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Science
by Pat Murphy & Paul Doherty

Glimpsing Titan

FOR MORE than a hundred years, those of us in the speculative fiction business have been speculating like mad about Titan, Saturn's largest moon. Back in 1894, in Journey in Other Worlds, John Jacob Astor wrote of a group of travelers whose spacecraft crossed the orbit of Titan on its way to Saturn. (On Saturn, the travelers wore their winter clothes, as it was rather cold.) Over the passing years, science fiction authors have written of Titan as a mining colony (Arthur C. Clarke's Imperial Earth, Alan E. Nourse's Trouble on Titan) and as a source of aliens (Philip K. Dick's The Game-Players of Titan). These days, authors seem more inclined to consider the possibilities of life on Saturn's largest moon (Stephen Baxter's Titan and Michael Swanwick's Hugo Award-winning novelette "Slow Life.")

Titan is the only moon known to have a thick atmosphere. That atmosphere is hazy and hides the surface from the view of Earth observers. From a science fiction writer's point of view, that's just great. Nothing fuels speculation like something you can't see!

But the surface of Titan is growing less and less mysterious—and more and more intriguing. In 2004, the Cassini-Huygens spacecraft went into orbit around Saturn, providing us with information direct from Saturn's moon. Science began to catch up with science fiction.

Launched in 1997, Cassini-Huygens took a circuitous route to Saturn, looping around the Sun twice. It gained momentum as it passed close by Venus once in 1998 and again in 1999. It gained more momentum as it passed by Earth in 1999 and Jupiter in 2000. In each close encounter with a planet, or "flyby," the spacecraft got a push that sped it on its way to Saturn. (If you want to know how a spacecraft gains momentum from a flyby, check out our 1998 column "Close Encounters of the Gravitational Kind" (http://www.sfsite.com/fsf/1998/pmpd9812.htm).)

In July 2004, Cassini-Huygens swung into orbit around Saturn, where it will remain for the next four years (barring alien intervention or other mishap). During this time, the spacecraft will complete seventy-five orbits of Saturn, forty-four close encounters with Titan, and numerous close encounters with Saturn's other moons. Equipped with infrared and radar "eyes," Cassini can see the surface of Titan through the moon's hazy atmosphere. In early 2005, the Huygens probe actually penetrated the haze and made a safe parachute landing onto the surface.

Each time Cassini passes by Titan, its instruments reveal a small portion of the moon's surface. This episodic series of glimpses is a bit like the old science fiction serials, where each installment left you hungry for more.

To help you appreciate this strange world, Paul and Pat would like to take you on a journey that begins on Earth and proceeds into and through the atmosphere of Titan to the surface of the moon. Along the way, we'll share some of the hands-on explorations Paul developed for Exploratorium webcasts about Titan.

DOING THE MATH

Christian Huygens discovered Titan in 1655. Viewed with human eyes, Titan is a smoggy orange blur, a bit like Los Angeles seen from an airplane on a bad day.

As moons go, Titan is huge. Misled by its smoggy atmosphere, Earth observers originally thought that it was the largest moon in the solar system. But after scientists subtracted the 200 km (120 mile)-thick atmosphere, it dropped into second place behind Jupiter's moon Ganymede. That still leaves Titan bigger than the planet Mercury, and only slightly smaller than Mars.

When the Voyager spacecraft passed by Titan, observations of how much the moon's gravity deflected the spacecraft allowed scientists to calculate Titan's mass (an estimate that will be refined based on information from the Cassini spacecraft's multiple encounters with the moon). Titan has 1/44 the mass of Earth. But its diameter is forty percent that of Earth—and therefore it has 1/16 the volume of the Earth. Do that math and you'll discover that Titan has a density of about 1/3 that of Earth.

The Earth has a rocky mantle and crust wrapped around an iron core. Scientists think that Titan has a dirty water ice mantle and crust over a rocky core. Since ice is less dense than rock, that would explain the discrepancy in mass.

The combination of the mass and radius of Titan means that the acceleration of gravity on the surface of Titan is 1/7 of the acceleration of gravity at the surface of the Earth. On Titan, your weight (measured with a spring scale) would be 1/7 your weight on Earth.

HOW COLD IS IT?

