On the Shores of Titan's Farthest Sea

by Michael Carroll

  Titan’s rain precipitates out of methane clouds at an altitude of 12.5 miles (20 km). Above that cloud deck, thinning haze underlies a high altitude layer of methane ice crystals. The methane precipitation may increase closer to the poles, where the lakes have been found.

  Titan’s rains may come in intense, seasonal waves. Many river channels have been charted across Titan’s surface, and these typically have morphologies indicating intense floods that produced them. Planetary meteorologists reason that a steady drizzle would not do it. Titan's methane storms may be most like storms over terrestrial deserts that wet the ground and even carve arroyos but don't result in any significant buildup of bodies of liquid. Despite active methane cloud systems and carved floodplains, it appears that methane rain falls far less often on Titan than water-rain falls on Earth. Titan is very much a desert planet, where rainfall is extremely rare compared to Earth. Where there is precipitation, it is torrential. The question is, is that torrential rainfall seasonal, or is it a year-round phenomenon? Although the methane humidity at the equatorial Huygens site was 45 %, which would be enough to trigger rainstorms on Earth, the distant Sun’s heat at Titan is just too weak to drive moist air up and generate storms under current conditions. Large cloud systems would require higher humidity. The Cassini spacecraft has spied storm clouds forming over the equatorial regions, but the events have been rare. Typically, weather systems blossom rapidly to the southeast. As the storm clouds drift away, they sometimes leave in their wake a darkening of the landscape, implying a changed surface from rainfall.

  Researchers estimate that the annual rainfall on Titan amounts to about 2 in. (about 5 cm). This is the equivalent to annual precipitation in Death Valley. But another kind of precipitation falls from Titan’s skies: hydrocarbons. Titan’s upper atmosphere reacts with solar radiation to generate organic soot. This fine powder drifts down to the surface, building up enough to fill valleys and bank into drifts.

  Titan’s globe is split across its midsection by a belt of dunes. The equatorial regions seem to be desert, nearly devoid of the methane rains that fall in other areas. Here, dunes act as weather vanes, pointing along the direction of the global winds that blow from west to east. These dunes often interact with dry river valleys, canyons and ancient eroded impact craters. To the north and south, methane humidity increases until lakes form. Many lakes and seas have been mapped in the north, but the southern provinces have far fewer. The largest lake in the south is Ontario Lacus, covering an area equivalent to that of North America’s Lake Ontario. Near the north pole, Titan’s largest methane sea, Kraken Mare, is the size of the Eurasian Caspian Sea.

  Strange Seas on the Surface and Beneath

  Since the late 1960s, scientists theorized that Titan’s surface conditions might allow for seas or oceans of liquid methane. After the Voyager flybys in the early 1980s, the idea of a global ocean was in vogue. But remote sensing by Earth-based observatories, and data from the Hubble Space Telescope, along with the first Cassini orbiter data, suggested that Titan was a dry desert world. The hoped-for Titan seas were nowhere to be seen until 2004. In that year, the Image Science Subsystem aboard the Cassini spacecraft observed the first dark features with coastline-like shapes, although it was not immediately clear whether the dark regions were filled with liquid or simply damp. Images showed a dark expanse near the south pole, now known as Lacus Ontario. The lake was imaged in greater detail by the RADAR instrument, finally demonstrating that some dark areas were, in fact, covered by liquid. Later, Cassini’s radar eyes spotted hundreds of small lakes, mostly contained within hollows, in Titan’s northern polar regions. Although dark, flat features had been seen in other regions, these were different. Their morphology (physical shape) was similar to terrestrial lakes, as was their relationship to channels, deltas, and other river-like features. They had very low radar backscatter, implying that their surfaces were very smooth. The polar regions in which they were found had high levels of methane humidity, consistent with computer predictions using atmospheric and climate models. Finally, the radiometric “brightness”—or how the radar reflects energy off the surface—of the supposed lakes was consistent with the high emissivity expected for a smooth surface of liquids such as methane, ethane, butane and other related substances. This high emissivity showed that the lake areas were warmer than their surroundings.

