Is there really life under the ice layer of Enceladus?

A mysterious extraterrestrial ocean, who lives there? Let's take a look at the natives of Enceladus!

"We know from Cassini's measurements that Enceladus' ocean is habitable. We know there is liquid water, energy sources, and chemicals such as carbon, hydrogen, nitrogen, oxygen, phosphorus and sulfur."

Illustration: Icy plumes ejected from Enceladus.

(Image credit: NASA)

New research suggests there are places on the surface of Saturn's moon Enceladus where a spacecraft could land and pick up pristine traces of key ingredients for life. These biosignatures are believed to come from a subsurface ocean within this world's icy shell.

It has long been known that Enceladus's subsurface ocean harbors organic molecules - compounds made of carbon, oxygen and nitrogen. Before crashing into Saturn's surface in 2017, the Cassini spacecraft flew through plumes of material ejected from cracks in Enceladus' surface and detected organic molecules such as methane and ethane, as well as other complex compounds, reaching great altitudes.

Of that material, about 90 percent of the larger particles are launched thousands of miles above Titan, but they don't actually escape the Saturn system. Scientists now say the particles fall back to the surface of the Saturnian moon and could, in theory, be collected and examined by a spacecraft.

"We can learn about potential biosignatures in Enceladus' ocean by sending a mission to the surface. Previously, it was thought that to sample the freshest material from Enceladus' ocean, you had to fly through the plume and measure the plume's particles and gases," said study leader Amanda R. Hendrix, a senior scientist at the Planetary Science Institute, in a statement. "But now we know you can land on the moon's surface and be confident that your instruments can measure relatively pristine ocean-derived plume organics."

However, some organic molecules in Enceladus' plumes that could be fingerprints of biological life may be destroyed by the sun's ultraviolet (UV) light, which means we need to find these molecules when they are intact.

"We know that the ocean of Enceladus is habitable, thanks to Cassini's measurements. We know there's liquid water, energy, and chemicals like carbon, hydrogen, nitrogen, oxygen, phosphorus and sulfur," Hendrix said. "These are all necessary ingredients for life as we know it. If we want to find out if there are any ocean biosignatures in the plume particles, we need those particles to be as pure as possible and to be exposed to as little ultraviolet light as possible."

Searching for pristine matter on Enceladus

To find places on Enceladus with this pristine material, Hendrix and the team analyzed data from the Hubble Space Telescope and the Cassini spacecraft to see how deeply UV photons can penetrate the moon's surface. "What we found in this study is that there are places on the surface of Enceladus where we can land with a spacecraft and take a sample -- and the sample we measure is relatively pristine organic matter," Hendrix said. "That's because solar ultraviolet (UV) photons can't penetrate deeply enough to get to the icy surface." The team found that damaging UV photons can only penetrate about 100 microns, or the width of a few human hairs, through the icy surface of Enceladus.

"So the top-most part of the surface gets exposed to these damaging UV photons, but only a portion of the organic matter gets chemically transformed, and then very quickly that material gets covered over by fresher plume material," Hendrix explains. "Grains deeper down don't get transformed anymore -- because the UV photons can't interact with the deeper material. The freshly deposited plume particles act like a protective shield for the underlying material. They're like sunscreen!"

Illustration shows that ultraviolet light affects particles on the surface of Enceladus but not deeper. (Image credit: Amanda Hendrix/PSI)

The results the team gathered are useful because they tell scientists that missions to Enceladus will have plenty of organics to sample without having to study them in depth.

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"Because UV light can easily alter organic molecules, the depth to which this light penetrates the surface of an icy world is very important," study co-author Christopher House, a scientist at Pennsylvania State University, said in the statement. "Because the UV penetration depth found is very short, our results confirm that Enceladus' ice contains large amounts of organic material that can be traced back to the ocean. It is encouraging to think that large amounts of organic material could be easily retrieved from a habitable extraterrestrial ocean using known techniques."

The team's research results are published in the journal Communications Earth & Environment.

BY: Robert Lea

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