Even after Europe's Jupiter Icy Moon Explorer (JUICE) concludes its mission more than ten years from now, we are unlikely to know for sure whether life might be thriving in the vicinity of the solar system's largest planet.
Liquid water, a source of energy and nutrients are the three things scientists believe are necessary for life to emerge anywhere in the universe. Some of Jupiter's moons are more likely than others to provide all three of these elements. The European Space Agency's JUICE mission, set to launch this week, aims to help scientists better understand which of these moons possess the right stuff and could possibly support life, and which couldn't.
JUICE, however, is not going to detect life, nor its direct signatures, scientists say. And by the time the mission ends, most likely by crashing into Jupiter's largest moon Ganymede, we will still be decades away from knowing for sure whether life, even if only microbial, thrives on any of Jupiter's four main moons. So what exactly is JUICE going to tell us?
Underneath the ice crust
Many of the moons orbiting the giant planets of the solar system, such as Jupiter, Saturn and possibly Uranus, are quite different from Earth's moon. Data from probes that flew past them suggest that these moons may be harboring abundant oceans of water. Due to the extremely cold temperatures in these distant parts of the solar system, these oceans are covered by shells of ice dozens of miles thick, so peeking inside of those water bodies is not an easy task.
On some of these moons, such as Saturn's moon Enceladus, scientists have detected evidence of water geysers that spray upwards for miles into space through cracks in the ice. Measurements by the Hubble Space Telescope suggest that Jupiter's smallest moon Europa might also produce such plumes. These plumes indicate that some source of heat must be at work inside of those moons, increasing the possibility that conditions favorable for life might exist on these worlds.
Jupiter's largest moon Ganymede, the main target of the JUICE mission, is also believed to possess an ocean, and so might the most distant, crater-riddled Callisto.
JUICE will be studying these two moons from the altitude of several hundred miles, mostly measuring the physical properties that can help scientists confirm the existence of these oceans, determine their depth and gain some insights into their chemical composition. The probe will also make two flybys of Europa, the second closest of the four main moons to Jupiter. Despite the valuable measurements JUICE will take of these moons, scientists don't think that any signs of life could be detected from so high above.
"If life exists on these moons, we expect it to be in the water, and that's very hard to access," Adam Masters, senior lecturer in space and atmospheric physics at Imperial College London and a member of the team that built one of the scientific instruments for JUICE, told Space.com. "We don't expect life on the surface of these moons and it's not feasible yet to go down [underneath the ice crust] to where there might be life."
The wrong moon for life
Of the two missions set to study Jupiter's moons in the early 2030s, JUICE has a disadvantage when it comes to gathering evidence of extraterrestrial life. That's simply because JUICE's main target is Mercury-sized Ganymede, the solar system's largest moon and the third most distant from Jupiter out of the planet's four main natural satellites. NASA's Europa Clipper mission, which will launch next year but will reach the Jupiter system one year ahead of JUICE, has a better chance of producing ground-breaking insights in that respect, as its focus is the smaller Europa. This moon, closer to Jupiter than Ganymede, is much more likely to possess all three of the main prerequisites for the emergence and existence of life, scientists think.
"Evidence from previous missions as well as from theory and modeling suggests that on Europa, under the ice shell, if you keep going down into the water, you will eventually reach a rocky bottom," said Masters. "This rock is the source of nutrients. And because we think there might be water plumes on Europa, just like on Enceladus, we think there must be some source of energy. Ganymede, however, is a different category."
Ganymede, over 3,200 miles wide (5,200 kilometers), has an ocean that is likely much deeper than that of the smaller Europa. In fact, scientists think that Ganymede's ocean might hold 25 times as much water as Earth's ocean. But the immense depth of this body of water may be one of the reasons why life may not exist on this moon.
"What we think happens inside Ganymede if you go deep enough is that the water becomes ice again," said Masters. "So Ganymede's ocean does not only have an ice shell above but also below and that means that the water is likely not in touch with the rocky core."
