Although the Cassini mission at Saturn ended nearly four years ago, data from the spacecraft still keep scientists busy. And the latest research using Cassini’s wealth of data may be the most enticing yet.
Scientists say they have discovered methane in Saturn’s icy moon Enceladus. The process of how the methane is produced is not known at this time, but the study suggests that the surprisingly large amount of methane found is likely to come from activity at hydrothermal vents present on Enceladus’ inland seabed. These ventilation ducts can be very similar to those found in the earth’s garden, where microorganisms live, feed on the energy from the ventilation openings and produce methane in a process called methanogenesis.
“We do not conclude that there is life in Enceladus’ ocean,” said Régis Ferrière, an associate professor at the University of Arizona and one of the study’s two lead authors. “We would rather understand how likely it would be that Enceladus’ hydrothermal vents could be habitable for soil-like microorganisms. Most likely, the Cassini data tells us according to our models. ”
One of the biggest surprises of the 13-year-old Cassini mission came in Enceladus, a small moon with active geysers at its south pole. With only approx. 500 km in diameter, the bright and ice-covered Enceladus should be too small and too far from the sun to be active. Instead, this small moon is one of the most geologically dynamic objects in the solar system.
Stunning backlit images of the moon from Cassini’s camera show feathers erupting in Yellowstone-like geysers emanating from tiger stripe-shaped fractures in the moon’s surface. The discovery of the geysers became more important when Cassini later determined that the feathers contained water ice and organic matter. Since life as we know it is dependent on water, this small but energetic moon has been added to the short list of possible places of life in our solar system.
For the new study, the research team analyzed one of these plum materials that was pushed into space. They looked at Enceladus’ plume composition as the end result of several chemical and physical processes that take place inside the moon, where dihydrogen, methane and carbon dioxide are produced.
“We wanted to know: Could soil-like microbes that ‘eat’ the dihydrogen and produce methane explain the surprisingly large amount of methane that Cassini has discovered? ‘Ferrière said in a press release from the University of Arizona.
First, the researchers assessed which hydrothermal production of dihydrogen best suited Cassini’s observations, and whether this production could provide sufficient energy to sustain a population of soil-like hydrogenotrophic methanogens. To do so, they developed a model of population dynamics for a hypothetical hydrogenotrophic methanogen whose thermal and energetic niche was modeled after known strains from Earth.
Then, the research team ran the model to see if a given set of chemical conditions, such as the dihydrogen concentration in the hydrothermal fluid, and the temperature would provide a suitable environment for these microbes to grow. They also looked at the effect a hypothetical microbial population would have on its environment – for example, on the release rates of dihydrogen and methane in the cloud.
The team wrote in their paper, published in Nature:
“We find that the observed escape rates (1) can not only be explained by the abiotic change of the rocky core by serpentinization; (2) are compatible with the hypothesis of habitable conditions for methanogens; and (3) scores the highest probability under the hypothesis of methanogenesis, provided that the probability of life occurring is high enough. If the probability of life occurring on Enceladus is low, the Cassini measurements are consistent with habitable, yet uninhabited hydrothermal vents and point to unknown sources of methane (e.g. urmethane) awaiting discovery at future missions.
“And biological methanogenesis seems to be compatible with the data,” Ferrière said. “In other words, we cannot dismiss the ‘life hypothesis’ as very unlikely. To reject the life hypothesis, we need more data from future missions. ”
University of Arizona