Planetary

Does methane Scientists are eager to prove that the meandering river valleys and braided streambeds seen on ’s largest moon carry liquid methane to its vast lakes. by Robert Zimmerman flow onTITAN?

The probe captured ine ago, Europe’s Huy- new world,” raved Jean-Jacques Dordain, theorized that methane might be able to Now, nearly a later, planetary Before the two probes this 360° panorama of ’s surface from an altitude of 6 gens probe dropped through ESA’s director general. flow on Titan like water does on Earth. scientists remain as excited and baffled Dutch Christiaan Huygens miles (10 kilometers) as it de- the atmosphere of Saturn’s “I’m shocked! It’s remarkable!” Once they got close enough to get a good by Titan as they did before Huygens discovered Titan in 1655. For , scended through the saturnian moon Titan and landed on enthused Carolyn Porco, leader of the look, the thinking went, probes might arrived. In the years since, the Cassini scientists thought this moon was the moon’s atmosphere January the surface. Planetary scien- imaging team for NASA’s Cassini - detect methane rainstorms feeding riv- spacecraft repeatedly has flown ’s largest. In the mid-20th 14, 2005. Dark drainage chan- nels in the brighter highland Ntists reacted with unbridled joy to the craft, which delivered Huygens to Titan ers, lakes, and even oceans on that cold this giant moon, detecting what look , however, observations revealed terrain appear to feed into a mission’s success. and continues to orbit Saturn. and distant moon, the largest in like numerous additional phenomena that the moon has a thick atmosphere, darker region that scientists “I have to say I was blown away by Porco then proceeded to describe the solar system. that resemble things we find on Earth suspect could be a lakebed. what I saw,” said David Southwood, then what all these scientists had found so And that is exactly what Huygens — large rainstorms, river channels, and When not writing articles for , ESA/NASA/JPL/UNIVERSITY OF ARIZONA director of science programs for the Euro- astonishing: “There are river channels. apparently had found in January 2005 in lakes all produced not by water but by Robert Zimmerman reports on science and pean Space Agency (ESA) and now presi- There are channels cut by something … practically its first images: meandering liquid methane. culture at his website, Behind the Black (http:// dent of the Royal Astronomical Society. a fluid of some sort is my best guess.” river channels flowing into what looked It is as if Titan is a frozen and dark behindtheblack.com). His first book, Genesis: “The scientific data that we are col- For almost three before Huy- like a large lake. For these researchers, it twin of Earth, similar in many ways yet The Story of Apollo 8, has just been re-released lecting now shall unveil the secrets of this gens’ triumph, planetary scientists had was almost too good to believe. also completely and weirdly alien. in a new e-book edition.

22 ASTRONOMY • FEBRUARY© 2014 Kalmbach 2014 Publishing Co. This material may not be reproduced in any WWW.ASTRONOMY.COM 23 form without permission from the publisher. www.Astronomy.com Huygens took this series of images as it neared Titan’s surface January 14, 2005. From left to right, the probe captured the moon from altitudes of 95 miles (150 kilometers), 12 miles (20km), 4 miles (6km), 1.2 miles (2km), 0.4 mile (0.6km), and 0.12 mile (0.2km). ESA/NASA/JPL/UNIVERSITY OF ARIZONA and its presumed large size was in part an conditions on Titan might allow oceans of Until the arrival of Cassini and Huy- A lake by any other name illusion caused by that atmosphere. Jupiter’s liquid methane to exist, but few listened to gens, however, these possibilities remained This is just one of Titan’s strange parallels moon Ganymede, which has a diameter of this wild idea. mere hypotheses. Although planetary sci- with Earth. Cassini observations also have 3,270 miles (5,262 kilometers), ranks as the Then in November 1980, entists knew that liquid methane could revealed hundreds of dark patches on the solar system’s biggest moon. zipped just 4,033 miles (6,490km) above the exist on Titan’s surface, no one had seen it. moon’s surface that scientists interpret as Still, with a diameter of 3,200 miles moon’s surface. Images revealed a feature- lakes. Some of them appear filled with liq- (5,150km), Titan is larger than . If less orange ball, the surface shrouded by a Alien uid while others seem to have dried up par- it were in an independent orbit around the smog-filled atmosphere 