An icy Saturn satellite, Enceladus, has been a topic of increasing interest in recent years since Cassini captured jets of water and other material ejected from the moon’s south pole. One particularly tempting hypothesis, supported by the sample composition, is that there may be life in the oceans beneath the Enceladus ice sheets. To assess Enceladus ‘habitability and how best to study this icy moon, scientists need to better understand the chemical makeup and dynamics of Enceladus’ ocean.
In particular, adequate salinity could be important for habitability. Like the mush of the three bears, the salinity of the water must be just right for life to flourish. Too high a salt content can be life threatening, and too low a salt content can indicate a weak water-rock reaction and limit the amount of energy available to life. When life exists, ocean circulation, which is also indirectly dependent on salinity, determines where the heat, nutrient and potential biosignatures are transported and is therefore the key to detecting biosignatures.
UT video about the successes of the Cassini mission.
A team of scientists working with Dr. Working with MIT, Wanying Kang approaches these questions by numerically simulating the likely ocean circulations for various possible salinity levels and assessing the likelihood of each scenario by asking if it is able to maintain the ice shell geometry observed by Cassini on the icy moon pictured.
The circulation of the oceans depends on differences in the density of the water constituent in different parts of the ocean. Water that is denser flows to water that is less dense to achieve equilibrium. These differences in density are themselves controlled by two key factors, the location of the lunar heat source and the salinity of the ocean, both of which are currently poorly understood.
UT video about the chemical composition of Enceladus.
Enceladus has two potential sources of heat: in the silicate core or in the lower ice shelf, where it meets the upper part of the ocean. If a significant amount of heat is generated in the silica core by tidal bending under the ocean, scientists would expect convection, just like boiling a pot of water. When the sea freezes, salt is expelled from the ice, increasing the local water density and triggering convection from above.
The salinity also plays a key role in these density calculations. At relatively low salt levels, the water contracts when it is heated to near freezing, making it denser. Because the Enceladus Ocean is in contact with a global ice shell, it is almost frozen. This is not intuitive to understand how most people think about heating – which generally implies that the material becomes less dense as the temperature increases. At higher salt levels this comes true and water begins to behave normally and expands when heated.
Cutaway shows the interior of Saturn’s moon Enceladus. Photo credit: ESA
Given the uncertainty of the salinity of the Enceladus ocean (between 4 and 40 grams of salt per kilogram of water) and the percentage of the planet warming at either source, Dr. Kang and her co-authors used MIT’s ocean model to simulate ocean circulation under various combinations, assuming that the observed ice shell is preserved by freezing in the thick ice regions and melting elsewhere. This is largely true of icy worlds, as the ice shelves would naturally flatten over time due to the flow of ice if no other process maintained a difference.
The team diagnosed heat transport under different scenarios and found that few of them can maintain a “balanced” heat budget, i.e. how the various heat sources (the amount of heat flow from the ocean to the ice plus the heat) produced in the ice due to tidal bending and the release of latent heat can precisely compensate for the conductive heat loss through the ice shell.
Image from the paper showing the cycle of water and ice in the oceans of Enceladus.
Photo credit: Kang et al
According to the model, by and large, such equilibrium can be achieved when the ocean’s salinity is at an intermediate level (10-30 g / kg) and when the ice shell is the dominant source of heat. When both of these conditions are met, ocean circulation is weak. As a result, the warm polar water is not mixed too efficiently towards the equator, so that equatorial melting does not occur. This results in an ice shelf that is thicker around the equator of the moon, as observed by Cassini. This also means that the pressure at the water-ice interface is lower at the poles, which means that it also has a higher freezing point than water at the equator.
In scenarios with an “unbalanced” heat budget, ie part of the heat generated on the moon is not dissipated, the heat transport directed towards the equator is too efficient and the equatorial ice shell tends to melt. Meanwhile, the pressure gradient force drives an ice flow from the equator to the poles. Together, the melting and the flow of ice inevitably reduce the thickness of the ice near the equator. In this scenario, the observed ice geometry cannot be maintained over the entire lifespan of the moon.
Artist rendering showing an internal cross-section of Enceladus’ crust showing how hydrothermal activity can cause the plumes of water on the lunar surface. Credits: NASA-GSFC / SVS, NASA / JPL-Caltech / Southwest Research Institute
Ultimately, Dr. Kang and her colleagues point out that the ice shell and ocean circulation on icy satellites should be viewed as a coupled system: the ocean circulation redistributes the heat and reshapes the ice shell and thus the freezing of the ice shell. Melting and fluctuations in thickness drive the ocean circulation. A nice result of this research is that it shows the possibility of inferring one another, which can be useful well beyond Enceladus. As part of this effort to understand the icy moons in our solar system, a group known as the Exploring Ocean Worlds program will work together to deepen our understanding of the habitability of icy moons and how to optimally explore them.
Learn more:
arXiv – How does salinity affect ocean circulation and ice geometry on Enceladus and other icy satellites?
UT – Complex organic molecules gush out of Enceladus
UT – Radioactive core could explain geysers on Enceladus
Mission statement:
Enceladus interior illustration – thicknesses not to scale.
Photo credit: NASA / JPL – Caltech
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