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One of the rare and brief bursts of cosmic radio waves that have puzzled astronomers since they were first detected nearly 10 years ago has finally been tied to a source: an older dwarf galaxy more than 3 billion light years from Earth.

Fast radio bursts, which flash for just a few milliseconds, created a stir among astronomers because they seemed to be coming from outside our galaxy, which means they would have to be very powerful to be seen from Earth, and because none of those first observed were ever seen again.

A repeating burst was discovered in 2012, however, providing an opportunity for a team of researchers to repeatedly monitor its area of the sky with the Karl Jansky Very Large Array in New Mexico and the Arecibo radio dish in Puerto Rico, in hopes of pinpointing its location.

Thanks to the development of high-speed data recording and real-time data analysis software by a University of California, Berkeley, astronomer, the VLA last year detected a total of nine bursts over a period of a month, sufficient to locate it within a tenth of an arcsecond. Subsequently, larger European and American radio interferometer arrays pinpointed it to within one-hundredth of an arcsecond, within a region about 100 light years in diameter.

Deep imaging of that region by the Gemini North Telescope in Hawaii turned up an optically faint dwarf galaxy that the VLA subsequently discovered also continuously emits low-level radio waves, typical of a galaxy with an active nucleus perhaps indicative of a central supermassive black hole. The galaxy has a low abundance of elements other than hydrogen and helium, suggestive of a galaxy that formed during the universe's middle age.

The origin of a fast radio burst in this type of dwarf galaxy suggests a connection to other energetic events that occur in similar dwarf galaxies, said co-author and UC Berkeley astronomer Casey Law, who led development of the data-acquisition system and created the analysis software to search for rapid, one-off bursts.

Extremely bright exploding stars, called superluminous supernovae, and long gamma ray bursts also occur in this type of galaxy, he noted, and both are hypothesized to be associated with massive, highly magnetic and rapidly rotating neutron stars called magnetars. Neutron stars are dense, compact objects created in supernova explosions, seen mostly as pulsars, because they emit periodic radio pulses as they spin.

"All these threads point to the idea that in this environment, something generates these magnetars," Law said. "It could be created by a superluminous supernova or a long gamma ray burst, and then later on, as it evolves and its rotation slows down a bit, it produces these fast radio bursts as well as continuous radio emission powered by that spindown. Later on in life, it looks like the magnetars we see in our galaxy, which have extremely strong magnetic fields but rotate more like ordinary pulsars."

In that interpretation, he said, fast radio bursts are like the tantrums of a toddler.

This is only one theory, however. There are many others, though the new data rule out several suggested explanations for the source of these bursts.

"We are the first to show that this is a cosmological phenomenon. It's not something in our backyard. And we are the first to see where this thing is happening, in this little galaxy, which I think is a surprise," Law said. "Now our objective is to figure out why that happens."

Law, team leader Shami Chatterjee of Cornell University and other astronomers on the team will present their findings today at the American Astronomical Society meeting in Grapevine, Texas, in the scientific journal Nature, and in two companion papers to appear in the Astrophysical Journal Letters.

Looking for transients

Fast radio bursts are highly-energetic -- though not energetic enough to blow a star apart -- and very short-lived, lasting one to five milliseconds. These bursts of radio waves have remained a mystery since the first one was discovered in 2007 by researchers scouring archived data from Australia's Parkes Radio Telescope in search of new pulsars. The burst they found occurred in 2001.

There now are 18 known fast radio bursts, all discovered using single-dish radio telescopes that are unable to pinpoint the object's location with sufficient precision to allow other observatories to identify its host environment or to find it at other wavelengths. The first and only known repeating burst, named FRB 121102, was discovered in the constellation Auriga in November of 2012 at the Arecibo Observatory in Puerto Rico, and has recurred numerous times.

Law has been working for the past few years on methods to quickly find transient radio bursts like these, which require collecting about one terabyte of data every hour. At the VLA, he currently uses 24 computer central processing units (CPUs) in parallel, both to record and search the data for brief radio bursts.

"The overall theme, first with the Allen Telescope Array and now with the VLA, is to use these interferometers as high-speed cameras, taking the sensitive imaging capabilty of the telescope, cranking up the data rate and improving our algorithms to get access to these millisecond time-scale transients," he said. "We really pushed hard to capture this terabyte-per-hour datastream reliably and set up a real-time platform for extracting these very faint fast bursts from that massive datastream."

The first burst was found in the data just a few hours after it was recorded on Aug. 23, Law said.

"We observed for about 40 hours earlier last year and saw nothing," he said. "Then we started a new campaign in the fall of 2016, and in our first observation we saw one. Then we observed for another 40 hours or so and saw eight more bursts. So this thing just suddenly turned on."

