* Astronomy

'Follow the Water." That is NASA's motto for the future of the space program. Where there is water, there could be life.
And the discovery of life outside Earth could be the biggest story of the 21st century.
Mars is a good place to look, but astronomers believe Europa, a tiny moon of Jupiter - one of its 63 known moons - may harbour more water than all of the oceans on Earth.
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Scientists have produced a global geological map of Jupiter's moon Europa, which has been proposed as a destination for a future space mission.
Interest in Europa has been fuelled by indications that a liquid water ocean lurks beneath its outer shell of ice.
The mapping effort will help build a geological history of the enigmatic moon and target future explorations.
A team at Arizona State University compiled the maps from data sent back by the US-European Galileo probe.
Galileo explored the Jupiter system from 1995 to 2003.



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While NASA and the European Space Agency focus on Mars rovers and future missions to search for life on the Red Planet, a determined core of scientists is lobbying for equal attention to a place they feel is just as likely to harbour life - Jupiter's icy moon Europa.

"Because of the well-supported presence of water ice on Europa and the probability that there are briny oceans, Europa has to be a major target for the search for life in the solar system. Many of us are proposing that there is habitat there where we can expect to find evidence of life" -  paleobiologist Jere H. Lipps, professor of integrative biology at the University of California, Berkeley.

Lipps took up the issue with three other scientists on a panel Sunday (Feb. 18) at the annual meeting of the American Association for the Advancement of Science in San Francisco. The group, organized by Lipps, reviewed what is known about Europa and focused on the problems that need to be solved before undertaking a search for life on the frozen moon.

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A new study suggests that most of the craters on Jupiter's moon Europa are formed by chunks of rock and ice splashing back down onto the moon's surface after a meteor strike.

It was previously thought that most of the craters seen on moons and planets were the work of direct, or "primary" impacts from asteroids and comets. The new finding suggests that most of those craters might instead be "secondaries," impacts that formed by the material ejected from primary impacts.

For Europa, secondaries account for as much as 95 percent of all the small craters—those less than a mile in diameter—observed on the moon. The finding has implications for how astronomers date the ages of planetary surfaces.
Asteroids, comets and chunks of cosmic debris routinely bombard the surface of planets and moons. Earth's atmosphere protects us from most of these impacts, incinerating most objects before they hit the ground. Even so, Earth has experienced countless meteor impacts throughout its long history. The evidence for most of those impacts has been erased by erosion from wind and rain and by constant turnover of the Earth's crust.

Earth's Moon, on the other hand, is pockmarked with millions of craters because it lacks both atmosphere and geologic activity. Similarly, Mars has thin atmosphere and relatively little geologic activity.
On both the Moon and Mars, teasing out the primary impacts from the secondaries is difficult because the craters are just too numerous.

The researchers instead turned to Europa, Jupiter's smallest moon and a world covered in a thick crust of ice. More importantly, Europa is geologically active like Earth. Its surface is constantly being repaved with new ice and as a result, Europa has very few craters.

Using high-resolution images from NASA's Galileo spacecraft, the researchers measured the number, size and distribution of craters on Europa. They then ran a computer simulation of meteors randomly striking Europa but with the condition that the number and size of the craters had to match the real number and size observed in the images.
After running the simulations hundreds of times and comparing the results to the images, they found that the crater distributions were not similar as would be expected if most of the craters were caused by primary impacts.

The finding is important because scientists typically use crater counts to date the ages of planet and moon surfaces. When comparing two similar regions on a moon, for example, scientists generally assume that the region with more impact craters is older. Scientists can also use a region's crater density to calculate its absolute age. They usually use our own Moon as a reference because scientists have reliably dated the age of some its craters based on rocks brought back by astronauts.

If "…it turns out that most of these small caters are secondary and not primary, then that means the calibrated age from (Earth's) Moon is not right" - Edward B. Bierhaus, researcher at Lockheed Martin's Space Exploration Systems in Denver, Colorado and an author on the study.

Bierhaus stresses, however, that large primary craters can still be reliably used to date a region. It's only in regions where large, primary impacts are scarce or nonexistent that dating becomes difficult.
Most of the objects that strike Jupiter and its moons come from a region of the Solar System known as the Kuiper Belt. Therefore, another implication of the finding may be that there are fewer small asteroids in the Kuiper Belt than previously thought, the researchers wrote. It may be that small asteroids are rarely made or perhaps some process depletes them before they can reach Jupiter and its moons.

The finding was reported in the Oct. 20 issue of the journal Nature.