A classic: Evidence of an ocean beneath the surface of Europe
A classic: Evidence of an ocean beneath the surface of Europe
Note: In this article, instead of reporting on a recent result in astro-ph, we discussed a historical discovery of a work that is now a classic.
1979
In January 1979, Voyager 1 took its first image of Jupiter during its interplanetary cruise through gas giants and their moons. In the few months that circled the ship's maximum approach to the largest planet in the solar system, Voyager 1 took a series of breathtaking images of Jupiter and its moons Ganymede, Io, Europa, Callisto and Amalthea. These images have been compiled in a 21-page work in the magazine Science that was published that same year and that summarizes the discoveries of the first phase of Voyager's journey 1. (If you have access to the original work, it's worth checking out if it's even to see the images.) They are not as detailed as those that were taken by the Juno probe in orbit around Jupiter today, but they are certainly a classic!)
While the article is full of pictures of red and yellow clouds on Jupiter, and of the surface torn and rugged Io, let's focus on the little moon that was only worth a couple of images in this work in 1979: Europe (see Figure 1).
Figure 1. The highest resolution image of Europe currently available. Although we know that the surface of this moon is mostly water in the solid state, we do not know the composition of the reddish material. Image credit: NASA / JPL-Caltech / SETI Institute.
The beginning of the section of the publication dedicated to Europe is compressed into four sentences above a full page of relief maps, where it would certainly be easy to miss. In this brief section, the authors note that the surface of Europe is crossed in random directions by dark lines of 50 to 200 km wide, which can be seen in Figure 2. They propose that this particular pattern is due to tectonic activity , but they do not give a hypothesis about a possible cause of this activity. The most interesting part of this short paragraph is the last sentence, written almost without paying much attention: "An outer layer of water in liquid and solid state up to 100 km deep is entirely possible. (!!!!!!!!!!!) "(The exclamation signs, I attribute them to me.)
Figure 2 A relief map of the surface of Europe, generated from multiple images of the moon taken by Voyager 1. Figure 14b of the work.
Under his Fragile and mobile ice layer, Europe could have a huge ocean. Is it possible that the effort caused by the tides that Jupiter exerts on this small moon, acting continuously to deform and heat it by friction, can produce enough energy for life to be in those waters? Or is that ocean full of compounds that would make life impossible? And how deep is that ice sheet exactly? Could we ever dig a well deep enough to reach the underlying liquid? Before we begin to answer-or even sketch-these questions, we need more evidence of this supposed ocean. To understand what is happening below the frozen surface of Europe, we will still need another mission.
1997
It is now 1997. The first satellite to go into orbit on Jupiter, Galileo, has been exploring the planet and its moons for two years. Among its objectives is Europe, which orbits Jupiter in just 3.5 days, well inside the confines of its magnetosphere (see Figure 3) (Jupiter's magnetosphere is so large that if it were visible to our eyes, it would be the object bigger in the sky - several times bigger than the full moon.)
Figure 3 Jupiter's magnetosphere and Earth's magnetosphere shown to scale in the small upper left box. The tail of Jupiter's magnetosphere extends away from the Sun almost as far as Saturn's orbit. Credit: Fran Bagenal and Steve Bartlett.
During the first passage of Galileo near Europe, its magnetometer detected a slight change in the magnetic field. This small disturbance was interpreted as the presence of a magnetic field of Europe - a discovery that, if correct, would be very interesting. However, subsequent analyzes revealed something completely different: the results were consistent with the presence of a conductive layer below the surface of Europe, as would be the case of a salty ocean.
A work presented in the magazine Science in the year 2000 he details how this works in Europe. While the amplitude of the magnetic field of Jupiter in the position of Europe remains more or less constant, its direction changes as Jupiter rotates. This field that changes over time induces a current in the conductive liquid layer, which in turn generates its own magnetic field, which is detected by the magnetometer as a small disturbance. So we have detected the presence of an ocean on a distant moon, even without the help of visible feathers of water, due to the way it responds to an external magnetic field. This is very exciting, but there is still much we do not know about the ocean of Europe.
2020 - 2030?
What does the future exploration of Europe hold for us? Luckily, two new missions to Jupiter are foreseen in the next two decades: the mission of the NASA, Europe Clipper, which will be launched in the 2020s, and the mission of the ESA, Jupiter IC and moons Explorer (JUICE), which should take off in 2022. Although the main objective of JUICE is to explore Ganymede, another of Jupiter's moons - the largest in the solar system and the only one with a known magnetic field of its own -, the ship will carry a radar capable of penetrating the ice from the top layer of the crust of Europe. Clipper will also carry one of these instruments, as well as a magnetometer, which will allow us to determine if Europe has an ocean or not, how salty it is and how wide the ice cap is. Until then, we will have to content ourselves with imagine what is under the ice.
Cover image credit: NASA / JPL-Caltech.
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