One day on Europa lasts as long as one year — 3. Europa is tidally locked to Jupiter. Europa has a very weak atmosphere.
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The dark band that originates at the bottom center of the image and runs to the left center is wedge-shaped. As in image 1 , there is a noticeable absence of impact craters. This Galileo image is 42 kilometers across and is illuminated from the right. Although we saw indications of surface motions in image 2 , this image is by far the clearest evidence for large motions of blocks of material across the surface of Europa.
The ocean interpretation rests on the belief that the existence of so much lateral motion across the surface requires the presence of some sort of layer to lubricate the flow at depth. These scientists assume that this lubrication requires a liquid, and hence favor the existence of an ocean.
As a possible counter-example, consider the physics controlling plate tectonics on Earth. As a general rule, temperature increases with depth inside a planet, and as materials increase in temperature, they tend to become less viscous less rigid, or more colloquially, softer.
These plates move over a mantle which is solid virtually everywhere we know this because of the way seismic waves travel through the mantle. This Galileo image is kilometers across and is illuminated from the left. The major feature is a mitten-shaped region of chaotically disrupted terrain in the center of the image. This chaos region is superimposed on the surrounding plains and ridges, so it must be the youngest feature in this region.
Based on the pattern of sunlight and shadows around the edge of the chaos region, the chaos region is slightly elevated compared with the surrounding plains. On the west left side of the structure, there is a narrow trough separating the plains from the uplifted chaos terrain. Similar chaos units are found in many parts of Europa.
Some scientists believe that these regions form when the subsurface ocean melts through a relatively thin outer ice shell. Numerous ridges also cross this image. The relative ages of these ridges can be determined by observing the intersections between ridges the younger ridge will appear to cut the older ridge.
This image shows four of the largest impact craters found on Europa. Because impact craters excavate into the crust of a planet, they serve as natural core samples into the structure of the upper crust. Generally, the excavation depth of a crater increases as the size of the crater increases.
In other words, small craters make shallow holes and larger craters make deeper holes. Based on the known depths of the largest craters on Europa, it appears that the ice shell of Europa remains solid to a depth of at least 19 to 25 kilometers. The pattern of crater depths as a function of crater diameter suggests that either an ocean or a layer of warm and thus soft and weak ice occurs below this depth.
Our current knowledge of the interior of Europa comes from observations of its gravitational and magnetic fields. This material has probably separated into a metal-rich core and a rock-rich mantle, with the core having a radius of to kilometers. The surface of Europa is known to be predominantly water ice, probably with some rock mixed in, based on spectroscopy studies. This outer shell of water ice is to kilometers thick. The right side of the image highlights two fundamentally different views about the nature of the ice shell on Europa.
The available gravity observations do not indicate whether this layer is entirely solid or if there is a subsurface ocean on Europa. This effect has been observed by Galileo and is the strongest present evidence for a subsurface ocean inside Europa. This ocean must be globally distributed. Solid ice and rock can not explain the observed magnetic signature.
The magnetic evidence requires that the ocean be at least 10 kilometers thick, but does not tightly constrain the depth at which this ocean begins. The presence of water beneath the moon's frozen crust makes scientists rank it as one of the best spots in the solar system with the potential for life to evolve.
The icy depths of the moons are thought to contain vents to the mantle much as oceans on Earth do. These vents could provide the necessary thermal environment to help life evolve. If life exists on the moon, it may have gotten a kick from deposits from comets. Early in the life of the solar system, the icy bodies may have delivered organic material to the moon.
In , a study suggested that Europa produces 10 times more oxygen than hydrogen , which is similar to Earth. This could make its probable ocean friendlier for life — and the moon may not need to rely on tidal heating to generate enough energy.
Instead, chemical reactions would be enough to drive the cycle. In , the U. Exploration of Europa was ranked as the highest-priority mission. In , the mission was officially called Europa Clipper after several years of the researchers and media informally using the moniker. According to NASA, this mission — which will leave sometime in the s, perhaps late in the decade — would perform 40 to 45 flybys of Europa with a spacecraft orbiting Jupiter.
There will be nine scientific instruments on board including cameras, radar to peer beneath the ice and try to figure out its thickness, a magnetometer to measure the magnetic field and by extension, how salty the ocean is , and a thermal instrument to search for signs of eruptions.
The flybys would range in height between 16 miles 25 km and 1, miles 2, km. This brings the flybys well into the radiation-heavy zone of Europa, which is tough for a spacecraft to survive.
Bringing the spacecraft in and out of the zone will extend its lifetime and make it easier to transmit data back to Earth. One of Europa Clipper's priorities will be to follow up on the Hubble observations of plumes. The mission is expected to launch in and arrive at Jupiter's neighborhood in for at least a three-year mission. Once it gets to Europa, the mission will look at organic molecules and other components that could make the moon friendly to life.
Also, the spacecraft will probe how thick the crust is, particularly over any active regions it finds. Join our Space Forums to keep talking space on the latest missions, night sky and more!
And if you have a news tip, correction or comment, let us know at: community space. Elizabeth Howell is a contributing writer for Space.
She is the author or co-author of several books on space exploration. Elizabeth holds a Ph. She also holds a bachelor of journalism degree from Carleton University in Canada, where she began her space-writing career in Besides writing, Elizabeth teaches communications at the university and community college level, and for government training schools.
To see her latest projects, follow Elizabeth on Twitter at howellspace.
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