125 miles (200km) Then Huygens descended through Titan’s tially. Sinuous channels lead into some of Sun, scientists would consider it a planet. thick with an additional haze layer that atmosphere and landed on what appeared them, but others look like lakes that have By the 1970s, just before the twin Voy- varies in altitude between roughly 240 and to be a dry lakebed. On its way down, the filled ancient impact craters or calderas — ager spacecraft flew past Saturn, scientists 310 miles (380 and 500km). The atmo- probe snapped some amazing pictures of depressions at volcanoes’ centers created were still unsure what made up Titan’s sphere itself turns out to be about 95 per- what looked like ordinary meandering riv- when their magma chambers empty and thick atmosphere. Some believed methane cent nitrogen with methane making up ers draining into a dark area that appeared the overlying surfaces collapse. was the major component while others most of the rest. The surface temperature just like a lake. In one case, Cassini images show that argued that nitrogen reigned supreme. A Huygens photographed two different captured Titan from 1.4 million the southwestern shoreline of hovers at a bone-chilling –290° Fahrenheit miles (2.3 million kilometers) away as it flew handful of scientists even proposed that (–178° Celsius) while the surface pressure riverlike complexes. The more exciting past in August 1981. Unfortunately, the — at 146 miles (235km) across, the largest measures 50 percent higher than on Earth. image showed what seemed to be a river filters used on Voyager’s camera could not lake known in Titan’s southern hemisphere The Voyager measurements showed that with many branches merging to form a sin- penetrate the hazy atmosphere to show any — retreated by several miles (10km) surface detail. NASA/JPL conditions on Titan were perfect for the gle large channel that emptied into a lake. between 2005 and 2009. This suggests that existence of both liquid methane and eth- The channels themselves meandered back the lake is drying up slowly. In another ane. It even seemed possible that these mol- and forth like rivers do on Earth. instance, repeated observations of one area ecules could mimic the evaporation cycle In the second image, the main channel for mapping surface details. The radar showed what looked like several new lakes of water on Earth, which might result in appeared as straight as an arrow, with its mapper, however, can penetrate the haze forming shortly after a storm burst. Radar methane storms that produce methane rain tributaries joining it at sharp right angles. and see objects as small as about 1,150 feet images taken several years later showed falling into methane rivers that flow across It seemed almost as if the flowing liquid, (350 meters) in diameter. that these dark patches had disappeared, the moon’s surface into methane lakes. rather than meandering, was following What these observations have shown once again implying that they had dried up. ­natural cracks in the as it surged scientists is that Titan is an alien environ- Figuring out the exact nature of these This close-up view of the downhill to the lake. ment that mimics Earth only in the most geological features remains a difficult chal- south polar vortex shows Since the Huygens’ mission, the Cassini superficial ways. For example, the moon’s lenge, however. Looking at an image pro- the swirling atmospheric feature from a distance orbiter has flown past Titan nearly 100 crust is made of water ice, which is as solid duced by radar is not the same as looking at of 301,000 miles (484,000 . It has snapped visible-, infrared, as granite at the frigid surface temperature. an ordinary photograph. With radar, the kilometers). Taken June 27, and radar images that cover more than 50 Moreover, liquid methane does not dissolve brightness of the reflection correlates with 2012, the true-color image percent of the moon’s surface. water, so the methane flows over this fro- the roughness of the surface. Thus, the shows the moon’s south pole as winter approaches. The three key instruments used during zen water without eroding it much. The ice smooth surface of a Titan lake looks dark in NASA/JPL-CALTECH/SSI these flybys have been the Imaging Science acts like bedrock. radar images, while bright areas usually Subsystem (ISS), the Visual and Infrared Yet geologists know that flowing water suggest rough hilly terrain. Mapping Spectrometer (VIMS), and, most on Earth erodes bedrock — it just takes a The problem is that the data don’t importantly, the