"The reason I like Europa so much is that it’s a world whose orbital dynamics with Jupiter, its orbital resonances with the other Galilean moons, and its own rigid-body dynamics have a strong hand in creating its surface features – and giving it the potential to harbor life. It’s one of perhaps two or three extraterrestrial places in the Solar System where we might hope to find life. Europa is also easier to get to than Enceladus or Titan. As such, I think it ought to be one of the highest-priority exploration targets for robotic space probes. (Human exploration would be nice, too, but if you think radiation exposure on the way to Mars is hard, you don’t even want to consider putting people in the Jovian system!)"
Joseph Shoer, Cornell University
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, 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.
A PhD student at Cornell University named Joseph Shoer, reported wired.com, has designs on Europa. At his Quantum Rocketry blog, he laid out all the requirements that a mission to explore the moon would need, complete with sketches of exactly what the robotic landers would look like.
A manned mission to Europa has some huge obstacles right off the launching pad: first, it will take about five years to reach the satellite, with the same amount of time being required for the return journey. Second, Jupiter's radiation would mean that any trips beyond a thick set of shielding would be deadly.
But Europa, says Shoer, “ought to be one of the highest-priority exploration targets for robotic space probes”, mainly because it’s “one of perhaps two or three extraterrestrial places in the Solar System where we might hope to find life“.
“It’s a world whose orbital dynamics with Jupiter, its orbital resonances with the other Galilean moons, and its own rigid-body dynamics have a strong hand in creating its surface features,” he adds.
Astronomers believe, based on images from the Galileo mission and magnetometer readings, that Europa has an icy shell over a liquid ocean, with a solid rocky core at the centre. There’s some disagreement between scientists on how thick the ice is — estimates range from 10 to 100,000 metres — but observations have yielded reports of a number of “double-ridges” on the surface that are believed to be cracks in the crust.
These are thought that they’re caused by huge gravitational forces — the Jovian equivalent of tides on Earth, but amplified many times due to Jupiter’s considerably-greater mass. Once a crack forms, it gets squeezed back together and pulled apart again every time the moon rotates, which is approximately once every three and a half Earth days.
The cracks are the most likely place for life to take root. They get sunlight (unlike the rest of the ocean below the ice) and are also subjected to strong curr
Shoer’s plan to investigate Europa's cracks-likely to be the largest energy sources available to any life that exists on the planet- is right on the mark. A probe would enter orbit around the moon, scanning for these double-ridges. Once located, a lander would be despatched to the inside surface of the ridge, which would then monitor the crack, verify that it’s opening and closing, and work out the exact timing of the cycle.
Then, when the crack is closed, it would inflate cushions around itself and roll down the slope until it comes to rest at the bottom, centred over the crevasse. The cushions deflate, and tethers would be attached to either side of the walls, holding the probe in place. Then, once the crack opens again, a smaller vehicle could be dropped down inside to take measurements. Eventually, it’ll hit the ocean below, and could keep one part at the surface while another section dives as deeply as possible.
The difficulty, however, as he points out, is in the timing. The probe will have less than a quarter of a day to operate before it gets squished by the crack closing again. Making sure that the orbiting satellite is overhead at the last possible moment to receive any recorded data is crucial.
“Though it is one of the very few places in the Solar System that we can imagine harbouring life, the mission design to explore the Europan biosphere is very difficult and requires many stretches of space technology," Schoerr adds. "Those are challenges that I would love to see the space program address, though — because the discovery of extraterrestrial life would have a profound impact on our science and society.”