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TOPIC: Enceladus


L

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RE: Enceladus
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In this image Enceladus is seen against Saturn and its rings. Visible on Saturn is the region where daylight gives way to dusk. At the top, the rings throw thin shadows onto the planet.


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Images taken using red, green and blue spectral filters were combined to create this natural colour view. The images were taken using the Cassini spacecraft wide-angle camera on January 17, 2006 at a distance of approximately 200,000 kilometres from Enceladus.
The image scale is 10 kilometres per pixel.

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Wrinkles and cracks have reworked the surface of Enceladus, perhaps due to the influence of tidal stresses. The monochrome view also makes it clear that certain geological provinces on the moon have been altered by the activity, erasing ancient craters, while other places have retained much of the cratering record.

This false colour look reveals subtle details on Enceladus that are not visible in natural colour views.
The now-familiar bluish appearance (in false colour views) of the southern "tiger stripe" features and other relatively youthful fractures are almost certainly attributable to larger grain sizes of relatively pure ice, compared to most surface materials.


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On the "tiger stripes," this coarse-grained ice is seen in the coloured deposits flanking the fractures as well as inside the fractures. On older fractures on other areas of Enceladus, the blue ice mostly occurs on the exposed wall scarps.
The colour difference across the moon's surface (a subtle gradation from upper left to lower right) could indicate broad-scale compositional differences across the moon's surface. It is also possible that the gradation in colour is due to differences in the way the brightness of Enceladus changes toward the limb, a characteristic which is highly dependent on wavelength and viewing geometry.

Terrain on the trailing hemisphere of Enceladus is seen here. North is up.
The view was created by combining images taken using ultraviolet, green and infrared spectral filters, and then was processed to accentuate subtle colour differences. The images were taken with the Cassini spacecraft narrow-angle camera on Jan. 17, 2006 at a distance of approximately 153,000 kilometres from Enceladus and at a Sun-Enceladus-spacecraft, or phase, angle of 29 degrees. Image scale is 912 meters per pixel.

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This global digital map of Saturn's moon Enceladus was created using data taken during Cassini and Voyager spacecraft flybys. The map is an equidistant projection and has a scale of 110 meters per pixel.


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The mean radius of Enceladus used for projection of this map is 252 kilometres. The resolution of the map is 40 pixels per degree.

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Recent Cassini images of Saturn's moon Enceladus backlit by the sun show the fountain-like sources of the fine spray of material that towers over the south polar region. The image was taken looking more or less broadside at the "tiger stripe" fractures observed in earlier Enceladus images. It shows discrete plumes of a variety of apparent sizes above the limb of the moon.


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The greatly enhanced and false colour image shows the enormous extent of the fainter, larger-scale component of the plume.

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Enceladus Plumes
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A fine spray of small, icy particles emanating from the warm, geologically unique province surrounding the south pole of Enceladus was observed in a Cassini narrow-angle camera image of the crescent moon taken on January 16, 2005.
The fine spray of icy particles is believed to supply the material comprising Saturn's E ring


Taken from a high phase angle of 148 degrees - a viewing geometry in which small particles become much easier to see - the plume of material becomes more apparent in images processed to enhance faint signals.

Imaging scientists have measured the light scattered by the plume's particles to determine their abundance and fall-off with height. Though the measurements of particle abundance are more certain within 100 kilometres of the surface, the values measured there are roughly consistent with the abundance of water ice particles made by other Cassini instruments at altitudes as high as 400 kilometres above the surface.
At present, it is not clear if the plume emanating from the south pole arises because of water vapour escaping from warm ice that is exposed to the surface, or because at some depth beneath the surface, the temperatures are hot enough for water to become liquid which then, under pressure, escapes to the surface like a cold Yellowstone geyser.
The image at left was taken in visible green light. A dark mask was applied to the moon's bright limb in order to make the plume feature easier to see.
The image at right has been colour-coded to make faint signals in the plume more apparent. Images of other satellites (such as Tethys and Mimas) taken in the last 10 months from similar lighting and viewing
geometries, and with identical camera parameters as this one, were closely examined in order to demonstrate that the plume towering above Enceladus' south pole is real and not a camera artefact.

The images were acquired at a distance of about 209,400 kilometres from Enceladus. Image scale is about 1 kilometre per pixel.


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Recent Cassini images of Enceladus at high phase show the fountain-like sources of the fine spray of material that towers above the south polar region of Enceladus.
The image was taken on November 27, 2005 and received on Earth November 28, 2005. Cassini was approximately 147,488 kilometres away. The image was taken using the CL1 and CL2 filters.

-- Edited by Blobrana at 23:35, 2005-11-28

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RE: Enceladus Life
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Saturn's tiny moon Enceladus is "absolutely a highlight" of the Cassini mission and should be targeted in future searches for life according to Robert H. Brown of The University of Arizona, leader of the Cassini visual and infrared mapping spectrometer team.