Cassini Radar Mapper. long . Similarly, as liquid methane always follow this pattern. For example, a The first two instruments take relatively flows downhill across Titan’s bedrock, it large number of the detected river valleys The only image taken from Titan’s surface Titan’s south polar vortex appears at the bottom of low-resolution images and spectra (which slowly erodes the rock-hard ice and picks and lakes appear bright compared to the shows the dry riverbed the Huygens probe this true-color Cassini image. The spacecraft took break the light down into its component up small pebbles, transporting it all down- surrounding terrain. Scientists think that landed in. The foreground rocks measure the photo July 25, 2012, from a distance of about colors, or wavelengths) from the near ultra- stream as sediment. In the process, the the radar brightness in such images sug- some 6 inches (15 centimeters) across while 64,000 miles (103,000 kilometers). NASA/JPL-CALTECH/SSI those in the distance are roughly 3 feet (1 violet to the infrared. Titan’s thick hazy methane has carved a variety of Earth-like gests a generally dry riverbed or lakebed meter) in diameter. Huygens took this photo atmosphere limits the value of these tools valleys, channels, and canyons. filled with gravel, cobble, and rocks typically January 14, 2005. ESA/NASA/JPL/UNIVERSITY OF ARIZONA

24 ASTRONOMY • FEBRUARY 2014 The mere existence of the meandering scale. Such megastorms would produce channels poses an even more perplexing horrendous flash floods that could easily puzzle. On Earth, plants help hold the carve out the many meandering channels banks of a meandering river in place. There and canyons that Cassini and Huygens are no plants — and no life that we know of have detected. — on Titan, so the presence of meandering These are extremes. Cassini observa- river valleys requires some other, as yet tions indicate that methane rain on Titan unknown mechanism to keep the shoreline also can occur in a gentle steady drizzle. stable for long periods. It remains unclear how frequently and with what strength these rainstorms occur. Rain and floods So far, most of the observed storms that To create Titan’s river channels and fluctu- have taken place have been in the high lati- ating lakes requires rainfall. Although sci- tudes of the southern hemisphere, which entists think the moon’s methane cycle of experienced more direct sunlight during evaporation, precipitation, and runoff Cassini’s first several years in orbit. Some mimics the water cycle on Earth, most of clouds built and disappeared in as little the details — how much rain falls how as two . often and where — remain uncertain. Spring arrived in Titan’s northern hemi- “Because Titan is so far away from the sphere in 2009. Scientists expect that as Sun and doesn’t get as much as sunlight grows stronger in the north, the Earth, there isn’t as much rainfall or pre- rainstorms will migrate there as well. But cipitation,” explains Burr. “When it does the changes will come slowly because each Cassini’s Visible and Infrared Mapping Spectrometer took this infrared view of Titan on September 12, occur, however, it has been storing itself of the moon’s four seasons lasts a bit more Titan appears silhouetted against Saturn’s edge-on rings and massive atmosphere in this true-color 2013. Green represents the water-ice bedrock that covers most of the moon. Orange reveals areas where up for a long time and therefore can be than seven years. ­image taken May 6, 2012. Cassini’s Imaging Science Subsystem wide-angle camera captured this view liquid methane likely evaporated, similar to salt flats on Earth. NASA/JPL-CALTECH/UNIVERSITY OF ARIZONA/UNIVERSITY OF IDAHO very energetic.” from a distance of 483,000 miles (778,000 kilometers) from Titan. NASA/JPL-CALTECH/SSI Based on Cassini and Huygens data, Fuzzy vision scientists estimate that the heaviest thun- Unfortunately, the reality of Titan’s rivers bigger than an inch (a of centimeters) thus has a smooth beachlike surface made derstorms on Titan can drop as much as remains uncertain. The chief problem is the ­meandering valleys, scientists have not yet Although budget cuts at NASA have across. In this scenario, methane flows only up of a dry soft snow — sediment laid down 100 inches (250cm) of methane in only two haze-choked atmosphere and the resulting been able to see them. slashed the agency’s planetary program for in narrow braided streams that wind and by a methane river that flows intermittently. hours. Compare that with the largest Earth lack of