With its enticingly secretive surface blanketed by a dense golden-orange fog, Titan–the largest moon of Saturn–was long regarded as a mysterious, frigid moon-world. However, the Cassini-Huygens mission changed all that when the Huygens lander floated down to the foggy moon's surface in 2004, and gazed at Titan's well-hidden face behind its strange orange mask. Although the Cassini-Huygens mission ended in 2017, planetary scientists are still pouring over the treasure trove of information that it sent back to Earth before it was intentionally destroyed by mission scientists. In October 2019, a team of scientists led by a University of Hawaii (Manoa) chemistry professor and researcher, announced that they have been able to provide answers to important questions about the strange surface of Titan. The researchers say that they have unraveled the origin and chemical composition of Titan's alien dunes.
Physical chemist, Dr. Ralf I. Kaiser, and his colleagues, examined remote sensing data provided by Cassini-Huygens regarding Titan. Titan is the only Solar System body, other than Earth, that sports a solid surface, lakes and a substantial atmosphere with a pressure of approximately 1.5 atmospheres at surface level. Images and data provided by Cassini-Huygens revealed the existence of vast longitudinal dunes on that foggy orange moon-world. The dunes are located across Titan's equatorial deserts, and they reach lofty heights of up to 100 meters–making them similar in size to the Egyptian pyramids of Giza. However, while Earth's dunes are composed primarily of silicates, imaging studies revealed that Titan's dunes are made up of a different material. Titan's dunes were shown to contain dark organics that, until this new study, were of undetermined origin and chemical composition.
The University of Hawaii at Manoa (UH) team, which also included Dr. Matthew Abplanalp, then a graduate student at UH, discovered the existence of acetylene ice in Titan's dunes. Acetylene is a chemical used on Earth in welding torches. It exists in Titan's equatorial regions, but at low temperatures to proxies of high-energy Galactic cosmic rays. The scientists found a rapid cosmic-ray-driven chemistry which causes simple molecules like acetylene to experience a sea-change into more complex organic molecules like benzene and naphthalene. Naphthalene is a compound which is found in familiar mothballs on Earth, and it exists on Titan's exotic surface. These processes also occur in the space between stars (interstellar medium) on hydrocarbon rich layers of interstellar nanoparticles.
This new research has been published in the journal Science Advances under the title Low-temperature synthesis of polycyclic aromatic hydrocarbons in Titan's surface ices and on airless bodies.
Polycyclic aromatic hydrocarbons (PAHs) are organic compounds that contain only carbon and hydrogen, and are composed of multiple aromatic rings.
The Strange Face Behind The Smoggy Orange Mask Of A Distant Moon
Titan is the largest moon of the ringed gas-giant planet, Saturn, as well as the second-largest moon in our Solar System. Only Ganymede of Jupiter is larger than Titan. Here, in the frigid realm of the quartet of majestic, giant, and gaseous outer planets–Jupiter, Saturn, Uranus, and Neptune–our Sun shines with a weaker fire than it does in the inner Solar System where Earth is situated–along with the three other solid and relatively small planets: Mercury, Venus, and Mars.
Titan is a hydrocarbon-tormented world, that bears an eerie resemblance to the way our own planet was, very long ago, before life emerged and evolved here (prebiotic). Like Earth, Titan experiences frequent downpours of rain, swirling seas, and eroding organic material. However, on Titan, it is not liquid water that fills its alien seas, rivers, and lakes with lovely little raindrops. On this strange smoggy moon-world, life-sustaining liquid water is replaced by weird, lazy, large drops of methane. The surface of Titan is pelted with a “rain of terror”–on Titan, it rains gasoline. On this strange moon, atmospheric molecules composed of methane are perpetually being torn apart by sunlight, and the resulting atmospheric smog floats down to the surface, where it accumulates as organic settlements that rapidly rob the atmosphere of methane. The surface of Titan is well-coated with the material of old atmospheres that have long since vanished.
There is no obvious source of Titan's methane, with the exception of the evaporation of methane originating from polar hydrocarbon-filled lakes. The problem is that Titan's strange lakes contain only approximately one-third of the methane in Titan's atmosphere. This methane will be depleted soon on geological time scales.
Titan itself is the sixth gravitationally rounded moon from Saturn, and this large moon-world is often described as being “planet-like”. Titan is twice as large as Earth's own comparatively large Moon, as well as being 80% more massive. Indeed, Titan is larger than the planet Mercury–the innermost major planet from our Star. However, Titan is only about 40% as massive as Mercury.