No rocky bottom means no source of nutrients. However, Masters adds, all may not be lost for Ganymede as cracks may exist in the bottom ice layer, created perhaps by some form of thermal activity within the planet-sized moon's core. The size of Ganymede, coupled with its greater distance from Jupiter (665,000 miles or 1,070,000 km compared to Europa's 417,000 miles or 671,000 km), means the intensity of the tide-producing gravitational forces that Ganymede is subject to is much weaker than those that Europa experiences, which in turn might result in less thermal activity inside the moon and therefore a lower likelihood of life.
Scientists are so confident that there is no chance for JUICE to encounter anything alive around or on Ganymede's surface that they plan to dispose of the spacecraft by crashing it into the moon at the end of the mission. This maneuver is intended to prevent JUICE from turning into an out-of-control piece of space junk that could potentially collide with the more promising Europa in the future and contaminate it with Earthly germs.
"There would need to be a real shake-up of knowledge about Ganymede to put it into a different category," said Masters. "If we for instance find that there isn't such a thick ice shell or if we find that there is material on the surface coming from below and that Ganymede is more like Enceladus and Europa, that might change things."
So what is JUICE likely to find?
Even though JUICE will not touch down on Ganymede and attempt to reach down below the moon's crust of ice (which may be up to 90 miles, or 150 km, thick), its package of ten scientific instruments will allow scientists to glean a surprising amount of information about the moon's interior ocean.
One of the key questions scientists want the mission to answer is what the chemical composition of the water below Ganymede's icy crust might be, as that could hint at the moon's ability to sustain life.
Remote analysis of the composition of the ice crust surface might provide some hints, as might remote measurements of the magnetic field around the moon and its variations. The magnetic field of Jupiter is more than 10,000 times stronger than that of Earth. In the 1990s, NASA's Galileo mission detected odd disturbances in this magnetic field that scientists eventually traced to the effects of oceans inside Jupiter's moons.
Magnetometers on JUICE are more sensitive than those on Galileo, and Masters hopes they will provide much more detailed views of the water body under Ganymede's icy shell. These sensitive instruments could even reveal, even through the dozens of miles of ice, how much salt is dissolved in the subsurface ocean's waters.
"If there is an ocean that has salt in it, it makes it electrically conducting," said Masters. "Then we expect to detect a sort of pulsing magnetic signal of a very low frequency. It's a very weak signal that is quite hard to resolve. But that's where JUICE fits in. If it's very, very salty, it's going to be more electrically conductive and the signal will be stronger. If it's got very little salt in it, it will be a weaker signal."
The chemical composition of the hidden ocean may help scientists decide whether searching for life or its direct signatures with a future mission would be a worthwhile endeavor.
When will we find whether there is life on Jupiter's moons?
Data gathered by both JUICE and Europa Clipper will help scientists determine how to best look for direct traces of life in the Jupiter system in the future. Masters, however, cautions that a mission that might provide the ultimate answer about life on Jupiter's moons may still be a "generation away."
"JUICE will set the scene for future missions that will look for biomarkers on these moons," said Masters. "We don't even know how these biomarkers should look like and there will be a lot of work done by our colleagues in life sciences and chemistry, who will try to understand the implications of what JUICE is going to find and how that perhaps fits in with what we know about life in extreme environments on Earth, such as the bottom of Earth's oceans."
Even without the discovery of life, the JUICE mission is expected to produce a slew of fascinating scientific discoveries. In addition to the Galileo mission, NASA's Juno spacecraft has studied the Jupiter system, but that spacecraft's main focus has been on the planet itself, rather than its moons. In the 1970s, the Pioneer 10 and Pioneer 11 and the Voyager 1 and Voyager 2 mission made brief visits to Jupiter, as did the Saturn explorer Cassini and the New Horizon probe bound for Pluto and beyond.