Law hopes soon to switch to 64 dedicated and more powerful GPUs -- graphics processing units -- so that real-time analysis is possible.

While Law has his pet hypothesis about the origin of fast radio bursts -- a magnetar surrounded by either material ejected by a supernova explosion or material ejected by a resulting pulsar -- there are other possibilities. One alternative is the galaxy's active nucleus, with radio emission coming from jets of material emitted from the region surrounding a supermassive black hole. The source of the fast radio burst is within 100 light years of the continuous radio emissions from the core of the galaxy, suggesting they are the same or physically associated with one another.

"Finding the host galaxy of this FRB, and its distance, is a big step forward, but we still have much more to do before we fully understand what these things are," Chatterjee said.

Other members of the team are the National Radio Astronomy Observatory, a facility of the National Science Foundation operated under a cooperative agreement by Associated Universities, Inc.; West Virginia University; McGill University in Montreal, Canada; and the Netherlands Institute for Radio Astronomy.

Science and Information / Role of supernovae in clocking the universe
« on: January 15, 2017, 08:06:36 PM »
How much light does a supernova shed on the history of universe?

New research by cosmologists at the University of Chicago and Wayne State University confirms the accuracy of Type Ia supernovae in measuring the pace at which the universe expands. The findings support a widely held theory that the expansion of the universe is accelerating and such acceleration is attributable to a mysterious force known as dark energy. The findings counter recent headlines that Type Ia supernova cannot be relied upon to measure the expansion of the universe.

Using light from an exploding star as bright as entire galaxies to determine cosmic distances led to the 2011 Nobel Prize in physics. The method relies on the assumption that, like lightbulbs of a known wattage, all Type Ia supernovae are thought to have nearly the same maximum brightness when they explode. Such consistency allows them to be used as beacons to measure the heavens. The weaker the light, the farther away the star. But the method has been challenged in recent years because of findings the light given off by Type Ia supernovae appears more inconsistent than expected.

"The data that we examined are indeed holding up against these claims of the demise of Type Ia supernovae as a tool for measuring the universe," said Daniel Scolnic, a postdoctoral scholar at UChicago's Kavli Institute for Cosmological Physics and co-author of the new research published in Monthly Notices of the Royal Astronomical Society. "We should not be persuaded by these other claims just because they got a lot of attention, though it is important to continue to question and strengthen our fundamental assumptions."

One of the latest criticisms of Type Ia supernovae for measurement concluded the brightness of these supernovae seems to be in two different subclasses, which could lead to problems when trying to measure distances. In the new research led by David Cinabro, a professor at Wayne State, Scolnic, Rick Kessler, a senior researcher at the Kavli Institute, and others, they did not find evidence of two subclasses of Type Ia supernovae in data examined from the Sloan Digital Sky Survey Supernovae Search and Supernova Legacy Survey. The recent papers challenging the effectiveness of Type Ia supernovae for measurement used different data sets.

A secondary criticism has focused on the way Type Ia supernovae are analyzed. When scientists found that distant Type Ia supernovae were fainter than expected, they concluded the universe is expanding at an accelerating rate. That acceleration is explained through dark energy, which scientists estimate makes up 70 percent of the universe. The enigmatic force pulls matter apart, keeping gravity from slowing down the expansion of the universe.

Yet a substance that makes up 70 percent of the universe but remains unknown is frustrating to a number of cosmologists. The result was a reevaluation of the mathematical tools used to analyze supernovae that gained attention in 2015 by arguing that Type Ia supernovae don't even show dark energy exists in the first place.

Scolnic and colleague Adam Riess, who won the 2011 Nobel Prices for the discovery of the accelerating universe, wrote an article for Scientific American Oct. 26, 2016, refuting the claims. They showed that even if the mathematical tools to analyze Type Ia supernovae are used "incorrectly," there is still a 99.7 percent chance the universe is accelerating.

The new findings are reassuring for researchers who use Type Ia supernovae to gain an increasingly precise understanding of dark energy, said Joshua A. Frieman, senior staff member at the Fermi National Accelerator Laboratory who was not involved in the research.

"The impact of this work will be to strengthen our confidence in using Type Ia supernovae as cosmological probes," he said.

On Monday, Aug. 21, 2017, millions in the U.S. will have their eyes to the sky as they witness a total solar eclipse. The moon's shadow will race across the United States, from Oregon to South Carolina. The path of this shadow, also known as the path of totality, is where observers will see the moon completely cover the sun. And thanks to elevation data of the moon from NASA's Lunar Reconnaissance Orbiter, or LRO, coupled with detailed NASA topography data of Earth, we have the most accurate maps of the path of totality for any eclipse to date.

