A rough guide to Titan

Titan Unveiled: Saturn's Mysterious Moon Explored

Princeton University Press: 2008. 296 pp. $29.95, £17.95 9780691125879 | ISBN: 978-0-6911-2587-9

A future tourist guidebook to this remote destination would warn us to bring our heavy-duty rain gear, but be prepared not to need it. Droughts may last many years there, but when a hurricane-sized storm sweeps across the sky, the rainfall is torrential. At high latitudes, the landscape is dotted with thousands of lakes, some mere ponds and others inland seas. Networks of channels and canyons are etched into the terrain, over which huge volcanic domes loom. Other regions harbour vast fields of dunes, some 100 metres high. Welcome to Titan, Saturn's largest moon.

Our guidebook would go on to explain that the dune particles are not sand, but hydrocarbons, totalling more than all the coal reserves on Earth. The magma flowing from the volcanoes is not liquid rock, but a mix of ammonia and water, similar to antifreeze. Liquid ethane fills the lakes. And liquid methane carved the gullies at rates far in excess of the worst flash-flooding on Earth.

Liquid ethane fills lakes on the surface of Titan, Saturn's largest moon. Credit: AP PHOTO/NASA

Titan Unveiled, by planetary scientist Ralph Lorenz and astronomy writer Jacqueline Mitton, presents a good overview of the state of our knowledge of this curious moon, and is accessible to most. Lorenz is closely involved with the Cassini mission to Saturn and the Huygens probe it dropped onto Titan's surface in 2005. The book focuses on his key interests, which include Titan's surface and lower atmosphere, regions that parallel Earth and are thus the most engaging for readers.

Titan Unveiled describes how most of what we once hypothesized about Titan has been proved wrong. The story of how we gained our current knowledge is fascinating; even more intriguing is what remains to be learned. Larger than Mercury, Titan is the only moon in our Solar System that is enveloped in a thick atmosphere. Analogous to Earth's water-based weather system, Titan's atmosphere experiences weather based on the phase changes of methane, shifting between its gas, liquid and solid states. At the extremely cold temperatures on Titan's surface (94 K, or −179 °C), water is frozen and acts like rock. The moon is geologically active, including volcanism and uplifting of mountain ranges. Deep under the icy surface, evidence for an ocean of liquid water and ammonia has been found.

Scattered throughout the text are personal anecdotes by Lorenz, labelled “Ralph's Log”. Key to the book's success, these sections convey how planetary exploration, and science in general, progresses as a human enterprise. Lorenz communicates what it is like to be a scientist involved with a current space mission, working with diverse colleagues and following your curiosity to make new discoveries.

Advances may come serendipitously, but they are usually hard-won following years of intense work, carried out with the risk of failure and research dead-ends. Some obstacles to progress are simple to overcome. For example, Lorenz recounts how, while working alone at night at an observatory, he was once held back by a crucial piece of equipment that lay behind a locked storage-room door. His eventual solution was to remove the door's hinges. Other challenges are greater, such as the discovery of an engineering problem with the radio transmitter on the Huygens probe after its launch. It required a major effort to retarget and replan nearly the entire mission, involving hundreds of people and thousands of hours of work.

With the Cassini mission flying past Titan every few weeks and astronomers observing it from Earth nearly every night, new discoveries are regular. It is inevitable that any book on Titan is a little out-of-date before it is released, but this reflects the vitality of the research.

We won't be able to book a ticket to Titan in the next few decades, but further robotic spacecraft will be sent to explore. A Titan orbiter could map the surface, observe the seasonal weather patterns and study the subsurface ocean. Balloon-borne detectors could examine the atmosphere and surface up close. And a new mission will add detail to our guidebook to Titan. Hopefully, someone working on that mission will write an insider's account, like Titan Unveiled, to tell us how it all happened.

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Roe, H. A rough guide to Titan. Nature 453, 453–454 (2008). https://doi.org/10.1038/453453a

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