Sonntag, 21. Dezember 2014 - 13:25 Uhr
Signs of Europa Plumes Remain Elusive in Search of Cassini Data
-- Data from Cassini's 2001 Jupiter flyby show Europa contributes less material to its surrounding environment than previously thought.
-- Unlike Saturn's known-active moon Enceladus, Europa is surrounded by very tenuous hot, excited gas.
A fresh look at data collected by NASA's Cassini spacecraft during its 2001 flyby of Jupiter shows that Europa’s tenuous atmosphere is even thinner than previously thought and also suggests that the thin, hot gas around the moon does not show evidence of plume activity occurring at the time of the flyby. The new research provides a snapshot of Europa's state of activity at that time, and suggests that if there is plume activity, it is likely intermittent.
The Europa results are being presented today at the American Geophysical Union fall meeting in San Francisco and published in the Astrophysical Journal. Europa is considered one of the most exciting destinations in the solar system for future exploration because it shows strong indications of having an ocean beneath its icy crust.
Members of Cassini's ultraviolet imaging spectrograph (UVIS) team analyzed data collected by their instrument during the brief time it observed Europa in 2001, as Cassini sped through the Jupiter system en route to Saturn. The observations show that most of the hot, excited gas, or plasma, around Europa originates not from the moon itself, but from volcanoes on the nearby moon Io. In fact, from their data, the researchers calculated that Europa contributes 40 times less oxygen than previously thought to its surrounding environment.
"Our work shows that researchers have been overestimating the density of Europa's atmosphere by quite a bit," said Don Shemansky, a Cassini UVIS team member with Space Environment Technologies in Pasadena, California, who led the study. The team found that the moon's tenuous atmosphere, which was already thought to be millions of times thinner than Earth’s atmosphere, is actually about 100 times less dense than those previous estimates.
A downward revision in the amount of oxygen Europa pumps into the environment around Jupiter would make it less likely that the moon is regularly venting plumes of water vapor high into orbit, especially at the time the data was acquired.
Scientists would expect that ongoing plume activity at Europa, as Cassini has observed at Saturn's moon Enceladus, would inject large amounts of water vapor into the area around Europa's orbit if the plumes were large enough, but that is not what UVIS observed.
"We found no evidence for water near Europa, even though we have readily detected it as it erupts in the plumes of Enceladus," said Larry Esposito, UVIS team lead at the University of Colorado at Boulder.
"It is certainly still possible that plume activity occurs, but that it is infrequent or the plumes are smaller than we see at Enceladus," said Amanda Hendrix, a Cassini UVIS team member with the Planetary Science Institute in Pasadena, who co-authored the new study. "If eruptive activity was occurring at the time of Cassini's flyby, it was at a level too low to be detectable by UVIS."
Indications of possible plume activity were reported in 2013 by researchers using NASA's Hubble Space Telescope, launching a wave of interest in searching for additional signs, including this effort by the UVIS team. Cassini's 2001 Jupiter flyby provided UVIS the opportunity to directly measure the environment near Europa, which is not possible with Hubble.
For more than a decade, Cassini's UVIS has observed the cold, dense doughnut of gas that encloses the orbit of Enceladus. There, the massive amount of gas being breathed into orbit around Saturn by the Enceladus plumes acts like a brake on electrons being dragged through it by Saturn's magnetic field, which rotates with the planet. This braking helps to keep down the temperature of the plasma. Apparently there is no such brake at Europa.
Since UVIS saw a hot plasma, rather than a cold one, around Europa's orbit, it suggests Europa is not outputting large amounts of gas -- including water.
Snapshots provided by missions that visited Jupiter prior to Cassini provided strong indications that Io is the major contributor of material to the environment around Jupiter, and indicated a hot, low density plasma surrounding Europa. The new results confirm that. "Io is the real monster here," Shemansky said.
“Europa is a complex, amazing world, and understanding it is challenging given the limited observations we have,” said Curt Niebur, Outer Planets program scientist at NASA Headquarters in Washington. “Studies like this make the most of the data we have and help guide the kinds of of science investigations NASA should pursue in the future.”
Scientists are currently using the Hubble Space Telescope to conduct an extensive six-month long survey looking for plume activity, and NASA is also studying various possible Europa missions for future exploration.
The Cassini-Huygens mission is a cooperative project of NASA, the European Space Agency and the Italian Space Agency. NASA's Jet Propulsion Laboratory, a division of the California Institute of Technology in Pasadena, manages the mission for NASA's Science Mission Directorate, Washington, D.C. JPL designed, developed and assembled the Cassini orbiter. The UVIS team is based at the University of Colorado, Boulder, where the instrument was designed and built.