Titan is ten times farther from the sun than the Earth is. According to the good old inverse square law (which predicts how light spreads out from a point source with distance), the sunlight that reaches Titan is 100 times dimmer than sunlight on Earth. Titan's thick smog blocks some of that sunlight, preventing it from reaching the moon's surface. As a result, Titan's surface is about 1000 times dimmer than a sunny day on Earth. That's about as bright as an average household living room at night with incandescent lighting.

Without sunlight to warm it up, Titan is cold! How cold? Well, cold enough that a winter coat just wouldn't cut it, no matter what John Jacob Astor says. The temperature is just over 94 kelvins at the surface. (That's 180 C or 290 F.) For those of you who have seen demonstrations with liquid nitrogen, the temperature of the liquid nitrogen on Earth is 77 kelvins, slightly colder than Titan. If Titan were much colder, the nitrogen of the atmosphere would turn into a liquid nitrogen ocean.

The effects of cold temperatures on Titan can be modeled using liquid nitrogen.

Paul likes to use a hot dog to simulate human flesh on Titan. If you put a hot dog into liquid nitrogen, then hit the hot dog with a hammer, it shatters. Of course Paul does this by surreptitiously slipping the hot dog into the index finger of a rubber glove, and then putting his fingers into the rest of the glove with his index finger folded against his palm. After warning the audience about the dangers of liquid nitrogen, he puts the hot-dog-finger into the liquid nitrogen. Then he removes the glove from the liquid and shatters the hot-dog-finger with a hammer. (Paul's performances are not for people with heart trouble!)

ABOUT THAT SMOG

Titan's smoggy atmosphere is mostly nitrogen (a distinction Titan shares with Earth.) Nitrogen is a dull gas—it's not very reactive. It's the impurities in the atmospheres of Earth and Titan that make things exciting.

In Earth's atmosphere, the major impurity is oxygen, which makes up twenty-one percent of the dry atmosphere. Oxygen defines an exciting gas. Combine it with fuel and a spark and you get fire. Living things use it to generate energy.

The second major impurity on Earth is water vapor, which makes up less than three percent of the atmosphere. Water at Earth temperatures can exist as a gas, a liquid, or a solid, forming clouds, rain, snow, oceans, and glaciers. As you probably remember from your elementary school science textbook, water evaporates from oceans, then forms clouds and falls as rain, which flows back into the ocean (carving the land as it goes). This is known as the water cycle.

Titan's version of the water cycle might be called the "methane cycle." On Titan the major impurity is methane or natural gas (CH4 to a chemist). Methane makes up about six percent of Titan's atmosphere. At the temperatures common on Titan, methane can liquefy, falling as rain and running over the moon's surface to create rivers and seas.

The methane also accounts for Titan's smog. Los Angeles and Titan have photochemical smog for similar reasons. Both have sources of hydrocarbons. In Los Angeles, the source is car exhaust. On Titan, it's methane in the atmosphere. Both Los Angeles and Titan are irradiated by ultraviolet radiation from sunlight. On Titan, the ultraviolet light breaks apart the methane. The fragments of methane recombine to form heavier hydrocarbons. Liquid drops and solid particles of these hydrocarbons make up the smog.

Spacecraft have identified a number of gaseous hydrocarbons on Titan, including ethane, butane, and acetylene. On Titan's surface, scientists expect to find tholin, a hydrocarbon that can be made in the lab by subjecting mixtures of methane, ammonia, and water vapor to simulated lightning discharges. At Titan's temperatures, tholin will be a waxy solid, probably impure and dark in color. They postulate that methane rain washes tholin from the atmosphere onto the moon's surface where it colors the icy surface dark. The dark tholin accumulates at the bottoms of valleys and depressions and contributes to the darkness of river channels in the Huygens images.

There is so much atmosphere on Titan that the surface pressure is one point five times the surface pressure on Earth. That's the pressure you would experience on Earth at the bottom of the deep end of a swimming pool, fifteen feet down. The high pressure plus the cold temperature makes the air on Titan about five times denser than the atmosphere of Earth.

