Cassini's final breathtaking close views of Dione

A pockmarked, icy landscape looms beneath NASA's Cassini spacecraft in new images of Saturn's moon Dione taken during the mission's last close approach to the small, icy world. Two of the new images show the surface of Dione at the best resolution ever.

This view from NASA's Cassini spacecraft looks toward Saturn's icy moon Dione, 
with giant Saturn and its rings in the background, just prior to the 
mission's final close approach to the moon on August 17, 2015 
[Credit: NASA/JPL-Caltech/Space Science Institute]

Cassini passed 295 miles (474 kilometers) above Dione's surface at 11:33 a.m. PDT (2:33 p.m. EDT) on Aug. 17. This was the fifth close encounter with Dione during Cassini's long tour at Saturn. The mission's closest-ever flyby of Dione was in Dec. 2011, at a distance of 60 miles (100 kilometers).

Dione hangs in front of Saturn and its icy rings in this view, 
captured during Cassini's final close flyby of the icy moon 
[Credit: NASA/JPL-Caltech/Space Science Institute]

"I am moved, as I know everyone else is, looking at these exquisite images of Dione's surface and crescent, and knowing that they are the last we will see of this far-off world for a very long time to come," said Carolyn Porco, Cassini imaging team lead at the Space Science Institute, Boulder, Colorado. "Right down to the last, Cassini has faithfully delivered another extraordinary set of riches. How lucky we have been."

Saturn's moon Dione hangs in front of Saturn's rings in this 
view taken by NASA's Cassini spacecraft during the
 inbound leg of its last close flyby of the icy moon 
[Credit: NASA/JPL-Caltech/Space Science Institute]

The main scientific focus of this flyby was gravity science, not imaging. This made capturing the images tricky, as Cassini's camera was not controlling where the spacecraft pointed.

NASA's Cassini spacecraft captured this parting view showing
 the rough and icy crescent of Saturn's moon Dione following the 
spacecraft's last close flyby of the moon on Aug. 17, 2015 
[Credit: NASA/JPL-Caltech/Space Science Institute]

"We had just enough time to snap a few images, giving us nice, high resolution looks at the surface," said Tilmann Denk, a Cassini participating scientist at Freie University in Berlin. "We were able to make use of reflected sunlight from Saturn as an additional light source, which revealed details in the shadows of some of the images."

NASA's Cassini spacecraft gazes out upon a rolling, cratered
 landscape in this oblique view of Saturn's moon Dione 
[Credit: NASA/JPL-Caltech/Space Science Institute]

Cassini scientists will study data from the gravity science experiment and magnetosphere and plasma science instruments over the next few months as they look for clues about Dione's interior structure and processes affecting its surface.

This view from NASA's Cassini spacecraft shows terrain on Saturn's moon 
Dione that is entirely lit by reflected light from Saturn, called Saturnshine 
[Credit: NASA/JPL-Caltech/Space Science Institute]

Only a handful of close flybys of Saturn's large, icy moons remain for Cassini. The spacecraft is scheduled to make three approaches to the geologically active moon Enceladus on Oct. 14 and 28, and Dec. 19. During the Oct. 28 flyby, the spacecraft will come dizzyingly close to Enceladus, passing a mere 30 miles (49 kilometers) from the surface. Cassini will make its deepest-ever dive through the moon's plume of icy spray at this time, collecting valuable data about what's going on beneath the surface. The December Enceladus encounter will be Cassini's final close pass by that moon, at an altitude of 3,106 miles (4,999 kilometers).

This view of Dione from Cassini includes the mission's highest-resolution
 view of the icy moon's surface as an inset at center left 
[Credit: NASA/JPL-Caltech/Space Science Institute]

After December, and through the mission's conclusion in late 2017, there are a handful of distant flybys planned for Saturn's large, icy moons at ranges of less than about 30,000 miles (50,000 kilometers). Cassini will, however, make nearly two dozen passes by a menagerie of Saturn's small, irregularly shaped moons—including Daphnis, Telesto, Epimetheus and Aegaeon—at similar distances during this time. These passes will provide some of Cassini's best-ever views of the little moons.

A region on Dione where features in shadow are illuminated by reflected 
light from Saturn. Inset above center is one of Cassini's 
highest-resolution views of Dione's surface 
[Credit: NASA/JPL-Caltech/Space Science Institute]

During the mission's final year—called its Grand Finale—Cassini will repeatedly dive through the space between Saturn and its rings.

Author: Preston Dyches | Source: Jet Propulsion Laboratory [August 21, 2015]

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Do comet fractures drive surface evolution?

