Ocean Worlds: A New Horizon
Enceladus’ oceans contain phosphorus (P) and in larger amounts than expected. This is the conclusion of a study published in September 2022 by an international research team*. If the news caused a stir, it was because it added this element to the carbon (C), hydrogen (H), nitrogen (N), oxygen (O) and sulfur (S) already found in Saturn’s small moon ice. These six atoms, which complete the CHNOPS list, make up all the molecules of living organisms as we know them on Earth.
If this is not enough to conclude that there is life on Enceladus, “This confirms that its subglacial ocean has potentially very interesting chemistry… and the satellite is the best target for searching for signs of life in the Solar System today.” Planetologist Alice Le Gall of the University of Versailles-Saint-Quentin-en-Yvelines exhibits.
However, Saturn’s small moon isn’t the only moon of interest to researchers. They have their eyes on another gas giant moon, Titan, and especially on Europa, which orbits Jupiter. What they have in common? All contain oceans of liquid water, more or less deep beneath the ice sheet. Their discovery dates back to data collected by the Galileo and Cassini probes that explored the Jupiter and Saturn systems at the end of the millennium.
“Due to the gravitational properties of these objects, there must be a large amount of liquid water in their internal structure” asks Olivier Grasset, director of the laboratory of planetology and earth sciences in Nantes. So much so that some scientists are now talking about the world’s oceans. “This is the new Eldorado for exobiology” Alice Le Gall confirms.
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THREE CONDITIONS FOR LIFE
If we follow the trail of liquid water, it is because it is the best known natural solvent on earth. “It is a universal ‘chemical compatibilizer’ that facilitates a variety of reactions,” says Hervé Cottin, an astrochemist at the University of Paris-Est Créteil. However, this is only one of the three conditions generally considered necessary for the creation of life.
“It also requires complex chemistry as well as an available energy source.” the researcher recalls. However, these other conditions may be met on Enceladus as they are on Europa. Reason ? Their interior oceans are likely in direct contact with their rocky cores.
“The exchange between water and minerals contributes to the rich chemistry, while heat from the core can escape in the form of small underwater volcanoes called hydrothermal vents.” Olivier Grasset explains. However, on Earth, ocean hydrothermal vents are teeming with life!
So the crucial question is: is this also the case in ocean worlds? Biochemists are cautious about this. Recent research suggests that, in fact, the thermal or chemical energy found in deep water may not be sufficient to fuel all of the reactions that can drive life. “Only those carried by photons [particules de la lumière, NDLR] appears to be a sufficient concentration to trigger certain syntheses of complex organic molecules” Hervé Cottin defines. This would mean that hydrothermal vents on Earth are not actually a place where life could have originated, but a place where life that had previously appeared elsewhere would adapt after the fact. Also, there is no light in the oceans of Enceladus and Europa, imprisoned under a thick layer of ice.
Enceladus is currently the best target for searching for signs of life in the Solar System
Titan’s case further divides researchers because its oceans are trapped between two thick sheets of ice. “That’s why it’s so chemically weak” Underlines Olivier Grasset. In addition, the giant moon of Saturn has an advantage: a very dense atmosphere rich in methane, in which many advanced reactions of organic chemistry take place. This multiplicity of reactions even results in liquid hydrocarbon lakes on the surface! However, all these organic substances were able to occasionally come into contact with the waters of the inner oceans thanks to the fall of meteorites whose traces have been identified. “The heat generated by these impacts could have kept liquid water on the surface for thousands of years, despite the average outside temperature being -180°C.” Nathalie Carrasco, an astrochemist at the University of Versailles-Saint-Quentin-en-Yvelines, emphasizes. But still, the debate between optimists and skeptics remains alive: “Thousands of years are too short for the development of any life form, even prebiotic chemistry” so believes Olivier Grasset.
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Everyone agrees on at least one point: these debates remain highly theoretical. More field data is needed to move them forward… which the next generation of missions is about to collect.
NASA’s Europa Clipper and Esa’s Juice (Jupiter Icy Moon Explorer) should be launched to attack Europa in 2023-2024. The Dragonfly mission from the American agency should go to Titan by 2027. Ideally, the ice would be drilled to access these moons’ famous oceans, but that’s technologically unthinkable at the moment. : “It is difficult to reach Lake Vostok under the ice 4 km deep in Antarctica, even though it is on Earth” Reminds me of Alice le Gall. Therefore, these missions will have to be content with analyzing potentially detectable surface areas: craters on Titan, fissures on Europa, and geysers on Enceladus.
