The first time we saw Europe closely, one of Jupiter’s 79 moons, was forty years ago, when a Voyager spacecraft flew over it.
The images showed brown cracks that cut the icy surface of the satellite, which gave it the appearance of a venous eyeball.
Since then, the information obtained by different missions has made that mysterious world a priority objective of NASA in the search for life outside the Earth.
It is possible that Europe has all the necessary ingredients for life.
Scientists have long known that one of them, liquid water, is present beneath the icy surface and is sometimes spit into space by gigantic geysers.
But no one had been able to directly confirm the presence of water in these plumes.
Now, an international team led by NASA’s Goddard Space Flight Center in Greenbelt, Maryland, has succeeded for the first time.
It has detected water vapor on the surface of Europe through one of the world’s largest telescopes in Hawaii.
This confirmation helps scientists better understand the inner workings of the moon.
For example, it helps support the idea that there is an ocean of liquid water, possibly twice as large as Earth’s, that spills beneath a layer of ice kilometers thick.
Some scientists suspect that another source of water could be shallow deposits of melted water ice not far below the surface of Europe.
It is also possible that Jupiter’s strong radiation field is removing water particles from the ice sheet of Europe, although a recent investigation argued against this mechanism as the observed water source.
“Essential chemical elements (carbon, hydrogen, oxygen, nitrogen, phosphorus and sulfur) and energy sources, two of the three requirements for life, are found throughout the solar system. But the third, liquid water, is something difficult to find beyond Earth,” explains Lucas Paganini, a NASA planetary scientist who led the research.
“While scientists have not yet detected liquid water directly, we have found the following best option: water in the form of steam.”
The team published in the journal “Nature Astronomy” the detection of sufficient water release in Europe (2,360 kilograms per second) to fill an Olympic pool in minutes.
However, scientists also discovered that water appears infrequently, at least in quantities large enough to detect it from Earth.
“For me, the interesting thing about this work is not only the first direct detection of water on Europe, but also the lack of it within the limits of our detection method,” says Paganini.
In fact, Paganini’s team detected a weak but distinct water vapor signal only once during 17 nights of observations between 2016 and 2017.
Upon observing the moon from the WM Keck Observatory on the top of the inactive Mauna Kea volcano in Hawaii, the scientists saw water molecules in the main hemisphere of Europe, or the side of the moon that is always oriented in the direction of the moon’s orbit around Jupiter.
(Europe, like the Earth’s moon, is gravitationally locked to its host planet, so the main hemisphere always looks in the direction of the orbit, while the posterior hemisphere always looks in the opposite direction).
For the analysis, they used a spectrograph at the Keck Observatory that measures the chemical composition of planetary atmospheres through the infrared light they emit or absorb.
Molecules such as water emit specific frequencies of infrared light as they interact with solar radiation.
Before the recent detection of water vapor, there have been many tempting findings in Europe.
The first came from NASA’s Galileo spacecraft, which measured disturbances in Jupiter’s magnetic field near Europe while orbiting the giant gas planet between 1995 and 2003.
The measurements suggested to scientists that the electrically conductive fluid probably a salty ocean beneath the ice sheet of Europe, was causing magnetic disturbances.
When researchers analyzed magnetic disturbances more closely in 2018, they found evidence of possible feathers.
Meanwhile, the scientists announced in 2013 that they had used the NASA Hubble Space Telescope to detect the chemical elements hydrogen (H) and oxygen (O), water components (H2O), in pen-like configurations in the atmosphere of Europe.
And a few years later, other scientists used Hubble to gather more evidence of possible feather eruptions when they took pictures of finger-like projections that appeared in silhouette when the moon passed in front of Jupiter.
The research, along with other previous findings from Europe, has only measured water components on the surface.
The problem is that detecting water vapor in other worlds is a challenge.
Existing spacecraft have limited capabilities to detect it, and scientists who use terrestrial telescopes to search for water in deep space must take into account the distorting effect of water on Earth’s atmosphere.
To minimize this effect, Paganini’s team used complex mathematical and computational models to simulate the Earth’s atmosphere conditions so that they could differentiate Earth’s atmospheric water from that of Europe in the data returned by the Keck spectrograph.
Still, “we will have to approach Europe to see what is really happening,” suggests Avi Mandell, a planetary scientist at Goddard in Paganini’s team. Your wish may come true soon.
The Europa Clipper mission will be launched in the mid-2020s to conduct a detailed study of this moon’s surface, deep interior, thin atmosphere, subsurface ocean and active vents.
The spacecraft will try to photograph the plumes and take samples of the molecules it finds in the atmosphere with its mass spectrometers. It will also look for a suitable site for a future landing module that can collect a sample.
As they claim from NASA, these efforts should further unlock Europe’s secrets and their potential for life.