven away from the equator by prevailing winds.

Cassini is also equipped with infrared instruments, which the current study used to exploit three atmospheric windows to study the tropical region of Titan. The researchers studied the ratio of reflected light intensity to the incident light flux from the Sun, known as the incident flux (I/F). A high I/F value indicates either a highly reflective surface or scattering of light off atmospheric molecules, while a low value indicates absorption.

The authors found a sharp cutoff in I/F values in all three wavelengths in a small region at 14° S latitude and 175° longitude, indicating a dark spot. Such a consistent cutoff was not expected, based on measurements elsewhere on Titan's surface. They followed up by comparing the spectrum with a model of a methane lake, and found consistent results. The oval-shaped region is approximately 40km by 60km in size, indicating a small but substantial lake, at least 2 meters deep. This depth is comparable to other seas on Titan, including Kraken Mare. The researchers also identified four other possible lakes, though the data was far less clear. The authors suggest they may be very shallow pools, less than 10cm deep.

The non-reflective oval region was present in Cassini data from 2004 through 2008, so it is a persistent feature. Since the tropics of Titan are arid, if this dark spot is truly a lake, it cannot be fed by rain. Instead, it must be a desert oasis, supplied by an underground methane source. Interestingly, if this result is borne out, it might help solve the problem of how Titan renews its atmospheric methane, which is depleted as the Sun's light changes it into other hydrocarbons. Underground sources of methane, if they are sufficient to feed tropical lakes, may also vent elsewhere on the surface.

Nature, 2012. DOI: 10.1038/nature11165 (About DOIs).