After twenty years of ups and downs, development and downsizing, scientists are on the cusp of sending missions to explore Europe's oceanic world. Could this be our best chance of finding life anywhere in the solar system? After all, Europe is a very tiny world orbiting the giant planet Jupiter, even smaller than the Earth's Moon. From a distance, Europe looks like a jagged web of dark stripes, like a messy pencil drawing of a toddler. Long linear cracks in the ice are found close by, extending in some cases for thousands of kilometers. Many are filled with an unknown contaminant that scientists call "brown mud." Elsewhere, the surface is uneven and shattered, as if massive slabs of ice were drifting, spinning, and flipping in slush.
Jupiter's powerful gravity helps generate tidal forces that stretch and weaken the moon many times over. But the stresses that have created Europe's fragmented landscape are best explained by the ice shell floating in an ocean of liquid water.
“The fact that there is liquid water beneath Europa's surface, as we know from previous missions, in particular from magnetometer observations collected by Galileo in the 1990s, makes it one of the most interesting potential targets for the search for life,” says Professor Andrew Coates. from the Mullard Space Research Laboratory in Surrey, UK.
The salty depth of Europa can reach 80-170 kilometers deep into the satellite, which means that it can contain twice as much liquid water than all of the Earth's oceans.
While water is one of the most important prerequisites for life, Europa's oceans may have others, such as a source of chemical energy for microbes. Moreover, the ocean can interact with the surface through a number of means, including warm drops of ice that rise up the ice shell from the bottom up. Therefore, studying the surface can provide clues to what is happening in the ocean.
Now NASA is launching two missions to explore this intriguing world. Both were discussed at the 48th Lunar and Planetary Science Conference (LPSC) in Houston.
The first is a flyby mission called Europa Clipper, which is likely to take place in 2022. The second is a landing mission that will follow a few years later.
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Dr. Robert Pappalardo of NASA's Jet Propulsion Laboratory is a Clipper Scientist.
“We are trying to understand the potential habitability of Europe, its ingredients for life: water and the availability of possible chemical energy for life,” he says. “We do this by trying to understand the ocean and ice shell, composition and geology. And all together they demonstrate the level of the current activity of Europe”.
The Clipper carries a payload of nine tools, including a camera that will capture most of the surface; spectrometers to understand its composition; ice-permeable radar for mapping the ice shell in three dimensions and finding water under the ice shell; magnetometer to characterize the ocean.
However, since the Galileo spacecraft provided evidence of the ocean in the 1990s, we know that Europe is not the only one of its kind.
“Over the past ten years, we have been surprised to find that it is impossible to travel to the outer solar system and not collide with the ocean world,” says Clipper scientist Kurt Niebuhr.
On Saturn's moon Enceladus, for example, ice from the subsurface ocean erupts into space through cracks at the south pole.
The Saturnian moon may also see a special mission in the 2020s, but Dr. Niebuhr thinks Europe is a more attractive target: “Europe is much larger than Enceladus and has the most: more geological activity, more water, more space for that water, more heat. more raw materials and more stability in the environment."
There is something else that makes this moon stand out: its surroundings. Europa's orbital path goes deep into Jupiter's magnetic field, which captures and accelerates particles.
The result is intense radiation belts that roast the electronics of spacecraft, limiting mission durations to months or even weeks. However, this radiation also causes reactions on Europa's surface, creating oxidants. On Earth, biology uses chemical reactions between oxidants and compounds known as reducing agents to provide the necessary energy for life.
However, oxidants created on the surface are beneficial to Europa's microorganisms only if they can descend into the ocean. Fortunately, the convection process that pushes warm ice droplets upward can also erode surface material. Once in the ocean, oxidants can react with reducing agents produced by seawater, reacting on the hard ocean floor.
“You need both poles of the battery,” explains Robert Pappalardo.
For scientists like Dr. Pappalardo, the missions ahead are a dream come true for two decades. Since the first concepts for a mission to Europe were developed in the late 1990s, proposals have been thwarted one by one.
In the 2000s, the United States and Europe even pooled resources for a mission that would send separate spacecraft to Europe and Jupiter's moon Ganymede. But the plan was canceled due to budget cuts, and the European part spilled over into the Juice mission.
