Last fall, as NASA’s celebrated Cassini spacecraft spiraled toward its final, fatal descent into Saturn’s clouds, astrochemist Morgan Cable couldn’t help but shed a tear for the school-bus-size orbiter, which became a victim of its own success.
Early in its mission, while flying past Saturn’s ice-covered moon Enceladus, Cassini discovered jets of ice and saltwater gushing from cracks in the south pole — a sign that the body contained a subsurface ocean that could harbor life. When the orbiter began to run low on fuel, it smashed itself into Saturn rather than risk a wayward plunge that would contaminate the potentially habitable world.
Now, from beyond the grave, the spacecraft has offered yet another prize for scientists. New analysis of Cassini data suggests that those icy plumes shooting into space contain complex organic compounds — the essential building blocks of living beings.
The fact that an aging orbiter not designed for life detection was able to sense these molecules — which are among the largest and most complex organics found in the solar system — makes the icy moon an even more tantalizing target in the search for extraterrestrial life, said Cable, a research scientist at the Jet Propulsion Laboratory who was not involved in the new research.
“This is a powerful study with a powerful result,” she said.
The findings published Wednesday in the journal Nature rely on data collected by two Cassini instruments — the Cosmic Dust Analyzer and the Ion and Neutral Mass Spectrometer — as the spacecraft flew through Saturn’s outermost ring and the plumes of Enceladus (pronounced “en-SELL-a-dus”).
Previous research using these instruments has detected small organic molecules such as methane, which consists of four hydrogen atoms attached to a single carbon. The INMS has also detected molecular hydrogen — a chemical characteristic of hydrothermal activity that provides important fuel for microbes living around seafloor vents on Earth.
The molecules reported in the new Nature paper are “orders of magnitude” larger than anything that’s been seen before, according to lead author Frank Postberg, a planetary scientist at the University of Heidelberg in Germany. There were stable carbon ring structures known as aromatics as well as chains of carbon atoms linked to hydrogen, oxygen, maybe even nitrogen.
Some of the molecules sensed by the CDA were so large that the instrument couldn’t analyze them. This suggests that the organics Cassini found are only fragments of even bigger compounds, Postberg said. There may well be huge polymers — many-segmented molecules such as those that make up DNA and proteins — still waiting to be discovered.
“We astrobiologists get excited about larger molecules and that sort of thing because it means that something is building upon itself and making itself more complex,” said Kate Craft, a planetary scientist at Johns Hopkins University Applied Physics Laboratory who was not involved in the research.
The molecules Cassini has detected may be produced abiotically — without the involvement of life. But they are also the kinds of compounds that microbes on Earth like to eat, and they might even be byproducts of microbial metabolisms.
“Put it this way, if they did all these tests and didn’t see these larger molecules, [Enceladus] wouldn’t seem to be habitable,” Craft said. “But these findings . . . are reason to say, ‘Hey, we need to go back there and take a lot more data.’ ”
Scientists believe that the gravitational influence of Saturn squeezes and flexes the porous, rocky material at Enceladus’s core, generating heat. That heat allows for chemical interactions between the salty ocean and the seafloor. On Earth, such water-rock interactions provide fuel for chemotrophs — organisms that obtain energy by breaking down chemicals in their environments – and support vast ecosystems in the ocean’s deepest, darkest depths.
