Water, they say!


In the issue of 26 July of Nature, it is published that Callisto, one of Jupiter's moons, may hold an underground ocean and evidence of water ice has been detected on Mars.
Up to now, a watery environment has been required for life.

I. Callisto's watery secret


One of Jupiter's largest moons, Callisto, may hold watery secrets beneath its surface, suggests a new analysis. The satellite's icy crust may be the planetary equivalent of a blanket, insulating an underground ocean.

Radioactivity at Callisto's core provides ample heat to keep water from freezing. But scientists believed that the heat would escape through the satellite's crust of ice and rock.

"It was thought that a body of liquid water in a big icy satellite like Callisto would freeze within a few hundred million years," says Javier Ruiz, a geologist at the Complutensian University in Madrid, Spain. This seems like a long time but is short compared with the age of the Solar System (Ruiz, J. Nature, 412, 409 - 411, (2001).

Yet beneath Callisto's heavily cratered surface, a treasure may be hiding. Ruiz calculates that the ice holds in heat better than was previously thought. The temperature and pressure conditions on Callisto make the ice less conductive than normal, trapping in heat from the satellite's core that could keep water in a liquid form.

The finding offers "a new and remarkable insight into the icy rock world", says Kristin Bennett, a geophysicist at Los Alamos National Laboratory in New Mexico. Callisto's surface lacks volcanoes, mountains, rifts or any other sign of geologic activity, so the satellite has long been considered the 'ugly duckling' of Jupiter's large, icy moons, "Frankly, from a geologist's point of view, Callisto was boring," she says. Now "it might just be the 'swan' of the Solar System".

The first hints of Callisto's watery secret emerged in 1998. Data collected by sensors on the spacecraft Galileo revealed that the moon's magnetic field fluctuates as Jupiter turns. An underground ocean of salt water seemed the most plausible explanation, as salt water conducts electrical current, which could interact with Jupiter's magnetic field to produce the fluctuations.

It looked like indications of an ocean at depth, says Jeffrey Kargel of the US Geological Survey in Flagstaff, Arizona. "But people didn't quite believe it. How could Callisto have an ocean?"

Applying the analysis by Ruiz to other moons, such as Jupiter's Ganymede and Saturn's Titan, could also reveal underground seas. "This makes it a lot easier to have liquid water in icy satellites," Kargel says.

Whether such oceans could provide a home for hardy forms of life is now up for debate. Suggestions of life on icy Callisto have previously seemed incredible, says Kargel. But in recent years, researchers have found bacteria thriving in hostile environments on Earth, he points out, such as toxic thermal vents on the ocean floor. "Where we've found liquid water on Earth, we've found life," he says.

But the presence of life on Callisto should not be presumed, cautions Ruiz. The ocean's depths are probably packed with dense balls of ice and rock, which would block the circulation of heat and make it an inhospitable environment.

reproduced from :http://www.nature.com/nlink/v412/n6845/abs/412409a0_fs.html

II. Evidence of icy region and recent climate change observed on Mars

New images of the surface of Mars provide the first direct evidence that the climate of Mars changed during the last 100,000 years, much more recently than the hundreds of millions of years scientists had previously thought, according to Brown University geologist John Mustard (John F. Mustard, Christopher D. Cooper & Moses K. Rifkin, Nature 412, 411 - 414 (2001).



The high-resolution images show evidence of water ice closer to the equator than had previously been observed. They show a unique surface terrain of pits and hummocks that appears to have been soil once impregnated by water ice. The ice has since evaporated, leaving a five-meter thick mantle of porous terrain that is being broken up by wind and other factors.


“Where the mantle is still intact, you could conceive of water ice not far below the surface,” Mustard said. “Where it’s broken into pits and hummocks, the water is gone.”

The geologically young terrain was observed in two bands north and south of the equator at 30 to 60 degrees latitude. The location is significant because it shows that ice in the soil was once present as close to the equator as 30 degrees and as recently as 100,000 years, while previous observations had shown that ice had been present in the polar regions. Due to climate change, it now appears the icy region moved from the planet’s poles to nearer its equator, and has now retreated to the poles, the most recent of probably many such cycles, Mustard said.

“While we have always thought the climate of Mars has changed over time, this is direct evidence for change. And that change fits with the theory of periodic climate change, similar to the change that causes the ice ages on Earth,” Mustard said.

“Maybe we don’t have to go to the poles to find water ice. It’s easier for a spacecraft to survive at the equator,” Mustard said.

The work was supported by grants from NASA.

reproduced from: (http://www.brown.edu/Administration/News_Bureau/2001-02/01-006.html)

transmitted by Jérôme

30 juillet 2001

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