“I don’t think there has been a mission to Europe in the past 18 years that has passed my fingers and eyes,” says Niebuhr. “It's been a long journey. The road to launch has always been a thorny one, and it has also been full of disappointments. We felt it most of all on the example of Europe”.
Exploring Europe is costly - though no more than other NASA flagship missions such as Cassini or Curiosity.
There are complex engineering challenges, such as working in Jupiter's radiation belts. The spacecraft's instruments must be shielded with materials such as titanium metal, Pappalardo says, but "they must be able to see Europa."
Therefore, to keep Clipper safe, NASA will deviate somewhat from the rules. “It was supposed to be like this: Galileo flew past Europa, so the next mission should be in orbital. This is how we do business,”says Niebuhr. But instead of entering Europa's orbit, Clipper will reduce the impact of mission-shortening radiation by entering Jupiter's orbit and make at least 45 close missions to the icy moon in three and a half years.
"We realized that we could avoid these technical problems of entering the orbit of Europe, make the mission more feasible and at the same time fulfill all the scientific tasks."
The intensity of sunlight near Europa is thirty times weaker than on Earth. But NASA decided that it could power Clipper's solar panels, so it wouldn't have to use radioisotope generators like other missions. “All these years of research have forced us to abandon old concepts and focus on what is actually achievable, not desired,” says Kurt Niebuhr.
In 2011, following the cancellation of the US-European mission, a report from the National Research Council confirmed the importance of studying the icy moon. Despite this, NASA is still cautious about the cost.
The lander did not receive funding in the president's 2018 budget request for NASA. But Dr. Jim Green, director of planetary sciences at the agency, says that "this mission is extremely exciting because it will tell us about the science we could be doing on the surface of a satellite."
“We have to go through a long process to understand what measurements we need to take. Then we have to work with the administration and schedule the right time, agree on the budget to move forward."
Over the past twenty years, highly innovative lander concepts have been proposed, reflecting the scientific generosity that can be used after landing. Gearyne Jones of the Mullard Space Research Laboratory has been working on a concept called a "penetrator."
“They haven’t gone into space before, but the technology is very promising,” he explains. The projectile, fired from the satellite, hits the surface "very hard, at a speed of about 300 meters per second, 1000 km / h", throwing out ice for further analysis by onboard instruments that should be able to resist the fall.
Conversely, NASA's future lander will land softly using the "sky crane" technology that was used to safely drop the Curiosity rover on Mars in 2012. During landing, it will use an autonomous landing system to detect and prevent surface hazards in real time.
Clipper will be able to provide reconnaissance for the landing site. “I love the idea that he will find a suitable oasis where the water is close to the surface. Maybe it will be warm and there will be organic materials,”says Pappalardo.
The vessel will be equipped with sensitive instruments and a revolving saw that will provide fresh samples from under the radiation-treated surface ice.
“The lander will have to get to the freshest, pristine ice sample. To do this, he will have to dig deep or erupt at the surface - create a geyser - that will dump a lot of fresh material to the surface,”says Kurt Niebuhr.
In recent years, the Hubble Telescope has made preliminary observations of eruptions of water ice erupting from under Europa, similar to those of Enceladus. But there is no point in visiting the places of ten-year eruptions - the device needs to visit a place with a relatively fresh ejection.
Therefore, scientists need to understand what drives these geysers: for example, Clipper will determine if geysers are associated with any hot spots on the surface.
The sea expanses of the Earth are teeming with life, so it is difficult for us to imagine a sterile 100 km deep ocean in Europe. But the scientific threshold for detecting life is set very high. Will we be able to recognize alien life if we find it?
“The goal of the landing mission is not just to discover life (to our satisfaction), but to convince everyone else that we did it,” explains Niebuhr. "It won't be very good for us to invest in this mission if all we create is scientific controversy."
Thus, the team suggested two ways. First, any detection of life must be based on multiple independent data lines from direct measurements.
“You cannot make one measurement and say: yes, there is eureka, we found it. You're looking at the total,”says Niebuhr. Second, scientists have developed a framework for interpreting these results, some of which may be positive and others negative. “A decision tree is created that goes through all the different variables. By following all these different paths, we get the end result, one of two things: either we found life, or we didn't,”he says.
ILYA KHEL