Brown and other Cassini scientists attended a meeting in London last week and are at the 37th annual Division of Planetary Sciences meeting at Cambridge University this week.

Enceladus turns out to have a primarily water vapour atmosphere tinged with nitrogen, carbon dioxide and other simple carbon-based molecules (organics) concentrated at its south pole. Its south pole is a hotspot, hovering at a relatively balmy minus -183 degrees Celsius compared to the expected temperature of -203 degrees Celsius.

"The kind of geophysical activity we see is quite likely being driven by liquid water below the surface" - Robert H. Brown.

Cassini hasn't seen ice geysers or ice volcanoes, but the lack of ammonia, and the sheer volume of water vapour escaping suggests there's pure-water volcanism on Enceladus.

"We detected simple organics in the tiger stripes. Methane (basically natural gas) has probably been locked up inside Enceladus since the solar system formed and is now bubbling up through the vents " - Robert Brown.

The simple organics include carbon dioxide and hydrogen-and-carbon-containing molecules like methane, ethane and ethylene.

The visual and infrared mapping spectrometer can't detect nitrogen, but Cassini's ion neutral mass spectrometer may have found nitrogen in Enceladus's atmosphere. All other results from these two very different instruments are entirely consistent, which gives Cassini mission scientists confidence in their results.

"[I]So you've got subsurface liquid water, simple organics and water vapour welling up from below. Over time - and Enceladus has been around 4.5 billion years, just like Earth and the rest of the solar system - heating a cocktail of simple organics, water and nitrogen could form some of the most basic building blocks of life. Whether that's happened at Enceladus is not clear, but Enceladus, much like Jupiter's moon Europa and the planet Mars, now has to be a place where we eventually search for life[/I]" - Robert Brown.

Source

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On July 11, 2005, the Cassini ultraviolet imaging spectrograph observed the star Bellatrix as it passed behind Enceladus, as seen from the spacecraft.
The starlight was observed to dim when it got close to Enceladus, indicating the presence of an atmosphere, as illustrated in figure A. The ultraviolet imaging spectrograph team was able to identify water vapour as the composition of the atmosphere from absorption features in the spectrum of the star.


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From the depth of the absorption features, it was also possible to estimate the quantity of water vapour the starlight passed through. The colours show the undimmed star signal (blue) versus the dimmed star signal (pinkish).

Enceladus atmosphere is localised, not global in extent. As Bellatrix re-emerged from behind Enceladus, there was no dimming of the starlight observed. An occultation of the star Lambda Scorpius in February also showed no sign of an atmosphere, as illustrated in figure B.

In figure A and B, the arrow marks the path of the star as it was blocked from view by Enceladus. In figure A, the dimming of the starlight shows as a gradual decrease in brightness, while in figure B the starlight drops abruptly just at the point in time that the star goes behind Enceladus.

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RE: Enceladus south ploe
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The southern hemisphere of Enceladus is seen in this polar stereographic map, mosaiced from the best-available Cassini and Voyager clear-filter images.
The map is centred on the South Pole and surface coverage extends to the equator. Gridlines show latitude and longitude in 30-degree increments.


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The map show that the character of terrains near the North Pole differs strongly from those near the South Pole. Terrain near the North Pole is among the most heavily cratered and oldest on the surface of Enceladus.
In the southern hemisphere map, the band of cratered terrain at 0 and 180 degrees longitude extends southward from the equator. However, pole ward of about 55 degrees south latitude, the cratered terrain is interrupted and replaced by a conspicuously fractured terrain around the pole that is nearly devoid of impact craters. In contrast to the very old north polar terrain, the southern polar terrain is among the youngest on the surface of Enceladus.
Within the southern polar region is a group of prominent parallel "stripes" made up of fractures that are delineated by relatively dark reflective (albedo) markings flanking the sides of each fracture.
An interesting property of the parallel fracture system is that each appears to turn back at its westernmost segment as if it has been "bent" or "folded" into a hook-like curve. Similar patterns of folded or kinked fractures can be found throughout the region -- a unique feature of the south polar terrains.

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RE: Enceladus north pole
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Fractures on the surface of Enceladus record a long and complex history of tectonic activity. Many of the geologically youngest fractures define remarkably systematic patterns relative to Enceladus' axis of rotation, as well as to its tidal orientation (that is, the longitudes that point toward and away from Saturn).

These fracture patterns offer clues to global changes of shape that the satellite has undergone over time, possibly in response to tidal forces exerted by Saturn or as a result of changes in the internal structure of the icy moon.
This map is a polar stereographic projection that was mosaiced from the best-available Cassini and Voyager clear-filter images. The map is centred on the North Pole and coverage extends to the equator. Gridlines show latitude and longitude in 30-degree increments.
Terrain near the North Pole is among the most heavily cratered and oldest on the surface of Enceladus. The conspicuously fractured southern polar region is nearly devoid of impact craters, making this terrain among the youngest on the moon's surface.


