Cloaked mysteriously in a thick shroud of golden hydrocarbon fog, Titan, Saturn’s largest moon, is a mesmerizing world. For centuries, this captivating moon sang a magical sirens’ song to the curious among us who wanted to peer beneath Titan’s veil to view its frigid surface, hidden beneath its alien clouds. However, this moist hydrocarbon-slashed world was finally forced to reveal its mysterious face, well-hidden beneath its obscuring veil of mist, when the Cassini Spacecraft’s Huygens Probe landed on its strange surface back in 2004, promptly dispatching back to waiting astronomers on Earth some tantalizing and revealing images. Although Cassini’s mission is over, scientists are still pouring over the cornucopia of revealing pictures of the distant Saturn-system that it sent back to Earth. In April 2019, while hunting for the origins of Titan’s methane and organics that coat its surface, University of Arizona (Tucson) planetary scientists made the surprising discovery of a mysterious large ice feature on Saturn’s largest moon.
Downpours of rain, sloshing seas, and a surface composed of eroding organic material can be seen both on our own planet and on Titan. However, on Titan, it is not water that fills its seas, rivers and lakes with raindrops. On this distant moon, water is replaced by exotic, lazy, large and heavy drops of methane rain. On Titan, atmospheric molecules of methane are constantly being ripped apart by sunlight, and the resulting atmospheric fog settles down to the surface and accumulates in the form of organic sediments that quickly rob the atmosphere of methane. This organic coating is composed of the material of long-gone atmospheres.
There is no obvious source of methane on Titan, except from the evaporation of methane originating from polar lakes. The problem is that Titan’s hydrocarbon lakes hold only about one-third of the methane in Titan’s atmosphere. This methane will be used up soon on geological time scales. While trying to discover the mysterious origin of Titan’s methane, University of Arizona planetary scientist Dr. Caitlin Griffith and her team experienced an episode of “scientific serendipity” and discovered something else instead–the unexpected long ice feature that snakes nearly half way around Titan. Dr. Griffith, who is of the University of Arizona’s Lunar and Planetary Laboratory, is the lead author on a paper published in the April 29, 2019 issue of the journal Nature Astronomy.
One theory proposes that the surface methane could be replenished by subsurface reservoirs that shoot methane into Titan’s golden-orange atmosphere. Indeed, earlier studies of Titan do suggest the existence of a region named Sotra, which looks like an ice volcano (cryovolcano), with slushy ice flow features.
However, the large ice feature that the scientists discovered was a complete surprise. It is composed of a linear ice corridor that coils around 40 percent of Titan’s circumference.
“The icy corridor is puzzling, because it doesn’t correlate with any surface features nor measurements of the subsurface. Given that our study and past work indicate that Titan is currently not volcanically active, the trace of the corridor is likely a vestige of the past. We detect this feature on steep slopes, but not on all slopes. This suggests that the icy corridor is currently eroding, potentially unveiling the presence of ice and organic strata,” Dr. Griffith explained in an April 29, 2019 University of Arizona Press Release.
Dr. Griffith and her team’s analysis also suggests that there is a diversity of organic material in certain areas. These surface deposits are important because laboratory simulations of Titan’s alien atmosphere indicate that some biologically interesting compounds, such as amino acids, may have formed.
A Beautiful Ringed Planet And It’s Mysterious Moon
With its sparkling frozen retinue of icy moons and moonlets, its gossamer system of rings, and its circling tidbits of glittering ice, Saturn is arguably the loveliest planet in our Sun’s family. As the second-largest planet in our Solar System (after the banded behemoth Jupiter), Saturn orbits our Sun about ten times farther than Earth. However, as enormous as Saturn is, it is so light that it could easily float on water–that is, if a swimming pool exists that is large enough to contain it.
Saturn is a denizen of the cold outer region of our Solar System, along with its giant, majestic, and gaseous planetary siblings: Jupiter, Uranus and Neptune. As the sixth major planet from our Star, Saturn is famous for its large population of attendant moons that, for the most part, are small icy objects. However, Titan, is certainly not small. As the second-largest moon in our Solar System–after Ganymede of Jupiter–Titan is a hydrocarbon tormented, smoggy moon that is a little larger than Mercury, the innermost major planet from our Sun.
