Saturday, 22 November 2014

NASA's New View of Jupiter's Ocean Moon --Is Europa the Solar System's Best Bet for Life?


The puzzling, fascinating surface of Jupiter's icy moon Europa looms large in this newly-reprocessed color view, made from images taken by NASA's Galileo spacecraft in the late 1990s. This is the color view of Europa from Galileo that shows the largest portion of the moon's surface at the highest resolution.

Scientists have produced this new version of what is perhaps NASA's best view Europa. The mosaic of color images was obtained in the late 1990s by NASA's Galileo spacecraft. This is the first time that NASA is publishing a version of the scene produced using modern image processing techniques.

An earlier, lower-resolution version of the view, published in 2001, featured colors that had been strongly enhanced. The new image more closely approximates what the human eye would see. Space imaging enthusiasts have produced their own versions of the view using the publicly available data, but NASA has not previously issued its own rendition using near-natural color.

The image features many long, curving and linear fractures in the moon's bright ice shell. Scientists are eager to learn if the reddish-brown fractures, and other markings spattered across the surface, contain clues about the geological history of Europa and the chemistry of the global ocean that is thought to exist beneath the ice.

The view was previously released as a mosaic with lower resolution and strongly enhanced color. To create this new version, the images were assembled into a realistic color view of the surface that approximates how Europa would appear to the human eye. The scene shows the stunning diversity of Europa's surface geology. Long, linear cracks and ridges crisscross the surface, interrupted by regions of disrupted terrain where the surface ice crust has been broken up and re-frozen into new patterns.

Color variations across the surface are associated with differences in geologic feature type and location. For example, areas that appear blue or white contain relatively pure water ice, while reddish and brownish areas include non-ice components in higher concentrations. The polar regions, visible at the left and right of this view, are noticeably bluer than the more equatorial latitudes, which look more white. This color variation is thought to be due to differences in ice grain size in the two locations.

Hidden beneath Europa's icy surface is perhaps the most promising place in our solar system beyond Earth to look for present-day environments that are suitable for life. The Galileo mission found strong evidence that a subsurface ocean of salty water is in contact with a rocky seafloor. The cycling of material between the ocean and ice shell could potentially provide sources of chemical energy that could sustain simple life forms.

Over the centuries, Europa, has provided an abundance of mysteries. These culminated in what may have been a literal explosion in December 2012, when a cloud of water vapor was seen 20 miles over its south pole. This eruption was tiny on the cosmic scale, but enormous in its importance to astrobiology.

Outside of Earth, Europa may be the most hospitable home for life inside the Solar System. Four billion years of tidal heating and a liquid ocean may have given rise to something we can identify as life. A man-made satellite in the Jovian system could potentially capture traces of that life in the water vapor shooting from Europa’s surface. Yet, in spite of the exciting science, a dedicated mission to Jupiter hasn’t launched in a generation.

Though Europa was discovered more than 400 years ago, it wasn’t until deep space satellites came along that we had our first good look at one of the most luminous objects in the Solar System. Between 1973 and 1993, eight satellites flew past Europa. Each dispelled some of the uncertainties surrounding this mysterious body orbiting 390.4 million miles (628.3 million kilometers) away.

The first arrived in 1973. The Pioneer 10 satellite sent back the first close-up photograph of that bright moon. Europa reflects back into space 64 percent of the light that falls on its surface. By contrast, Earth’s light reflectivity, or albedo, is 33 percent. Venus’ is 76 percent. In other words, Europa’s brightness falls somewhere between Earth’s liquid oceans and Venus’ constant cloud cover.

But what creates the brightness? With the Sun 2,000 times further away, Europa probably isn’t covered in liquid water the way that the Earth is. As for clouds, Europa is slightly smaller than our Moon. It lacks the gravity to maintain a substantial atmosphere. A planet coated in solid ice would explain Pioneer’s observations, but it doesn't account for one big effect: Jupiter’s tidal force. Europa’s proximity to Jupiter means that it might very well be heated from the inside out, melting some of the ice at least near the center.

Shortly before the arrival of the next satellite into the Jovian system, another suggestion was made: Europa might have three layers. In this model, the innermost core would be silica. The outermost core would be ice. The pressure of being slung around Jupiter every 3.5 days might generate enough tidal heating to maintain a liquid ocean in between. If this model were true, even though the third layer is solid, Jupiter’s tidal forces might be strong enough to crack the ice shell covering of Europa as it moves rapidly around the gas giant.

Thanks to Pioneer, Voyager and Galileo, we had learned more in three decades than in the preceding five centuries. The brightness of the ice was known to be the result of continual surface renewal. The enormous cracks, sometimes referred to as “flexi”, appear to originate when the solid ice shell flexes as Jupiter pulls on Europa.

In a nod towards the three-layer-model, Galileo’s measurements also indicated that a large, salt water ocean might well exist beneath the ice shell. While all this was being discovered half a billion miles away, things were being uncovered in our own backyard that made the possibility of Europa’s oceans even more exciting.

In 1977, hydrothermal vents teeming with life were discovered deep within Earth’s oceans. This was the first proof that life could thrive in the absence of light, using heat as a source of chemical power. This led to the current understanding that life can prosper as long as there is heat and water. With a probable ocean and definite heat source, Europa suddenly became a leading candidate in the search for habitability.

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