domingo, 3 de marzo de 2013

Could the Solar System's Deepest Ocean Harbor Life?



The deepest ocean on Earth is the Pacific Ocean's Marianas Trench, which reaches a depth of 6.8 miles awesomely trumped by the depth of the ocean on the Jupiter's moon, Europa, which some measurements put at 62 miles. 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 sub-Europan 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.

A recent study of Europa, however,  shows life most likely doesn’t exist, says a report co-authored by a USF astrobiologist. Matthew Pasek. The ocean underneath the ice layer of Europa is likely too acidic to support larger life forms, dousing hopes by those who thought the indication of water on the Jupiter moon might be a place where extraterrestrial life could be found. The new chemical analysis by Assistant Professor of Geology Matthew Pasek and Richard Greenberg of the University of Arizona’s Department of Planetary Science, says the Europan sea environment is probably not friendly to marine life as we know it.

Oxygen is a major component of Europa’s crust. But unlike Earth with its photosynthetically-derived oxygen, the oxygen on Europa is formed by the high-energy bombardment of Europa’s surface by radiation, Pasek explains.

The scientists used computer models to calculate the predicted chemistry of Europa – the same technology that’s designed to predict Earth’s groundwater chemistry or water chemistry at phosphate mines. After generating data on the inputs of oxygen descending into the crust, the scientists estimated the amount of material coming out of the rocks beneath the ocean.

The oxygen reacts with sulfur and other materials emanating from rocks at the bottom of the ocean. “When the two meet, they generate acid - sulfuric acid in this case,” Pasek said. That would produce water with a pH of about 2.6, "about the same as your average soft drink. Just as soft drinks are bad for your teeth as they are quite acidic, fish, corals, whales, or other large animals would find it difficult to live within the ocean of Europa.” 

Those acidic levels would make it impossible for marine organisms like the ones found on Earth to grow shells or develop the way early marine organisms have here. That said, if there is life on Europa it might look more like the microbes found near acid mines in Spain who thrive in the Rio Tinto. Those organisms have evolved to oxidize iron and sulfide as energy sources, and tolerate environments even more acidic than those predicted for Europa.

"The microbes there have figured out ways of fighting their acidic environment," Pasek said. "If life did that on Europa, [Jupiter's moon] Ganymede, and maybe even Mars, that might have been quite advantageous."*
Pasek's paper can be read here.

Image: NASA / JPL / University of Arizona

Source: The Daily Galaxy via University of Arizona

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