Ever since NASA’s Voyager 1 spacecraft soared over Titan back in 1980, astronomers have wanted to peer through its blanket of hydrocarbon fog in order to explore this planet-size-moon’s hidden surface. The now successfully completed Cassini/Huygens voyage to the Saturn-system was a collaborative NASA/European Space Agency/Italian Space Agency mission. The robotic spacecraft was made up of two components. The first was the European Space Agency’s (ESA’s) Huygens Probe, that was named after the Dutch astronomer and mathematician Christiaan Huygens (1629-1695), who discovered Titan. The Huygens Probe also peered at Saturn’s beautiful system of rings and ringlets. The second component, the NASA-designed Cassini Orbiter, was named after the Italian-French astronomer Giovanni Dominico Cassini (1625-1712), who discovered a quartet of Saturn’s other moons.
Following a very dangerous and long voyage through interplanetary space, the Cassini/Huygens Spacecraft arrived at Saturn on July 1, 2004. On December 25, 2004, the Huygens Probe was deliberately severed from the Cassini Orbiter– that had been carrying it piggyback through space–and started its historic dive down through the dense golden clouds to see Titan’s hidden face.
After spending twenty years in operation, Cassini successfully finished its remarkable voyage of exploration and discovery. Cassini’s end came on September 15, 2017, when it approached Saturn for the last time, and was deliberately sent crashing down into the gas-giant’s banded atmosphere by mission scientists. Cassini burned up and disintegrated “like a meteor”, becoming part of the distant planet that it had been observing for so long.
Cassini/Huygens revealed that Titan is a strange world–a fascinating and frigid oddball, with an eerie and haunting resemblance to the primordial Earth before life emerged and evolved on our own planet (prebiotic). Indeed, Titan has the distinction of being the only moon in our Solar System that sports a significant atmosphere. It is also the only known world–other than our own–where stable bodies of liquid pool on its surface.
But there is one significant difference. On Earth, lakes and seas are filled with swirling, sloshing water, while Titan’s lakes and seas contain mostly methane and ethane, that flow within these very exotic reservoirs. In this unearthly cycle, the hydrocarbon molecules evaporate and condense into atmospheric clouds that then pour down a “rain of terror” onto this bizarre moon’s hydrocarbon-tormented surface.
Like Earth, Titan’s atmosphere is mostly composed of nitrogen–with a smaller quantity of methane added to the mix. It is the only other world in our Solar System known to possess a cycle of liquids that shower back down to the surface again–this time in the form of downpours of gasoline. However, even though the ingredients of this exotic witch’s broth are different from those on Earth, this cycle on Titan is similar to our own planet’s water cycle.
An Icy Feature Snakes Around Titan
Dr. Griffith analyzed tens of thousands of spectral images that Cassini’s Visible and Infrared Mapping Spectrometer had obtained of the uppermost layer of Titan’s surface, using a method that provided for the detection of weak surface features.
This was accomplished by using Dr. Griffith’s application of the principal components analysis (PCA). Using the PCA application, Dr. Griffith and her team were able to distinguish subtle features, resulting from the ice and organic sediments on Titan’s surface, from the ubiquitous fog and the more obvious surface features. Rather than measuring the surface features separately for each pixel in an image, the PCA uses all of the pixels to pick out the main and more subtle features.
Dr. Griffith’s team compared their results with earlier studies, including the Huygens Probe. The comparison validated both the technique and the results. Currently, there are plans to use the technique to explore Titan’s poles where strange methane seas are located.
“Both Titan and Earth followed different evolutionaary paths, and both ended up with unique organic-rich atmospheres and surfaces. But it is not clear whether Titan and Earth are common blueprints of the organic-rich of bodies or two among many possible organic-rich worlds,” Dr. Griffith explained in the April 29, 2019 University of Arizona Press Release.