Saturn's largest moon has truly strange geography. Astronomers working on the Cassini mission recently observed the sudden appearance of a bright, mysterious object in one of Titan's northern seas — only to watch it disappear again. Uncertain as to what it is, the researchers offered four explanations.
Last year during a routine flyby, the Cassini spacecraft spotted the feature, which the team members have since dubbed the "magic island." But by the time the probe had returned for a second pass, it was gone.
The feature appeared in Ligeia Mare, the second-largest sea on Titan. Unlike Earth and its seas of liquid water, Titan's seas are comprised of liquid methane and ethane, and are about the size of our planet's Great Lakes. And like Earth, Titan features a substantial atmosphere (comprised of nitrogen-methane) and a seasonal cycle. In fact, because the moon is currently transitioning from spring into summer (which it will do by 2017), the change of seasons might very well have something to do with it; more of the Sun's energy is being channeled in the northern hemisphere, resulting in dynamic weather and previously unobserved geologic conditions.
Despite the name, however, this transient feature is probably not an island. The Cassini team suspects one of four possibilities:
- Floating solids
- Suspended solids
These results now appear in the latest edition of Nature Geoscience. It's considered the first observation of dynamic, geological processes in Titan's northern hemisphere.
"This discovery tells us that the liquids in Titan's northern hemisphere are not simply stagnant and unchanging, but rather that changes do occur," noted Cornell's Jason Hofgartner in a statement. "We don't know precisely what caused this 'magic island' to appear, but we'd like to study it further."
Prior to this observation, this particular region of Ligeia Mare had been completely devoid of any features, including waves. But as astronomers learned earlier this year, Titan does seem capable of producing waves — or at least ripples — on its seas.
In this case, winds in the northern hemisphere may be kicking up and forming waves on Ligeia Mare. Cassini's radar imaging system may have picked up these waves as a kind of "ghost" island. But given just how tiny these waves are, this may be the most unlikeliest of the explanations.
Alternately, Cassini may have captured gas bubbling up to the surface from a subsea volcanic vent. And indeed, the abundant methane in Titan's atmosphere must be continually replenished; the molecule can only exist for a short time before it's destroyed by UV rays.
Or, it could truly be a solid object floating on the surface, namely an iceberg. As spring arrives on Titan, sunken solids formed by the winter freeze could become buoyant. But these objects wouldn't be made of ice-water, which, owing to its density, would sink in a liquid hydrocarbon sea. Rather, icebergs on Titan would be comprised of a frozen mixture of methane and ethane.
Lastly, it's conceivable that Ligeia Mare has suspended solids, which are neither sunken nor floating, but act like silt in a terrestrial delta.
"Likely, several different processes – such as wind, rain and tides – might affect the methane and ethane lakes on Titan," noted Hofgartner. "We want to see the similarities and differences from geological processes that occur here on Earth. Ultimately, it will help us to understand better our own liquid environments here on the Earth."
Excitingly, there's probably more to come as Titan transitions to full-on summer. The moon's seasons change on a longer time scale than Earth's. Its northern hemisphere is transitioning from the vernal equinox (August 2009) to summer solstice (May 2017).
"We suggest that our observations are an initial glimpse of dynamic processes that are commencing in the northern lakes and seas as summer nears in the northern hemisphere," conclude the authors in their study.
Images Credit: NASA/JPL-Caltech/ASI/Cornell.
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