Moonlets created and destroyed in Saturn's F ring

Cassini spied just as many regular, faint clumps in Saturn's narrow F ring (the outermost, thin ring), like those pictured here, as Voyager did but it saw hardly any of the long, bright clumps that were common in Voyager images. 

Credit: NASA /JPL-Caltech /SSI

There is an ongoing drama in the Saturnian ring system that causes small moons to be born and then destroyed on time scales that are but an eyeblink in the history of the solar system.

SETI Institute scientists Robert French and Mark Showalter have examined photos made by NASA's Cassini spacecraft and compared them to 30 year-old pictures made by the Voyager mission.

They find that there is a marked difference in the appearance of one of the rings, even over this cosmologically short interval, a difference that can be explained by the brief strut and fret of small moons.

"The F ring is a narrow, lumpy feature made entirely of water ice that lies just outside the broad, luminous rings A, B, and C," notes French.

"It has bright spots. But it has fundamentally changed its appearance since the time of Voyager. Today, there are fewer of the very bright lumps."

The bright spots come and go over the course of hours or days, a mystery that the two SETI Institute astronomers think they have solved.

"We believe the most luminous knots occur when tiny moons, no bigger than a large mountain, collide with the densest part of the ring," says French.

"These moons are small enough to coalesce and then break apart in short order."

The F ring is at a special place in the ring system, at a distance known as the Roche limit, named for French astronomer Edouard Roche who first pointed out that if a moon orbits too close to a planet, the difference in gravitational tug on its near and far side can tear it apart.

This happens at a distance dependent on the mass of the planet, and in the case of Saturn, happens to be at the location of the F ring.

Consequently, material here is caught between the yin and yang of forming small moons, and having them pulled apart.

The moons in question are typically no more than 3 miles (5 km) in size, and consequently can come together quickly.

This chaotic region is given additional stir by Prometheus, a moon that's roughly 60 miles (100 km) in size that orbits just inside the F ring.

Every 17 years, Prometheus aligns with the F ring in a way that emphasises its gravitational influence on the ring's particles, precipitating the formation of the mini-moons, or moonlets.

"These newborn moonlets will repeatedly crash through the F ring, like bumper cars, producing bright clumps as they careen through lanes of material," says Showalter.

"But this is self-destructive behaviour, and the moons, being just at the Roche limit, are barely stable and quickly fragmented."

This scenario can explain the rapid variation in the number of bright clumps in the F ring, but is it true?

If the periodic influence of Prometheus is causing the waxing and waning of the clumps, then there should be an increase in their prevalence over the next few years, a prediction that the astronomers will be checking with Cassini data.

In addition to the drama of moons that come and go over less than a human lifetime, studies of the ring system give insight into how solar systems in general are built.

"The sort of processes going on around Saturn are very similar to those that took place here 4.6 billion years ago, when the Earth and the other large planets were formed," notes French. "It's an important process to understand."

This research was published in the online edition of the journal Icarus on July 15, 2014.

More information: Robert S. French, Shannon K. Hicks, Mark R. Showalter, Adrienne K. Antonsen, Douglas R. Packard, "Analysis of clumps in Saturn's F ring from Voyager and Cassini," Icarus, Volume 241, October 2014, Pages 200-220, ISSN 0019-1035, dx.doi.org/10.1016/j.icarus.2014.06.035 . Preprint: arxiv.org/abs/1408.2548

Randolph Glacier Inventory (RGI): First globally complete glacier inventory

This image shows the Zhadang glacier south of lake Nam Tso on the nortern ridge of the Nyainqentanglha mountain range (Tibet). 

Credit: Tino Pieczonka (TU Dresden)

For the first time ever, we have a complete inventory of all the glaciers on Earth.

Now we know how many glaciers there are, where they are, and what their extents and volumes are.

The now available digital outlines allow for the first time reliable calculations of the glaciers' future development and hence their contributions to regional hydrology and global sea-level rise.

Thanks to the efforts of an international group of scientists, one of them is Tobias Bolch from Technische Universität Dresden, Germany, who have mapped all of the world's glaciers, glaciologists can now study with unprecedented accuracy the impacts of a changing climate on glaciers worldwide, and determine their total extent and volume on a glacier-by-glacier basis.

Nyainqentanglha mountain range

Overall, glaciers cover an area of about 730,000 km2 and have a volume of about 170,000 km3.

The scientists found nearly 200,000 of them, but they say that this is the least important result of the mapping exercise as the number constantly changes due to disappearing small and fragmenting larger glaciers.

More importantly, each glacier in the new inventory is represented by a computer-readable outline, making precise modelling of glacier–climate interactions much easier.

Graham Cogley

"This boost to the infrastructure means that people can now do research that they simply couldn't do properly before", said Graham Cogley of Trent University, one of the coordinators of the new Randolph Glacier Inventory (RGI), which is named after one of the group's meeting places in New Hampshire.

The now published article is presenting the RGI and first statistical analysis of the global glacier distribution.

The main stimulus for completing the inventory was the recently-published Fifth Assessment of the Intergovernmental Panel on Climate Change (IPCC). Several studies that relied on earlier versions of the RGI were essential sources for that assessment.

Tad Pfeffer

"I don't think anyone could have made meaningful progress on projecting glacier changes if the Randolph inventory had not been available", said the University of Colorado's Tad Pfeffer, lead author of the study just published in the Journal of Glaciology.

Like several of his co-authors, Tad Pfeffer was also involved in the IPCC assessment.

More information: Pfeffer, W.T., Arendt, A. A., Bliss, A., Bolch, T., Cogley, J. G., Gardner, A. S., Hagen, J.-O., Hock, R., Kaser, G., Kienholz, C., Miles, E. S., Moholdt, G., Mölg, N., Paul, F., Radic, V., Rastner, P., Raup, B. H., Rich, J., Sharp, M. J. and the Randolph Consortium: The Randolph Glacier Inventory (2014): a globally complete inventory of glaciers. Journal of Glaciology 60(221), DOI: 10.3189/2014JoG13J176

Nuclear fission algorithm is created

Nuclear fission algorithm is created

U.S. Department of Energy scientists say they've created a computer algorithm that allows a substantially enhanced view of nuclear fission.

The Argonne National Laboratory scientists said the algorithm, known as the neutron transport code, enables researchers for the first time to obtain a highly detailed description of a nuclear reactor core.

"The code could prove crucial in the development of nuclear reactors that are safe, affordable and environmentally friendly," laboratory officials said in a statement.

To model the complex geometry of a reactor core currently requires billions of spatial elements, hundreds of angles and thousands of energy groups -- all of which lead to problem sizes with quadrillions of possible solutions, the researchers said. Such calculations exhaust computer memory of the largest machines, they said, and therefore reactor modeling codes typically rely on various approximations.

"The (neutron transport code) is intended to reduce the uncertainties and biases in reactor design calculations by progressively replacing existing multilevel averaging techniques with more direct solution methods based on explicit reactor geometries," said Andrew Siegel, leader of Argonne's reactor simulation group.

Officials said the code has run successfully in some of the world's fastest supercomputers, including the IBM Blue Gene at Argonne and the Cray XT5 at the Oak Ridge National Laboratory.