Repeated Flybys Yield a Pole-to-Pole View of Europa
Date: 1 Feb 1999
This view of Jupiter's moon Europa features several regional-resolution mosaics overlaid on a lower resolution global view for context. The regional views were obtained during several different flybys of the moon by NASA's Galileo mission, and they stretch from high northern to high southern latitudes. Prominent here are the long, arcuate (or arc-shaped) and linear markings called lineae (Latin for strings or threads), which are a signature feature of Europa's surface. Color saturation has been enhanced to bring out the subtle red coloration present along many of the lineae. The color data extends into the infrared, showing bluish ice (indicating larger ice grains) in the polar regions.
The terrain in this view stretches from the side of Europa that always trails in its orbit at left (west), to the side that faces away from Jupiter at right (east). In addition to the lineae, the regional-scale images contain many interesting features, including lenticulae (small spots), chaos terrain, maculae (large spots), and the unusual bright band known as Agenor Linea in the south.
The regional-resolution mosaics enhance the amount of detail visible in a previously released view of the same region on Europa, [see PIA02590]. While the earlier image uses much of the same low-resolution data, its images are projected from a different angle and are processed with greater color saturation.
This view is an orthographic projection centered on 5.53 degrees south latitude, 214.5 degrees west longitude and has a resolution of 1600 feet (500 meters) per pixel. An orthographic view is like the view seen by a distant observer looking through a telescope.
The mosaic was constructed from individual images obtained by the Solid State Imaging (SSI) system on NASA's Galileo spacecraft during six flybys of Europa between 1996 and 1999 (flybys designated G1, E11, E14, E15, E17, and E19).
Ruddy "Freckles" on Europa
Reddish spots and shallow pits pepper the enigmatic ridged surface of Europa in this view combining information from images taken by NASA's Galileo spacecraft during two different orbits around Jupiter.
The spots and pits visible in this region of Europa's northern hemisphere are each about 10 kilometers (6 miles) across. The dark spots are called "lenticulae," the Latin term for freckles. Their similar sizes and spacing suggest that Europa's icy shell may be churning away like a lava lamp, with warmer ice moving upward from the bottom of the ice shell while colder ice near the surface sinks downward. Other evidence has shown that Europa likely has a deep melted ocean under its icy shell. Ruddy ice erupting onto the surface to form the lenticulae may hold clues to the composition of the ocean and to whether it could support life.
The image combines higher-resolution information obtained when Galileo flew near Europa on May 31, 1998, during the spacecraft's 15th orbit of Jupiter, with lower-resolution color information obtained on June 28, 1996, during Galileo's first orbit.
Thera and Thrace
Thera and Thrace are two dark, reddish regions of enigmatic terrain that disrupt the older icy ridged plains on Jupiter's moon Europa. North is toward the top of the mosaic obtained by NASA's Galileo spacecraft. The sun illuminates the region from the northeast.
Thera (left) is about 70 kilometers wide by 85 kilometers high (43 by 53 miles) and appears to lie slightly below the level of the surrounding plains. Some bright icy plates which are observed inside appear to be dislodged from the edges of the chaos region. The curved fractures along its boundaries suggest that collapse may have been involved in Thera's formation. In contrast, Thrace (right) is longer, shows a hummocky texture, and appears to stand at or slightly above the older surrounding bright plains. Thrace abuts the gray band named Libya Linea to the south and appears to darken Libya. One model for the formation of these and other chaos regions on Europa is complete melt-through of Europa's icy shell from an ocean below. Another model is that warm ice welled up from below and caused partial melting and disruption of the surface.
To produce this image, two regional images obtained at a resolution of 220 meters (240 yards) per picture element during Galileo's 17th orbit of Jupiter were colorized with lower resolution (1.4 kilometers or 1526 yards per picture element) images of the region obtained during the 14th orbit. The color image is generated from the violet, green, and near-infrared (968 nanometers) filters of the Galileo Solid State Imaging system and exaggerates the subtle color differences of Europa's surface. The mosaic, centered at about 50 degrees south latitude and 180 degrees longitude, covers an area approximately 525 by 300 kilometers (325 by 186 miles). The images from the 17th orbit were acquired at 2:51 Universal Time on September 26, 1998.
New Horizons Spies Europa
NASA's New Horizons spacecraft took this image of Europa hanging above Jupiter's cloud tops with its Long Range Reconnaissance Imager (LORRI) at 11:48 Universal Time on February 28, 2007, six hours after the spacecraft's closest approach to Jupiter.
The picture was one of a handful of the Jupiter system that New Horizons took primarily for artistic, rather than scientific, value. This particular scene was suggested by space enthusiast Richard Hendricks of Austin, Texas, in response to an Internet request by New Horizons scientists for evocative, artistic imaging opportunities at Jupiter.
