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Ethane lakes in a red haze: Titan's uncanny moonscape

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SUMMERTIME is on its way. A thick smog hangs in the air. Storms are on the move, bringing rare rains to dry areas, and elsewhere lakes are shrinking. Only this is not Earth. This is the view on Saturn's largest moon, Titan, more than a billion kilometres away.

We knew very little about this strange world before NASA's Cassini spacecraft arrived at Saturn seven years ago. Since then, the ringed planet has completed a quarter of an orbit around the sun and Titan has passed through its spring equinox. In that time, Cassini has swooped by Titan almost 80 times and has released the Huygens lander onto its surface.

Together they have penetrated the haze that hides Titan's surface to reveal modest mountains, vast fields of sand dunes, rocks, and even streams, lakes and weather.

Titan looks surprisingly familiar, although it is a cold, dimly lit world made from unfamiliar materials, says planetary scientist Elizabeth Turtle of Johns Hopkins University Applied Physics Laboratory in Baltimore, Maryland. "The rocks are water ice and the lakes are methane and light hydrocarbons, yet we see processes very similar to what we see on Earth."

So far, there are no recognisable signs of organic life. That's not surprising: by terrestrial standards, Titan is a deep freeze with surface temperatures at a chilly -180°C. Yet Titan is very much alive in the sense that its atmosphere and surface are changing before our eyes. Clouds drift through the haze and rain falls from them to erode stream-like channels draining into shallow lakes. Vast dune fields that look as if they were lifted from the Sahara sprawl along Titan's equator, yet the dark grains resemble ground asphalt rather than sand. It is a bizarrely different world that looks eerily like home. Or as planetary scientist Ralph Lorenz puts it: "our prototype weird-world exoplanet".

By exploring Titan, we can hope to understand more about planets around other stars. NASA's Kepler telescope has found over 1200 possible exoplanets and we would dearly love to find out what these worlds are like. So far our understanding of geological processes, such as how the landscape is sculpted by winds and waves, is based on empirical evidence from just one planet - our own. That means any explanation of the underlying processes either on other planets or on a changing Earth are beyond us. "If we can test theories on both Earth and Titan, we can be much more confident in extending these theories to other conditions," says Lorenz, who is also at Johns Hopkins.

What an atmosphere

Titan was discovered in 1655 by Christiaan Huygens. At about 5150 kilometres across, it is slightly larger than the planet Mercury and a touch smaller than Jupiter's largest moon Ganymede. But what sets it apart from other large moons is its atmosphere, discovered in 1944 by Gerard Kuiper. Wisps of gas surround some satellites, but Titan's atmosphere is the real thing - and is many times thicker than Earth's.

This atmosphere made Titan a uniquely tempting target when spacecraft began exploring the solar system. Pioneer 11 took the first snaps in September 1979 and Voyager 1 travelled within 22,000 kilometres of Titan a year later. The results were spectacular, but disappointing: a beautiful high-altitude blue haze hanging over a thick orange haze that obscured the surface. NASA and the European Space Agency soon began planning the Cassini mission to get a closer look.

Cassini finally arrived in 2004, after a seven-year journey from Earth, and released the Huygens lander. This transmitted images and data for 2.5 hours as it descended through Titan's murky atmosphere and for 1.5 hours after landing. Cassini has continued to monitor Titan ever since, flying past as low as 880 kilometres above its surface. With each pass, Cassini reveals a little more about the only body in the solar system besides the Earth known to have liquid on its surface.

The picture we have built up is intriguing. Like Earth's atmosphere, Titan's is largely nitrogen plus one more-reactive compound and some trace gases (see diagram). Titan's weaker gravity lets its atmosphere extend far higher than on the Earth, with the stratosphere starting at 40 kilometres and reaching up to a few hundred kilometres.

Standing on Titan, you would see a dense orange haze similar to a really bad smog day in Los Angeles. It comes from the same basic process, too - photochemical reactions in the air. LA's smog is caused by sunlight driving chemical reactions between oxygen, nitrogen compounds and hydrocarbons in the lower atmosphere. On Titan the photochemistry starts at the top of the atmosphere, about 1000 kilometres up, where energetic ultraviolet photons from the sun and cosmic rays trigger reactions among methane and nitrogen molecules. That produces hydrocarbons and cyanide compounds, which drift downwards and react further to produce a poorly understood group of complex compounds called tholins.

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Issue 2820 of New Scientist magazine
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