can’t see the forest

Touchdown Titan

Posted in astronomy, ecology, Science by Curtis on 9/9/06


Titan is the largest moon of the planet Saturn and is the second-largest of all moons in the solar system, exceeding the planet Mercury in size (though not in mass—Mercury is much more dense.) Titan is about 50% larger than Earth’s Moon, and was once thought to have been our solar system’s largest moon until it fell second to Jupiter’s Ganymede when it was discovered that some of Titan’s apparent size is accounted for by a thick, dense atmosphere; Titan is the only moon to have retained an atmosphere of anything more than trace amounts of gases. In fact, Titan’s atmosphere is denser than that of Earth and the atmospheric pressure at Titan’s surface is about 1.5 times that at the surface of Earth. Titan’s atmosphere is composed mainly of nitrogen, as is Earth’s, but of different isotopes and at a higher concentration (98% to our 71%). Methane dominates the upper atmosphere of Titan, and the effect of the solar wind upon the upper atmosphere is responsible for some peculiar chemical effects in Titan’s sky and at its surface. The moon is enveloped in a thick yellowish haze which completely obscures its surface.

This picture of Titan was taken by NASA’s Voyager 1 probe in late 1980. Voyager did not possess instrumentation capable of collecting data from beneath the surface of Titan’s atmosphere. The Cassini-Huygens spacecraft, a joint venture between NASA and the ESA, reached the vicinity of Saturn in July 2004. High-resolution photographs of Titan from relatively close-by were produced in October 2004, through which some elementary contour and other surface information was gathered. On January 14, 2005, the Huygens probe was dispatched into Titan’s atmosphere; it landed upon the surface of Titan and produced pictures of the surface and other valuable data. This is the most remote landing of a manmade craft; the Saturn system lies, on average, about 1.3 billion kilometres (800 million miles) from Earth. We now have photographs taken on the surface of a body that far away.

The discovery of Titan

Picture of Titan's surface from high in the atmosphereThe Huygens probe was named for Dutch astronomer Christiaan Huygens, who discovered Titan in 1655. It was the fifth moon of another planet to be identified; the four largest moons of Jupiter had earlier been discovered by Galileo. Huygens named Titan simply “Saturn’s moon.” It was John Herschel, son of British astronomer William Herschel, who gave the body its present name in 1847.

The astronomer Gerard Kuiper used spectroscopy in 1944 to infer the presence of a significant atmosphere on Titan. His experiments indicated the presence of methane, which forms the most superficial layer of the atmosphere floating high above the thick blanket of nitrogen which envelopes the moon in a smog somewhat similar to that of Los Angeles, but much worse. It was not until the Voyager 1 flyby in 1980 that astronomers discovered just how thick Titan’s atmosphere really is, and relatively little was known of its composition or of the features of Titan’s hidden surface until the arrival of Cassini-Huygens in 2004-5. This black-and-white picture of Titan’s surface was snapped during the descent of the Huygens probe. Contrasts in surface contour are evident, and the dark “rivers” are likely composed of now mostly frozen liquid material ejected from low-temperature volcanos at the surface. Scientists are trying to determine whether or not there is ongoing tectonic activity on Titan; no one is yet sure about that.

Titan’s weird chemistry

Titan is several times farther away from the Sun than Earth, and its methane- and hydrocarbon-frosted atmosphere produces a reverse greenhouse effect so that the majority of what solar radiation reaches Titan is reflected back into space. The mean temperature at Titan’s surface is about -179 degrees Celsius (-290 Fahrenheit).

This moon is thought to have a solid rocky core which is certainly much smaller and cooler than the core of Earth but could still be thermally active. It is currently thought that a layer of liquid, mostly water, cushions this rocky core from several outer layers of different crystal states of water and ammonia ice. There are some volcanic features on the surface of Titan, and scientists believe that these volcanos emit plumes of liquid and vaporized water and ammonia in much the same way as the volcanos of Earth emit magma and gaseous discharge. Water appears to be something like the magma of Titan.

The emissions from Titan’s volcanos collect in the upper atmosphere. Sunlight acts upon these compounds and changes them into hydrocarbons like ethane and propane, which periodically rain down from the sky and collect in lakes, particularly near Titan’s polar regions. It was once hypothesized that Titan might be covered with vast seas of these hydrocarbon liquids, but the evidence from Cassini-Huygens indicates that condensation at the surface is largely, if not completely, limited to pools near the poles. However, it has also been proposed that Titan may go through long wet and dry seasons, so that, at other times, the low-lying mushy areas of its surface may be covered with hydrocarbon seas that evaporate relatively quickly. The Cassini craft continues to take radar measurements of the surface from its orbit above the clouds of Titan.

Titan’s rocky surface is made mostly of water ice. The chunks of ice are, at least in the lower-lying regions, embedded in a mushy “soil” of organic compounds which have fallen to the surface as rain over very long periods of time. Water ice does not sublimate at the high pressures and extremely low temperatures on Titan, so there is almost no water vapor in the atmosphere; what vapor is present is emitted from Titan’s cryovolcanos.

A sunny day on Titan

Titan's surface, artifically brightenedIf you were standing on the surface of Titan on a sunny day, the Sun would appear as a bright area in the sky, though it would be not nearly as bright as seen from Earth and might be hard to locate specifically. This would be not only because of the larger distance between Titan and the Sun, but because the atmosphere of the planet is very thick and hazy. The sky would appear a dim yellow; if Saturn were visible through the haze, more likely at night, it would take up much of your field of vision in the sky as a peacefully looming giant with big, goofy rings.

The ground would be grainy and mushy beneath your feet—to get a sure footing, you might have to stand atop the ice rocks which predominate throughout the surface. There are some highland regions on Titan which are probably composed almost completely of water ice rocks. The mushy “soil” seems to be prevalent only in the lower areas since it collects as condensation.

There are vast stretches of sand dunes across parts of Titan’s surface; this sand is composed of ice particles shuffled about the surface by slow and steady winds. The dunes are arranged in very neat rows parallel to Titan’s equator, their distribution and organization being governed by the magnetic tidal forces of the gigantic Saturn acting upon Titan’s surface.

It is a very cold, very weird, and very mysterious place. But as the only moon in the solar system with a significant atmosphere, an atmosphere which shares certain fundamental characteristics with that of Earth, Titan may one day become a sort of “base camp” for human exploration of the outer planets of our solar system—and beyond. While the pressure is somewhat greater at the surface than on Earth, the presence of a largely nitrogenous atmosphere makes Titan an attractive destination for human exploration. Just be sure to bring extra-thick spacesuits, lots of blankets, and a camera with a very bright flash. Scientists said that imaging Huygens’ photos from the surface was much like photographing “asphalt at dusk” because the dense, organically rich sky allows so little light from the Sun to reach the surface. This surface photograph, in which small rocks of water ice can be seen in the foreground and throughout the landscape beyond, has been artifically brightened for contrast.



2 Responses

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  1. Skip Conover said, on 9/10/06 at 10:57 pm

    Curt: Take a look at the proposal on my site, in honor of 9/11 victim Greg Buck, and his fellow first responders. Sorry, I don’t have an e-mail address for you. Best regards, Skip

  2. Drima aka SudaneseThinker said, on 9/13/06 at 11:31 pm

    Dude, I’m liking your blog more and more.

    I love the way you choose the different topics.

    Politics, Science and Philosophy all in one. Nice!

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