September 5, 2019 : A New Study Proposed By Krista Soderlund Reveals The Potential Influence Of The Rotation Speed Of The World On The Potential Dynamics Of The Presumed Subsurface Ocean Of Europa, Ganymede, Enceladus And Titan

A new research work entitled  Ocean Dynamics of Outer Solar System Satellites , published on July 29, 2019 in the journal Geophysical Research Letters and proposed by Krista Soderlund reveals the potential influence of the rotation speed of the world on the dynamics of the presumed subsurface ocean of four moons of the Solar System. In the study, Krista Soderlund who is a specialist on planetary fluid dynamics at the University of Texas in Austin focused her attention on Europa and Ganymede which are two moons of the Gas Giant Jupiter and on Enceladus and Titan which are two moons of the Ringed Planet Saturn. Our world is dominated by oceans whose dynamics has been intensively studied by meteorologists or planetologists on the basis of satellite data in particular. No other world in the Solar System contains any ocean of liquid water on its surface. However, Titan, the largest moon of Saturn, contains lakes, seas or rivers of liquid hydrocarbons in its high latitudes or in its polar regions.

Most moons in the Solar System are devoid of any atmosphere and are heavily cratered. Yet, we have become aware that many worlds of the Solar System may contain a subsurface layer dominated by liquid water. In other words, they may contain an internal ocean. The Dwarf Planet Pluto, the largest moon of Neptune Triton, Enceladus and Titan, Europa and Ganymede and even the Dwarf Planet Ceres may contain a subsurface ocean dominated by liquid water beneath their external crust. We've clearly observed that worlds of the Outer Solar System like Enceladus or Triton can be remarkably dynamic in terms of geology or internal activity. We now know that there are geysers on Triton or Enceladus. The fractured surface of Europa implies the potential presence of a liquid layer beneath the icy shell. We try to gather clues regarding the hypothetical subsurface ocean of worlds like Europa or Enceladus. The new study shows the potential relationship between the rotation rate of the world and the ocean currents of the presumed subsurface ocean on the basis of the rotating convection theory and numerical simulations.

The emergence and the development of any lifeform may be more likely in a dynamic ocean with strong currents than in an ocean where the currents are weak. Krista Soderlund is fascinated by the remarkable internal activity of worlds like Europa, Ganymede, Enceladus or Titan. The complex landscape features observed on the surface of the icy moon Europa can tell us a lot regarding the characteristics of the presumed internal ocean. The surface of a world like Enceladus seems almost uniform and boring from outer space but the geysers found in the south polar region of the tiny moon remind us that there is probably an internal ocean dominated by liquid water beneath that icy crust. The ocean of salty water found on the surface of our planet may exist elsewhere. The prospect of exploration of an extraterrestrial ocean of water in our Solar System is really captivating. It fuels our imagination and allows us to anticipate what we could find or discover in the Outer Solar System in exotic environments beneath the external crust of worlds orbiting Gas Giants or the Sun. A cratered world is not always a dead world. A world without any significant atmosphere is not always a dead world as well.

The Dwarf Planet Ceres, located in the Asteroid Belt between Mars and Jupiter, has unveiled us bright features on its surface that are likely to demonstrate that there is a subsurface ocean of liquid water or that there are pockets of liquid water beneath the dark crust or the dark soil of the small world. Planetologists are particularly interested in the internal dynamics of worlds like Europa, Ganymede, Enceladus, Titan, Pluto, Charon or other icy worlds or moons. The new calculations of Krista Soderlund clearly show that several moons of the Solar System are remarkably dynamic in their interior. The movement of liquid water is influenced by several factors and researchers have to take into account the rotation rate or the rotation speed of the world, the thickness of the crust of the world as well as the density of the type of water. Krista Soderlund was in a position to determine that the presumed oceans of Enceladus and perhaps Titan could unveil currents in alternating bands as well as particularly strong heat flow close to the poles. By contrast, the calculations suggest that the complex world Europa may reveal the most significant heat flow close to the equator due to the weaker impact of spin.

