As probe comes to an end, scientists plan to use its findings to search for extraterrestrial life
NASA’s aging Cassini spacecraft plunged like a falling torch into the atmosphere of Saturn early Friday morning, ending in a blaze of burning plastic and molten aluminum its 13 years of exploration around the ringed planet.
At the Jet Propulsion Laboratory in Pasadena, Calif., which manages mission operations, people greeted the predawn moment when Cassini ceased transmitting radio signals to Earth with groans, applause and sighs. In the control room, somber technicians hugged each other. At the California Institute of Technology nearby, hundreds of scientists, some of whom worked on the project for 27 years, watched with their families as Cassini’s final moments played out on outdoor video screens.
“I am going to call this the end of mission,” Cassini program manager Earl Maize announced to the JPL control room.
NASA mission managers intentionally steered the probe to its doom in order to avoid contaminating Saturn’s icy moons with Earthly bacteria that might have survived the rigors of space or radioactive plutonium-238 fuel from its small nuclear power generators. But the mission’s end also marks a beginning.
For the past 13 years, the probe has been studying the ringed planet and its moons. Among its most surprising and influential findings were the discoveries of liquid oceans on two of its moons: Enceladus and Titan.
That has emboldened the National Aeronautics and Space Administration to embark on new interplanetary missions to examine if simple life-forms exist in outer space. Under the agency’s Ocean Worlds program launched in 2016, NASA will explore the universe’s diverse oceans, with Enceladus and Titan, along with Jupiter’s moon Europa, the leading contenders for potential life-breeding environments.
Oceans are among the most habitable places scientists know about, according to Curt Niebur, NASA’s lead program scientist for outer-planet exploration. Here on Earth, some posit life may have evolved at hydrothermal vents, geysers at the bottom of the ocean.
Not long ago, astronomers thought oceans were an anomaly, found on only Earth and Jupiter’s moon Europa. Now, scientists say there has been a paradigm shift in how they see the universe, in large part thanks to Cassini.
“Maybe these ocean worlds are not the one-off places that we thought,” said Dr. Niebur. “Maybe, they’re actually common in not just the solar system, but in the universe. That’s where the implications get really profound.”
When Cassini flew by Enceladus, it gave planetary scientists and astrobiologists a peek at water-containing plumes squirting out of the icy moon’s southern pole. These plumes, which scientists concluded were spewing material from a liquid ocean trapped under miles of ice, also contain other compounds that scientists say are important building blocks of life: hydrogen, salt and ammonia.
“This is very important. Since the discovery, we are thinking that Enceladus may be a good place to search for extraterrestrial life, and this was not expected at all before Cassini,” said François Raulin, a scientist who worked on Cassini.
Astronomers have put together a proposal for a mission called Enceladus Life Finder to go back and study its plumes more closely, according to Linda Spilker, a Cassini mission project scientist at NASA. The team expects a decision in December, she said. Scientists have proposed missions to Titan, but none are scheduled so far.
Meanwhile, Jupiter’s Europa is up first in the Ocean Worlds life-finding tours. In the 1990s, data from the Galileo orbiter suggested that the moon had an ice-capped ocean and plumes, much like Enceladus. A mission to the Jovian moon, dubbed Europa Clipper, is now scheduled for the 2020s. Researchers and engineers are working to build the sensors and instruments necessary to study this moon in greater detail.
Cassini wasn’t equipped to find life, but Dr. Niebur said the data it acquired has been “invaluable” in helping scientists figure out what life-detecting instruments future spacecraft should carry. Those tools include mass spectrometers to allow scientists to measure complex molecules, such as proteins and cellular compartments; DNA sequencers that work in deep space; and ground-penetrating radars to explore beneath Europa’s icy crust. These would help scientists figure out where it might be easiest to sample.
While these technologies should, in theory, help astrobiologists sleuth out signs of life on other planets, questions remain. To start, scientists have yet to define what might constitute “life” on other planets, according to Mary Voytek, NASA’s senior scientist for astrobiology, so there is some debate about what to measure.
Dr. Voytek developed the “Life Detection Ladder,” which describes general indicators of life on Earth. The giant spreadsheet includes basic elements like water, amino acids, genetic material and energy, as well as more complex components like metabolic and reproductive systems.
But what if extraterrestrial life doesn’t mimic Earth’s?
Titan, Saturn’s largest moon, is one of the most Earthlike worlds in the solar system. Its atmosphere is made up of nitrogen, which is the same major component of Earth’s. It has valleys, rocky hills, craters, surface oceans and rivers. But water doesn’t flow through the red moon’s rivers and oceans. Instead, they’re made up of methane and ethane.
“Could those seas support a very exotic biology, a very exotic kind of life?…It would push the boundaries for the definition of life,” said Jonathan Lunine, a co-investigator for Europa Clipper and a Titan expert at Cornell University. In computer simulations, his team found that, in theory, methane could support life, though that still needs to be tested in lab experiments and then eventually by going back to Titan, he said.
Cassini’s destruction may have left planetary scientists in 19 countries bereaved, but the spacecraft’s sensors gathered so much data that researchers said it could take decades for them to analyze it all. So far, they’ve published more than 3,900 technical papers on findings from its five billion miles of space travel and they are conceiving return voyages.
Appeared in the September 16, 2017, print edition as ‘Cassini Probe Around Saturn Comes to a Fiery End.’