To simulate Titan's atmosphere at the Exploratorium, Paul fills a balloon with sulfur hexafluoride gas. This dense gas gives the balloon five times the density of the Earth's atmosphere. When you wave the dense balloon around, it feels quite strange as the inertia of the dense gas makes the balloon feel almost as if it is full of a liquid. The atmosphere of Titan is dense enough that it would really slow you down if you tried to run.

For another demonstration, Paul uses a clear plastic butane lighter. You can see the liquid butane inside the lighter. When the lighter releases this flammable hydrocarbon into the atmosphere, the butane becomes a gas. Supply a spark and the butane gas ignites and burns in the presence of oxygen. To make a lighter on Titan, you would need to fill the lighter with oxygen. Release the oxygen gas into the atmosphere of Titan and provide a spark. With the oxygen, methane in the atmosphere would burn as a flame.

WET T-SHIRT CONTESTS AND CASSINI

Human eyes can't see through the smoggy haze that covers Titan's surface. That's because our eyes detect visible light, a very narrow range of wavelengths in the spectrum of electromagnetic radiation. Titan's smog blocks visible light. But there are other forms of electromagnetic radiation that penetrate the smog. The Cassini spacecraft has eyes that detect infrared light. Some wavelengths of infrared light pass through Titan's atmosphere as if it were clear.

In the comfort of your own home, using a few technological toys in ways that their manufacturers did not intend, you can do an experiment that lets you examine things that are transparent and opaque in the infrared.

Remote controls for televisions and other audio and video devices emit pulses of infrared light from an infrared light-emitting diode. Most digital cameras are sensitive to infrared light. Have a friend point the end of the remote control that you normally point at the television toward your digital camera while you look at the monitor screen of the camera. When your friend presses a button on the remote control, you'll notice a light flashing on the monitor screen of the camera. But if you look directly at the remote control, you won't see anything. The camera can see infrared light, but your eyes can't.

Some things that are opaque to visible light are transparent in the infrared. Find a thin dark plastic trash bag. You can't see much through it. But look at the TV remote control inside such a bag with a digital camera held outside the bag and you can see its infrared light easily. The trash bag material is transparent to infrared light and opaque to visible light, just like the atmosphere of Titan.

If your digital camera doesn't detect infrared from the remote, there's a reason for that. Thin white cotton—like the cotton of a T-shirt—is also transparent to some wavelengths of infrared. When the infrared viewing capabilities of digital cameras were first discovered, some people realized that cameras could see right through a thin cotton T-shirt. Sales of these cameras boomed (for the same reason that wet T-shirt contests are popular). Camera manufacturers added infrared filters to block out most of the infrared. (Sony, in particular, now has infrared blocking filters on most of their video cameras.)

The infrared cameras on the Cassini spacecraft, unfettered by privacy concerns, can see through Titan's smog, revealing dark regions and light regions. Scientists still don't know exactly what the dark and light areas mean. The dark regions look like oceans or lunar mare. They appear to lap up against white coastlines. Light islands protrude from the dark mare. But, like mare on the moon, which were once molten lava but are now solid rock, the dark regions of Titan may be solid. We'll have to wait to find out exactly what these dark regions of Titan really are. In the meantime, there's plenty of room for speculation.

WHAT WE DON'T SEE

In Sir Arthur Conan Doyle's short story "Silver Blaze," Sherlock Holmes comments on the "the curious incident of the dog in the night time." What was curious about the dog's activity was the absence of activity. When a racehorse was stolen, the stable dog never barked.

Scientists, like Sherlock Holmes, must always be aware of what isn't there. The most interesting features on the surface of Titan are the features that are not there. It's notable that Titan has few visible craters. (Just one large crater has been seen on Titan during a flyby in February 2005.) This is interesting since craters are found on every old solid surface in the solar system. The lack of craters on Titan (like the lack of craters on the volcanically active moon Io and on the Earth) means that the surface of Titan is young on a geologic timescale. Something removes craters on Titan. Some hints of the processes that remove craters can be gleaned from the images taken by the Huygens probe that parachuted under the clouds of Titan and returned clear photographs of some of the surface. We'll pause for a moment to discuss these photos, and then return to a discussion of the missing craters.

