I Must Go Down to the Seas Again…
When the poet, John Masefield, wrote this in 1902, the world had no idea that the seas in question might not be on earth. We now know that the moons of Jupiter and Saturn possess vast amounts of water. Data gathered from the Cassini probe, which explored the Saturn system, reveals the presence of large underground oceans in its three major moons: Titan, Enceladus, and Mimas.
A recent study of Titan, slightly larger than Mercury, estimates that the underground ocean may be 12 times greater than all the oceans of the earth. Not only does it possess an underground ocean, but it also has a hydrocarbon-based ocean and atmosphere on its surface. See the NASA video showing the 2005 soft landing of the Huygens lander on Titan’s surface.
Even though the Cassini mission ended in 2017, analysis of Titan continues to provide us with a portrait of a moon whose surface seems surprisingly familiar.
These images from the Radar instrument aboard NASA’s Cassini spacecraft show the evolution of a transient feature in the large hydrocarbon sea named Ligeia Mare on Saturn’s moon Titan. Credit: NASA/JPL-Caltech
Titan and Earth are the only worlds in the solar system which have liquid surfaces in the form of oceans and lakes. But the liquid on Titan is not water. It is methane mixed with nitrogen. In the frigid atmosphere of the surface, methane gas liquefies and pools. In addition, the methane evaporates and forms clouds leading to rain. But strangely enough, the UV rays of sunlight reaching the surface set off a chemical reaction that turns methane into ethane. When the methane/ethane drops fall to the surface, the methane liquid evaporates to form clouds again, but the ethane does not. There is no ethane cycle on Titan.
Radar images from NASA’s Cassini spacecraft reveal many lakes on Titan’s surface, some filled with liquid, and some appearing as empty depressions. Credit: NASA/JPL-Caltech/ASI/USGS
Recent reports have provided a newer picture of the surface. Seas are mostly concentrated towards the poles. There are some smaller ones in the deserts.
Interestingly, the dynamism around the methane seas produces fractal style land profiles similar to that of earth. Most of the other surface of Titan is desert with frozen water acting as rock. Recent observations by the giant radio dish telescope in Canberra Australia also found that there are ripples on the surface of Titan’s lakes. The height of these ripples varies by latitude. At the poles, the wave height is close to three millimeters, while in the desert they are closer to one millimeter (good surfing for ants).
Scientists also observed that Titan has seasons, just like Earth. Its orbit around the Sun is 29 years (like Saturn) and, like Earth, it has a tilt of about 23 degrees relative to its orbital plane around the Sun. Thus, it has four seasons, each lasting about 7.5 years. This implies an atmospheric dynamism just like earth. How it came to be this way is a story yet to unfold.
Saturn’s other moon, Enceladus, also contains vast amounts of water. Using doppler technology to study the gravitational wobbles of Enceladus, scientists conclude that the underground ocean, which lies approximately 19 miles under the southern pole, is about 6 miles deep. Scientists believe that this particular ocean may be more thermally active and more likely to support life.
Dramatic plumes, both large and small, spray water ice out from many locations along the famed “tiger stripes” near the south pole of Saturn’s moon Enceladus. Credit: NASA/JPL/SSI
The Cassini probe detected the venting of large amounts of water, as a result of subsurface heating. Embedded within these escaping water vapors, moving at an astonishing speed of 1 kilometer a second, are a host of organic molecules: primary constituents of life.
Finally Mimas, whose enormous Star Wars “Death Star” type crater also possesses large quantities of subsurface water (more than half of its mass).
Mimas Credit: NASA/JPL-Caltech/Space Science Institute
From observing the precession of its elliptical orbit, scientists were able to infer that the ocean on Mimas is unusually large for such a small planet, as demonstrated in this animation. The ocean lies 20 to 30 kilometers under its icy crust. It is thought to be relatively young (25 million years) compared to the oceans of other moons. That is because the smooth surface of Mimas does not exhibit the kind of fracturing that, on other older moons, results from underground dynamism. All these observations lead one to conclude that water has a ubiquitous presence on the moons of the Solar System. It may be that moons, not planets, are the more likely candidates to support extraterrestrial life.
