Could Ganymede, Callisto or Europa, three large moons of Jupiter discovered by Galileo in 1610, be habitable, and perhaps harbor some form of life in subterranean oceans? This is the big question that the Juice mission of the European Space Agency (ESA) that should be launched on Thursday 13 April and NASA's Europa Clipper that will be launched next year will try to answer.
Juice, the first European mission to Jupiter, will fly by all three moons before orbiting Ganymede to explore it in detail. With a diameter of 5,250 kilometers, almost half that of Earth, Ganymede is larger than the planet Mercury and the only moon in the solar system with its own magnetic field. What makes it more attractive to ESA is that under its icy crust there could be between six and eight times more liquid water than in all the seas and oceans on Earth, according to observations made by the Hubble Space Telescope and missions. before Jupiter sent by NASA.
The Europa Clipper mission, for its part, will focus on the moon Europa, which has a diameter of 3,140 kilometers, only 10% less than Earth's moon. With an ocean of liquid water below its surface, NASA describes it as "one of the most promising places where we could find currently habitable environments in the solar system." The subterranean ocean appears to be in contact with Europa's rocky core, which could have given rise to complex chemical reactions like those that gave rise to life on Earth.
By contrast, the ocean on Ganymede and possibly Callisto appear to be wedged between layers of water ice, making it more unlikely that the chemistry of life could evolve there, since the available ingredients would be reduced to hydrogen and oxygen. .
Finding out what Ganymede, Callisto and Europa are like on the inside is precisely a main objective of the Juice mission when it arrives in the Jupiter system after eight years of travel. “We will look at the thickness of the ice and oceans below the surface of icy moons,” says Jan-Erik Wahlung of the Swedish Institute for Space Physics in a video produced by ESA.
Stas Barabash, also from the Swedish Institute for Space Physics, hopes to "study the plumes on Europa, which probably come from subterranean oceans" and which are expected to rise to the surface through cracks in its icy crust. “If we are lucky enough to access the original material from the oceans, we might see some distinctive signs that indicate the existence of life,” says Barabash.
Juice is due to leave for Jupiter on April 13 at 2:15 p.m. (Spanish peninsular time) with an Ariane 5 rocket from the Kurú space base in French Guiana. Once the ship has reached its destination in July 2031, it will spend three and a half years in orbit around the planet, during which it will fly over Callisto 21 times and come within 200 kilometers of its surface; it will visit Ganymede 12 times and will come within 400 kilometers; and Europe, twice and it will also come within 400 kilometers.
In December 2034 it will go into orbit around Ganymede and will study it for at least nine months from 500 kilometers above sea level. It will be the first ship to orbit a moon from another planet. If it has enough fuel left by the end of 2035, ESA plans to extend the mission and lower its orbit to around 200 km altitude to obtain more detailed data from Ganymede.
How much fuel it has left will largely depend on the launch date from Kourou. If takeoff has to be delayed for a few days for technical or weather reasons, Juice will have to use more fuel to get to Jupiter and have less left over to make trajectory corrections when he gets there.
Juice's observations will be complemented by those from NASA's Europa Clipper mission, which is due to leave Earth in October 2024 and arrive at Jupiter in April 2030 to study the moon Europa. Since Jupiter's magnetic field is stronger on Europa than on Ganymede, NASA has ruled out placing its spacecraft in orbit around the moon so as not to damage its instruments. The mission plan calls for flying over Europe 44 times over four years.
Juice and Europa Clipper were born as part of a project by NASA and ESA to send a joint mission to the moons of Jupiter. Initially called Laplace, it was to be launched around 2020 and consist of two complementary ships, one destined for Ganymede and the other for Europa. Seeing that NASA froze the project for budgetary reasons, ESA decided in 2011 to go ahead with its own mission to Ganymede. Subsequently, NASA recovered in 2015 the project of a mission to Europe.
Although they are now two separate missions, Juice and Europa Clipper scientists have continued to collaborate and plan to work together when they arrive at Jupiter. "We reached the Jupiter system together," said Norbert Krupp of the Max Planck Institute for Solar System Research in Göttingen, Germany, in an ESA video. "Taking data with two spacecraft will allow us to do unprecedented science on Jupiter."