  Titan is now confirmed as the only world besides our own with an active cycle involving rain fed by evaporation from surface lakes and rivers. Its river valleys drain into liquid-filled basins, some as large as the Black Sea. Because of their size, the largest are referred to as “maria,” Latin for “seas.” To date only three have been classified as such. From largest to smallest, they are Kraken Mare, Ligeia Mare, and Punga Mare. Titan’s lakes and seas vary greatly in size, from the limits of Cassini’s resolution up to about 400,000 km2 for Kraken Mare. For comparison, North America’s Lake Superior is 82,000 km2 in extent, and Europe/Asia’s Black Sea is 436,400 km2.

  Rugged coastlines resembling the fjords of Scandanavia or the flooded valleys of Lake Powell scar the edges of Titan’s seas. A few of the largest lakes also have some rough features, but the smaller ones are of a very different character. They seem to have mostly circular, oblong or curving shorelines, and their margins are often quite steep. Because of their cliff-like borders, some researchers suggest that the lakes are the result of collapse or melting, much like the rounded lakes caused by melting ice blocks left behind by retreating glaciers on Earth. Terrain dissolved by water is called karstic. On Earth, similar regions are often fractured and porous, with groundwater flowing beneath their surfaces. It may well be that on Titan, these lake regions drain into a web of underground methane aquifers that make their way to the coasts, eventually feeding the seas. If so, this underground river network may be a major contributor to Titan’s atmospheric methane (Fig. 61.3).

  Fig. 61.3 Left: Mayda Insula is a rugged island within the great methane sea Kraken Mare. It is roughly the size of the island of Hawaii (north is to the right). Right: Ontario Lacus spreads across 235 km in the southern hemisphere (Images courtesy NASA/JPL/Space Science Institute)

  The makeup of the methane brew through which Troy navigated his little blue submersible is somewhat mysterious. The exact blend of hydrocarbons in Titan’s lakes is unknown, but it is probably a mixture of ethane and methane. Although the methane rainfall is thought to be at least a 100 times greater than that of ethane—which may make up part of the precipitation—methane is far more volatile than ethane. Any surface methane would evaporate more quickly. Over time, a standing body of liquid would become enriched with the more stable ethane, in which case ethane is probably dominant. Ligeia Mare, which is farther north than Kraken, may be filled with more fresh methane rainfall than Kraken, as higher latitudes appear to receive more rainfall. In this case, a recent paper proposes,4 Ligeia Mare may be methane-enriched, with up to 80 % of its liquid in this form. But currents within the basins of Kraken Mare mix methane with the more dense ethane, which might leave Kraken dominated by ethane, by as much as 60 %. The variations in the two hydrocarbon seas, says author Ralph Lorenz, are analogous to the salinity gradient between Earth’s Black Sea and the Mediterranean.

  Methane and ethane are intrinsically more transparent than is water, so visibility might be excellent along a Titan shoreline. However, there may be material suspended within the liquid. Titan’s descending haze of very fine particles, as small as a third of a micron, is very similar to smog found hovering over Earth’s major cities. But its precipitation is less snowstorm and more constant, invisible settling. Those hydrocarbons drift out of the sky and land in the seas. The density of liquid methane and ethane is such that almost anything sinks in it. Under Titan’s low gravity, material would sink very slowly, so Troy’s cruise through Kraken Mare might be like a trip through Miso soup. The particles eventually settle out, but even convection currents are enough to lift the particles up. This means there are a lot of opportunities fo
r particles to clump together in Titan’s seas before they settle, and those larger particles, once stranded on drying beaches, might become the material that builds into sand dunes.

  It is difficult to tell how full some of Titan’s lakes are. The lakes and seas of Titan are somewhat transparent to radar. The surface of the liquid is smooth, so it acts like a mirror. Incoming radar hits it at a slight angle and reflects away, leaving a dark image. But the liquid methane also lets some of that radar pass through into its depths, like light passing into clear water. This clearness makes it challenging to tell just where the methane/ethane sea ends and the shoreline begins.

  Within the seas of Titan, investigators observed several subsurface channels. The researchers were able to measure depth—or at least general slope—along their lengths. Because rivers flow downhill, they expected that if these were indeed drowned river valleys, the channels would get progressively darker downstream, since the spacecraft would observe them through progressively deeper liquid. This has, in fact, been confirmed.

  In 2009, the visual and infrared mapping spectrometer (VIMS) observed a glint of sunlight reflected off the sea Jingpo Lacus, resolving any remaining doubt that Titan’s dark areas are bodies of liquid. Researchers are now searching for wave action using Cassini’s VIMS in areas where the sunlight might glint off the surface, and also radar that might uncover telltale signatures of waves.

