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Does Methane Flow on Titan?

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Does Methane Flow on Titan? Planetary science Does methane Scientists are eager to prove that the meandering river valleys and braided streambeds seen on Saturn’s largest moon carry liquid methane to its vast lakes. by Robert Zimmerman flow onTITAN? The Huygens probe captured ine years ago, Europe’s Huy- new world,” raved Jean-Jacques Dordain, theorized that methane might be able to Now, nearly a decade later, planetary Before the two probes this 360° panorama of Titan’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 astronomer 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 centuries, 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 space- detect methane rainstorms feeding riv- spacecraft repeatedly has flown past solar system’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 century, however, observations revealed terrain appear to feed into a mission’s success. and continues to orbit Saturn. and distant moon, the second 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 Astronomy, 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 decades 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, Voyager 1 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 Mercury. If less orange ball, the surface shrouded by a Alien planet 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 Voyager 2 captured Titan from 1.4 million the southwestern shoreline of Ontario Lacus 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 geology 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 times. It has snapped visible-light, 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 time. 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.
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