Regarding Ganymede, the largest moon in the Solar System, we don't have enough data at our disposal to determine the type of ocean dynamics or ocean currents potentially occurring beneath the external crust. So far away from the Sun, the level of energy received from the Sun is particularly weak but tidal forces or gravitational factors can have a strong influence on the internal activity of the moon. If we are in a position to determine that the subsurface ocean is relatively dynamic, we can better evaluate the potential for the development of any exotic lifeform. Alyssa Rhoden who is a researcher at the Southwest Research Institute, who works on the topic of ice shells and who is co-leading a new network for ocean worlds study pointed out that ocean dynamics and habitability are closely related. One has to figure out the potential movements of ocean currents from swirling or whirling phenomena to linear or convective movements. Any lifeform will need nutrients which must move anywhere in the subsurface ocean. There must be the right distribution of energy within the hypothetical subsurface ocean.

The ingredients of life or the complex molecules will more likely collide or interact if the ocean is dynamic with swirling or whirling phenomena. We don't have a lot of key empirical data at our disposal but our theoretical models or simulations allow us to anticipate what we could potentially encounter in those extreme environments beneath the external crust of those worlds. One has to keep in mind that if we go deeper, we have to undergo higher pressures and the environmental temperature is likely to significantly increase. We try to develop theories whose conclusions are in line with reality but reality is always or often too complex for the development of a perfect model or theory. That's what Alyssa Rhoden argued. Most models are not perfect or are simply wrong because nature is based on a huge amount or an infinity of factors. We always simplify in planetology. In meteorology, for instance, one can accurately predict the weather up to approximately a week. One can't predict the development of a specific cyclone or hurricane that will take shape two months later for instance.

Our studies of the presumed internal oceans found in the Outer Solar System in particular are fed by the clues we gather thanks to ambitious missions like the Galileo mission or the Cassini-Huygens mission. Europa, Ganymede, Enceladus or Titan represent giant puzzles for planetologists. We've never landed onto Europa, Ganymede or Enceladus. We have only landed onto the surface of Titan and on the basis of what we've seen on Titan, one can say that the Opaque Moon is a particularly complex world. Our models of those exotic worlds can be significantly improved if we gather new data. We've only explored the tip of the iceberg of those worlds. We can study an internal ocean from orbit with a satellite or a spacecraft. The Cassini orbiter allowed us to conclude that there are probably internal oceans dominated by water beneath the external crust of Enceladus and Titan for instance. Two ambitious missions to Europa have been planned by NASA and ESA for the next decade. NASA has planned Europa Clipper and the European Space Agency has planned JUICE or Jupiter Icy Moons Explorer.

Krista Soderlund is part of the group developing one of Europa Clipper's instruments. That device is REASON or the Radar for Europa Assessment and Sounding: Ocean to Near-Surface device. The planetologists will study the surface of Europa and evaluate the potential interactions between the icy crust and the presumed subsurface ocean because the topography may be influenced by the dynamics of the hypothetical subsurface ocean. If the model of Krista Soderlund is correct regarding the distribution of heat in the presumed subsurface ocean, one can anticipate the determination of a thinner crust at the level of the equator because the heat flow is supposed to be stronger at the level of the equator. One can be enthusiastic about the outcome of the calculations or simulations for the four moons studied by Krista Soderlund because the presumed internal ocean of those exotic worlds where the environmental temperature at the level of the surface is particularly low is far from being passive and is probably remarkably dynamic. Those hypothetical internal oceans dominated by liquid water probably unveil strong currents implying a lot of chemical interactions or a potentially complex chemistry. Are there similarities between the abyss of those hypothetical internal oceans and the abyss of our own ocean ?

The image above shows a mosaic of views of Titan and Enceladus represented at scale. The original view of Titan was generated on the basis of data acquired with the Wide-Angle Camera of the Cassini orbiter on January 30, 2012. Images obtained using red, green and blue spectral filters were mobilized to generate the original view of Saturn's largest moon in natural colors. Only a portion of the original view of Titan is visible in the mosaic. The original view of the tiny moon Enceladus which appears particularly bright was captured in visible light with the Narrow-Angle Camera of the Cassini spacecraft on October 14, 2009. Both moons may contain a subsurface ocean dominated by liquid water. Credit for the original view of Titan: NASA/JPL-Caltech/ Space Science Institute. Credit for the original view of Enceladus: NASA/JPL/Space Science Institute. Credit for the montage: Marc Lafferre, 2019.

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