The spacecraft was 2.3 million kilometers (1.4 million miles) from Jupiter and 3 million kilometers (1.8 million miles) from Europa when the picture was taken. Europa's diameter is 3,120 kilometers (1,939 miles). The image is centered on Europa coordinates 5 degrees south, 6 degrees west. In keeping with its artistic intent - and to provide a more dramatic perspective - the image has been rotated so south is at the top.
Conamara Color Closeup
This image shows a close-up view of terrain within the region of Europa's surface named Conamara. This region sports ice rafts that look like those at Earth's poles, where large chunks of ice break away and float freely on the ocean. Much of the region bears the reddish/brownish discoloration seen here - the same as seen along many of Europa's fractures. Scientists believe this material may contain clues about the composition of an ocean beneath the icy surface, if it is proven to exist.
The data used to produce this color view was originally released by the Galileo mission as PIA01403.
This view from NASA's Galileo spacecraft shows a severely fractured surface on Europa - with dark linear, curved, and wedged-shaped bands. These fractures have broken the crust into plates as large as 20 miles (about 30 kilometers) across. Areas between the plates are filled with dark material that was probably icy slush contaminated with rocky debris. Some individual plates were separated and rotated into new positions.
This image covers part of the equatorial zone of Europa and was taken on June 27, 1996 from a distance of about 96,300 miles (156,000 kilometers) from Europa by Galileo's Solid-State Imaging subsystem. North is to the right and the sun is almost directly overhead. The area shown is about 310-by-600 miles (510 by 989 kilometers) across, and the smallest visible feature is about 1 mile (1.6 kilometers) wide.
Ridges, Plains and Mountains
is high resolution mosaic shows ridges, plains and mountains on Europa. The sun illuminates the scene from the left, showing hundreds of ridges that cut across each other, indicating multiple episodes of ridge formation either by volcanic or tectonic activity within the ice.
The mosaic, centered at 35.4 degrees north latitude and 86.8 degrees west longitude, covers an area of 66 miles by 55 miles (108 kilometers by 90 kilometers). North is to the top of the image. The smallest distinguishable features in the image are about 223 feet (68 meters) across. These images were obtained by Galileo's Solid State Imaging (SSI) system on November 6, 1997, when the spacecraft was approximately 1,983 miles (3,250 kilometers) from Europa.
Icy Cliffs at High Resolution
Europa: Water World Infographic
In our solar system there is an ocean twice the size of all Earth's oceans combined.
Is it Inhabited?
Moon of Jupiter
Diameter: 12,742 km
Average Ocean Depth: ~4 km
Volume: ~1.4 billion km3
Liquid Saltwater Surface Ocean
Earth's surface is 29% land, 71% liquid water
Polar Ice Cap
Regional Liquid Salt Water Ocean
Rocky Sea Floor
Diameter: 3,120 km
Average Ocean Depth: ~100 km
Volume: ~3 billion km3
Liquid Saltwater Sub-Surface Ocean
Europa's surface is covered in a global water ice crust/ Estimates range from 3 km - 30 km thick
Global Liquid Saltwater Ocean
Rocky Sea Floor
So Why Does This Matter?
Because Earth's Oceans are teeming with life!
The Oceans are Home to:
50-80% of all life on Earth
At least 224,000 named species
Plants, bacteria, fungi, reptiles, mammals, reefs, algae, fish, mollusks, and many more
Extremophiles, surviving from the freezing temperatures below arctic sea ice to the boiling temperatures near deep sea hydrothermal vents
On Earth, wherever we find water, we find life.
And Europa has twice the water of Earth, so...
Is There Life?
Reddish Bands on Europa
Date: 6 Nov 1997
This colorized image of Europa is a product of clear-filter grayscale data from one orbit of NASA's Galileo spacecraft, combined with lower-resolution color data taken on a different orbit. The blue-white terrains indicate relatively pure water ice, whereas the reddish areas contain water ice mixed with hydrated salts, potentially magnesium sulfate or sulfuric acid. The reddish material is associated with the broad band in the center of the image, as well as some of the narrower bands, ridges, and disrupted chaos-type features. It is possible that these surface features may have communicated with a global subsurface ocean layer during or after their formation.
Part of the terrain in this previously unreleased color view is seen in the monochrome image, PIA01125.
The image area measures approximately 101 by 103 miles (163 km by 167 km). The grayscale images were obtained on November 6, 1997, during the Galileo spacecraft's 11th orbit of Jupiter, when the spacecraft was approximately 13,237 miles (21,700 kilometers) from Europa. These images were then combined with lower-resolution color data obtained in 1998, during the spacecraft's 14th orbit of Jupiter, when the spacecraft was 89,000 miles (143,000 km) from Europa.
