We’ve sent unmanned spacecraft to inspect all the planets of the solar system save distant Pluto, and have landed on several of them, including moons. And what we’ve found everywhere we’ve looked is wonderful—even if it’s not always what we expected (though that in itself is great news for a scientist).
Where might we go, not in search of resources, but for the sheer wonder of exploration?
Even the humble moon has its wonders. For example, at the moon’s north pole, at a crater called Peary, there are mountains where the sun never sets. These “Peaks of Eternal Light” are believed to be the only site in the solar system where this is true. It comes about because the moon’s axis isn’t tilted relative to the plane of its (and Earth’s) orbit around the sun, unlike Earth’s tilt, which is the cause of our seasons.
Mars may have no egg-laying princesses as in Burroughs’ books. But it is a small, strange world, very unlike the Earth, with volcanic mountains so tall they stick out of the atmosphere, and a canyon system that stretches around half the planet, and valleys that look as if they were carved by flowing water. Right now there are robots working up there, machines built by human hands rolling across the desiccated seabeds. And, we’re increasingly suspecting, maybe there’s life there after all. (In the Avatar universe there are human colonies on Mars.)
Venus is a world only a little smaller than Earth, but swathed in a monstrous ocean of atmosphere, almost all of it carbon dioxide, a bright layer that utterly blankets the ground from our view. It’s so hot down there that at night the ground glows in the dark. But, astonishingly, despite the (literally) infernal conditions, human craft have made it here too. The Soviets achieved a landing with Venera 7 in 1970, a spacecraft as tough as a miniature AMP suit. A very Russian achievement!
One of the general wonders of the age of planetary exploration is that the solar system is turning out to be full, if not of Earths, at least of abodes where some form of life is conceivable. Consider Jupiter’s second moon out. Europa is close enough to its parent for tides to have melted a deep layer of the moon’s water-ice mantle. Its cracked icy crust looks like nothing so much as ice floes on Earth’s frozen-over Arctic Ocean, beneath which is a sea, tremendously deep, perhaps reaching all the way to the moon’s rocky core. And hydrothermal vents on that black-as-night seabed could provide nutrients for some form of life. A world with a roof.
And, a little further out, is a mysterious world that may be the solar system’s greatest wonder of all.
The furthest any craft from Earth has landed, so far, is on Titan, sixth moon of the sixth planet Saturn, nearly ten times as far from the Earth as the sun. It was an astounding achievement.
And the world the Huygens probe found is the solar system’s own Pandora.
Titan, Saturn’s largest moon, was discovered by the Dutch astronomer Christianus Huygens in 1655. To him it was just a dot of light, glowing dull orange. But in 1944 Gerard Kuiper, another Dutch astronomer, discovered methane gas there. This was a moon with air! Titan turned out to have the most massive atmosphere of any rocky world after Venus. Bigger than our moon but only half the diameter of Earth, Titan is able to retain a fat layer of air because of its extreme cold.
Our first close-up views of Titan came in 1980 and 1981, when Voyagers 1 and 2 flew past Saturn. But Titan was just a ball of smog; we could see nothing of the surface. Then, in 2004, the Cassini spaceprobe arrived, with the Huygens lander, named for the pioneering astronomer, clinging to its side.
Titan really is like Pandora in many ways. Like Pandora it is a low-gravity moon of a giant planet, and, superficially, remarkably Earthlike. Huygens came down on what looked like a relic of a flash flood, a plain littered by worn pebbles. On Titan there are mists and clouds, and slow-falling rain; there are branching river valleys that lead to oceans crossed by waves hundreds of metres tall. One ocean, called the Kraken Mare, is as big as the Caspian Sea.
But Titan is an Earth reimagined in different materials. On Titan water-ice plays the role silicate rock does on Earth, and methane plays the part of liquid water. Those pebbles Huygens saw were ice, not rock. The methane cycle isn’t quite like Earth’s water cycle, so the weather isn’t the same; evaporation is slow, but the air can hold a lot of vapour. The result is long periods of drought punctuated by intense rainstorms. There could even be “cryovolcanoes,” volcanoes spewing liquid water; there is evidence of lava flows in the past. If you stood on Titan you would be a monster of molten lava!
And, like Pandora, Titan is full of opportunities for life.
Out of those layers of clouds, complex organic molecules—the stuff of life itself—continually drift down to the surface below. They are created by electrical storms in the atmosphere, and the reaction of sunlight and Saturn’s magnetism with the upper air. These organic molecules could be the basis of an Earthlike life: carbon-water life, maybe anaerobic (that is, oxygen-hating) methane-eating bugs.
But there could be other kinds of life. Maybe a more exotic sort of carbon-based life form, using ammonia as its solvent rather than water and a metabolism based on carbon-nitrogen bonds, could be found in the stuff bubbling out of the cryovolcanoes. This is the sort of life that might live in the oceans of “roof worlds” like Europa. Most exotic of all could be a community of slime-like organisms that use silicon compounds as their basic building blocks, not carbon as we use; they might live in the surface ethane lakes, so cold they favour the long but fragile silicon-silicon molecular chains on which this form of life depends. Such forms might also find a home on Triton, the even colder moon of Neptune, where there are lakes of liquid nitrogen.
Nowadays we envisage many kinds of life, and many diverse habitats in the solar system. But Titan is extraordinary, for it may be a junction for life forms related to types from deep within the solar system’s warm heart, and from its chill edge. Huygens only glimpsed this; we must go again.
But Titan, like the other bodies of the solar system, might have a value beyond science—and that’s what might put it at risk. Titan is a natural organic-synthesis machine, way off in the outer system. It could become a factory for future colonists, churning out fibres, plastics, even synthetic food, manufactured from carbon, hydrogen, oxygen, nitrogen. Further out in time, it may be possible to export Titan’s volatiles to inner planets lacking them; Titan nitrogen, taken away on a massive scale, could be used to terraform Mars, to make it like the Earth. Just as some once hoped the moon could be a stepping stone to the planets, so Titan, a vital resource pool on the fringe of interstellar space, may some day be a key refuelling dump for ships like Venture Star, on their way to the stars.
But surely the worlds of the solar system are more than just mines in the sky. There are already proposals to preserve the unique value of other worlds. Radio astronomer Claudio Maccone of Turin advocates a “protected antipodal circle” of radio silence covering the moon’s far side. This is the only place in the solar system permanently shadowed from Earth’s clamorous broadcasts and so ideal for radio astronomy, and worth preserving as a park of silence.
Certainly, I personally hope that by the time we get to Titan we will treat it with more respect than RDA treats Pandora.
So we have reached the effective edge of the solar system, and there have been wonders aplenty—but no true Pandora, nothing like Earth. To find life like ours we will have to go on beyond the sun’s family.
But how are we going to get there? Could we ever build a ship capable of reaching the stars? Will a Venture Star ever fly?