Evidence of Cryovolcanism on Europa in the Argadnel Regio

Surface composition of pull-apart bands in Argadnel Regio, Europa: Evidence of localized cryovolcanic resurfacing during basin formation

Authors:

Prockter et al

Abstract:

We combine Galileo Solid State Imager (SSI) and Near-Infrared Mapping Spectrometer (NIMS) data to investigate the composition of pull-apart bands in Europa's Argadnel Regio. Using spectral linear mixture modeling employing cryogenic laboratory reference spectra, we find that bands of intermediate age (“grey” bands) are compositionally distinct from bands that are stratigraphically younger (“dark” bands). The grey bands have higher abundances of larger ice grains and lower abundances of hydrated salts than the dark bands; both of these tendencies are statistically significant at the 1% level. The grey and dark bands have similar abundances of hexahydrite, a material which is relatively stable under irradiation; however, the derived abundances of frozen magnesium sulfate brine and of mirabilite, which are more susceptible to fragmentation by radiation, are significantly higher in the dark bands than in the grey bands. These results are consistent with a physical model in which the differences in composition and in ice grain sizes are linked to space weathering and radiolytic processing levels; the grey bands have presumably undergone higher levels of processing, due to being exposed on Europa's surface for a longer period of time. One prominent wedge-shaped band exhibits an anomalous albedo variation across its northern portion, appearing dark in its top third, and grey in its southernmost two-thirds. We find that the dark part of the band has a modeled composition that is in-family with other dark bands, while the grey portion has a modeled composition that is indistinguishable from other grey bands in the study area. Because these variations cannot easily be attributed to the band's formation mechanism (bands open sequentially along a central axis), we surmise that the northern part has been resurfaced, probably in response to the formation of a large topographic basin that cuts through the band. Faulting accompanying basin formation may provide conduits allowing transport to the surface of materials from Europa's interior. We hypothesize that the formation of the basin resulted in fresh cryovolcanic material being deposited across the northern portion of the band, effectively “resetting” its surface age. If, as has been suggested, the giant arcuate basins resulted from an episode of true polar wander, our study may help to more tightly constrain the age of that event within Europa's geologic column.

NASA Releases Galileo Pictures of Europa to Public

The picture above, showing the icy moon’s creased and cracked surface, was made from images acquired by NASA’s Galileo spacecraft during its exploration of Jupiter and its family of moons in 1997 and 1998. While the data itself isn’t new per se the view seen here has never been released by JPL, and so it’s new to you! (And to me too.)

The original high-resolution images were acquired on Nov. 6, 1997, in greyscale and colorized with data acquired during a later pass by Galileo in 1998. The whiter areas are regions of relatively pure water ice, while the rusty red bands are where ice has mixed with salts and organic compounds that have oozed up from deeper within Europa.

The entire image area measures about 101 by 103 miles across (163 km x 167 km).

Europa has long been one of the few places we know of outside our own planet where life could very well have evolved and potentially still exist. Getting a peek below the icy moon’s frozen crust — or even a taste of the recently-discovered water vapor spraying from its south pole — is all we’d need to further narrow down the chances that somewhere, something could be thriving in Europa’s subsurface seas. Get a planetary scientist’s perspective in a video interview with Dr. Mike Brown here.

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Global Map of Ganymede Created

Scientists, including Brown University geologists and students, have completed the first global geological map of Ganymede, Jupiter's largest moon and the largest in the solar system.

With its varied terrain and possible underground ocean, Ganymede is considered a prime target in the search for habitable environments in the solar system, and the researchers hope this new map will aid in future exploration. The work, led by Geoffrey Collins, a Ph.D. graduate of Brown now a professor at Wheaton College in Massachusetts, took years to complete. The map was published today by the U.S. Geological Survey.

"It is very rewarding to see the results of all of our efforts here at Brown come together into this integrated global compilation that will now be used to plan the next phase of scientific exploration of the Galilean satellites," said Jim Head, the Scherck Distinguished Pprofessor of Geological Sciences at Brown and one of the map's co-authors.

The researchers combined images from the Voyager and Galileo spacecraft to put the map together. Voyager was the first mission to fly through the Jupiter satellite system and passed by the icy surface of Ganymede in 1979. Those first images revealed a complex surface, segmented and fractured into dark and light terrain. In 1995, the Galileo spacecraft was placed in orbit around Jupiter and began to return high-resolution images of the surface that help to understand many of the features seen at low-resolution by Voyager.

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Europa Has Traces of Clay...from an impact?

A new analysis of data from NASA's Galileo mission has revealed clay-type minerals at the surface of Jupiter's icy moon Europa that appear to have been delivered by a spectacular collision with an asteroid or comet. This is the first time such minerals have been detected on Europa's surface. The types of space rocks that deliver such minerals typically also often carry organic materials.

"Organic materials, which are important building blocks for life, are often found in comets and primitive asteroids," said Jim Shirley, a research scientist at NASA's Jet Propulsion Laboratory, Pasadena, Calif. Shirley is giving a talk on this topic at the American Geophysical Union meeting in San Francisco on Friday, Dec. 13. "Finding the rocky residues of this comet crash on Europa's surface may open up a new chapter in the story of the search for life on Europa," he said.

Many scientists believe Europa is the best location in our solar system to find existing life. It has a subsurface ocean in contact with rock, an icy surface that mixes with the ocean below, salts on the surface that create an energy gradient, and a source of heat (the flexing that occurs as it gets stretched and squeezed by Jupiter's gravity). Those conditions were likely in place shortly after Europa first coalesced in our solar system.

Scientists have also long thought there must be organic materials at Europa, too, though they have yet to detect them directly. One theory is that organic material could have arrived by comet or asteroid impacts, and this new finding supports that idea.

Shirley and colleagues, funded by a NASA Outer Planets Research grant, were able to see the clay-type minerals called phyllosilicates in near-infrared images from Galileo taken in 1998. Those images are low resolution by today's standards, and Shirley's group is applying a new technique for pulling a stronger signal for these materials out of the noisy picture. The phyllosilicates appear in a broken ring about 25 miles (40 kilometers) wide, which is about 75 miles (120 kilometers) away from the center of a 20-mile-diameter (30 kilometers) central crater site.

The leading explanation for this pattern is the splash back of material ejected when a comet or asteroid hits the surface at an angle of 45 degrees or more from the vertical direction. A shallow angle would allow some of the space rock's original material to fall back to the surface. A more head-on collision would likely have vaporized it or driven that space rock's materials below the surface. It is hard to see how phyllosilicates from Europa's interior could make it to the surface, due to Europa's icy crust, which scientists think may be up to 60 miles (100 kilometers) thick in some areas.

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