The fascinating results were presented at the Goldschmidt conference (virtual this year due to Covid-19). The associated abstract is available on the conference website. These are preliminary results, and not yet peer reviewed.
First, the findings support what scientists had predicted about ocean composition, as planetary scientist and lead researcher Mohit Melwani Daswani explained in a statement:
We were able to model the composition and physical properties of the core, the silicate layer, and the ocean. We find that different minerals lose water and volatiles at different depths and temperatures. We add these volatiles that are estimated to have been lost from the interior, and we discover that they are consistent with the predicted mass of the current ocean, which means that they are probably present in the ocean.
To reach this conclusion, the researchers modeled geochemical deposits in the interior of Europe using data from the ancient Galileo mission. The results are significant, as they show that oceans like that of Europe can be formed by metamorphism, that is, by a change in minerals or a different arrangement of minerals (a change in what geologists call texture) in pre-existing rocks (protolites ) without the protolyte melting into liquid magma.
In Europe, warming and increased pressure caused by early radioactive decay or subsequent movement of subsurface tides would have caused a decomposition of the minerals that contain water. Trapped water would be released and Europe's underground ocean would be born.
So what about habitability?
The study also found that Europe's ocean would have been slightly acidic at first, with high concentrations of carbon dioxide, calcium, and sulfate. But over time, it became rich in chloride, resembling Earth's oceans (seawater on Earth contains 1.94% chloride). According to Daswani:
In fact, it was thought that this ocean could still be quite sulfuric, but our simulations, along with data from the Hubble Space Telescope, showing chloride on the surface of Europe, suggest that the water probably became rich in chloride. In other words, its composition was more like Earth's oceans. We believe that this ocean could be quite habitable for life.
Europe is one of our best opportunities to find life in our own solar system. NASA's Europa Clipper mission will launch in the coming years, so our work aims to prepare for the mission, which will investigate the habitability of Europe. Our models lead us to think that the oceans on other moons, such as Europe's neighbor Ganymede, and Saturn's moon Titan, may also have been formed by similar processes.
However, we still need to understand several points, such as how fluids migrate through the rocky interior of Europe.
Another interesting possibility is that the volcanic vents on the sea floor of the European ocean could have contributed to the process of turning the water rich in chlorides. That would be exciting, as such vents on Earth provide heat and nutrients to a wide variety of life in our world's deep oceans.
The Cassini spacecraft has already found evidence of such vents on the seabed of Saturn's moon Enceladus, which also has a global ocean beneath its icy crust.
These or other sources of energy would be necessary for life to evolve and prosper in an underground ocean: a dark, sunless environment. As Steve Mojzsis, professor of geology at the University of Colorado Boulder, noted:
A long-standing question about whether a `` hidden ocean '' world like Europe could be livable boils down to whether it can maintain a flow of electrons that can provide the energy to fuel life. What is not clear is whether such icy moons could generate enough heat to melt rocks; Certainly interesting chemistry is produced within these bodies, but what reliable flow of electrons could extraterrestrial life use to feed in the cold, dark depths?
A key aspect that makes a world 'livable' is the intrinsic ability to maintain these chemical imbalances. Arguably, icy moons lack this ability, so it should be tested on any future mission to Europe.
The results of the new study are tantalizing, but much work remains to be done. How much chloride is really in the ocean of Europe? As mentioned in a June 25, 2020 article on Centauri Dreams, a 2019 article stated the following about sodium chloride (NaCl) in Europe:
The presence of NaCl in Europe has important implications for our understanding of internal chemistry and its geochemical evolution over time. While aqueous differentiation of chondritic material and long-term leaching of a chondritic seabed can result in a sulfate-rich system, more extensive hydrothermal circulation, as on Earth, can lead to a NaCl-rich ocean. The chemistry of the Enceladus plume, which is perhaps the best analog in Europe, suggests an ocean dominated by NaCl and a hydrothermally active seabed. However, the compositional relationship between the Europa Ocean and its endogenous material is unknown, and the surface may simply represent the end result of compositional stratification within the ice sheet ... Regardless of whether the observed NaCl is directly related to the composition of the ocean, its presence guarantees a reevaluation of our understanding of the geochemistry of Europe.
Scientists can learn more about Europe's ocean through modeling, but obtaining solid evidence will require sending a mission there. Fortunately, NASA's Europa Clipper mission is poised to do just that, slated to launch in 2023.
Europe is one of the largest moons in the solar system with a diameter of 1,926 miles (3,100 km), just slightly less than Earth's moon. While the subsurface ocean is relatively warm (the exact temperature is not yet known), on Europe's nearly airless surface, temperatures are always bitter minus 256 degrees Fahrenheit (minus 160 degrees Celsius) or less.
There is also mounting evidence of water vapor columns in Europe, similar to those of Saturn's moon Enceladus. If they are there, Europa Clipper could fly through them, just like Cassini did on Enceladus, and take samples of the steam for analysis. If connected to the ocean, as Enceladus is believed to be, that would provide valuable clues to conditions in the Europan ocean and, perhaps, even evidence of life itself.
The latest analysis of Europe's ocean is tempting, but we'll know much more after Europa Clipper, and ESA's Jupiter Icy Moon Explorer (JUICE) mission, take a closer look in the years to come.
Bottom line: A new study by NASA scientists shows that Europe's ocean is habitable.
Source: Evolution of volatiles from the interior of Europe to its ocean.