A Misty, Moisty Moon-World: So Close, And Yet So Far Away
By Judith E Braffman-Miller
Enshrouded in a thick, secretive orange mist, tormented Titan–Saturn’s largest moon–for centuries kept its mysterious face well-hidden beneath this alien veil composed of obscuring hydrocarbons. An inhabitant of the outer Solar System–far from our Sun–this frigid moon-world finally was forced to reveal its hidden face to the prying eyes of astronomers using the Huygens Probe–that had been carried piggy-back, through interplanetary space, by NASA’s Cassini spacecraft. Cassini’s highly successful mission of exploration to the Saturn system is now over, but astronomers are still pouring over the treasure trove of important data that Cassini/Huygens sent back to Earth. In December 2017, using the now-complete Cassini data set, astronomers published two papers describing newly discovered features on Titan. The papers present a new global topographical map of Saturn’s misty, moisty, mysterious orange enshrouded moon-world, and this map has opened intriguing new windows into the many mysteries of Titan’s surface–its strange hydrocarbon liquid flows, as well as its hydrocarbon-slashed terrain, and bizarre seas of liquid ethane and methane. Although Titan is nearly a billion miles from Earth, the two topographical studies, based on data obtained from the Cassini spacecraft, reveal a new way that this distant moon and our own planet are eerily, hauntingly similar.
Just as the surface of oceans on our own planet are situated at an average elevation that we commonly refer to as “sea level”, Titan’s liquid hydrocarbon seas also lie at an average elevation. This is the most recent discovery showing intriguing likenesses between our Earth and this distant moon-world. Titan is the only other world that astronomers know of in our own Solar System that has stable liquid pooling on its surface. However, the difference is that Titan’s lakes and seas are filled with hydrocarbons instead of the liquid water that fills Earth’s familiar lakes, rivers, seas, and oceans.
Using the Cassini data set, astronomers at Cornell University in Ithaca, New York, were able to create the new topographical map of Titan. Creating the map required about a year, according to Cornell doctoral student Paul Corlies, who is first author on a paper describing the research. This study is published under the title: Titan’s Topography and Shape at the End of the Cassini Mission.
The new map combines all of the Titan topography data collected from many different sources. Because only about 9 percent of Titan has been observed in relatively high-resolution–compared to the 25 to 30 percent of its topography imaged in lower-resolution–the remainder of Titan was mapped, by the Cornell team, using an interpolation algorithm and global minimization process. This method reduced errors such as those resulting from the location of the Cassini spacecraft.
The second paper, published under the title: Topographic Constraints on the Evolution and Connectivity of Titan’s Lacutrine Basins, finds that Titan’s three known seas share a common equipotential surface. This means that they form a sea level, just like that on our own planet. This is either because there is a flow through the subsurface between the three liquid hydrocarbon-filled seas or because the channels between them permit sufficient liquid ethane and methane to flow through. The oceans on Titan are all on the same elevation.
Both papers describing the map, and the discoveries resulting from it, are published in the December 2, 2017 Geographical Review Letters. The new research is based on data obtained by Cassini’s radar instrument until only months before the spacecraft was intentionally crashed into the whirling atmosphere of Saturn in 2017–where it burned up in the magnificent Grand Finale of a highly successful mission.
The Cassini-Huygens mission was a collaborative NASA/European Space Agency/Italian Space Agency robotic spacecraft that was made up of two components: One was the European Space Agency’s (ESA’s) Huygens Probe, that had been named in honor of the Dutch mathematician and astronomer Christiaan Huygens (1629-1695), who discovered Titan. Huygens also studied Saturn’s beautiful system of gossamer rings. 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 numerous–and mostly icy–moons. After a dangerous, long journey through interplanetary space, the Cassini-Huygens spacecraft finally reached Saturn on July 1, 2004. On December 25, 2004, the Huygens Probe was intentionally separated from the Cassini Orbiter. Huygens then began its historic descent down through the dense orange clouds of Titan, to reach, at last, the never-before-seen secretive surface of this mysterious moon-world. Titan’s hidden face was finally unveiled.