The icy months are far away
WORLDS-OCEANS: TRACKING THE BIOCIES
Know what to look for. “Small organic molecules are ubiquitous in the Universe and not necessarily associated with life. On the contrary, the determination is ‘big “The building blocks of life on Earth, like DNA or proteins, require sophisticated tools that are difficult to get on board today.” details Nathalie Carrasco. Bringing samples back to perform such analysis on Earth is an option. ESA Director Josef Aschbacher mentioned this for a hypothetical mission to Enceladus in early 2022… But again, in the short term, it seems unrealistic: “Ice Moons Are Far Away” Julie Castillo, a researcher at NASA’s Jet Propulsion Laboratory (JPL). In fact, Jupiter is eight times as far away from us as Mars… and Saturn is sixteen times.
“In the near future, it will be necessary to carry out the analyzes on the spot” the planetologist concludes. For all these reasons, researchers are increasingly interested in intermediate carbon molecules known as prebiotics, such as amino acids, building blocks of proteins, and even lipids and sugars. Although these molecules have already been found in meteorites, they are not necessarily from living organisms. “If many different species are identified in the same location, this can constitute a biosignature” Alice Le Gall says.
To measure them, Europa Clipper and Dragonfly will carry analytical instruments, including spectrometers that separate molecules by their mass and chromatographs that discriminate between them by their chemical properties. If these two techniques were already on the Cassini-Huygens and Rosetta missions, as well as on the Curiosity rover that landed on Mars in 2012, they have made great progress since then: “Today we can distinguish, for example, the spatial orientation (chirality) of two molecules of the same composition. “But this is another interesting clue, because on Earth, for reasons we don’t know, the molecules of living things share the same chirality.” Nathalie Carrasco continues.
With the OWLS (Ocean Worlds Life Surveyor) instrument to be launched in October 2022, NASA’s JPL wants to approach the problem from a different angle. It will carry a kind of ultra-high-definition camera responsible for capturing all the movements of the sample at the molecular level. An automatic image recognition system based on artificial intelligence will then be responsible for identifying “organized” movements that can be associated with living organisms. A system completed too late to launch on Europa Clipper or Dragonfly could power future missions to Enceladus.
* The abundant phosphorus expected for possible life in Enceladus’ ocean, Glen et al., Materials of the National Academy of Sciences, 2022.
© NASA – RON MILLER/ SP
© NASA – RON MILLER/ SP
Ocean Worlds: Europe
Features: Jupiter’s moon is slightly smaller than our Moon (diameter 3122 km).
Composition : metallic core, rocky mantle, liquid water oceans. Intermediate, superficial ice sheet (20 km to 100 km thick).
Recent intelligence: It was flown by the Galileo mission between 1997 and 1999.
Areas of interest: the presence of a liquid ocean in contact with a rocky core. Probable hydrothermal activity at depth.
Further explorations: Europa Clipper missions (launch in 2024, about fifty flights in 2030) and Juice (launch in 2023, two flights in 2030).
Ocean worlds: Enceladus
© MIKKEL JUUL JENSEN/ SPL
Features: Saturn’s moon is ten times smaller than our Moon (diameter 504 km).
Ingredients: rocky core, intermediate liquid water ocean, surface ice crust (5 km to 50 km thick).
Recent intelligence: Between 2004 and 2017, more than twenty flights were made by the Cassini probe.
Areas of interest: geysers with salts and organic matter. The presence of a liquid ocean in contact with a rocky core. Probable hydrothermal activity at depth.
Further research: not planned to date, likely goal for missions after 2030.
Features: Saturn’s largest natural moon (diameter 5150 km).
Ingredients: a rocky core, an inner liquid water ocean trapped between two thick sheets of ice, a dense methane-rich atmosphere.
Latest explorations: 127 Cassini flybys between 2004 and 2017. The landing of the Huygens module in 2005.
Areas of interest: surface liquid hydrocarbon lakes. Complex carbon chemistry in the atmosphere. The possibility of liquid water on the surface after large meteorite impacts.
Further research: Dragonfly mission (launch in 2027, landing in 2034).
© RON MILLER/ SPL. Artist’s impression of hypothetical hydrothermal vents beneath the icy surface of Saturn’s moon Enceladus.
Liquid water, which is not so rare in the solar system
This was thought to be possible only in the temperate zone of the Sun. But liquid water now exists on Jupiter’s moons – Europa, Io, Callisto or Ganymede – as well as on Saturn’s moons – Titan, Enceladus and possibly Mimas and Dinoe. However, the anemic radiation of our star plays a minor role in this part of the solar system. Therefore, we have to look at the side of the huge gravitational force exerted by the giant planets on their satellites. “This gravity deforms their rocky cores. It is the frictional heat that allows liquid water to exist.” Alice Le Gall explains.
Even more surprising: other “independent” bodies would also contain liquid water, such as Ceres, the largest object in the asteroid belt, and Pluto, far away. “In these cases, it is the decay of radioactive elements in the core of the nucleus that provides the heat” Julie Castillo says.