SELFI: NASA’s Pursuit of Life on Saturn’s Enceladus

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A new instrument will study the plumes of Saturn’s small moon, telling scientists more about the possibility of life in its global ocean.

Enceladus with its tiger stripes (blue) in false color. Image Credit: NASA/JPL/Space Science Institute

Scientists didn’t initially believe that Enceladus, Saturn’s small moon, could harbor life. It was too far from the sun and appeared to be frozen solid. However, the recently ended Cassini mission hinted at the possibility when it flew by and captured images of plumes spewing water from the surface of Enceladus at a rate of 358 meters per second. Scientists hadn’t anticipated this, so instruments onboard Cassini were not designed to study the plumes thoroughly. NASA scientists are now building an instrument to measure the chemical composition of the plumes. It’s called the Submillimeter Enceladus Life Fundamentals Instrument, or SELFI for short.

Cassini helped scientists answer many questions. First, Cassini found that the plumes were coming from the southern hemisphere of Enceladus, from fissures that are referred to as “tiger stripes.” It is unknown why, but the northern hemisphere of Enceladus’ surface appears to be older than its southern hemisphere, covered with many impact craters compared to the smooth, nearly crater-free surface of its other face.

Second, Cassini confirmed the presence of a subsurface global ocean under Enceladus’ icy crust. There was a slight wobble in the moon’s orbit that could not exist if the moon was frozen solid. The plumes are coming from this ocean. Cassini also determined that the plumes were mostly water vapor. Other elements of the plumes are methane, ammonia, carbon dioxide and silica grains. This was an exciting discovery for scientists, but it also raised questions. Consider this: Enceladus is covered in an icy shell that is so white that it reflects nearly 100 percent of the light that reaches it. So what mechanism was heating the subsurface water to result in such large plumes spraying from its surface?

Scientists concluded that there must be hydrothermal vents heating the ocean from below. The size of the silica grains also pointed to this possibility. If these hydrothermal vents were anything like hydrothermal vents found in Earth’s oceans, Enceladus could very well have thriving ecosystems. And that, ultimately, is what scientists are trying to find out.

Artist rendering of the Enceladus subsurface ocean and hydrothermal vents. Image Credit: NASA/JPL-Caltech/Southwest Research Institute

“You have an environment that seems as though it may be habitable. You have liquid water and you have temperatures that are warm, but what else could be going on?” said Gordon Chin, the principal investigator for SELFI. “The only way to know is to go back there with instruments that are capable of sampling and seeing if it is capable of life.”

The aim of SELFI itself is not to determine if there is life, DNA or complex proteins. Instead, SELFI use a highly sensitive submillimeter spectrometer to detect certain molecules which may exist in the plumes. Each chemical compound has a unique submillimeter signature. A submillimeter spectrometer can detect these signature wavelengths and determine exactly what molecules there are, even if they are present in trace amounts. SELFI will use remote-sensing to identify 13 molecular species, including water and its isotopes, methanol and sodium chloride. This will tell scientists about the oxidation state of the ocean, if the subsurface ocean is salty and whether there are subsurface volcanoes. In addition, SELFI will determine characteristics of the plumes such as their velocities and temperatures.

Essentially, SELFI will determine the possibility of how habitable Enceladus’ subsurface ocean might be.

SELFI was chosen by NASA’s Maturation of Instruments for Solar System Exploration (MatISSE) program to advance its technical readiness for a space flight project. The sensor electronics are being built and tested to make it sure that the instruments can survive operating in Saturn’s environment. SELFI is on track to be completed by 2022.

Chin says SELFI is an ambitious instrument, with much potential.

“Enceladus is a unique place,” he said. “You don’t have to drill through kilometers of ice to get to the ocean. Here, nature has provided samples of the ocean content in space for you. It’s the easiest place for us to get into another world’s ocean without drilling or a submarine or any other difficult-to-deploy technology. I think that’s why Enceladus is such an attractive place to look for exobiology. It’s just so exciting to think about that.”

Cassini’s capture of the plumes erupting from Enceladus. Image Credit: NASA/JPL/SSI; Mosaic: Emily Lakdawalla

IEEE Earthzine Editor-in-Chief Paul Racette is a member of the senior technical staff at the NASA Goddard Space Flight Center and lead system engineer on SELFI.

Sanna Darwish is a student science writer at NASA Goddard Space Flight Center and a senior hearing and speech sciences major at the University of Maryland.