Early map-making

Eclipse maps have long been used to plot the predicted path of the moon's shadow as it crosses the face of Earth. Friedrich Wilhelm Bessel and William Chauvenet, two prominent 19th century astronomers and mathematicians, developed the math still used to make eclipse maps -- long before computers and the precise astronomical data gathered during the Space Age.

Traditionally, eclipse calculations assume that all observers are at sea level and that the moon is a smooth sphere that is perfectly symmetrical around its center of mass. The calculations do not take into account different elevations on Earth and the moon's cratered, uneven surface.

For slightly more accurate maps, people use elevation tables and plots of the lunar limb -- the edge of the visible surface of the moon as seen from Earth. Until recently, astronomers have used the limb profiles published in 1963 by astronomer Chester Burleigh Watts to create eclipse maps of the moon's path of totality. To produce his profiles, Watts designed a machine that traced 700 photographs covering every angle of the moon visible from Earth.

However, eclipse calculations have gained even greater accuracy based on topography data from LRO observations.

A new look at an ancient phenomenon

Using LRO elevation maps, NASA visualizer Ernie Wright at Goddard Space Flight Center in Greenbelt, Maryland, created a continuously varying lunar limb profile as the moon's shadow passes over the United States as it will during the upcoming eclipse. The mountains and valleys along the edge of the moon's disk affect the timing and duration of totality by several seconds. Wright also used several NASA data sets to provide an elevation map of Earth so that eclipse observer locations were depicted at their true altitude.

The resulting visualizations show something never seen before: the true, time-varying shape of the moon's shadow, with the effects of both an accurate lunar limb and the Earth's terrain.

"We couldn't have done visualizations like this even 10 years ago," Wright said. "This is a confluence of increasing computing power and new datasets from remote sensing platforms like LRO and the Shuttle Radar Topography Mission."

The lunar umbra is the part of the moon's shadow where the entire sun is blocked by the moon. On an eclipse map, this tells you where to stand in order to experience totality. For centuries, eclipse maps have depicted the shape of the moon's umbra, or darkest part of its shadow, as a smooth ellipse.

As evidenced in the new visualizations, the umbral shape is dramatically altered by both the rugged lunar terrain and the elevations of observers on Earth.

"We've known for a while now about the effects of the lunar limb and the elevation of observers on the Earth, but this is the first time we've really seen it in this way," Wright said. "I think it'll change how people think about mapping eclipses."

The true shape of the umbra is more like an irregular polygon with slightly curved edges. Each edge corresponds to a single valley on the lunar limb, the last spot on the limb that lets sunlight through. As these edges pass over mountain ranges, they are scalloped by the peaks and valleys of the landscape. The moon's umbra will cross the Cascades, Rockies and Appalachians during the 2017 eclipse.

"Solar and lunar eclipses provide an excellent opportunity to talk about the moon, since without the moon there would be no eclipses," said Noah Petro, deputy project scientist for LRO. "Because we know the shape of the moon better than any other planetary body, thanks to LRO, we can now accurately predict the shape of the shadow as it falls on the face of the Earth. In this way, LRO data sheds new light on our predictions for the upcoming eclipse."

The total solar eclipse on Monday, Aug. 21, 2017 will cross the continental United States beginning in Oregon and ending in South Carolina. The last time a total solar eclipse spanned the United States was in 1918, when the path of totality entered through the southwest corner of Washington and passed over Denver, Colorado, Jackson, Mississippi, and Orlando, Florida before exiting the country at the Atlantic coast of Florida.

Science and Information / Astronomers discover cosmic double whammy
« on: January 15, 2017, 08:05:50 PM »
An international team of astronomers, including Lancaster's David Sobral, has discovered a cosmic one-two punch never seen before.

Two of the most powerful phenomena in the Universe -- a supermassive black hole and the collision of giant galaxy clusters -- have combined to create a stupendous cosmic particle accelerator.

By combining data from some of the best X-ray, optical and radio telescopes in the world, researchers have found out what happens when matter ejected by a giant black hole is swept up in the merger of two enormous galaxy clusters.

"We have seen each of these spectacular phenomena separately in many places," said Reinout van Weeren of the Harvard-Smithsonian Center for Astrophysics (CfA), who led the study that appears in the inaugural issue of the journal Nature Astronomy. "This is the first time, however, that we have seen them clearly linked together in the same system."

This cosmic double whammy is found in a pair of colliding galaxy clusters called Abell 3411 and Abell 3412 located about two billion light years from Earth.

The two clusters are both very massive, each weighing about a quadrillion - or a million billion - times the mass of the Sun.

Optical data from the Isaac Newton Telescope, in La Palma and Keck Observatory and Japan's Subaru telescope, also on Mauna Kea, Hawaii detected the galaxies in each cluster.

