By JoEllen McBride, PhD
Our solar system is full of potential. From Earth to the frozen surface of Pluto, hydrocarbons and other complex organic molecules are surprisingly common. With every new space mission, we find the ingredients of life on more of our celestial neighbors.
The newest location to add to our list of places with potential for life comes from NASA’s Cassini spacecraft which began its study of Saturn in 2004. In the 13 years that Cassini has studied Saturn and its moons, it solved many mysteries and discovered some startling similarities to our own planet.
Saturn, at first glance, seems nothing like Earth. It is a gas giant, full of hydrogen and helium, with a possible Earth-sized core at the center. But Cassini revealed that there are phenomena occurring in the gas giant’s atmosphere that also occur on Earth. Cassini recorded video of lightning strikes on Saturn— the first taken on a planet other than our own. Since Saturn doesn’t have interference from mountains and other land features, jet streams can flow unimpeded forming a continuous hexagonal shape at the poles. But scientists are still unsure why that specific shape is created. Saturn also develops a planet-wide storm every 30 years that just happened to show up while Cassini was around in 2011– 10 years early. From the data collected by Cassini, scientists were able to determine that the storms form in a similar way to thunderstorms on Earth. Instead of adjacent hot and cold fronts mixing on Saturn, layers of warm water vapor and cool hydrogen gasses mix. The storms take time to develop because water vapor is much heavier than hydrogen so it is normally positioned below the hydrogen fog. This gives the elevated hydrogen gas time to cool. Once it cools down enough, it becomes more dense which causes it to sink into the warmer water vapor. The two mix and voila!, a Saturnian thunderstorm is born. The storm also kicked up hydrocarbons from the lower atmosphere which surprised scientists.
Although Saturn probably can’t harbor life, two of Saturn’s moons, Titan and Enceladus, are ripe with the ingredients. The Cassini spacecraft made numerous orbits around Titan and even sent a probe (Huygens) down to the surface. Titan has land features similar to Earth, with lakes, mountains, ice caps, and deserts. The difference is methane and ethane are the chemical building blocks of the complex molecules found on the moon instead of carbon.
Enceladus was the biggest surprise to come out of the Cassini mission. This moon is essentially a smaller version of Jupiter’s moon Europa. Both are covered in a liquid ocean topped with a thick layer of ice that surrounds the moon. There is one big difference: Enceladus has hydrothermal vents deep within its oceans, just like on Earth, and these vents violently force liquid through cracks in the ice. The plumes are huge and powerful, extending hundreds of miles into space and traveling at hundreds of miles an hour. The Cassini spacecraft revealed that these plumes are chock full of hydrocarbons, which are the building blocks necessary for life. This tells scientists that there is the potential for life in the oceans of Enceladus and possibly Europa.
The other moons that Cassini visited revealed some startling information. Tethys has bright arcs of light which can only be seen at infrared wavelengths. Scientists are puzzled as to what they are and what is causing them. The spongy-looking moon Hyperion builds up a static charge as it tumbles around Saturn. Mimas, aka the Death Star Moon, was thought to be a dead world but shows evidence of a liquid ocean underneath its cratered surface. The moon is the same size as Enceladus but has no visible jets or plumes, so the liquid is trapped beneath the surface. Why these two moons are so different and whether Mimas’ ocean is full of hydrocarbons is something scientists hope to study in the future.
The potential of life in the Saturnian system is the main reason Cassini’s mission will come to a destructive end. The spacecraft is running out of fuel, meaning that scientists on Earth will eventually lose the ability to control the spacecraft. Our own planet is surrounded by defunct satellites whizzing around our planet– just waiting to crash into other orbiting objects. The scientists in charge of the mission worry that if Cassini were left to orbit Saturn, it could potentially crash into Enceladus. This could introduce foreign microbes and chemicals, devastating any microbial life on the moon or ruining the chances of it ever forming. Instead, Cassini is performing its last dance with Saturn, orbiting the planet so closely that it is between the rings and the gaseous atmosphere. After 22 orbits, the spacecraft will take a dive into Saturn’s clouds on September 15, 2017, sacrificing its own metallic body for the sake of billions of potential life forms on the moons of Saturn.