Scientists report definitive evidence of the presence of lakes filled with liquid methane on Saturn's moon Titan in this week's journal Nature cover story.
Radar imaging data from a July 22, 2006, flyby provide convincing evidence for large bodies of liquid on Titan today. A new false-color radar view gives a taste of what Cassini saw. Some highlights of the article follow below.
- Radar-dark patches are interpreted as lakes based on their very low radar reflectivity and morphological similarities to lakes, including associated channels and location in topographic depressions.
- Radar-dark surfaces are smooth and most likely liquid, rock, ice or organics. More than 75 radar-dark patches or lakes were seen, ranging from 3 kilometers (1.8 miles) to more than 70 kilometers (43 miles) across.
- Some lakes appear partly dry, while others seem liquid-filled. Some of the partly filled lakes may never have filled fully, or may have partly evaporated at some point in the past. The dry lakes have margins or rims and a radar brightness similar to the rest of the surrounding terrain, making them appear devoid of liquid.
- The varying states of how full the lakes are suggest that lakes in this region of Titan might be temporary on some unknown timescale.
- Approximately 15 of the dark patches seem filled and show no clear evidence of erosion. These dark patches resemble terrestrial lakes confined within impact basins (for example, Clearwater Lakes in Canada) or within volcanic calderas (for example, Crater Lake, Oregon). The nest-like nature of these lakes and their limited range of sizes make it unlikely that they originated from an impact. A volcanic origin for the depressions is possible, given their appearance.
- Some lakes have steep margins and very distinct edges, suggesting a topographic rim. These lakes are consistent with seepage or groundwater drainage lakes.
- Other lakes have diffuse, more scalloped edges, with a gradual decrease in radar brightness towards the center of the lake. These lakes are more likely to be associated with channels, and may be either drainage lakes or groundwater drainage lakes.
- Yet other lakes have curvy channel-like extensions, similar in appearance to terrestrial flooded river valleys (for example Lake Powell).
- Bright patches near the lake edges could be small islands peeking through the surface. Floating “icebergs” are unlikely because most materials would not float in liquid hydrocarbons.
- Based on the lake characteristics, Cassini scientists think they are observing liquid-filled lakes on Titan today. Another possibility is that these depressions and channels formed in the past and have now been filled by a low-density deposit that is darker than any observed elsewhere on Titan. However, the absence of wind-blown features in this area makes the low-density hypothesis unlikely.
- These northern hemisphere lakes are the strongest evidence yet that Titan's surface and atmosphere have an active hydrological cycle, though with a condensable liquid other than water. In this cycle, lakes are filled through methane rainfall or intersect with a subsurface layer saturated with liquid methane.
- As Titan's seasons progress over the 29-year cycle of Saturn's orbit around the sun, lakes in the winter hemisphere should expand by steady methane rain, while summer hemisphere lakes shrink or dry up entirely.
To read more about the radar imaging data from the July 2006 flyby, go to Cassini Finds Lakes on Titan's Arctic Region.
I'm sure there must be some, but I can't call to mind any fiction set on Titan featuring the dark, thick and frigid seas of scientific speculation. Anyone want to offer up a title or two? It is interesting how Cassini and Huygens have reshaped planetologists' thinking about Titan's climate. Previously, the models were somewhat Earthlike in behavior, with regular hydrocarbon "rainfall" consisting of methane and perhaps ethane, which then collects in rivers, lakes and seas before evaporating and starting the cycle over again. Now, however, it appears that Titan leans toward more of a "deluge" model, in which large bodies of liquid (the lakes in the latest finding) gradually evaporate, leaving the landscape in a somewhat arid condition. Once the atmosphere reaches saturation point, a vast amount of liquid hydrocarbons precipitate, flooding the landscape--a sort of desert monsoon season.
It will be interesting to see how this model is refined as researchers collect more data from Cassini, and even more interesting to see how SF writers take these new wonders from the Saturn system and apply them in a fictional context.