The European Space Agency has selected a satellite study of Jupiter's icy moons for its next large scale launch. The mission is called JUICE (JUpiter ICy moon Explorer), and would see a probe launch towards the Jovian system in 2022 if the billion euro mission is approved by the Science Programme Committee in May.
The satellite would make close flybys past the ocean-bearing moons of Callisto and Europa, before orbiting Ganymede, the largest moon in the Solar System. The ESA plans to closely study the environments of those worlds, and assess whether the moons could host life.
Juice was part of the ESA's Cosmic Vision roadmap, which invited the science community to come up with ideas for future missions.
Although Europa is covered in a thick crust of scarred and cross-hatched ice, measurements made by NASA's Galileo spacecraft and other probes strongly suggest that a liquid ocean lies beneath that surface. The interior is warmed, researchers believe, by the tidal stresses exerted on Europa by Jupiter and several other large moons, as well as by radioactivity.
Most scientists believe that the subEuropan seas are locked under tens of kilometers of ice. Heat is then conducted from the warm core by bulk convective motion of ice - huge chunks of frozen material literally carrying the heat away with them as they move up through the icy layer, shuffling and refreezing as they dump heat into space.
But Jupiter's Europa might not only sustain, but foster life, according to the research of University of Arizona's Richard Greenberg, a professor of planetary sciences and member of the Imaging Team for NASA's Galileo Jupiter-orbiter spacecraft.
Europa, similar in size to Earth's moon, and has been imaged by the Galileo Jupiter-orbiter spacecraft. Its surface, a frozen crust of water, was previously thought to be tens of kilometers thick, denying the oceans below any exposure. The combination of tidal processes, warm waters and periodic surface exposure may be enough not only to warrant life, but also to encourage evolution.
With Jupiter being the largest planet in the solar system, its tidal stresses on Europa create enough heat to keep the water on Europa in a liquid state. More than just water is needed to support life. Tides also play a role in providing for life. Ocean tides on Europa are much greater in size than Earth's with heights reaching 500 meters (more than 1,600 feet). Even the shape of the moon is stretched along the equator due to Jupiter's pull on the waters below the icy surface.
The mixing of substances needed to support life is also driven by tides. Stable environments are also necessary for life to flourish. Europa, whose orbit around Jupiter is in-sync with its rotation, is able to keep the same face towards the gas giant for thousands of years. The ocean is interacting with the surface, according to Greenberg, and "there is a possible that extends from way below the surface to just above the crust."
"The real key to life on Europa," Greenburg adds, "is the permeability of the ice crust. There is strong evidence that the ocean below the ice is connected to the surface through cracks and melting, at various times and places. As a result, the , if there is one, includes not just the liquid water ocean, but it extends through the ice up to the surface where there is access to oxidants, organic compounds, and light for photosynthesis. The physical setting provides a variety of potentially habitable and evolving niches. If there is life there, it would not necessarily be restricted to microorganisms."
Tides have created the two types of surface features seen on Europa: cracks/ridges and chaotic areas, Greenberg said.The ridges are thought to be built over thousands of years by water seeping up the edges of cracks and refreezing to form higher and higher edges until the cracks close to form a new ridge.
The chaotic areas are thought to be evidence of the melt-through necessary for exposure to the oceans.
The tidal heat, created by internal friction, could be enough to melt the ice, along with undersea volcanoes - a combination of factors would give organisms a stable but changing environment -- exactly the type that would encourage evolution.
Ganymede is thought to have a thicker ice shell, with its interior ocean sandwiched between ice above and below. ESA's Jupiter Ganymede Orbiter would investigate this different kind of internal structure. The Jupiter Ganymede Orbiter would also study the intrinsic magnetic field that makes Ganymede unique among all the solar system's known moons.
The Daily Galaxy via ESA
Image credit: NASA artist's vision of the moons Ganymede, Europa, Callisto near Jupiter
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