Scientists with expertise in geophysics at Washington State University have developed an alternative pathway for the transport of nutrients to the deep ocean under Europa’s exterior surface. Europa, a huge moon orbiting Jupiter, holds great promise as a site for life beyond Earth. WSU researchers developed this hypothesis, indicating that salty surface ice would slowly descend through Europa’s heavy ice shell and deliver nutrients into the ocean below.
Austin Green is the primary author of this study and was also a postdoctoral researcher at Virginia Tech. He completed much of this work while earning his Ph.D. degree from WSU under the mentorship of Dr. Catherine Cooper, associate professor of geophysics in the School of Environment and associate dean in the College of Arts and Sciences, and her research team.
Dr. Green explains, “The new concept we are proposing is unique to planetary science but based on a well-established theory in Earth science. Importantly, this new notion will potentially help fill in the gaps related to a long-standing issue regarding Europa’s habitability as it pertains to possible life in the ocean.”
Europa’s ocean contains more water than all of the oceans on Earth combined. The ocean that lies under Europa’s ice shell may be approximately 10 kilometers thick. Because of the thickness of the ice shell on Europa, the ice acts as a barrier to most types of sunlight.
As a result, any life that may exist there would require energy from chemical reactions and could not survive using photosynthesis. For many decades, scientists have had difficulty developing answers as to how this ocean could get nutrients to sustain even the simplest forms of biological life.
High-energy radiation from Jupiter constantly bombards and modifies the surface of Europa. Radiation creates chemical compounds, including hydrogen peroxide and oxygen, on Europa’s surface, much like oxygen fuels microbes on Earth. However, in order to provide energy for likely organisms living in Europa’s ocean, these chemicals must travel through the ice and into the water below.
The ice shell overlying Europa’s ocean is dynamic and behaves like a glacier. However, due to the extreme weight of Jupiter’s gravitational pull on Europa, the ice shell mostly moves sideways. This motion makes the transport of surface-sourced material to the ocean extremely difficult. Cracks extending all the way to the ocean may be rare, especially with a thick ice shell present.
The idea for researchers Green and Cooper to investigate Europa’s ice and the processes that may help bring salt and hydrogen peroxide into contact with liquid water came from studying the geology of Earth. On Earth, gravity can pull heavy or unstable portions of the crust down into the mantle in a process known as crustal delamination. The researchers wondered if Europa’s ice might similarly undergo crustal delamination due to the weight and instability of the ice above it.
Certain areas of Europa’s surface are rich in salts, remnants of salts from the ocean that once existed beneath Europa’s surface. Due to the addition of salts to the ice, Europa’s ice becomes denser than surrounding areas and extremely weak. As a result, salty ice is significantly more ductile than regular ice, with this ductility increasing under repeated application of stress.
Using a computer model, Green and Cooper tested this theory by creating a model of the potential for dense, weak ice in the upper region of Europa’s ice shell to sink to the ocean below. The model consisted of a 30-kilometer-thick ice shell with a rigid, cold top layer and a warmer, very slowly deforming internal portion.
The researchers created models based on observations of various types of ice on Earth, including frozen salt and other unique forms of salt, to determine how ice formation could lead to the migration of organic materials into oceans beneath Europa’s icy crust. Initially, small patches of salty frozen ice became porous, causing instability in their mass. These regions were eventually pulled down by gravity, forming “drips.” By pulling down surface materials from above, the upper ice layers could be funneled down into the lower oceanic water.
The modeling included a variety of scenarios. It demonstrated that if the salt concentration was at least 1 percent and the ice formation was weak enough, there was a likelihood the ice would sink. In these cases, it could reach the ocean between a few tens of thousands and a few million years, both significantly shorter timescales than the age of Europa’s current surface.
The final results suggest that the major difference between fast and slow sinking events was due to the physical properties of the ice itself. This behavior depended on the strength of the ice layer rather than how much denser it was than surrounding regions. Ice of lower density can migrate to the depths of oceans much more rapidly than slightly heavier surrounding layers.
Once the lower-density ice layer breaks free from the surface, it migrates quickly into the warmer portions of the ice shell below. Understanding the thickness of Europa’s ice shell will be instrumental in predicting how organic materials may have migrated into the ocean beneath Europa over geological time.
The new research indicates that even with a thicker ice shell, this type of sinking flow and delivery of surface materials to the ocean may still be possible.
In summary, the current study shows that while differences in ice thickness on Europa might appear to limit material transport from the surface to the ocean, sinking ice could potentially resolve this issue across much of the moon’s surface. This process would allow downward transport of materials without heating and across a wide range of temperatures throughout Europa’s ice shell.
Surface features detected on Europa indicate that the moon’s ice moves in complex ways. The presence of long ridges, pits, and bands suggests zones of ice convergence and deformation. The new model better explains the physical processes that allow ice motion to be directed downward through the ice shell.
The downward motion of salty ice may regularly deliver abundant oxidants and other nutrients to Europa’s ocean over long periods of time. According to estimates from this study, a single sinking event of salt-laden ice, roughly 15 kilometers in scale, would significantly contribute oxygen to the ocean.
The results align with the goals of NASA’s 2024 Europa Clipper mission, which aims to study Europa’s chemistry, ice shell, and habitability, as well as to obtain evidence of salt-laden sinking ice.
The possible effects of salt-laden sinking ice are not limited to Europa. Similar processes may exist on other icy worlds, such as Titan and Pluto. On Titan, the density of ice may be high enough to sink due to hydrocarbons on the surface. On Pluto, recycling of ice may explain large-scale surface features.
Though the model simplifies how slow, steady sinking of ice may occur, it demonstrates that such motion is possible. Future studies will need to refine these models further and perform laboratory experiments with icy materials at temperatures similar to those found on Europa.
Overall, this research provides new answers to a longstanding question. While occasional catastrophic sinking events may occur, there is likely a more subtle and consistent delivery of essential ingredients to support life on Europa through salt-laden sinking ice over millions of years.
This research provides scientists with a better understanding of how life-supporting materials may be transported through ice on icy worlds. The findings can guide future exploration efforts by identifying Europa’s most promising regions for study.
Additionally, the results will assist NASA in identifying processes that may help determine whether Europa’s ocean could support life. Beyond Europa, this work broadens scientific understanding of ice recycling on other moons and planetary bodies throughout the solar system.
Research findings are available online in The Planetary Science Journal.
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