Thursday, April 13, 2017

New Signs of an Environment Favorable for Life on Saturn’s Enceladus

In the search for extraterrestrial life, one of the most promising candidates so far is the tiny moon Enceladus. Research appearing today in the AAAS journal Science includes exciting new evidence of this promise—the detection of molecular hydrogen.

A view of icy Enceladus from the Cassini spacecraft.
Image Credit: JPL-Caltech/Space Science Institute.

One of Saturn’s 62 moons, Enceladus is just over 300 miles in diameter (the Earth is nearly 8,000) and covered in an icy crust. It may not sound like an a very inviting place to live, but data from the Cassini spacecraft’s flybys suggest that Enceladus is home to a global ocean that sits between the frozen exterior and a rocky core. Streams of water and ice spew forcefully out from something akin to icy volcanoes near the south pole of Enceladus, providing clues to the chemistry of this ocean.

In 2015, the Cassini spacecraft flew close to Enceladus for the last time. As in previous flybys, one of the instruments on board, called the Ion Neutral Mass Spectrometer, collected and analyzed samples emitted by one of the geysers. This instrument sorts the contents of a sample by mass and charge to determine the types of atoms and molecules it includes. Previously, the results have indicated the presence of organic compounds, volatile gases, carbon dioxide, carbon monoxide, salts, and silica in the water.

This time, the Cassini scientists ran the mass spectrometer in a different mode than in previous runs. While much less sensitive than the normal mode, this special mode made it possible for them to look for low levels of molecular hydrogen that can be easy hidden by noise when the instrument operates in its normal mode. An analysis of the results by scientists from the Southwest Research Institute, Johns Hopkins University, and Cornell University found it—a statically significant signal of hydrogen.

The existence of hydrogen on Enceladus doesn’t necessarily mean that there is or was life there, but it does suggest that the ocean contains the chemical energy to support some forms of life.

In the journal article, the scientists lay out the possible origins for this molecular hydrogen, such as preexisting reservoirs in the ocean and chemical reactions taking place in the icy crust. One-by-one, the scientists discuss whether the chemical concentrations of molecular hydrogen and other elements Cassini detected support each possibility. The most likely source, they reveal, comes from hydrothermal reactions between water and the rocky core that forms the bottom of the ocean.

People usually consider sunlight to be a key ingredient for life. This might seem like a sticking point, since the icy surface of Enceladus reflects nearly 100% of the light that reaches it and the ocean is hidden beneath many miles of ice. However, life exists in even the deepest, darkest parts of the Earth’s oceans—life sustained by chemical energy. One example of this is something called methanogenesis, a process in which microbes take carbon dioxide and hydrogen formed in water-rock reactions near the ocean floor, and convert it into methane to receive a jolt of energy. Methanogenesis plays a key role in supporting ecosystems at the bottom of the ocean, where many scientists believe terrestrial life developed in the first place.

The fact that the Cassini sample contained hydrogen created by hydrothermal activity occurring near the ocean floor, in addition the fact that the ocean includes carbon dioxide, suggests a chemical environment on Enceladus that can support methanogenesis—and therefore life.

Interplanetary tourism poster for Enceladus.
Image Credit: NASA Jet Propulsion Laboratory.

The scientists caution that inferring what the ocean is like based on the materials detected in a geyser plume is tricky. At the time Cassini captured the sample, the sample had already been shot into space through cracks in the icy surface and it’s hard to say how this process affects the relative amounts of different chemicals. In addition, there could be very different geochemical processes at work on Enceladus than on Earth, maybe even processes that are unique to ice-covered bodies that scientists on Earth have never considered.

Despite these warnings, there is reason for cautious optimism. With this discovery, scientists have identified nearly all of the key ingredients for life on Enceladus. In a statement, Hunter Waite, the lead author of the Cassini study, put the new results this way, "Although we can't detect life, we've found that there's a food source there for it. It would be like a candy store for microbes.”

Kendra Redmond

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