Artist concept of NASA's Europa Clipper making a close flyby of Jupiter's moon Europa. (NASA/JPL-Caltech)
NASA has taken a big step closer to testing the waters of the ocean hiding under the icy crust of Europa, Jupiter's most enigmatic moon.
The Europa Clipper mission, in development at the Jet Propulsion Laboratory in Pasadena, has just been approved for its final design and construction phase. It's on track for a 2025 launch.
"Clipper" is the culmination of decades of dreaming and years of conceptual and preliminary design. It is only the second mission NASA has dedicated to exploring a moon in the solar system—our own moon was the first. The target, Jupiter's icy Europa, is very different from Earth's moon.
Believed to possess a heated rocky core and mantle surrounded by an ice-topped ocean of liquid water up to 100 miles deep, Europa is arguably the best place in our solar system to look for life beyond Earth.
Why Are We Interested in this Icy Jovian Moon?
Astrobiologists' mouths water at the prospect of an ocean of liquid water -- particularly a salty one -- in contact with a rocky ocean floor.
They theorize that heat from within Europa's rocky interior, generated by tidal forces of Jupiter's gravity, powers eruptions of hot, mineral-laden water on Europa's ocean floor. Such "hydrothermal vents" could supply all the ingredients necessary to sustain some form of life.
Hydrothermal vents dot Earth's own oceans in volcanically active areas. Since their discovery, researchers have found communities of life forms that thrive around hydrothermal vents, subsisting entirely on thermal and chemical energy emerging from Earth's interior.
How life arrived at these deep ocean oases is still open to scientific debate. One theory poses the idea that life on Earth could have gotten its start at hydrothermal vents and migrated later to the surface.
The Challenge of Exploring a Concealed Ocean Half a Billion Miles Away
You might wonder, if there's a saltwater ocean on Europa, and the strong possibility of a life-friendly environment, why don't we already have robot submarines in the water sending us images of beautiful bioluminescent jellyfish, or something?
Easier said than done. Even landing a robot on Europa's unexplored surface would be a great engineering challenge. Designing a mission capable of boring through miles of ice and descending through a hundred miles of water to reach the ocean floor, and still able to communicate with us back on Earth, is presently an adventure of science fiction.
Although earlier mission concepts flirted with dropping robots onto Europa's surface, the Clipper mission won't do that. It won't even orbit Europa.
That moon resides within bands of intense radiation that surround Jupiter, an environment where even a radiation-hardened spacecraft might survive only a few weeks. Such a short visit wouldn't allow much time to explore, let alone transmit the huge volumes of collected scientific data back to Earth before a fatal failure brought an end to the mission.
Instead, Clipper will follow a looping trajectory around Jupiter that will send it careening past Europa on 45 close flybys. Some will pass as close as 16 miles near the surface.
Between flybys the spacecraft will retreat to the far end of its elongated orbit, away from Jupiter and into safer climates beyond the deadly radiation zone. The longer mission time and extended orbits will ultimately let Clipper collect and send home up to three times as much data as a Europa-orbiting spacecraft could.
Europa Clipper Will See Under Europa's Skin
Europa Clipper will carry nine scientific instruments designed to offer a detailed look at the moon, particularly the vast ocean lurking beneath its icy crust.
Apart from the usual cameras and spectrometers that will take high-resolution pictures and analyze the composition of Europa's surface, Clipper will carry instruments to investigate what lies below that surface.
An ice-penetrating radar will probe the frozen crust to determine its thickness and map its structure. Scientists will look for any subsurface lakes in chambers closer to the surface, which may be sources of water plumes detected by the Hubble Space Telescope.
A magnetometer will measure the disturbance of Jupiter's magnetic field by Europa's salty ocean, divining its salinity and depth.
Two different instruments will analyze particles "sniffed" during very close flybys. The composition of particles and gases in Europa's tenuous atmosphere and possibly plumes of water and chemicals erupting from its surface could help explain what Europa's ocean is made of, if those plumes originate from the ocean's waters.
How Long Have We Known About Europa's Ocean?
We caught our first scent of Europa's ocean in 1979 when the Voyager 1 and 2 spacecraft flew through the Jupiter system. The spacecraft captured images of Europa's fractured surface. Its patterns of cracks and fissures were best explained by a thin icy crust floating on a body of liquid.
Starting in 1995 the Galileo spacecraft made 11 close flybys of Europa, capturing images of much higher detail and measuring Europa's effects on Jupiter's magnetic field. The images further confirmed the presence of the hidden ocean, and Europa's magnetic disturbances suggested that ocean is salty.