First published in The Tablet in January, 2005, just after the Huygens probe landed on Saturn's moon Titan. This text is based on what I submitted, which was edited for length when it was printed.
The most remarkable thing about the images from Saturn’s moon, Titan, which ESA’s Huygens probe has just sent us, is how “familiar” they look. They remind me of Nadar’s 19th century Collodion “wet plate” photographs taken from a balloon above Paris. Black and white, a bit fuzzy, these patterns of light and darkness can be grasped and processed by imagination and memory until we impose meaning on them. I picture myself floating through the clouds, gazing across at an intricate pattern of riverbeds cutting through rough light-colored mountains, flowing down to a flat, wine-dark sea.
If these are riverbeds – and they certainly look like textbook examples of the many-branched “dendritic channels” made by rainfall flowing down hills on Earth – the fluid in them can’t have been water. The temperature at the surface of Titan is several hundred degrees below zero. The only common substance that flows at that temperature is methane: liquified natural gas. And indeed, there is no guarantee that the flat regions are seas. As we have learned, the flat dark areas on Earth’s Moon once named “maria” are dry as dust.
The image taken from the surface, once the probe had landed, shows a field strewn with well rounded pebbles and boulders just like the rocky shore of a mountain lake. The rough edges have been worn off the stones, presumably when they were tumbled about while carried downstream by flowing liquids. But Titan's pebbles are made not of stone, but water ice; and at these temperatures, ice is as hard as steel. It must have taken a lot of rubbing to work these pebbles smooth.
The mission scientists have done their best to try to describe what they and their instruments have seen. “Titan may be typical of arid regions of Earth like Arizona, where riverbeds are dry most of the time,” said Marty Tomasko, quoted in the New Scientist last week. “Perhaps there’s a wet season once a year. We just don’t know.” He knows about Arizona and its river beds and rainy seasons; Dr. Tomasko teaches planetary sciences at the University of Arizona, where I was one of his students nearly thirty years ago.
Dry riverbeds are outside the experience of most of the European researchers. A few years ago, one of the Dutch members of the Huygens science team gave a talk at Arizona describing his results modeling the atmosphere of Titan. He described with amazement how, under the right circumstances, droplets of methane could rain out of the clouds but completely evaporate before they hit the ground. He was astonished at the concept; his Arizona audience smirked at his astonishment. Unlike in the Netherlands, that happens in the Arizona desert (it’s called verga) all the time.
On the other hand, a researcher working with John Zarnecki’s team at the Open University, which built a probe on Huygens to measure the strength of Titan’s surface, reported that the surface had the consistency of “creme broulee.” That is a substance not commonly known in Tucson, Arizona.
We describe the unknown in terms of the things we know. That’s the only way we can describe it. All description is metaphor. Remember the dendritic channels mentioned above? “Dendritic” comes from the word for “tree,” an apt metaphor for many-branched mountain streams. Thus dark patterns on Titan are analogized with river beds on Earth, themselves described by analogy with the shape of a tree.
Even a scientific equation is a metaphor. The laws of gravity and hydrodynamics that described the Huygens probe’s descent through Titan’s thick atmosphere are not literally true, just very good approximations. The path of that space probe was very much like the solutions to those equations. But the descent was a unique event, distinct from those general solutions. The equations describe; they do not replace.
Those who demand that the Bible (or the Law, or any other description) must be read literally, can’t literally mean what they say. God is bigger than any human words; only poetry can do him justice. The same is true for God’s creation.
Alas, the ability to understand and appreciate poetry is something too often lacking in a technical education. Without the ability to see how metaphor conveys truth (often better than more literal descriptions) one loses the chance to learn from the metaphors that life gives us. More seriously, without understanding how metaphor works, we are liable to be misled by analogies that are pushed too far, especially if we fail to realize that they are indeed analogies. The poetry of creme broulee is a delicious metaphor for Titan’s gooey surface with its thin hard crust, but the next visitor to Titan will be very disappointed if he brings a spoon and an appetite.
But the final lesson to be realized is that, when our experience is limited, our store of analogies is also limited, which makes it that much harder to comprehend anything new. Tamasko’s experience of dry river beds let him see patterns in the images of Titan that a rain-drenched European might have missed; but his analogy doesn’t convey the sponge-like surface layers so dear to creme broulee aficionados. We read into a scene (be it on a planet’s surface, or in life) the things we expect to see, which is to say the things we’ve seen before. The more we see, the more we are prepared to see.
For that reason alone, the human race grows when we lift our eyes beyond our terrestrial horizons.