Analyze the chances of living beings existing on Jupiter's moons

In addition to Earth, Europa is one of three worlds in our solar system - along with Saturn's two moons Enceladus and Titan - often thought to possess the three ingredients that make up life: liquid water, Energy and chemical molecular building blocks nourish life.

Europa hardly harbors life

In particular, Europa is of more interest than Enceladus and Titan because it is closer to Earth. Jupiter's orbit is only 4 astronomical units from Earth (1 astronomical unit corresponds to the distance from Earth to the Sun of 150 million kilometers) while Saturn's orbit is 9 astronomical units from Earth. .

Analyze the chances of living beings existing on Jupiter's moons Picture 1

Furthermore, Europa is thought to be about 4.5 billion years old, about the same age as Earth. In other words, life took the same amount of time to appear on Europa as it did on Earth.

As a way to highlight all those promising convergences, NASA is expected in October to launch Europa Clipper - the largest spacecraft it has ever developed for planetary missions.

But as Clipper's maiden voyage approaches, it remains unclear whether the ice-covered seas on Europa's moon can sustain life. As University of Washington planetary scientist Paul Byrne and other researchers question whether the seafloor on Europa is a dead zone, mysterious earthquakes are detected on Earth's moon suggests that mysterious mechanisms may also be at work inside Europa. And even if Jupiter's icy moon is uninhabitable today, that wasn't always the case.

Robert Pappalardo, a planetary scientist at the Jet Propulsion Laboratory in Pasadena, California, who worked on the Clipper mission, said the geology of Europa's seafloor and its ability to harbor life could is the crux of this moon's habitability problem. Pappalardo commented: 'That is a vexing question for us. Regardless, it's important to understand what life is like out there in general.'

All is quiet at the bottom of the ocean

Europa's ocean is in darkness. It is thought to lie beneath a layer of ice estimated to be at least 20 kilometers thick that envelops Jupiter's entire moon. And the depth of the ocean on Europa cannot be measured, but is only estimated to be about 60 to 150 km deep. For comparison, it is necessary to know that the average depth of the ocean on Earth is 4 km.

Whatever lives in that darkness must rely on chemical synthesis. While plants and phytoplankton synthesize food from light, water and carbon dioxide, chemosynthetic organisms harvest carbon-bearing molecules and energy released from chemical reactions in the environment of them. At the bottom of Earth's oceans, microbes of this type populate hydrothermal vents and methane seeps, chemical oases maintained by tectonic forces and volcanic activity.

For such organisms to exist in Europa's ocean, Byrne said, there would need to be geologically stable environments similar to those on Earth, or at least chemical reactions between the water and the surface. new stone. Therefore, the basic question for scientists is: How likely is it that this will happen?

Byrne and his colleagues built computer simulations of Europa's seafloor, taking into account the moon's gravity, the pressure of the ocean above, and the pressure of water inside the seafloor. From simulations, the team calculated the strength of rocks about 1 kilometer below the seafloor surface, or the stress needed to force faults on the seafloor to move and expose fresh rock to the surface. seawater.

Compared to the stress exerted on the seafloor by Jupiter's gravity and the convection of material in Europa's lower mantle, the rocks that make up Europa's seafloor have greater inertia, Byrne said. at least 10 times. From there, Byrne concluded: 'The take-home message is that the seabed in Europa is likely to be geologically inert', meaning it is very tight and does not easily create gaps for solvents to automatically erupt.

Not enough lava to revolutionize life

Austin Green, a planetary scientist at the Jet Propulsion Laboratory, shares this pessimistic view. He and his colleagues simulated lava flows originating from inside Europa, to test whether magma from below could rise to reach the seafloor and come into contact with water.

For that to happen, magma must first rise strong enough to breach the rock above. And second, the magma source must be large enough to steadily supply molten rock to the upwelling streams, otherwise they will cool and solidify during upwelling.

Simulations show that the first condition is impossible. Europa's low gravity, which reduces its ability to produce large chunks of molten rock, limits the force of magma's upwelling, Green said. Instead, magma only forms small clumps and is dispersed throughout the mantle.

The small volume of magma also eliminates the second condition: there must be enough supply. Assuming upwelling forces were satisfied, the team simulated magma flows rising in the mantle. They found that pockets of diffuse molten rock formed at a depth of about 200 km below the seafloor. From that depth, the magma flow rising to its highest point only advances 5% of the way to the sea floor before solidifying.

Green said: 'Volcanism on Europa's seafloor is very unlikely today. If volcanoes are necessary to support life, then Europa's ocean is uninhabitable.'

David Pac

Update 20 April 2024