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A unique feature of the south polar terrains is that broad networks of folded or kinked fractures can be found throughout the region. A prominent "chain" of these fractures abruptly separates the youthful south polar terrains from the older terrains that lie closer to the equator and appears to encircle the South Pole near about 55-degrees south latitude.

The wavy boundary of the south polar terrain is interrupted in numerous places by Y-shaped, or funnel-shaped, discontinuities that curve and taper northward. Well-developed examples of the funnel-shaped discontinuities appear to transition to systems of north-south trending surface cracks.
These north-south trending cracks are best explained as if they formed in response to tension stresses that are parallel to lines of latitude. Such extensional "hoop stresses" would be expected to develop if the equator became wider, perhaps in response to a change in the moon's spin rate.

Globally systematic patterns of fracture on Enceladus occur in ancient, cratered terrain as well as youthful terrain. Shown in green on the maps -- and particularly apparent in the northern hemisphere projection -- are roughly orthogonal (or perpendicular) systems of north-south and east-west trending fractures that formed quite close to the tidal axis of Enceladus. The moon's tidal axis is a line extending from 0-degrees longitude to 180-degrees longitude. Such orthogonal fractures might have formed in response to tidal stresses exerted on Enceladus by Saturn.

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RE: Enceladus
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Four fissures in the south pole of Saturn's moon Enceladus are spewing out a plume hundreds of kilometres high, the Cassini probe has revealed, and the ejecta is leaving a vapour trail that rings Saturn.
The Cassini spacecraft discovered the long, cracked features dubbed "tiger stripes" on Saturn's icy moon Enceladus are very young. They are between 10 and 1,000 years old.



These findings support previous results showing the moon's southern pole is active. The pole had episodes of geologic activity as recently as 10 years ago. These cracked features are approximately 80 miles long, spaced about 25 miles apart and run roughly parallel to each another.

The cracks act like vents. They spew vapour and fine ice water particles that have become ice crystals. This crystallization process can help scientists pin down the age of the features.

"There appears to be a continual supply of fresh, crystalline ice at the tiger stripes, which could have been very recently resurfaced. Enceladus is constantly evolving and getting a makeover" - Dr. Bonnie Buratti, team member of the Cassini visual and infrared mapping spectrometer at NASA's Jet Propulsion Laboratory (JPL), Pasadena.

This finding is especially exciting because ground-based observers have seen tiny Enceladus brighten as its south pole was visible from Earth. Cassini allows scientists to see close up the brightening is caused by geologic activity. When NASA's Voyager 2 spacecraft flew over the moon's north pole in 1981, it did not observe the tiger stripes.

Cassini's visual and infrared mapping spectrometer shows water ice exists in two forms on Enceladus. The ice exists in pristine, crystalline ice and radiation-damaged amorphous ice.

The fissures are at least 90 Kelvin (-183C), 15 warmer than most of the moon's surface.
When ice comes out of the "hot" cracks, or "tiger stripes," at the south pole, it forms as fresh, crystalline ice. As the ice near the poles remains cold and undisturbed, it ages and converts to amorphous ice. Since this process is believed to take place over decades or less, the tiger stripes must be very young.
Scientists have speculated before that Enceladus might supply material for one of Saturn's rings, the E-ring, and the new observations seem to confirm it. Water is pouring out at a rate of half a tonne per second - enough to keep the E-ring topped up.
Cassini has also seen 20-metre boulders near the moon's south pole, that could possibly have been blown out of the fissures, like giant, icy lava bombs
"They are awfully large; but Enceladus' gravity is weak, so it doesn't take much to lift stuff off the surface- Torrence Johnson of the Cassini imaging team.


"One of the most fascinating aspects of Enceladus is that it is so very small as icy moons go, but so very geophysically active. It's hard for a body as small as Enceladus to hold onto the heat necessary to drive such large-scale geophysical phenomena, but it has done just that. Enceladus and its incredible geology is a marvellous puzzle for us to figure out" - Dr. Bob Brown. Brown is a team leader for the visual and infrared mapping spectrometer at the University of Arizona, Tucson.

Adding to the already mounting evidence for an active body is the correlation of results from multiple instruments. Cassini's cameras provided detailed images of the south polar cap, in which the tiger stripe fractures were found to be among the hottest features.

The timing of the craft's ion and neutral mass spectrometer and the cosmic dust analyzer observations seems to indicate the vapour and fine material are originating from the "hot" polar cap region. These data also indicate the production of water vapour and ejection of fine material are connected, as they are in a comet. This suggests that vapour and dust-sized icy material are coming from the tiger stripes.

Enceladus is on a short list of bodies in our solar system where scientists have found internal activity. The others are the volcanoes on Jupiter's moon Io and geysers on Neptune's moon Triton.

Data for these measurements were taken during Cassini's closest flyby on July 14, 2005. The spacecraft came within 173 kilometres of the surface of Enceladus. Enceladus is 314 miles across and has the most reflective surface in the solar system.

Adapted from source

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