Might still waters run deep?

By | Science & Technology
Karst landforms have given rise to some of Earth's most picturesque landscapes, such as the Lijiang River in Guilin, China. Credit@Chensiyuan

Saturn’s moon Titan beholds vast seas and lakes brimming with liquid hydrocarbons, however what causes this has proven a secret. Using data from the joint NASA and European Space Agency (ESA) Cassini mission, a new study from NASA finds that the same process that creates caves and sinkholes on Earth may also be responsible for these ‘lakes’.

Titan is the only body in the Solar System besides Earth known to have liquid on its surface, observed by the Cassini spacecraft. There are other places in the Solar System that house vast volumes of water and a potential for life, like Jupiter’s moon Europa. However in those cases the water hides beneath an icy surface.

Due to Titan’s chilly surface temperatures of approximately -180 degrees Celsius, the lakes are predominantly filled by liquid methane and ethane rather than water. Cassini has discovered two types of lakes on Titan’s surface. The first describes great seas towards the poles spanning hundreds of kilometres across and up to several hundred metres deep, connected to rivers. The second, a number of smaller, shallower lakes in flatter areas of Titan as well as some empty basins.

A joint study by the European Space Agency and NASA discovered lakes on the surface of Saturn's moon Titan caused by rainfall. Credit@NASA/JPL-Caltech

A joint study by the European Space Agency and NASA discovered lakes on the surface of Saturn’s moon Titan caused by rainfall. Credit@NASA/JPL-Caltech

While the seas are believed to be replenished by connected rivers, rainfall and liquids from underground feed the smaller lakes. Some lakes appear to run dry and then refill over the thirty-year seasonal cycle shared between Saturn and Titan. Though, how the indentations in Titan’s surface occurred in the first place was still a pressing question.

Mysteries of the universe may often be explained by examining Earth for answers. In this case, scientists observed that Titan’s seas are similar to karst landforms found on Earth – landscapes shaped when soluble rocks such as limestone and dolomite dissolve with rainfall or groundwater to eventually create sinkholes or caves as are found along the Puerto Princesa Underground River in the Philippines (pictured below).

While Titan and Earth experience significantly different climate conditions, researchers at NASA believe the underlying process to be the same. A team lead by Thomas Cornet of the European Space Agency figured out that it may take roughly 50 million years to create a 100 metre-deep imprint in Titan’s rainier polar regions. This timing is consistent with the relatively young age of Titan’s surface.

“We compared the erosion rates of organics in liquid hydrocarbons on Titan with those of carbonate and evaporite minerals in liquid water on Earth,” explained Cornet. “We found that the dissolution process occurs on Titan some 30 times slower than on Earth due to the longer length of Titan’s year and the fact it only rains during Titan summer. Nonetheless, we believe that dissolution is a major cause of landscape evolution on Titan and [may] be the origin of its lakes.”

The Puerto Princesa Underground River, Philippines. Credit@Mike Gonzalez

The Puerto Princesa Underground River, Philippines. Credit@Mike Gonzalez

At latitudes approaching the equator the scientists calculated that 375 millions years may be necessary in order to form lakes, which makes sense given the absence of lakes around the equator. “Of course, there are a few uncertainties: The composition of Titan’s surface is [unconstrained], and neither are the long-term precipitation patterns, however our calculations are still consistent with the features we see today on Titan’s relatively youthful billion-year-old surface,” said Cornet.

Nicolas Altobelli, ESA’s Cassini project scientist, calls the study an “intriguing comparative glimpse” between Earth and a dynamic world floating through space more than a billion kilometres away. Scientists have even theorised the design of a cell membrane that might be capable of supporting methane-based life on Titan. This pushes the search for extraterrestrial life beyond the Sun’s minimal hospitable zone where cells might be based on methane, which has a colder freezing point than water. A scientist who worked on the study calls it, “the first concrete blueprint of life [unlike] we know it.”

What may be learned from Earth about the conditions necessary for life in the universe?

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