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By Matt Williams
In the 1970s, the Jupiter system was explored by a succession of robotic missions, beginning with the Pioneer 10 and 11missions in 1972/73 and the Voyager 1 and 2 missions in 1979. In addition to other scientific objectives, these missions also captured images of Europa’s icy surface features, which gave rise to the theory that the moon had an interior ocean that could possibly harbor life.
Since then, astronomers have also found indications that there are regular exchanges between this interior ocean and the surface, which includes evidence of plume activitycaptured by the Hubble Space Telescope. And recently, a team of NASA scientists studied the strange features on Europa’s surface to create models that show how the interior ocean exchanges material with the surface over time.
The study, which recently appeared in the the Geophysical Research Letters under the title “Band Formation and Ocean-Surface Interaction on Europa and Ganymede“, was conducted by Samuel M. Howell and Robert T. Pappalardo – two researchers from the NASA Jet Propulsion Laboratory. For their study, the team examined both Ganymede and Europa to see what the moons surface features indicated about how they changed over time.
Using the same two-dimensional numerical models that scientists have used to solve mysteries about motion in the Earth’s crust, the team focused on the linear features known as “bands” and “groove lanes” on Europa and Ganymede. The features have long been suspected to be tectonic in nature, where fresh deposits of ocean water have risen to the surface and become frozen over previously-deposited layers.
However, the connection between this band-forming processes and exchanges between the ocean and the surface has remained elusive until now. To address this, the team used their 2-D numerical models to simulate ice shell faulting and convection.Their simulations also produced a beautiful animation that tracked the movement of “fossil” ocean material, which rises from the depths, freezes into the base of the icy surface, and deforms it over time.
Whereas the white layer at the top is the surface crust of Europa, the colored band in the middle (orange and yellow) represents the stronger sections of the ice sheet. Over time, gravitational interactions with Jupiter cause the ice shell to deform, pulling the top layer of ice apart and creating faults in the upper ice. At the bottom is the softer ice (teal and blue), which begins to churn as the upper layers pull apart.
This causes water from Europa’s interior ocean, which is in contact with the softer lower layers of the icy shell (represented by white dots), to mix with the ice and slowly be transported to the surface. As they explain in their paper, the process where this “fossil” ocean material becomes trapped in Europa’s ice shell and slowly rises to the surface can take hundreds of thousands of years or more.
As they state in their study:
“We find that distinct band types form within a spectrum of extensional terrains correlated to lithosphere strength, governed by lithosphere thickness and cohesion. Furthermore, we find that smooth bands formed in weak lithosphere promote exposure of fossil ocean material at the surface.”
In this respect, once this fossil material reaches the surface, it acts as a sort of geological record, showing how the ocean was millions of years ago and not as it is today. This is certainly significant when it comes to future missions to Europa, such as NASA’s Europa Clipper mission. This spacecraft, which is expected to launch sometime in the 2020s, will be the first to study Europa exclusively.
In addition to studying the composition of Europa’s surface (which will tell us more about the composition of the ocean), the spacecraft will be studying surface features for signs of current geological activity. On top of that, the mission intends to look for key compounds in the surface ice that would indicate the possible presence of life in the interior (i.e. biosignatures).
If what this latest study indicates is true, then the ice and compounds the Europa Clipper will be examining will essentially be “fossils” from hundreds of thousands or even millions of years ago. In short, any biomarkers the spacecraft detects – i.e. signs of potential life – will essentially be dated. However, this need not deter us from sending missions to Europa, for even evidence of past life would be groundbreaking, and a good indication that life still exists there today.
If anything, it makes the case for a lander that can explore Europa’s plumes, or perhaps even a Europa submarine (cryobot), all the more necessary! If there is life beneath Europa’s icy surface, we are determined to find it – provided we don’t contaminate it in the process!
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