Source Region for Possible Europa Plumes
This reprojection of the official USGS Europa basemap is centered at the estimated source region for potential plumes that might have been detected using the Hubble Space Telescope. The view is centered at -65 degrees latitude, 183 degrees longitude.
In addition to the plume source region, the image also shows the hemisphere of Europa that might be affected by plume deposits. This map is composed of images from NASA's Galileo and Voyager missions. The black region near the south pole results from gaps in imaging coverage.
3-D Cilix Crater on Europa
This view of Cilix impact crater on Europa was created in 2013 using 3-D stereo images taken by NASA's Galileo spacecraft, combined with advanced image processing techniques. The crater has a diameter of about 11 miles (18 kilometers).
This image, which combines a 3-D Digital Elevation Model, or DEM, with original imagery, shows that the crater rim rises steeply for about 980 feet (300 meters) above a flat crater floor that is interrupted by a central peak which has a height of about 660 feet (200 meters). Such central peaks are common on other bodies in the solar system. Young, well-preserved craters like Cilix are rare on Europa's surface, where ongoing geologic activity is thought to disrupt most surface features over timescales of tens of millions of years.
Cracks and ridges on Europa
Date: 2 Feb 1999
This enhanced color image shows cracks and ridges on Europa's surface that reveal a detailed geologic history. Some ridges, such as the prominent one at top right, develop into long, arc-shaped "cycloids" that may be related to changing tidal forces as Europa orbits Jupiter. The wall of this ridge stands perhaps a third of a mile (0.5 kilometer) above the surrounding ridged plains, although the edges are likely not as steep as they appear in this view.
The view was captured by NASA's Galileo spacecraft on February 2, 1999, during its E19 orbit, when the spacecraft was about 2500 miles (4000 km) from the surface of Europa. Resolution in the scene is 295 feet (90 meters) per pixel. North is toward bottom left. Images taken through near-infrared, green and violet filters were combined to create the view.
Thick or Thin Ice Shell on Europa?
Scientists are all but certain that Europa has an ocean underneath its icy surface, but they do not know how thick this ice might be. This artist's concept illustrates two possible cut-away views through Europa's ice shell. In both, heat escapes, possibly volcanically, from Europa's rocky mantle and is carried upward by buoyant oceanic currents. If the heat from below is intense and the ice shell is thin enough (left), the ice shell can directly melt, causing what are called "chaos" on Europa, regions of what appear to be broken, rotated and tilted ice blocks. On the other hand, if the ice shell is sufficiently thick (right), the less intense interior heat will be transferred to the warmer ice at the bottom of the shell, and additional heat is generated by tidal squeezing of the warmer ice. This warmer ice will slowly rise, flowing as glaciers do on Earth, and the slow but steady motion may also disrupt the extremely cold, brittle ice at the surface.
Europa is no larger than Earth's moon, and its internal heating stems from its eccentric orbit about Jupiter, seen in the distance. As tides raised by Jupiter in Europa's ocean rise and fall, they may cause cracking, additional heating and even venting of water vapor into the airless sky above Europa's icy surface.
3D View of Double Ridges on Europa
These images reveal the dramatic topography of Europa's icy crust. A double ridge with a deep intervening trough cuts across older background plains and a darker, wedge-shaped band. The numerous cracks and bands of such terrain may indicate where the crust has pulled apart and sometimes allowed dark material from beneath the surface to well up and fill the cracks.
A computer generated three-dimensional perspective (upper right) shows that bright material, probably pure water ice, prevails at the ridge crests and slopes while most dark material (perhaps ice mixed with silicates or hydrated salts) is confined to lower areas such as valley floors. The northernmost, north-facing slope (right side) has a larger concentration of dark material than south facing slopes.
The model on the lower right has been color coded to accentuate elevations. The red tones indicate that the crests of the ridge system reach elevations of nearly 1000 feet (more than 300 meters) above the surrounding furrowed plains (blue and purple tones). The two ridges are separated by a valley about a mile (1.5 kilometers) wide.
The stereo perspective combines high resolution images obtained from two different viewing angles. Such a three dimensional model is similar to the three dimensional scenes our brains construct when both eyes view something from two angles.
North is to the right, and the sun illuminates the scene from northwest. North is to the right. The images were taken by the Solid State Imaging (SSI) system on NASA's Galileo spacecraft. The regional context (left), centered at about 16 degrees south latitude, 195 degrees west longitude, was imaged on November 6th, 1996 at a range of about 41,000 kilometers (25,500 miles). The higher resolution stereo images were taken on December 16th, 1997, at ranges of 5,800 kilometers(3,600 miles) and 2,600 kilometers (1,600 miles) leading to a best resolution of 26 meters per picture element.