How the fate of Europe's JUICE Jupiter mission depends on the risk of biological contamination

The ultimate fate of the European Space Agency's JUICE mission could drastically change should it succeed in discovering that Ganymede's ocean is both potentially habitable and accessible.

When it launches in April 2023, JUICE (short for "Jupiter Icy moons Explorer") will set course for what could be the best chances to find alien life in the solar system. JUICE's primary targets — the Jupiter moons Europa, Ganymede and Callisto — are all thought to contain subterranean oceans beneath a thick crust of surface ice, and it is possible that conditions for life could exist within these oceans.

This means that special rules must be adhered to on the mission to minimize any possibility of contaminating those moons with Earthly biological material that could interfere with future searches for life. These rules are referred to collectively as "planetary protection," and COSPAR (the international Committee on Space Research) has outlined five categories of planetary mission depending upon the degree of planetary protection required. Category I is a mission to a lifeless body like Mercury. Category II missions are flyby or orbiter missions that venture to worlds where there may be some possibility of life, but the chances of contamination are slim. Then there's category III, where the chances of contamination are deemed greater. Category IV refers to landers on potentially habitable bodies, and category V is sample-return back to Earth.

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"Currently the only problematic body for JUICE is Europa, because Europa has been identified as a category III object," said Claire Vallat, who is the JUICE science operations scientist at the European Space Agency, in an interview with Space.com. "There is the risk that if the spacecraft crashes onto Europa, it might contaminate the surface and jeopardize future exploration. For Ganymede, it is slightly different because it's category II."

The plan at the end of JUICE's mission is therefore to crash the spacecraft into Ganymede. However, this plan might be derailed if JUICE actually finds exciting news at Ganymede.

Ganymede isn't just Jupiter's largest moon, it's the largest moon in the entire solar system with a diameter of 3,272 miles (5,268 kilometers) — greater even than the planet Mercury. Evidence for Ganymede's ocean is primarily found in the fact that Ganymede is the only moon in the solar system to possess its own magnetic field. This magnetic field is induced in Ganymede by Jupiter's own giant, powerful magnetosphere. To be able to induce a magnetic field within the moon requires something that can carry electric charge, namely salt ions in liquid water.

However, that ocean may contain more water than all of Earth's seas, but it's buried beneath an icy crust estimated to be 90 miles (150 km) thick. Europa's icy crust, by contrast, is thought to extend only 9 to 15 miles (15 to 24 km) deep, allowing the sharing of material between its ocean and the surface. The presence of sea salts on the surface and the detection of apparent water plumes also suggest that Europa's ocean may have some connection with its surface.

On Ganymede, "The thicker the icy crust, the less obvious is the process that will allow the connection between material coming from the surface of Ganymede to the ocean layer," said Vallat. This is the basis for JUICE's category II planetary protection status: If the crust is too thick for material to be shared between the surface and the ocean, then the risk of contamination of the ocean is minimal.

"It's also linked to the fact that, as far as we know, Europa is more potentially habitable than Ganymede," said Vallat.

However, estimates of the thickness of Ganymede's crust are based on theoretical modeling — it hasn't actually been directly measured yet.

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When JUICE arrives in the Jovian system in 2030, it will perform a couple of flybys of Europa, 21 flybys of fellow icy moon Callisto, and 12 close passes of Ganymede before settling into orbit around Ganymede in 2032. From this vantage point, "JUICE will be able to measure the thickness of the crust, because we will have very good coverage of the entirety of Ganymede," said Vallat.

Should there be a surprise — for example, perhaps JUICE will discover thinner parts of the crust, or some mechanism that is transporting material, perhaps convectively, between the ocean and the surface — then JUICE's planetary protection category could change. This would mean that JUICE could no longer be allowed to crash into Ganymede to end its mission. Some other method of safely disposing of the spacecraft would need to be found, without posing a contamination threat to either Europa or Ganymede, now or at any time in the future.

"If we find out that Ganymede is actually category III, then we will have to look from a navigation point of view how to solve this," said Vallat. "It would be fantastic from a science point of view, but scary at the same time because it could affect the end of the mission quite strongly."

What that solution would be has yet to be determined, but it would not be an unwelcome headache for the JUICE team, since part of the mission's remit is to probe the potential habitability of Jupiter's icy moons and better understand the conditions that could render them habitable. While JUICE will not be able to observe the oceans directly, it will be able to characterize them indirectly. For example, its magnetometer will observe the strength of Ganymede's magnetic field and therefore the saltiness of its ocean, while its laser altimeter will measure tides in the ice brought about by tides in the ocean as it sloshes about under Jupiter's gravitational pull.

"JUICE is the next step in understanding habitability," concluded Vallat. "It will help answer so many questions that are important for humankind."

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