But Mars and Earth aren’t the only places in the solar system that either have or have water. On October 14, a NASA spacecraft named Europa Clipper left Florida. As its name suggests, the mission’s target is Europa, the largest of Jupiter’s 95 known moons.
Europa is a snowball slightly smaller than Earth’s Moon. Its atmosphere is thin to the point of non-existence, there is a layer of water ice and the surface temperature is about -180 degrees Celsius. But scientists believe a vast ocean exists beneath the ice, kept liquid by the friction caused by Jupiter’s powerful gravity squeezing Europa. Over the past few decades scientists have become increasingly excited about the life-supporting potential of such “icy moons”. In addition to Europa, these include Ganymede and Callisto, two other Jovian moons; Enceladus, which orbits Saturn; and Triton, the largest satellite. Of Neptune.
Europa’s icy layer is believed to be tens of kilometers thick. Therefore, Europa Clipper will not be able to tell whether there is actually an alien swimming in the depths. Instead, its job is to assess whether the Moon is a place where life could potentially arise. One task of the investigation will be to characterize the size and salinity of the ocean. NASA’s current best estimate is that it varies in depth from 60 km to 150 km. If this is correct, then, despite its smaller size, Europa will have about twice as much liquid water as Earth.
Although water is considered extremely useful (and possibly even important) for the evolution of life, it is not sufficient in itself. To qualify as habitable, a world needs enough other elements to allow complex chemistry. In addition to hydrogen and oxygen in water, a common shortlist includes carbon, nitrogen, phosphorus, and sulfur. All of these have already been found on a different icy moon – Enceladus.
On Enceladus, plumes of ocean water flow into space through cracks in the crust. In 2008, Cassini, another US spacecraft, flew directly through one of those plumes, and discovered six elements in the astronomical catalogue.
Robert Pappalardo, chief scientist for Europa Clipper, says whether such plumes exist on Europa is an open question. They say Europa’s ice core is much thicker than Enceladus’s, making it less likely that surface cracks or fissures would reach the ocean. Some intriguing—but uncertain—images taken from telescopes nevertheless show things that look like feathers. But follow-up observations with the space-based James Webb Space Telescope have so far failed to detect any.
If plumes are not present, Europa Clipper will have to be content with examining the moon’s surface. That surface is remarkably smooth and relatively free of impact craters, suggesting that it is regularly renewed by processes similar to plate tectonics as on Earth. This, in turn, suggests that chemicals forming on Europa’s surface may have a way to the ocean, and vice versa. Therefore, studying Europa’s surface can provide valuable clues about what lies beneath.
The final ingredient for a habitable world is a source of energy for life to harness. Whatever may be on Europa – far from the Sun, and beneath many kilometers of ice – it will not be sunlight. This is a bit of a problem. Nearly every living thing on Earth ultimately depends on photosynthesis for its energy, including the rich ecosystems in the ocean depths that were discovered in the 1980s and helped solidify the idea of life on Europa. Was done. Their inhabitants do not benefit from direct sunlight, but their metabolism is powered by photosynthesis, chemicals produced in the oxygen-rich surface oceans.
There is nothing like this on Europa. But it doesn’t have to be there. Some microbes living in the cracks of the Earth’s ocean floor use chemicals that come entirely from below, rather than above. It is a small source of energy, but real. And Europa can offer the same. Analyzing the chemistry of Europa’s surface could provide clues as to whether, at least in theory, something similar might be happening on its ocean floor.
two come together
And Europa Clipper won’t be the only probe hanging out at Jupiter. Last year saw the launch of a European probe called the Jupiter Icy Moons Explorer (JUICE). It will also examine Europa as well as Callisto and Ganymede, two other moons that are also believed to have oceans. Uncertainties of orbital mechanics mean that, despite the later departure, Europa Clipper will arrive in 2030, a year before JUICE.
If the findings from both missions are exciting enough, the next step could be sending a lander. Searching for landing sites on Europa is one of the goals of Europa Clipper. But the probe will not be able to create an accurate map of the moon’s surface. Jupiter’s powerful magnetic field generates fields of intense radiation near the planet, enough to fry any spacecraft that stays on for too long. Instead, Europa Clipper will make 49 looping flybys, collecting as much data as possible each time before moving to a safe distance. The world’s alien-hunters will be hoping it survives.
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