If it's true that a picture is worth a thousand words, what NASA's Cassini mission has left for us is indeed a treasure.
Launched in 1997, the mission terminated dramatically last week with the probe's final plunge into Saturn's upper atmosphere.
During its 13 years near Saturn, the mission has revealed the unexpected and spectacular richness of Saturn and its 62 (known so far) moons. This compilation of 100 images by The New York Times is worth many visits.
Once we see the majestic planet and its rings, it's hard to avoid feelings of otherworldliness; we are so different and, yet, Saturn is like a mini-solar-system in itself, with its moons and asteroid belt (its rings). While Cassini has answered many questions, such as illuminating the remarkable structure of the rings, it has also opened new lines of inquiry. In particular, two of its moons, Titan and Enceladus, have jumped to the top of the list of best candidates for finding life outside Earth. (They tie with Jupiter's moon Europa.)
Titan is a hazy ball larger than Mercury, the only moon in the solar system with an atmosphere. Using infrared cameras, Cassini was able to peer through Titan's nitrogen-rich atmosphere to discover massive lakes of liquid methane and other carbon-rich compounds. Cassini released the Huygens probe into Titan to take in details of its atmosphere and surface. Amazingly, it found mountains and canyons carved by liquid methane and a surface punctuated by rocks. No two worlds are ever the same; but there are general features that repeat across the solar system if liquid is allowed to flow and hasten erosion, while exchanging heat with the atmosphere. Although extremely cold, Titan is a model for an alien environment, rich with a dynamic atmosphere-surface energy exchange.
Enceladus was an even bigger surprise. Beneath its frozen surface lies an ocean of water, warmed by Saturn's gravitational pull. (Think of a ball of Play-Doh and how it gets warm if you keep squeezing and stretching it.) Near Enceladus' south pole, Cassini found huge geysers that shot water into the atmosphere, releasing the heat from the moon's insides. If you scroll down the Times' photo gallery, you can watch a movie of a geyser erupting.
Now, pause a second to ponder this: a man-made probe shooting a movie of a geyser erupting in a world about 750 million miles away. And we can watch this on our laptops at home.
But Cassini's scientists wouldn't just take pictures. They maneuvered the spacecraft to fly through the plumes to collect samples of the erupting vapors. Chemical analysis found water vapor, carbon dioxide, methane, molecular nitrogen, propane, acetylene, formaldehyde and traces of ammonia, all chemicals related to life. This discovery propelled Enceladus to the top of the list of worlds where we should search for life. No need to drill through the surface into the underground ocean; just fly through the plumes and collect samples to search for some kind of simple life form.
We are moving to a new phase of space exploration, going beyond the visits and mapping, and into the let's-really-look-for-life-out-there phase. We have found the best candidates and what kinds of environments they offer. NASA's planned mission to Europa, named Europa Clipper, is expected to launch in the 2020s. As in Enceladus, Europa is also supposed to have hot geysers shooting from its surface.
Whatever is out there (or not), we will have much to learn from visiting an alien ocean with four times more water than all oceans on Earth combined. But even if we do find some kind of alien life, one essential lesson will remain the same: With every new world we discover — and explore if it harbors, or not, some form of simple life — we should look back at our own planet with awe. For worlds like our own will surely be few out there, in the frozen depths of space.
Marcelo Gleiser is a theoretical physicist and writer — and a professor of natural philosophy, physics and astronomy at Dartmouth College. He is the director of the Institute for Cross-Disciplinary Engagement at Dartmouth, co-founder of 13.7 and an active promoter of science to the general public. His latest book is The Simple Beauty of the Unexpected: A Natural Philosopher's Quest for Trout and the Meaning of Everything. You can keep up with Marcelo on Facebook and Twitter: @mgleiser