On a clear night during the right celestial season, any person with a simple backyard telescope can look toward Jupiter and see the planet’s four largest moons. In silence, they whirl about the colossal Jupiter night after night, indifferent to humans and our desire to understand ourselves and the cosmos upon which we gaze.

Jupiter seen in space from a distance surrounded by four moons that appear as labeled dots
NASA's Juno spacecraft captured this unprecedented view of Jupiter and its four largest moons a distance of 3.3 million miles (5.3 million kilometers) from the giant planet. Credit: NASA/JPL-Caltech/SwRI/MSSS

Because they are clearly seen to orbit another world, those moons helped to reveal that Earth was not the center of our universe when they were discovered 400 years ago. But they remain remarkable in other ways today. Ganymede is the largest moon in the solar system, exceeding planet Mercury in size. Io is the most volcanically active body in the solar system, while Callisto has one of the solar system's oldest and most cratered surfaces.

But one of those four moons — Europa — has for decades stood out as a lightning rod for scientific curiosity. Beneath a layer of ice several times thicker than any on Earth, scientists think there lurks a hidden ocean of water. And it may be the best place to look for extraterrestrial life in the solar system.

Europa stands out as a lightning rod for scientific curiosity. It may be the best place to look for extraterrestrial life in the solar system.

To determine if a world about 484 million miles (779 million kilometers) away has conditions favorable for life, scientists are sending a new spacecraft called Europa Clipper to conduct the first detailed study of an ocean world beyond Earth.

The mission’s main science goal is to investigate Europa’s habitability. In other words, the mission will determine if Europa has the capability to support life (a field of science called astrobiology), though the spacecraft isn’t designed to detect life.

Reddish bands on Europa.
Blue-white terrain indicates relatively pure water ice.The reddish areas contain water ice mixed with hydrated salts, potentially magnesium sulfate or sulfuric acid. The broad band in the center is a disrupted chaos-type features that may be signs of interaction with an subsurface ocean. Credit: NASA/JPL-Caltech/SETI Institute

To support life as we know it, a world must have three essential ingredients: liquid water, the right chemistry and a source of energy for life. Here’s how Europa stacks up on those ingredients:

  • Water: Europa's global ocean may have twice as much liquid water as Earth’s ocean. The salty ocean is thought to lie below Europa’s icy outer shell, which is estimated to be 10 to 15 miles (15 to 25 kilometers) thick. Some liquid water is also likely to exist within Europa's ice shell.
  • Chemistry: Several essential chemicals are critical for the molecules of life to form, including carbon, hydrogen, nitrogen, oxygen, phosphorus and sulfur. Comets and asteroids may have carried some of these chemicals to Europa when they crashed into the icy moon throughout its history. Other chemicals may have been deposited or created during the moon’s formation.
  • Energy: All life needs an energy source. Sunlight powers the chemical reactions that sustain most of the life on Earth, but Europa’s energy for life may be provided by chemical reactions on its surface and sea floor.

Scientists have three main objectives, or specific steps to take, to investigate how the ingredients for life might interact to produce habitable environments on Europa:

  • Ice shell and ocean: Scientists aim to determine the thickness of Europa’s icy shell — the moon’s outer layer that includes its surface. They will discover whether there’s water within and beneath the shell. Researchers will estimate the size, saltiness and other qualities of Europa's ocean. They also will determine how the ocean interacts with the surface: Does anything in the ocean rise up through the shell to the top? Does any material from the surface work its way down into the ocean?
  • Composition: Scientists will investigate the composition and chemistry of Europa's ocean to determine if it has the ingredients to permit and sustain life.
  • Geology: Scientists will determine how Europa’s surface features formed and locate any signs of recent activity such as sliding tectonic plates or plumes that are venting water into space. They will identify key locations on the surface that need more scientific study.

In addition to investigating Europa’s potential habitability, Europa Clipper has the ability to scout for sites where future spacecraft could safely land on the surface. If all goes well, the people of Earth may someday be able to look toward Jupiter with our same backyard telescopes, and have a new understanding of where life might have found a foothold in our universe.

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