Jupiter’s moon Europa is thought to have a vast internal ocean beneath its thick ice, and is attracting attention as one of the celestial bodies in the solar system that is likely to harbor life.
This time, a research team led by Steve Levin, who is aiming to be a project scientist for the Juno mission at NASA’s JPL (Jet Propulsion Laboratory), announced the results of an analysis of data obtained by NASA’s Jupiter probe Juno, which shows that the thickness of the ice shell in the observed area is likely to be 29 km. A paper summarizing the research team’s results has been published in Nature Astronomy.
A long-standing mystery surrounding Europa’s ice shell
The research team analyzed observation data from Juno’s flyby of Europe in September 2022, and worked to examine the structure of the ice shell, which has been a mystery for many years.
Europa’s surface is thought to be thick ice, but it is speculated that there is a layer of liquid salt water (seawater) beneath it. However, opinions were divided among researchers as to how thick the ice shell that can be seen on the surface is.
To settle this controversy, the research team focused on data from one of Juno’s observation instruments, the Microwave Radiometer (MWR).
Ice shell thickness determined by microwave “transparency”
MWR, which was the key to this research, captures microwaves emitted from objects and measures their temperature. Although it was originally designed to depict the atmospheric structure beneath the thick clouds of Jupiter, it has also become a powerful tool for exploring the internal structure of icy objects like Europe.
Also, since the “visible depth” differs depending on the frequency, high frequencies can detect the temperature near the surface, and low frequencies can detect the temperature in deeper layers.
During its 2022 flyby, Juno came within about 360 km of the surface of Europe. The research team analyzed the data obtained at this time and modeled the temperature gradient of the ice (temperature changes as it gets deeper). As a result, assuming pure water ice without convection, the ice shell thickness in the area observed by Juno during the flyby was approximately 29 km (with an error of ±10 km).
According to the research team, if the ice contained salt, its estimated thickness could be reduced by as much as 5 km. However, it is much thicker than the several kilometer thickness suggested by the “thin shell” theory.
Surface cracks may not reach deep
Furthermore, the MWR data not only described the ice thickness, but also the current state of its internal structure.
Europa’s surface has countless linear patterns (striations) and complex collapsed topography called chaotic topography, which are thought to be traces of ice cracking or moving.
Analysis of this observation data suggests that there are “scattering bodies” in the ice near the surface that diffuse and disrupt microwaves. The research team believes that these scatterers are cracks or voids with widths ranging from several centimeters to several tens of centimeters. What is important is that these structures do not extend deep underground, but are estimated to exist only a few hundred meters below the surface.
Impact on life support potential
The characteristics of the ice shell, such as “thick ice” and “shallow cracks,” demonstrated in this study have important implications when considering the possibility of life in the European oceans.
In order for life to exist, it requires not only water but also energy sources and nutrients. In the case of Europe, the process by which compounds containing oxygen and other substances separated from the ice by radiation from Jupiter pass through the ice shell and into the underlying internal ocean is thought to be important for sustaining life.
However, if an ice shell is found, and the cracks in the surface may not reach shallow parts, it may be more difficult for surface material to reach the inner ocean than previously thought.
“The thickness of the ice shell and the presence of cracks and voids within it are pieces of a complex puzzle for understanding Europe’s habitability,” said Scott Bolton of SwRI (Southwest Research Institute), Juno’s principal investigator.
A bridge to the next generation’s mission
The findings brought by Juno are a major step towards understanding Europe as a whole, but not all mysteries have been solved yet.
Currently, NASA’s Europa Clipper spacecraft and ESA’s (European Space Agency) Jupiter icy satellite probe JUICE are continuing their flights with the aim of arriving in the Jupiter system in the 2030s. The Clipper is equipped with an ice-penetrating radar to investigate in detail the thickness of the ice shell and the properties of the ocean inside, and it is expected that this will help verify the results of this research based on Juno’s observation data.
Text/Saki Sorano Edit/sorae Editorial Department