Lifting Titan’s Veil
Titan is a little larger than Mercury–the smallest major planet belonging to our Sun’s family–and it would certainly be classified as a planet if it orbited our Star instead of Saturn. The Huygens Probe images revealed a smooth, youthful surface, scarred by relatively few impact craters. The Probe also discovered that this frigid moon’s climate includes downpours of heavy hydrocarbon rain, as well as fierce, powerful winds. Some of Titan’s surface features were immediately recognized by planetary scientists to be hauntingly similar to surface features on Earth. Indeed, the scientists studying the first batch of images derived from Huygens stared at sand dunes, seas, rivers, lakes, and deltas–all strangely resembling features on our own planet. Many planetary scientists now propose that Titan may be similar to the way Earth was, very long ago, before life had emerged and evolved out of non-living substances.
Titan orbits its beautiful gas-giant parent-planet once every 15 days and 22 hours. Like our planet’s own large Moon, as well as a number of other moons circling the quartet of large gaseous planets in our Solar System’s outer limits, Titan’s rotation period is precisely the same as its orbital period. This means that the moon is tidally locked in synchronous rotation with its ringed parent-planet. As a result, Titan always shows only one face to Saturn.
Titan has three large seas. However, these seas are not filled with water, but contain liquid hydrocarbons. All three seas are located close to Titan’s north pole, and they are all surrounded by a large number of smaller hydrocarbon lakes in the northern hemisphere. In sharp contrast, there is only one solitary lake situated in Titan’s southern hemisphere.
The exact composition of these hydrocarbon lakes and seas was not known until 2014, when Cassini’s radar instrument revealed that Ligeia Mare–the second largest of Titan’s seas–is loaded with methane. Ligeia Mare is about the same size as two of Earth’s Great Lakes combined–Lake Michigan and Lake Huron. The seabed of Ligeia Mare is believed to be coated with a layer of sludge composed of organic-rich compounds.
The nitrogen and methane that swirl around together in Titan’s atmosphere react with one another to create a variety of organic compounds. Many planetary scientists propose that the heaviest materials sink down to the surface of this tormented moon-world. When these compounds enter the hydrocarbon seas–whether by directly somersaulting down from the air as heavy, large, and lazy drops of hydrocarbon rain, or by traveling along with Titan’s rivers–some are dissolved in the liquid methane. The compounds that do not dissolve, such as nitrites and benzene, float down to the floors of these alien seas.
Both our own planet and Titan sport atmospheres that are dominated by nitrogen–more than 95% nitrogen, in Titan’s case. However, unlike our own planet’s atmosphere, Titan’s atmosphere contains very little oxygen. Indeed, the remainder of Titan’s atmosphere is primarily composed of methane, along with scanty amounts of other gases, such as ethane. At the frigid temperatures that are present at Saturn’s great distance from the golden light and warmth of our Sun, Titan’s methane and ethane can pool on the surface as liquids.
For this very reason, astronomers, for years, considered the possibility that hydrocarbon lakes and seas might exist on the surface of this mysterious, veiled moon-world. The data obtained from Cassini/Huygens validated their expectations. Ever since it arrived at the Saturn-system, the Cassini spacecraft managed to unveil more than 620,000 square miles of Titan’s well-hidden surface. During its mission, Cassini showed that almost 2% of Titan’s entire surface is covered with liquid.
Titan’s Topography and Shape at the End of the Cassini Mission
The new map of Titan reveals several features on Titan that had not been seen before. The recently discovered features include mountains that are no higher than 700 meters. The map also provides a global view of the highs and lows of Titan’s topography, which helped the astronomers to confirm that two locations in Titan’s equatorial region are actually depressions, that could be either dried ancient seas, or ice volcanoes (cryovolcanoes).
The map also showed that Titan is a little bit more oblate (flatter) than was previously believed. This indicates that there is more variability in the thickness of Titan’s crust than planetary scientists originally proposed.
