Planetary scientist Carolyn Porco led the imaging team of the Cassini mission to Saturn, which is about to complete its 20-year exploration of the entire system of rings and moons. Porco has authored over one hundred scientific papers and also served as the imaging scientist of the Voyager mission to the outer solar system in the 01980s.

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Where will unambiguous signs of life most likely be found outside Earth? While telescopes squint at impossibly distant (but numerous) exoplanets, increasing numbers of increasingly brilliant robots are probing the wildly exotic potential environments for life nearby in our own Solar System. Which ones are the most promising candidates, and why, and what are the plans to check them out?

Is life in the universe a one-off freak, or the norm? If we find just one more instance, we can infer it’s the norm.

Planetary scientist Carolyn Porco became most famous as the dynamic leader of the Imaging Team for the Cassini mission to Saturn, which delivered no end of stunning photos of the planet, its gorgeous rings, and its extravagant moons. (Her team discovered seven new ones.)

Life nearby

If we find, anywhere in the universe, one more instance of life besides what evolved on Earth, then we are bound to conclude that life is common throughout the vastness of this galaxy and the 200 billion other galaxies. The discovery would change how we think about everything.

Most of the search for life beyond Earth, Porco explained, is the search for habitats. They don’t have to look comfy, since we know that our own extremophile organisms can survive temperatures up to 250°F, total desiccation, and fiercely high radiation, high pressure, high acidity, high alkalinity, and high salinity.

In our own Solar System there are four promising candidate habitats—Mars, Europa (a moon of Jupiter), Titan (a moon of Saturn), and Enceladus (“en-SELL-ah-duss,” another moon of Saturn). They are the best nearby candidates because they have or have had liquids, they have bio-usable energy (solar or chemical), they have existed long enough to sustain evolution, and they are accessible for gathering samples.

On Mars water once flowed copiously. It still makes frost and ice, but present conditions on Mars are so hostile to life that most of the search there now is focussed on finding signs of life far in the past. Europa, about the size of Earth’s Moon, has a salty ocean below an icy surface, but it is subject to intense radiation. Photos from the Hubble Space Telescope revealed that occasional plumes of material are ejected through Europa's ice, so future missions to Jupiter will attempt to fly by and analyze them for possible chemical signatures of life.

The two interesting moons of Saturn are Titan, somewhat larger and much denser than our Moon, and tiny Enceladus, one-seventh the diameter of our Moon. Both have been closely studied by the Cassini Mission since 2004. Titan’s hazy atmosphere is full of organic methane, and its surface has features like dunes and liquid-methane lakes “that look like the coast of Maine.” But it is so cold, at 300°F below zero, that the chemical reactions needed for life may be too difficult.

Enceladus looks the most promising. Cassini has sampled the plumes of material that keep geysering out of the south pole. The material apparently comes from an interior water ocean about as salty as our ocean, and silica particles may indicate hydrothermal vents like ours. “I hope you’re gettin excited now,” Porco told the audience, “because we were.” The hydrothermal vents in Earth’s oceans are rich with life. Enceladus has all the ingredients of a habitat for life—liquid water, organics, chemical energy, salts, and nitrogen-bearing compounds. We need to look closer.

A future mission (arriving perhaps by the 2030’s) could orbit Enceladus and continually sample the plumes with instruments designed to detect signs of life such as complexity in the molecules and abundance patterns of carbon in amino acids that could indicate no biology, or Earth-like biology, or quite different biology. You could even look for intact organisms. Nearly all of the material in the plumes falls back to the surface. Suppose you had a lander there. “It’s always snowing at the south pole of Enceladus,” Porco said. “Could it be snowing microbes?”

(A by-the-way from the Q&A: Voyager, which was launched 40 years ago in 1977, led the way to the outer planets and moons of our Solar System, and five years ago, Porco pointed out, “It went beyond the magnetic bubble of the Sun and redefined us as an interstellar species.”)