Feature ArticleOceanography on Saturn's Moon, Titan
By Dr. Ralph Lorenz
Oceanography is no longer just an Earth science. Saturn's moon Titan, whose equatorial deserts were visited by the Huygens probe in 2005, has been found to have lakes and seas of liquid hydrocarbons that give a new arena in which to contemplate the physics of ocean circulation, coastline processes and air-sea interaction.
Titan, next to its parent planet. Note the filigree shadows of the rings cast onto Saturn. Titan is perceptibly darker/redder than Saturn, and darkening near Titan's north polar region can be seen here.
Mapping by the Cassini spacecraft, which is in Saturn orbit but passes near Titan every few weeks, revealed in 2006 hundreds of lakes around Titan's north pole. Later, three bodies of liquid large enough to merit designation as seas were identified. One major south polar lake is known, and a few transient areas of liquid have been spotted at low latitudes. The northern seas are prime targets for future exploration, not only as reservoirs of organic material of astrobiological interest but as laboratories for air-sea exchange and other oceanographic processes.
A One-of-a-Kind Moon
Titan is a unique satellite in the solar system in that it has a dense (1.5 bar) atmosphere, which endows Titan with many processes and phenomena familiar to us on Earth. At Saturn's distance from the sun (10 astronomical units), the surface temperature on Titan is 94 Kelvins, in part due to the greenhouse warming of methane, which makes up a few percent of the atmosphere (the rest being nitrogen). Ninety-four Kelvins is close to the triple point of methane so it is a condensable greenhouse gas, just like water vapor on Earth. Similarly, methane forms clouds, hail and rain. The latter phenomenon carves river valleys on Titan's surface. The weak sunlight that drives Titan's hydrological cycle results in rare rainfall, averaging only a few centimeters per year, expressed probably as massive downpours depositing tens of centimeters or even meters of rain in a few hours, but interspersed with centuries of drought. In some respects, Titan is to Earth's hydrological cycle what Venus is to its greenhouse effect—a terrestrial phenomenon taken to a dramatic extreme.
Titan has a tilt of 26 degrees, so its climate has significant seasonal forcing. However, it takes 29.5 years to go around the sun, making the seasons long. In addition to seasonal rainfall, the annual cycle also manifests in Titan's stratospheric circulation, where wide swings in the abundance of various organic gases and hazes (produced by the action of ultraviolet light on methane) take place. These changes are particularly strong at the winter pole, with some analogies to polar stratospheric clouds and ozone hole dynamics on Earth. Among the gases produced by photochemistry is ethane, which is also a liquid at Titan conditions, and was expected to accumulate on the surface.
The Lakes of Titan
Although hydrocarbon seas were long speculated to exist on Titan, bodies of standing liquid were only confirmed (in northern winter darkness) by radar observations in 2006, some two years after Cassini arrived in the Saturnian system. Hundreds of radar-dark lakes, typically 20 kilometers across, were discovered at about 70°N. By international convention, lakes on Titan are named after lakes on Earth, while the three seas are named after sea monsters. Ligeia Mare, a 300-to-400-kilometer body, was first to be observed. The smaller Punga Mare is closer to the north pole, and the giant Kraken Mare sprawls over some 1,000 kilometers toward midlatitudes.
Strikingly, the southern hemisphere has only one modest body of liquid, Ontario Lacus, about 70 by 250 kilometers. This is one of the most-studied lakes, since the south was better illuminated in 2004 to 2010 allowing near-infrared remote sensing on Cassini to detect ethane.
Analysis of the near-infrared (IR) data suggests that Ontario Lacus may be muddy, and a bright margin is suggestive of a 'bathtub ring' of evaporite deposits. Of course, these are not salts familiar as solutes in terrestrial waters, but some organic analog where differential solubility in an evaporating basin has been preferentially deposited at the shrinking margins. In fact, a comparison between an optically measured outline and the margins in a radar image some years later suggested that Ontario may have shrunk in extent, due to seasonal evaporation and the very shallow regional slopes. Ontario is most likely only a few meters deep.
The preponderance of seas in the northern hemisphere is thought to be the result of the astronomical configuration of Titan's seasons in the current epoch. As a result, the northern summer is less intense, but longer in duration, than that in the south. This causes a longer rainy season in the north, such that methane and ethane accumulate there. This seasonal configuration lasts several tens of thousands of years, much like the Croll-Milankovich cycles that play a part in the Earth's ice ages and the Martian polar layered terrain. A drying south and accumulating north is consistent with the ria coastlines of the PLK seas (Punga, Ligeia and Kraken Mare), which suggests valleys flooded by rising sea levels, and a kidney-shaped outline, shallow (and possibly declining) depth of Ontario Lacus in the south.
One striking observation in the near-IR is of the sun glinting off the surface of the lakes. This told us little that wasn't already evident from the low radar reflectivity—that the roughness of the lakes must be exceptionally low—but is a very iconic observation. To continue this article please click here.
Dr. Ralph Lorenz is a planetary scientist at the Johns Hopkins University Applied Physics Laboratory. More information on his work can be found by visiting the website www.lpl.arizona.edu/~rlorenz.