Life Detection in Enceladus' Plume

Introduction

Saturn’s moon Enceladus has been a focal point in astrobiology for over a decade. This unique moon has fissures, or cracks in the surface, that spray plumes of salt-water and possibly other material out into space. Observations of the icy moon have revealed strong evidence to suggest that Enceladus has a subsurface liquid ocean, making it one of the few places where liquid water exists in our solar system.

Illustration of Cassini sampling the plumes of Enceladus | Credit: NASA/JPL-Caltech

The Cassini spacecraft sampled the water of the moon by flying through a plume escaping from cracks in the moon’s southern surface. The spacecraft used the Cosmic Dust Analyzer (CDA) to capture and analyze the material being ejected by the plume. The CDA was able to tell scientists back on Earth that the plume material contained Water (H2O), carbon dioxide (CO2), ammonia (NH3), molecular hydrogen (H2), methane (CH4), and simple hydrocarbons.

Why Enceladus?

Enceladus is worth exploring for a variety of reasons, largely due to the salt-water plume. The contents of the plume uphold scientific conclusions about Enceladus including the state of the material below the surface. The CDA detected salt-rich particles in the plume, which is strong evidence that some portion of the source is liquid. The plume spraying out into space allows for sampling to be done via flyby or orbiter. A factor in determining what type of mission is necessary to investigate topics such as the search for life. Landers are more costly and have a higher risk of complications compared to missions that don’t have to enter an atmosphere, land, or maneuvering on the surface.

Illustration of Enceladus' Interior | Credit: NASA/JPL-Caltech

The chemical components of the plume also give scientists a good reason to focus on Enceladus. Carbon dioxide and methane are both byproducts of respiratory pathways we know of on Earth, humans respire carbon dioxide and methanogenesis is an anerobic process that occurs in the ocean around hydrothermal vents on Earth. This combined with constraints about the environment allow scientists to focus on what specific type of life could exist in each place. Life on Enceladus is theorized to exist in a few locations, the rock-water interface at the bottom of the subsurface ocean, within the ocean, near the fissures where the famous Enceladus plumes extend from, or possibly on the icy surface. While our ability to detect life below the ocean is extremely limited, the chances that life from the surface or icy crust could be transported up into the atmosphere in small droplets is high.

Latest Research

Researchers at the University of Edinburgh in the UK theorized that signs of life could exist in small droplets within the plume of Enceladus. They believe the droplets could contain biological material, or some other biosignature and be detected from the plume materials. To test their theory, L.J. Perera and C.S. Cockell led an experiment using the bacteria bacillus subtilis to see if the bacteria would become trapped in the Enceladus plume like fluids under vacuum conditions. This experiment models the likelihood bacteria on Enceladus could become trapped in the plume and get ejected into space. Bacteria dispersion has been described in length on Earth and bacteria have even been found in the atmosphere above Antarctica.

The authors suggest that two methods are needed to fully describe the dispersion of bacteria by the droplets. The first is the accumulation of biological material on bubbles forming in the liquid. These bubbles rise, collect material, and burst. Then the rapid boiling of the liquid due to low pressure at the surface causes the liquid and material to disperse into the air as droplets. Once the droplets come to rest, the liquid evaporates, and the remaining material contains biological cells. Either method alone would not provide the correct mechanisms for dispersion but together produce bubbles at the right depths to accumulate biological material.

The results of the experiment showed that bacteria can be dispersed through boiling in an Enceladus like environment. This means if there is life on Enceladus near the fissures it’s possible that we could detect it in the plume material using similar methods. By conducting this experiment, the researchers are reminding the astrobiology field why Enceladus is a place of interest, and that it might be more reasonable to research than a place where our spacecraft are required to land. Limitations to experiments like this include lack of a model simulating forceful collection. As flyby orbiters collect material from the plume, they are traveling at nearly 10 km/s and the particles have the potential to change chemistry upon impact. Further, our ability to make accurate and definitive observations remotely adds difficulty to research done at this distance from Earth. However, Enceladus continues to present reasons why it should be studied in the search for life in our solar system.

Reference

Perera, L. J., and C. S. Cockell. "Dispersion of Bacteria by Low-Pressure Boiling: Life Detection in Enceladus' Plume Material." Astrobiology 23.3 (2023): 269-279.