Titan was discovered by the Dutch astronomer Christiaan Huygens on March 25, 1655, and it was the first moon of Saturn to be observed. It is the sixth known natural planetary satellite, after Earth's Moon and the quartet of Galilean moons of Jupiter (Io, Europa, Ganymede, and Callisto). The four Galilean moons were discovered by the great Italian astronomer Galileo Galilei in 1610. Titan circles its ringed parent-planet at 20 Saturn radii. From Titan's weird hydrocarbon-slashed surface, Saturn subtends an arc of 5.09 degrees, and would loom 11.4 times larger in its dense golden-orange sky than the Moon from Earth.
Titan is primarily made up of ice and rocky material, which is thought to be differentiated into a rocky core encircled by sundry layers of ice, including a crust composed of ice Ih, and a subsurface layer of ammonia-rich liquid water. Before the era of space exploration, the heavy and opaque atmosphere of Titan made it impossible of planetary scientists to study its mysterious surface–until the Cassin-Huygens mission. This joint NASA/European Space Agency/Italian Space Agency mission to the Saturn system, finally revealed the strange moon's face that had been well-hidden behind its smoggy orange mask since its discovery in 1655. Cassini-Huygens provided precious new information, including the discovery of the liquid hydrocarbon lakes pooling in Titan's polar regions. The geologically youthful surface is mostly smooth, sporting relatively few impact craters. Smooth surfaces are young, while heavily cratered surfaces are more ancient. This is because young surfaces, like Titan's, have been resurfaced. However, mountains, as well as several potential cryolcanoes (ice volcanoes), have been observed on this strange and distant moon-world.
Titan's atmosphere is largely composed of nitrogen; smaller components result in the formation of thick hydrocarbon clouds composed of methane and ethane, all of which cook of up a witch's cauldron of dense organic smog that blankets this tortured moon-world. Titan's climate, including its wind and rain, carves surface features that appear eerily similar to those on our own planet–such as its dunes, lakes, rivers, and seas (likely brimming with sloshing liquid methane and ethane). Titan also sports deltas, and is subject to seasonal weather patterns that are similar to those on Earth. With its liquids, both surface and subsurface, and its plentiful nitrogen atmosphere, Titan's exotic methane cycle is analogous to Earth's water cycle–but at much more frigid temperatures of about -290.5 degrees F.
Mysterious Dunes Of A Misty, Moisty Moon
“Titan's dunes represent the dominating surface sink of carbon in Titan's organic chemistry. Therefore, unraveling the origin and chemical pathways to form the organic dune material is vital not only to understand Titan's chemical evolution, but also to grasp how alike the chemistries on Titan and on Earth might have been before life emerged on Earth 3.5 billion years ago,” Dr. Abplanalp noted in the October 17, 2019 University of Hawaii Press Release. Dr. Abplanalp is now a researcher at the Naval Air Warfare Center Weapons Division at China Lake.
“These processes eventually furnish the molecular building blocks not only for Titan's organic dunes, but also for organics on airless bodies in general such as on Kuiper Belt Objects like dwarf planet Makemake. The low temperature synthesis of PAHs from acetylene ices represents a fundamental shift from currently accepted perceptions that PAH formation takes place solely in the gas phase at elevated temperatures of a few 1,000 K, such as in combustion processes,” he added.
The Kuiper Belt is a distant region of our Solar System situated beyond the orbit of the outermost planet Neptune. It is believed to contain, not only dwarf planets, but also myriad frozen comet nuclei, asteroids, and an assortment of other small bodies made primarily of ice that exist in our Solar System's deep freeze.
These new findings have unprecedented implications for future space missions to Titan. NASA is currently planning to land a flying robot, named Dragonfly, on the surface of Titan–the favored target in the hunt for life on worlds beyond Earth, as well as for life's molecular precursors. Dragonfly–which is about the same size as a car–is a quadcopter that is equipped with instruments that are able to identify large organic molecules. It is scheduled to launch aboard a rocket in 2026, arrive at its target in 2034, and then fly to numerous locations hundreds of miles apart. Dragonfly will land close to Titan's equator near the organic dunes, thus providing an in situ peek at potentially biorelevant organics existing in a state of deep freeze. In this way, the quadcopter will zip around in order to observe the unknown.
“Overall, this study advances our understanding of the complex organics and fundamental chemical processing of simple molecules in deep space and provides a scientifically sound and proven mechanism of formation of aromatic structures in extreme environments in low temperature ices. Since Titan is nitrogen-rich, the incorporation of nitrogen in these PAHs may also lead to carbon-nitrogen moieties (parts of a molecule) prevailing in contemporary biochemistry such as DNA and RNA-based nitrogen bases,” Dr. Ralf Kaiser explained in the October 17, 2019 University of Hawaii Press Release.