“The main point of the work was to create a map for use by the scientific community,” commented Paul Corlies in a January 3, 2018 Cornell University Press Release. Within only 30 minutes of the data being made available online, Corlies began to receive inquiries on how to use it. The data set is downloadable in the form of the data that was observed, as well as interpolated data that was not observed. The map will be valuable to those scientists modeling Titan’s climate, studying Titan’s shape and gravity, and testing interior models, as well as for those seeking to understand morphologic land forms on this frigid, distant moon-world.
Other Cornell authors on the paper are senior author Dr. Alex Hayes, assistant professor of astronomy, doctoral candidate Samuel Birch, and research associate Dr. Valerio Poggiali.
Topographic Constraints on the Evolution and Connectivity of Titan’s Lacustrine Basins
“We’re measuring the elevation of a liquid surface on another body 10 astronomical units away from the Sun to an accuracy of roughly 40 centimeters. Because we have such amazing accuracy we were able to see that between the two seas the elevation varied smoothly about 11 meters, relative to the center of mass of Titan, consistent with the expected change in the gravitational potential. We are measuring Titan’s geoid. This is the shape that the surface would take under the influence of gravity and rotation alone, which is the same shape that dominates Earth’s oceans,” Dr. Alex Hayes explained in the January 3, 2018 Cornell University Press Release. One astronomical unit (AU) is equal to the average distance between the Earth and Sun, which is about 93,000,000 miles.
Other authors on this second paper are Paul Corlies, Samuel Birch, Dr. Valerio Poggiali, research associate Dr. Marco Mastrogiuseppe and Dr. Roger Michaelides.
The second result, described in this paper, proves a hypothesis that Dr. Hayes proposed in his first paper, when he was still in graduate school: that Titan’s lakes connect with each other beneath its alien surface. Dr. Hayes and his team measured the elevation of lakes brimming with liquid as well as those that are ancient and, by this time, dry. The planetary scientists found that lakes exist hundreds of meters above sea level, and that within a watershed, the floors of the dry lake beds are all at higher elevations than the lakes that are still filled with liquid that are located nearby.
“We don’t see any empty lakes that are below the local filled lakes because, if they did go below that level, they would be filled themselves. This suggests that there’s flow in the subsurface and that they are communicating with each other. It’s also telling us that there is liquid hydrocarbon stored on the subsurface of Titan,” Dr. Hayes continued to explain in the Cornell University Press Release.
However, this paper’s final conclusion has created another mystery. The researchers found that most of Titan’s lakes are atop sharp-edged depressions that “literally look like you took a cookie cutter and cut out holes in Titan’s surface,” Dr. Hayes added. The hydrocarbon filled lakes are encircled by high ridges that are hundreds of meters high in some areas.
The lakes appear to have been formed the same way karst is on our own planet. On Earth, in places like the Florida Everglades, underlying material dissolves, causing the surface to collapse. Similar holes, to those seen on Earth, apparently form in the ground on Titan. The lakes of Titan, in a way that is akin to Earth’s karst, are topographically shut, with no inflow or outflow channels. However, Earth’s karst does not sport sharp, raised rims.
The shape of Titan’s liquid hydrocarbon lakes suggests a process termed uniform scarp retreat, where the edges of the lakes are expanding by a constant amount every time. The largest lake located in the south, for example, appears to be a series of smaller dry lakes that have coalesced or conglomerated into one large feature.
Dr. Hayes commented in the January 3, 2018 Cornell University Press Release: “But if these things do grow outward, does that mean you’re destroying and recreating the rims all the time and that the rims are moving outward with it? Understanding these things is in my opinion the lynchpin to understanding the evolution of the polar basins on Titan.”
Judith E. Braffman-Miller is a writer and astronomer whose articles have been published since 1981 in various magazines, newspapers, and journals. Although she has written on a variety of topics, she particularly loves writing about astronomy because it gives her the opportunity to communicate to others the many wonders of her field. Her first book, “Wisps, Ashes, and Smoke,” will be published soon.
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