resolution. Without sharp optical Moreover, the lack of crisp resolution the next decade, they have not prevented intertwine through different parts of the Finally, it is even possible that the dark storm on record, which dumped 12 inches images of the surface, scientists must means that many of Titan’s most important any of these missions from getting under- wider cobble-filled valley, similar to the dry channels are dark because they, like the (30.5cm) of water in one . depend on the radar data. And although surface details remain either unseen or way. Instead, planetary scientists decided washes seen in the American Southwest. lakes, are filled with liquid methane. Once again, the analogy of the dry radar provides better resolution, it leaves undetermined. For example, it is difficult that the for many of these mis- “A desert wash is a very good analogy,” Unfortunately, the observations cannot yet washes and flash floods of the American many questions unanswered. from radar measurements alone to deter- sions was simply not ready, so they chose says planetary scientist Devon Burr of the tell scientists which of these possibilities Southwest come to mind, though the ones The only high-resolution images of mine the downstream direction for many not to include them in their recommenda- University of Tennessee, Knoxville. “They might be true. on Titan occur on a much more violent Titan’s surface scientists can study are those of these valleys. In some cases, the valleys tions to NASA in 2011. The researchers have wonderful rounded stones that are sent back by Huygens during its descent wind from a bright area thought to be asked for the engineering design to transported during summer monsoons, but and landing. As the probe parachuted mountainous to a dark area thought to be a continue but wanted to defer mission fund- most of the time they are dry or the flows through the thick atmosphere, it took dry or wet lakebed. Although the radar ing until the following decade. are confined to the lowest parts of the wash.” images that revealed objects as small as 65 instrument includes an altimeter to mea- In 2017, the Cassini mission will end. As for the dark meandering river val- feet (20m) across. Once on the ground, sure elevations, the observations typically NASA will send the spacecraft into Saturn’s leys, researchers have devised several theo- Huygens took a single image that showed a don’t resolve features as small as the valleys. atmosphere, where it will collect its last ries. One idea is that they are deeply incised cobble-strewn flat plain fading away into As often is the case, the way to resolve data before the growing pressure crushes it. bedrock channels, which look dark in radar the distance with rocks and pebbles rang- these questions is to go back to Titan with Once that happens, scientists will have images because the valleys’ steep walls ing from 0.1 inch (2.5 millimeters) to about better equipment. Planetary scientists have no probe available to study Titan’s surface. shadow the signal. 6 inches (15cm) across. proposed several missions to accomplish It will then be years, and probably decades, A second possibility is that the eroded Thus, except for this handful of Huy- this: an orbiter to circle the moon, a hot air before another spacecraft returns. Until sediment in the riverbed possesses a much gens images, all the so-called river net- balloon designed to float through Titan’s then, the meandering rivers of Titan shall finer, sandlike consistency. The dry riverbed Methane rain falls from Titan’s clouds, producing the changes seen here. The left image shows an area works identified on Titan’s surface to date atmosphere for years, and a boat that would remain veiled beneath the moon’s hazy near the moon’s equator May 13, 2007, while the other two were taken 15 hours apart January 15, 2011. are not rivers but wide valleys. If rivers land in one of Titan’s lakes and remain atmosphere, unmapped and unseen by The bright points in the latter two appear to be low clouds above where rain fell recently. NASA/JPL/SSI of methane actually run down these afloat to gather data for up to 30 days. human eyes.

On April 10, 2007, Cassini’s radar mapper took this swath that spans more than 4,100 miles (6,700 kilometers). Dunelike features appear near the left edge, but the terrain changes as the view moves northward (to the right) to reveal several large lakes. NASA/JPL-CALTECH/ASI TO SEE A MOVIE OF HUYGENS’ DESCENT AND LANDING, VISIT www.Astronomy.com/toc.

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