Extreme thermal stresses experienced by a comet as it orbits around the Sun could explain the extensive fracturing thought to drive its long-term surface erosion, say Rosetta scientists analysing high-resolution images of Comet 67P/Churyumov-Gerasimenko's surface.

Variety of fracture networks on comet 67P/C-G [Credit: ESA/Rosetta/MPS 
for OSIRIS Team MPS/UPD/LAM/IAA/SSO/INTA/UPM/DASP/IDA]

The study, which is published online in the journal Geophysical Research Letters, is based on images taken between 6 August 2014 – when Rosetta first arrived at the comet – and 1 March 2015, and includes detailed images acquired from between just 8 and 18 km from the comet's surface.

Lead author M. Ramy El-Maarry and his team identified three distinct settings in which the fractures occur: networks of long narrow fractures, fractures on cliffs and fractured boulders. In addition, several unique features were identified: the parallel fractures running across Hathor's 900 m-high cliffs, an isolated 500 metre-long crevice in the Anuket region of the comet's neck, and a 200 m-long complex crack system in Aker on the large lobe.

"The fractures show a variety of morphologies and occur all over the surface and at all scales: they are found in the towering 900 m-high cliffs of Hathor right down to the surfaces of boulders a few metres across," describes El-Maarry from the University of Bern.

Fractures found on cliffs on comet 67P/C-G [Credit: ESA/Rosetta/MPS 
for OSIRIS Team MPS/UPD/LAM/IAA/SSO/INTA/UPM/DASP/IDA]

The most prevalent setting appears to be networks of narrow fractures that extend for a few metres to 250 m in length, typically on relatively flat surfaces. Interestingly, in some locations, the fractures appear to cross cut each other in polygonal patterns at angles of 90° – on Earth and Mars this is often an indicator of ice that has contracted below the surface.

Another family of cross-cutting fractures is observed on cliff faces, such as in the Seth region on the comet's large lobe, with debris deposits littering their bases. Fractured cliff faces were also observed at Abydos, the final landing site of Philae on the small lobe, as recorded in images from the lander. The fact that the fractures cut across each other in different directions suggests that the stress direction changes over time.

At the smallest scale, fractures are observed on boulders (in this study, boulders 20-60 m wide were analysed). In some cases it is clear that the fractures run through the boulders, pervasively fracturing them, while other fractures appear confined to the boulder's surfaces. Taken together, the fracturing points to an erosional sequence leading to the boulders' eventual fragmentation.

Fractured boulders on comet 67P/C-G [Credit: ESA/Rosetta/MPS 
for OSIRIS Team MPS/UPD/LAM/IAA/SSO/INTA/UPM/DASP/IDA]

Scientists think that the majority of these fracture patterns are most likely linked to the thermal history of the comet and result from stresses that stretch the comet's surface apart. On Earth and Mars at least, these 'tensile' fractures can develop through several common processes: loss of volatile materials, thermal contraction or contraction and expansion cycles, and tectonic processes.

In addition to throwing off volatile materials as they near the Sun, comets are known to undergo high fluctuations in surface and subsurface temperature on daily and seasonal timeframes. This continuous thermal 'shock' leads to weakening or "fatigue" of the surface both on the short term due to daily heating cycles, and on the long term as the seasons change along the comet's 6.5 year orbit around the Sun.

"But the presence of fractures in different settings, in addition to the isolated fractures in Anuket and Aker, suggests that other mechanisms may also be at work," comments Ramy. "For example, perhaps mechanical forces related to the comet's rotation or orbit around the Sun are responsible for the crack in Anuket, while the fractured cliffs of Hathor could have resulted from the comet's formation, perhaps when two smaller cometisimals collided."

A 200-m complex fracture system in the Aker region, on the comet large lobe 
[Credit: ESA/Rosetta/MPS for OSIRIS Team MPS/UPD/LAM/IAA/SSO/INTA/UPM/DASP/IDA]

Regardless of the origin of individual fracture systems, it is clear that fracturing plays an important role in the evolution of the comet's surface.

Fractures observed in cliffs, with debris observed at their bases, imply that this phenomenon represents the first stage in the overall 'mass-wasting' of the comet: the cliff top is weakened and a landslide-type event ensues.

But, over long periods this process would act to erode the surface and flatten the landscape. The fact that there are many rough terrains on the comet suggests that it has either not gone through very many erosional cycles or other processes are acting against this to roughen the surface, such as explosive jet activity.

"Monitoring for changes in these fracture systems after the current perihelion phase, and running models to simulate the evolution of the comet over time, will enable us to test our various hypotheses for fracture formation," adds Ramy.

Source: European Space Agency [August 21, 2015]

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