Hiding in Plain Sight
The Cassini probe was launched to study Saturn, its rings, and its largest moon, Titan. But then, the icy moon of Enceladus crossed its path – with unexpected consequences for research.
Jun 17, 2019
The thick layer of ice reflects the sun’s light, leaving the entire surface of Saturn’s moon Enceladus wrapped in gleaming white. Averaging nearly 500 kilometers in diameter, the icy moon is one of more than 60 moons around Saturn. For many years, it has been the subject of research for Frank Postberg, who became a professor of planetary sciences at Freie Universität Berlin in October 2018.
He has been analyzing and interpreting data collected by the Cassini space probe on its Saturn mission since 2004, when he was still a doctoral candidate at the University of Heidelberg. The probe, which is named for Giovanni Domenico Cassini, one of the best-known European astronomers of the 17th century, was launched in 1997. Seven years later, once it traveled the 1.5 billion kilometers needed to reach the gas planet, Cassini was finally able to start its work – which meant Postberg’s work started as well.
“Everything we now know about Saturn, its ring system, and its moons is all based on analysis of the data the space probe sent back to us. It was practically terra incognita before that,” Postberg explains. The Voyager 1 and Voyager 2 probes had traveled through Saturn’s area in the early 1980s, but, as Postberg says, “they just whizzed by.” Cassini, by contrast, aligned itself with Saturn’s orbit and allowed itself to be caught by the gas giant’s gravity, ultimately remaining there for 13 years. It circled Saturn a total of 294 times between July 2004 and September 2017.
A Vast Ocean under a Thick Coating of Ice
Today, Postberg and his colleagues know more about Saturn and its moons than they ever thought it would be possible to find out on the mission, which was led by NASA, the European Space Agency (ESA), and the Italian Space Agency (ASI). What started out as purely an exploratory flight around Saturn, its rings, and its moons ended up supplying data that suggest there is a good chance that the icy moon of Enceladus could harbor extraterrestrial life.
Enceladus is not, as long presumed, a dead, ice-covered chunk of rock. Instead, samples collected by Cassini have shown that underneath the coating of ice, which can be up to 30 kilometers thick, lies a gigantic global ocean reaching depths of as much as 60 kilometers. “The probe discovered geologic activity at the south pole of Enceladus – what are known as cryovolcanoes, or ice volcanoes. There are rifts in the icy shell where veritable fountains shoot up, blasting water vapor, gases, and ice particles into space at a rate of as much as one kilometer per second,” Postberg says.
Four of these rifts are located at the south pole, running parallel to each other. In pictures, they are clearly recognizable as stripes across the smooth icy shell, which has earned them the name of “tiger stripes.” Along this formation, the spray from the geysers is like four curtains. Cassini flew through them, which allowed the probe to take samples of tiny ice particles and gas. “The particles are about 30 times thinner than a strand of hair,” Postberg explains. “They’re finer than droplets of fog, more like cigarette smoke.” If they were larger, they would have damaged the probe, which crossed the fountains at a speed of 40,000 kilometers per hour.
First-ever Chance to Study Extraterrestrial Liquid Water
“The samples we studied in the process allowed us not only to prove the existence of the ocean under the layer of ice, but also draw conclusions about its properties,” Postberg says. For example, scientists now know that the water has a slightly elevated pH value and is more alkaline than the Earth’s oceans. “This was the very first time anyone had a chance to study extraterrestrial liquid water. It’s also the only time so far.”
But the new information about the ocean’s existence and composition brought up a new puzzle: How is it possible for there to be a liquid ocean on the inside of the moon while the surface is covered with a layer of ice that is kilometers thick? Cassini data also showed that Enceladus exudes tremendous heat around the “tiger stripes.” While the icy moon’s surface temperature otherwise hovers at around negative 200 degrees Celsius, it rises to negative 75 degrees along the tiger stripes, about 130 degrees warmer than on the rest of the moon.
Both the ocean and the heat detected are only possible if the moon itself generates energy, Postberg explains: “Without an internal heat source, the ocean would have frozen in a million years at most. How, exactly, this moon – which is ten times as far from the sun as the Earth – generates heat was one of the biggest puzzles in planetary research in the past ten years.”
It wasn’t until 2014 that researchers made a discovery that put them on track to solve it: Along with ice particles, the samples from Cassini were also shown to contain tiny particles of rock, suggesting that there was a rocky core at the bottom of the ocean. Detailed studies of the gravitational field followed, using the probe. In the process, it was discovered that the rocky core has a diameter of 380 kilometers and must be very low in density.
While a normal pebble on Earth has a density of about 3.5 grams per cubic centimeter, the core of Enceladus is calculated to have a density of only 2.4 grams per cubic centimeter. “The only explanation is that the rocky core must be very porous,” Postberg says. And that discovery helped explain in the end how the moon generates energy. “Water from the ocean penetrates into this porous rock, which we should probably think of as being more like a pile of rock or rubble.”
Hydrothermal Vents like Those on Earth
“Saturn’s elliptical orbit causes powerful tides on the icy moon that revolves around it, and these tides practically knead the inside of Enceladus,” the planetary researcher explains. “As they rub against each other, the rocks generate heat, heating up the water flowing through them – like in a continuous flow water heater.”
This heats the water to temperatures as high as 200 degrees Celsius. It flows in the form of hydrothermal vents out of the rocky core and back into the ocean, which is at zero degrees, and then rapidly cools again due to the contact. But the rising heat is still enough to melt the layer of ice under the south pole, where the cryovolcanoes are active and the water pushes up to the surface, Postberg explains. While the coating of ice is as much as 35 kilometers thick at the moon's equator, it thins to only a few kilometers at the south pole.
Enceladus’s hydrothermal vents are much like those found on Earth, like the Lost City Hydrothermal Field, which is composed of mineral pillars reminiscent of a sunken city. In this area on the Mid-Atlantic Ridge, an undersea mountain range that runs from north to south through the entire Atlantic, hot water emerges from a number of vents in the ocean floor.
Despite the absolute darkness, the vent openings in the deep sea are home to life – in the form of bacterial cultures, for example. “Many scientists believe that prehistoric thermal vents offered the most favorable conditions for life to emerge, and that it spread around the entire globe from there,” Postberg says. “That’s why right now, astrobiologists think it is certainly possible that there is simple life on Enceladus as well.”
Clear Signs of Organic Components
In September 2017, Cassini reached the end of its mission. The fuel supply was almost exhausted after 13 years orbiting Saturn, so the probe made a controlled crash landing. For Postberg and his team, though, the work goes on. They expect to spend at least another two years analyzing and interpreting the data from Cassini. “Some of the ice particles show clear signs of organic components. Now we want to find out what specific organic compounds these are, and what kind of organic chemistry within the ocean on Enceladus might be behind it.”
The discovery of the ocean on Enceladus supports the hypothesis that there could be simple life on the icy moon. Several teams are working on developing proposals for follow-up missions, Postberg says. He expects that one of these missions will be launched at some point in the next ten years with the goal of proving or disproving that hypothesis.
This text originally appeared in German on April 27, 2019, in the Tagesspiegel newspaper supplement published by Freie Universität.