Beneath the icy surface of Jupiter’s moon Europa is perhaps the most promising place to look for present-day environments suitable for life.
Slightly smaller than Earth's moon, Europa’s water-ice surface is crisscrossed by long, linear fractures. Like our planet, Europa is thought to have an iron core, a rocky mantle and an ocean of salty water. Unlike Earth, however, Europa’s ocean lies below a shell of ice probably 10 to 15 miles (15 to 25 kilometers) thick and has an estimated depth of 40 to 100 miles (60 to 150 kilometers). While evidence for an internal ocean is strong, its presence awaits confirmation by a future mission.
Europa orbits Jupiter every 3.5 days and is locked by gravity to Jupiter, so the same hemisphere of the moon always faces the planet. Because Europa's orbit is elliptical (slightly stretched out from circular), its distance from Jupiter varies, and the moon’s near side feels Jupiter’s gravity more strongly than its far side. The magnitude of this difference changes as Europa orbits, creating tides that stretch and relax the moon’s surface. Flexing from the tides creates the moon’s surface fractures. If Europa's ocean exists, it might also have volcanic or hydrothermal activity on the seafloor, supplying nutrients that could make the ocean suitable for living things.
Based on the small number of observable craters, the surface of this moon appears to be no more than 40 to 90 million years old, which is youthful in geologic terms (the surface of Callisto, another of Jupiter’s moons, is estimated to be a few billion years old). Along Europa's many fractures, and in splotchy patterns across its surface, is a reddish-brown material whose composition is not known, but may hold clues to the moon's potential as a habitable world.
NASA's Galileo spacecraft explored the Jupiter system from 1995 to 2003 and made numerous flybys of Europa. Galileo revealed strange pits and domes that suggest Europa’s surface ice could be slowly turning over, or convecting, due to heat from below. Galileo also found regions called "chaos terrain," where broken, blocky landscapes were covered in the mysterious reddish material. In 2011, scientists studying Galileo data proposed that chaos terrains could be places where the surface collapsed above lens-shaped lakes embedded within the ice.
In 2013, NASA announced that researchers using the Hubble Space Telescope found evidence that Europa might be actively venting water into space, which would mean the moon is geologically active in the present day. If confirmed by follow-up observations, the plumes of water could be studied by future spacecraft similar to how the Cassini spacecraft sampled the plume of Enceladus.
One of the most important measurements made by the Galileo mission showed how Jupiter's magnetic field was disrupted in the space around Europa. The measurement strongly implied that a special type of magnetic field is being created (induced) within Europa by a deep layer of some electrically conductive fluid beneath the surface. Based on Europa's icy composition, scientists think the most likely material to create this magnetic signature is a global ocean of salty water.
Scientists will begin studying Europa anew with the Europa Clipper mission. Scheduled to launch in the 2020s, Europa Clipper would arrive at Jupiter several years later and try to see whether the icy moon could harbor conditions suitable for life. The radiation-tolerant spacecraft will perform 45 flybys of Europa at altitudes varying from 1,675 miles to 16 miles (2,700 kilometers to 25 kilometers) from a long, looping orbit around Jupiter.
Clipper’s instruments will include cameras and spectrometers to produce high-resolution images of Europa's surface and determine its composition. An ice-penetrating radar will determine the thickness of the moon's icy shell and search for subsurface lakes similar to those beneath Antarctica. The mission will also carry a magnetometer to measure strength and direction of the moon's magnetic field, which will allow scientists to determine the depth and salinity of its ocean.
Galileo Galilei is credited with discovering what we now call the Galilean moons—
Including Europa—on Jan. 8, 1610. Simon Marius probably made an independent discovery of the moons about the same time that Galileo did, and he may have unwittingly sighted them up to a month earlier, but Galileo was given credit since he published an account of the discovery first
How Europa Got Its Name
Europa is named for the daughter of Agenor, in ancient Greek mythology. Europa was abducted by Zeus (the Greek equivalent of the Roman god Jupiter), who had taken the shape of a spotless white bull. Europa was so delighted by the gentle beast that she decked it with flowers and rode upon its back. Seizing his opportunity, Zeus rode away with her into the ocean to the island of Crete, where he transformed back into his true shape. Europa bore Zeus many children, including Minos.
About Europa: https://europa.nasa.gov/about-europa/overview/
About Europa Clipper: https://europa.nasa.gov/about-clipper/overview/
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