Cassini also revealed Saturn’s largest moon, Titan, to be a complex, shifting world where giant dune fields of organic molecules blow in nitrogen winds over icy plains, and where organic chemistry mirrors precursors to life on Earth. Titan has rivers and lakes of liquid methane and ethane, methane weather systems of clouds and storms that mirror Earth’s hydrologic cycle, and seasonal cycles that rival Earth’s in complexity. Titan, too, is a world that could harbor life, and another place we’ll surely return to, perhaps to explore with airships or boats. In the meantime Titan has already helped us see climate in new ways. Repeated measurements of the radiation, chemistry, and particles in that strange atmosphere have added to the body of understanding with which we predict and respond to the changing climate of our own planet.
After all these years and discoveries, the discord surrounding Titan’s launch now seems like ancient history. Fittingly, Cassini’s demise on Friday will be carried out as one final act of environmental responsibility. The spacecraft is being deliberately crashed into Saturn due to the possibility, however slim, of living creatures on Enceladus and Titan. NASA, and all the other spacefaring nations of the world, have agreed to a set of “planetary-protection” principles, aimed at preventing the accidental contamination of another habitable world with organisms from Earth. Once Cassini runs out of fuel, if it is still in orbit, it could conceivably crash into, and contaminate, one of the moons. To prevent that, it is being hurtled into Saturn, on purpose, before the fuel runs out, when we can still control its movements.
What are the chances that earthly organisms not only have somehow survived Cassini’s journey and all its years in Saturn’s punishing radiation belts, but also would survive and thrive in the alien environs of one of those moons, potentially threatening indigenous organisms? Extremely low, to say the least. Perhaps absurdly low. But we cannot say that they are zero. And the risk of making a mistake in this arena is incalculable. So Cassini must be sacrificed. (It’s worth noting that plutonium is a naturally occurring element, so Cassini’s crash won’t introduce anything to the chemistry of the planet that isn’t already there.)
Cassini’s fiery swan dive is an expression of applied environmental ethics, orchestrated out of concern for the environmental protection of Saturn’s potentially habitable moons. As the spacecraft plummets, it will be returning new data on the composition of the planet’s upper atmosphere, a machine proxy for our human curiosity, exploring with its last electronic breaths. Then, glowing with the heat of entry and buffeted by the increasing atmospheric pressure, it will go silent, begin to vaporize, and fall to pieces. As it comes apart, among the last parts to remain intact will be those containers of plutonium, encased in iridium, hardened to survive reentry. Eventually, as they descend into the crushing depths where pressures far exceed any conditions on Earth, these too will disintegrate and merge into the strange hyperbaric liquid soup of the Saturnian depths.