Geologists like to get "ground truth," to go along with their images. They like to touch surfaces, break open rocks, and analyze them to find out what they are made of. To get ground truth on Titan, NASA teamed up with ESA, the European Space Agency, to drop the Huygens spacecraft onto the surface of Titan. Suspended from parachutes, the Huygens probe took a couple of hours to drift leisurely down through the thick atmosphere of Titan. It took pictures all the way down. These pictures showed something not often seen on other worlds—patterns that looked like dark rivers cut into a lighter landscape.

The surface of Titan seems to be made of water ice. Scientists think that the patterns that look like river channels are carved by flowing methane. Some channels have the pattern of rivers carved by raindrops falling from the sky, like the patterns you can see from an airplane flying over the southwestern desert of the United States. Others have a shorter branching pattern like what you see at the seashore when a wave washes up onto the sand and wets the sand. When the water drains out of the sand, short branching patterns are formed in a process known as "sapping." On Titan, scientists think that these shorter patterns are made by methane springs.

And what about the missing craters? Erosion of the surface by rainfall or sapping will remove craters from the landscape.

The Huygens probe's camera also showed molten material leaking from linear cracks on Titan. You can think of this material as lava, like the lava that flows from the linear East Rift Zone in Hawaii. On Titan, however, the lava is a mixture of water and ammonia, which is hot compared to its surroundings.

To understand the surface of Titan, it helps to think of water ice as a mineral and liquid water as molten ice. The ice on Titan is not pure ice. It's dirty ice, made of water mixed with silicates and ammonia. The heat leaking from Titan's interior melts this dirty ice, and the resulting "lava" flows out of volcanic vents and cracks on the surface of Titan. The softening of the surface by melting and the flow of molten ice also helps erase craters from the surface of Titan.

LANDING ON TITAN

The Huygens probe drifted down on its parachute and settled onto the dark surface of a Titan mare. A rod sticking out below the base of the probe poked into the surface. This rod hit a thin stiff crust first. Once through the crust, it entered a substance with the texture of "wet sand." However, the "sand" on Titan is probably crushed water ice while the "wet" is liquid methane. The lander had a gas chromatograph/mass spectrometer on board, a device that's rather like a mechanical "nose." As the hot (compared to 94 Kelvin) lander sank into the crushed wet ice, it "smelled" methane gas vaporized by the heat of the lander itself. So we know that there is liquid methane just beneath the dark flat surface on which the Huygens probe landed.

During his webcast, Paul modeled the surface of Titan by pouring cooking oil onto crushed ice. He kept the level of the oil below the top of the ice. When this mixture was placed into the freezer a crust formed on top of the oily crushed ice. The oil on Earth played the role of the liquid methane on Titan.

The cameras on the lander looked out and saw rounded boulders, seemingly shaped by flowing fluids. These boulders were made from water ice. At the cold temperatures of Titan, water ice is as hard as granite is on Earth.

We're used to experiencing water ice from the freezer where it has a hardness of one point five on the Mohs scale, the geologist's hardness scale that runs from one for talc to ten for diamond. Water ice at 0 C is harder than talc but softer than gypsum at Mohs two. But as ice gets colder, it gets harder. By the time ice reaches 70C, it has hardness six on the Mohs scale. That's as hard as the mineral feldspar, which is a major component of granite. The landscapes of Titan are made from water ice that's as hard as granite. The landscape is carved by rivers of liquid methane laden with crushed ice sediment.

A DIFFERENT PERSPECTIVE

When methane flows on Titan, it erodes and shapes the surface, creating features that look remarkably like Earth landscapes. Take a quick glance at some of the images snapped by the Huygens probe on its way to Titan's surface and you might think you were looking at a river valley on Earth. A series of streams branch from a major river system and the land has obviously been shaped by flowing liquid. But the liquid, in this case, is methane.

In a way, this makes Titan a perfect science fiction metaphor. The surface looks familiar but is, in fact, completely different. And that is, after all, one of the things science fiction does best—finding strangeness in the familiar, and familiarity in the strange.

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The Exploratorium is San Francisco's museum of science, art, and human perception—where science and science fiction meet. Pat Murphy and Paul Doherty both work there. To learn more about Pat Murphy's science fiction writing, visit her web site at www.brazenhussies.net/murphy. For more on Paul Doherty's work and his latest adventures, visit www.exo.net/~pauld.

===THE END===

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