March 19, 2014
Discovery Of Waves On Titan Is A First For Planetary Science
Lawrence LeBlond for redOrbit.com - Your Universe Online
On Saturn’s largest moon Titan it has long been known that the surface has liquid oceans. However, the movements of these liquid hydrocarbon seas have eluded prying eyes for years. Now, the NASA/ESA/ASI Cassini spacecraft may have caught its first glimpse of movement on this faraway moon.
After years of searching, scientists believe they have finally spotted waves rippling on the seas of Titan. If this can be confirmed, it would be the first discovery of ocean waves elsewhere in the solar system.
The glimpses came in 2012 and 2013 while Cassini flew high above one of Titan’s hydrocarbon seas: Punga Mare. Sunlight reflections from the surface are believed to be tiny ripples less than an inch in height that are disturbing an otherwise flat ocean, according to Jason Barnes, a planetary scientist at the University of Idaho in Moscow.
The findings were presented by Barnes at the Lunar and Planetary Science Conference (LPSC) on Monday, where another talk hinted at possible waves in another of Titan’s seas.
Planetary scientists, including Barnes, expect that more waves will appear in the coming years as Titan emerges from winter and approaches spring. During this transition it is expected that winds will pick up in Titan’s northern hemisphere, where most of the moon’s seas are located.
“Titan may be beginning to stir,” Ralph Lorenz, a planetary scientist at the Johns Hopkins University Applied Physics Laboratory (JHUAPL) in Laurel, Maryland, said to Nature News. “Oceanography is no longer just an Earth science.”
While Titan is often considered a “looking-glass” version of Earth, as reported by the BBC, it is a strange place for sure. It has an atmosphere and a seasonal cycle much like Earth, where wind and rain shape the surface, forming “river channels, seas, dunes and shorelines.”
On Earth, water carves through rock and sand, forming mountains and dunes, respectively. On Titan similar mountains and dunes are made of ice, carved by the moon’s liquid hydrocarbons.
During its many flights past Titan, Cassini discovered small lakes and large seas of methane – in particular is one body of liquid known as Ligeia Mare that has been estimated to contain about 2,150 cubic miles of liquid methane, which is about 40 times the reserves of oil and gas known to exist on Earth.
But during many moon passes Cassini never caught any glimpses of wind rippling the surface of Titan’s seas. They always seemed as smooth as glass, according to Barnes. The reasoning behind this is simple: liquid hydrocarbons are more viscous than water and are much harder to get moving. Another reason could be that winds on Titan are just not strong enough to blow ripples across any of the moon’s seas.
But as Titan moves toward spring, as was suggested by Lorenz and others in 2010, the moon’s winds would strengthen, allowing scientists a better shot at viewing waves. Because Saturn and its moons are so far away from the sun, a year on these bodies are equivalent to about 29 Earth years.
EVIDENCE IN THE PIXELS
During the 2012 and 2013 Titan passes, Cassini did in fact catch sunlight glinting off Titan’s surface, suggesting that rippling was occurring in at least the sea of Punga Mare. Cassini’s images are likened to those that might be seen on Earth when an airplane flies low over a lake at dusk.
At the conference, Barnes reported that four pixels in the images are brighter than one might expect from reflecting sunlight. He concluded that they must represent something particularly rough on the surface – likely waves.
"We think we've found the first waves outside the Earth," Barnes told the meeting. "What we're seeing seems to be consistent with waves at just a few locations in Punga Mare [with a slope] of six degrees."
While he believes waves are the most likely cause of the glinting, he said that other possibilities, such as wet mudflat, cannot be ruled out.
But if they are indeed waves, Barnes calculated that a wind speed of about 0.75 m/s was required to produce ripples with a slope of six degrees – or roughly 0.75 inches high.
As previously noted by Lorenz, et al, spring is on the horizon in Titan’s northern hemisphere, a good indication that more rippling may be to come.
"The expectation is that any day now, the winds will start getting strong enough as we move into northern summer, and the waves will start picking up," Lorenz told BBC News. "You can also get a phenomenon known as wind set-up, where wind over a body of water will cause the liquid to pile up, potentially causing a storm surge."
"A metre of storm surge, a metre of tides, is certainly within the realms of possibility for Titan. Whether we can see that [with Cassini] is another matter," he noted.
Lorenz said he was hopeful that sea level rise of a meter (39 inches) in height could cause shorelines to migrate and perhaps be picked up by Cassini.
It could get interesting over the next several years as Titan moves toward summer solstice, which will occur in 2017.
Barnes explained at the conference that knowing how the waves form on Titan will help scientists get a better understanding of the physical conditions in the moon’s lakes and seas. NASA had proposed a mission that would send a probe to float in one of Titan’s lakes, however that mission proposal was bested by a future Mars mission.
If a lander was sent to Titan, however, Barnes asks: “Is it going to splat instead of splash?” If the mudflat theory is true, then the lander would likely not make a splash.
Barnes is hopeful that future flybys by Cassini will spot waves on Punga Mare again, but there is currently no guarantee that Cassini will pass by in the right position before the end of its mission, which is slated to occur as the summer solstice kicks into high gear in 2017. Cassini is planned to plunge into Saturn after the mission comes to a close.
Barnes was not the only presenter at the conference this week to touch on the topic of Titan’s waves.
A second report by Jason Hofgartner, a planetary scientist at Cornell University in Ithaca, NY, hinted that waves were captured in 2013 in Titan’s sea of Ligeia Mare by Cassini.
The team reported that Cassini spotted what appeared to be a “magic island” that appeared and then quickly disappeared. It appeared as a bright reflection in one image but was not visible 16 days later or in any photographs taken since.
The team ruled out a number of possibilities, including a possible island exposed by a change in sea level, and made the case that what they had observed was a set of waves, a group of bubbles rising from below, or a suspended mass, such as an iceberg.
The Ligeia Mare team may also have better luck than Barnes and his colleagues, as Cassini is set to fly past this same area of Ligeia Mare in August, giving the team the chance to hunt for waves once again.
Lorenz also presented his own findings at the conference this week, focusing on a narrow “throat” feature that separates the two main basins of Titan’s largest sea, Kraken Mare.
He dubbed this region as the “Throat of Kraken,” comparing its size to the Straits of Gibraltar and noting that it may generate fast-moving tidal currents through the narrow channel. Similar events on Earth can produce whirlpools and giant roars that can be heard miles away. Whether this phenomena similarly exists on Titan is only speculation at this point, according to Lorenz.
"It's really getting quite exciting, because we're starting to get a literal big picture, in the sense that the radar coverage [of Titan's surface] is close to complete. But because we're moving into northern summer, there's better lighting, which means the camera and the near-infrared spectrometer on Cassini are also able to map the northern seas," Lorenz said.
"Everything is really starting to come together, and the seas and lakes are very much becoming the central topic in Titan science," he concluded.
Titan also had a brief starring role in the new show Cosmos: A Spacetime Odyssey, which was shown Sunday night on Fox, and again Monday night on NatGeo.