  Another unusual incident may indicate wave-caused foam on the surface: Cassini imaged a bright “island” during a flyby in July of 2013. The irregular region in Ligeia Mare was completely missing in imagery from three previous encounters. The area disappeared as abruptly and mysteriously as it had appeared. It was absent again on the next flyby just 16 days later, leading researchers to brand it the “magic island.” Aside from frothy waves, the brightening may be caused by gas bubbling up from the seafloor or icy slush floating on the surface. Methane ice is denser than methane in liquid form, so the slush would need to be a lighter related material, perhaps chains of polyacetylene. Titan experts continue to search images for further sightings.

  Adding to the portrait of Titan as a vibrant world, the lakes seem to be changing shape. The coastline of Titan’s largest southern lake, Ontario Lacus, receded by at least 10 km over a period of 4 years. Several transient lakes in its vicinity completely vanished during the same period. Radar indicates that Ontario Lacus itself, while as vast as Lake Ontario in North America, may have an average depth of 0.4–3.2 m, with its deepest part diving to just over 7 m. Ontario Lacus may resemble terrestrial mudflats.

  The lakes in the north are a different story. Their coasts have been unchanged over a decade of observation. Clearly, they are more stable than those in the southern hemisphere. The more numerous northern lakes have steeper-sided shores, so as surface levels drop, changes may be harder to detect along the shorelines. Or, the northern ponds may simply be deeper and less prone to evaporation. Whatever the cause, most researchers agree that the lakes in the north are more stable and permanent than the weather-related lakes in the south. Depths of the great Ligeia Mare may reach 170 m, plenty of room for a little blue submarine.

  Kraken Mare is the largest body of liquid known on the surface of another planet. Named after the legendary sea monster that lives off the coasts of Norway and Greenland, Kraken covers over 400,000 km2, an area equivalent to Earth’s own Black Sea. One or more channels may link Kraken to the nearby sea Ligeia Mare. Toward the northern end of the sea lies the island called Mayda Insula. Fjord-like valleys cut its rugged coastline. The island is 100 × 170 km, covering just a bit more territory than the main island of New Caledonia.

  Not all of Titan’s seas are skin-deep. Careful study of features on Titan’s surface show that the moon’s crust has wandered, shifting positions of mountains or other landmarks by as much as 30 km (19 miles). Cassini orbital measurements also showed some telltale evidence of bulges in the moon’s surface. If Titan’s interior were composed of solid ice and rock, Saturn’s gravity would just barely shove the surface out of round, causing it to rise and fall about 3 ft. each day. But in fact, Saturn’s push and pull trigger bulges some ten times that high, suggesting that Titan’s interior has a layer of liquid inside, creating a global ocean.

  Additional clues come from Titan’s orientation as it circles Saturn. Titan’s axis is tilted by about 0.3°. This angle—or obliquity—is too high for a solid body with its weight centered at its core. But if some of that weight were slightly above the core, in the form of liquid water nested in the solid ice crust, the obliquity would make sense. Models indicate that Titan’s ice surface is part of a shell that stretches down to a liquid water ocean. This ocean, in turn, lies above a mantle of frozen water. Beneath that mantle is an ice/rock core. The ice shell on top may be anywhere from 90 to 125 miles thick (150–200 km), while the ocean far below may range from 3 to 265 miles (5–525 km).

  Researchers suspect that the subsurface water ocean on Titan is a witches’ brew of briny water and ammonia. To fit the gravity data, the water must be dense. The water is likely high in salt content with dissolved sulfur, sodium and potassium, elements common in the outer Solar System. This density would be at least as salty as water found in the Dead Sea or in the ponds of Badwater, Death Valley. Although this may not be a positive sign for present-day life, conditions may have been very different within these dark waters in Titan’s past.

  Dunes

  With an area of over 77,000 km2, the Dasht-e Kavir is Iran's largest desert. But there is another that rivals even this spectacular natural formation. Titan’s Belet sand sea has not seen liquid water for eons. Although dunes in the Dasht-e Kavir rise some 40 m, those in Belet tower 150 m high. Instead of silica, its dunes may be made up of pulverized ice or organic material that rains from the sky. The dunes of Africa’s Namib Sand Sea are nearly as tall.

