Enceladus, the sixth-biggest moon of Saturn, presents a remarkable opportunity in our Solar System for searching for evidence of extraterrestrial life, given its habitable ocean and plume that deposits organic-bearing ocean material onto the surface. Organic ocean material could be sampled by a lander mission at Enceladus. It is of interest to understand the amount of relatively pristine, unaltered organics present on the surface, given the ultraviolet and plasma environment.
Enceladus’ tiger stripes are known to be spewing ice from the moon’s icy interior into space, creating a cloud of fine ice particles over the moon’s south pole and creating Saturn’s mysterious E-ring. Evidence for this has come from NASA’s Cassini spacecraft that orbited Saturn from 2004 to 2017. Pictured here, a high resolution image of Enceladus is shown from a close flyby. Tiger stripes are visible in false-color blue. Image credit: NASA / ESA / JPL / SSI / Cassini Imaging Team.
“We can learn a lot about potential biosignatures in Enceladus’ ocean by sending a mission to the surface of Enceladus,” said Planetary Science Institute senior scientist Amanda Hendrix.
“Previously, it was thought that in order to sample the freshest material from the Enceladus ocean, you have to fly through the plume and measure plume grains and gases.”
“But now we know that you can land on the surface and be confident that your instruments can measure relatively pristine plume organics — sourced from the ocean.”
“We know that Enceladus’ ocean is habitable thanks to the measurements from NASA’s Cassini spacecraft,” she added.
“We know there is liquid water, energy, and the chemicals carbon, hydrogen, nitrogen, oxygen, phosphorus and sulfur. These are the ingredients necessary for life as we know it.”
“Enceladus is an ocean world: it harbors a liquid ocean below an icy surface.”
“There are at least several ocean worlds in our Solar System, but Enceladus is special because it is spraying its ocean material out into space via its south polar vapor-and-ice grain plume, which means that the instruments on Cassini were able to characterize the ocean as the spacecraft flew by and through the Enceladus plume.”
“Luckily, for this study, even though some of the plume grains are ejected out into the Saturnian system, close to 90% of the plume grains fall back onto the surface of the moon, which likely means that ocean material — including organics — is sitting right on the surface.”
Organic molecules found in the plume of Enceladus, include molecules like methane and ethane, along with more complex molecules.
Organics can be processed, or chemically transformed, by solar ultraviolet photons and by charged particles like electrons.
But if scientists want to find out if any ocean-derived biosignatures are present in the plume grains, they need these grains to be as pristine, and unexposed to ultraviolet light, as possible.
An artist’s impression of NASA’s Cassini spacecraft flying through plumes erupting from the south pole of Enceladus; these plumes are much like geysers and expel a combination of water vapor, ice grains, salts, methane and other organic molecules. Image credit: NASA / JPL-Caltech.
In the new study, Dr. Hendrix and her colleague, Pennsylvania State University researcher Christopher House, used data from the NASA/ESA Hubble Space Telescope and Cassini to estimate how deeply ultraviolet photons can penetrate into the plume grain-coated surface of Enceladus.
“What we find in this study is that there are places on Enceladus’ surface where we could land with a spacecraft and take a sample — and we’d be measuring relatively pristine organics,” Dr. Hendrix said.
“That’s because the solar ultraviolet photons just don’t penetrate very deeply into the icy surface.”
“Those damaging solar ultraviolet photons only penetrate some 100 micrometers into the icy surface. That’s the width of a couple of human hairs!”
“So that very top part of the surface gets exposed to those damaging ultraviolet photons, but only a percentage of the organics are chemically transformed, and then soon enough that material is covered up by fresher plume material.”
“And the deeper grains don’t undergo more transformation — because the ultraviolet photons are prevented from interacting with the deeper material.”
“The freshly deposited plume grains act as a shield for the underlying material. They act like a sunscreen!”
“Ideally, we want to one day sample relatively pristine ocean-derived organics by landing on the surface of Enceladus.”
“This result is important because it tells us that there will be plenty of relatively pristine organics available to sample, because the penetration depth of those damaging ultraviolet photons is so shallow.”
“The slightly deeper grains haven’t been exposed much to ultraviolet, so that means the organics have a low exposure age.”
“Because ultraviolet light readily alters organic molecules, the depth that such light travels into the surface of an ice-covered world really matters,” Dr. House added.
“With the short ultraviolet penetration depths found, our results ensure that there is ample organic material locked away and preserved in the ices of Enceladus that can be traced back to its ocean.”
“It is awe-inspiring to think that with known technology, we can readily access lots of organic material from a habitable extraterrestrial ocean.”
The findings were published in the journal Communications Earth & Environment.
_____
A.R. Hendrix & C.H. House. 2023. Low effective ultraviolet exposure ages for organics at the surface of Enceladus. Commun Earth Environ 4, 485; doi: 10.1038/s43247-023-01130-8