  At least 20 % of Titan’s surface is covered by dunes, and perhaps much more. By comparison, dunes cover about 5 % of Earth’s land, and less than 1 % of the Martian landscape. Titan’s atmosphere is 1½ times as dense as Earth’s at sea level. Chilled to 290° below zero F, the sluggish air moves across the face of Titan like a planetary tidal wave. The shape and orientation of the dunes indicates that Titan’s winds blow from west to east near the surface. Recent estimates put the number of recorded dunes at well over 20,000.

  Titan’s dunes may be one of the most alien features of the planet. If they were analogous to the Earth’s dunes—silica sand that comes from ground-up rock—Titan’s dunes should be ice (since frozen water is Titan’s “rock”). But Titan’s dunes may not be comprised of water-ice at all. They may, in fact, be composed of organic material that falls from the sky. Cassini’s visual and infrared mapping spectrometer sees all the dunes as dark. If they were water-ice, they would appear bright. Cassini’s radar offers another clue. In addition to providing a visual image, radar yields insights into how the material behaves. Radar waves bounced off the surface indicate fine-grained organic material, undoubtedly related to the soot-like hydrocarbon matter that precipitates out of the sky as a result of the interaction of the Sun’s ultraviolet radiation and methane in Titan’s atmosphere (Fig. 61.4).

  Fig. 61.4Dunes drape across many of the landscapes in Titan’s frozen wilderness. Here, linear dunes draw a fingerprint on low plains, while raised topography remains clear (Images courtesy NASA/JPL/Space Science Institute)

  However, the dunes are made of coarser stuff than comes from the sky. How does it get there? What creates it? As Saturn’s orbit precesses, the seasons on both Saturn and Titan lengthen in one hemisphere and shorten in the other. This change might lead to a drying out of seas in one Titan hemisphere, and a shift in methane humidity to the other. We find a similar phenomenon recorded in Earth’s geological record. Huge layers of salt lie beneath the Arabian and Mediterranean gulfs, because they were once ocean basins that became closed off, eventually drying out. In this process of drying out, fine hydrocarbon particles in a Titan sea may come tog
ether as grains to make sand. The sand gets blown out and makes its way to the equator, resulting in the great sand seas that spread across the equatorial regions of Titan today.

  The dunes imaged by Cassini are similar to the largest ones seen in Earth’s deserts. Although no coastal dunes have been seen near the lakes or across areas scored by river valleys, it is possible that small dunes may form in these regions, as they do on beach areas of Earth’s coastlines. Farthest Sea makes this assumption.

  Living on Ice

  As Kevin Nordsmitt observed in our first chapter, on the moons of the giant worlds, water freezes to the consistency of rock. This can be used to the advantage of settlers. Just as Inuit peoples of the arctic use ice to build igloos, so astronauts could use blocks from the icy surface to construct various structures. Under the conditions in the outer Solar System, ice will provide excellent masonry, or even material from which to carve out caverns, as our pirates did at their Northern Quadrant site. The only caveat—it must be insulated from the interior heat. Humans like their rooms warm, and at a temperature of 70 °F, their walls and floors will begin to melt. “You have to isolate things, using insulation,” says Lockheed Martin’s Ben Clark. “But we’ve been doing it on Earth for a long time. They insulate buildings from the cold surface in the arctic and Antarctica.” In an ice cavern, inflatable habitats could be erected without being in direct contact with the frozen surfaces. Insulation could also be sprayed onto ice surfaces that, once stabilized, could provide a permanent barrier for interior heat. Another option for living on an ice world such as Titan would be inflatable habitats. Inflatable domes would require anchoring, as there is enough wind to shift them. Titan’s winds move particles around on the surface, as the sand dunes attest, so a large-scale inflatable would be a big area to drag on. And like the ice caverns on our north shore or the hard-bodied cylinder habitats of Mayda Research Station, they would require insulating from the cold surface. “We don’t know for sure what the surface composition of most of Titan is,” says JHU Applied Physics Laboratory’s Ralph Lorenz. “In some places it may be water-ice. In most places it appears to be organic, so it’s stuff that could soften or even melt at what we call room temperature.” For the now-canceled Constellation program, NASA’s Johnson Space Center conducted formal studies on inflatable habitats for the lunar surface for over a decade. Work with inflatable habs continues in Antarctica as well, with several outposts making use of the equipment as part of their infrastructure. With some added insulation and reinforcement for pressurization, these technologies are directly applicable to the environments of the outer Solar System (Fig. 61.5).