Dr David Sobral from Lancaster University said: "It was exciting to take deep images of this merger of galaxy clusters with the 2.5m Isaac Newton Telescope so we could then point the 10-m Keck telescope to the right galaxies to learn more. It turns out that giant telescopes can also "see further by standing on the shoulders" of smaller telescopes like the Isaac Newton."

The comet-shaped appearance is produced by hot gas from one cluster ploughing through the hot gas of the other cluster.

First, at least one spinning, supermassive black hole in one of the galaxy clusters produced a rotating magnetic funnel. The powerful electromagnetic fields associated with this structure have accelerated some of the inflowing gas away from the vicinity of the black hole in the form of an energetic, high-speed jet.

Then, these accelerated particles in the jet were accelerated to even higher energies when they encountered colossal shock waves - cosmic versions of sonic booms generated by supersonic aircraft - produced by the collision of the massive gas clouds associated with the galaxy clusters.

"It's almost like launching a rocket into low-Earth orbit and then getting shot out of the Solar System by a second rocket blast," said co-author Felipe Andrade-Santos, also of the CfA. "These particles are among the most energetic particles observed in the Universe, thanks to the double injection of energy."

These results were presented at the 229th meeting of the American Astronomical Society.

NASA's two Voyager spacecraft are hurtling through unexplored territory on their road trip beyond our solar system. Along the way, they are measuring the interstellar medium, the mysterious environment between stars. NASA's Hubble Space Telescope is providing the road map -- by measuring the material along the probes' future trajectories as they move through space. Even after the Voyagers run out of electrical power and are unable to send back new data, which may happen in about a decade, astronomers can use Hubble observations to characterize the environment through which these silent ambassadors will glide.

A preliminary analysis of the Hubble observations reveals a rich, complex interstellar ecology, containing multiple clouds of hydrogen laced with other elements. Hubble data, combined with the Voyagers, have also provided new insights into how our sun travels through interstellar space.

"This is a great opportunity to compare data from in situ measurements of the space environment by the Voyager spacecraft and telescopic measurements by Hubble," said study leader Seth Redfield of Wesleyan University in Middletown, Connecticut. "The Voyagers are sampling tiny regions as they plow through space at roughly 38,000 miles per hour. But we have no idea if these small areas are typical or rare. The Hubble observations give us a broader view because the telescope is looking along a longer and wider path. So Hubble gives context to what each Voyager is passing through."

The astronomers hope that the Hubble observations will help them characterize the physical properties of the local interstellar medium. "Ideally, synthesizing these insights with in situ measurements from Voyager would provide an unprecedented overview of the local interstellar environment," said Hubble team member Julia Zachary of Wesleyan University.

The team's results will be presented Jan. 6 at the winter meeting of the American Astronomical Society in Grapevine, Texas.

NASA launched the twin Voyager 1 and 2 spacecraft in 1977. Both explored the outer planets Jupiter and Saturn. Voyager 2 went on to visit Uranus and Neptune.

The pioneering Voyager spacecraft are currently exploring the outermost edge of the sun's domain. Voyager 1 is now zooming through interstellar space, the region between the stars that is filled with gas, dust, and material recycled from dying stars.

Voyager 1 is 13 billion miles from Earth, making it the farthest human-made object ever built. In about 40,000 years, after the spacecraft will no longer be operational and will not be able to gather new data, it will pass within 1.6 light-years of the star Gliese 445, in the constellation Camelopardalis. Its twin, Voyager 2, is 10.5 billion miles from Earth, and will pass 1.7 light-years from the star Ross 248 in about 40,000 years.

For the next 10 years, the Voyagers will be making measurements of interstellar material, magnetic fields, and cosmic rays along their trajectories. Hubble complements the Voyagers' observations by gazing at two sight lines along each spacecraft's path to map interstellar structure along their star-bound routes. Each sight line stretches several light-years to nearby stars. Sampling the light from those stars, Hubble's Space Telescope Imaging Spectrograph measured how interstellar material absorbed some of the starlight, leaving telltale spectral fingerprints.

Hubble found that Voyager 2 will move out of the interstellar cloud that surrounds the solar system in a couple thousand years. The astronomers, based on Hubble data, predict that the spacecraft will spend 90,000 years in a second cloud before passing into a third interstellar cloud.

An inventory of the clouds' composition reveals slight variations in the abundances of the chemical elements contained in the structures. "These variations could mean the clouds formed in different ways, or from different areas, and then came together," Redfield said.

An initial look at the Hubble data also suggests that the sun is passing through clumpier material in nearby space, which may affect the heliosphere, the large bubble containing our solar system that is produced by our sun's powerful solar wind. At its boundary, called the heliopause, the solar wind pushes outward against the interstellar medium. Hubble and Voyager 1 made measurements of the interstellar environment beyond this boundary, where the wind comes from stars other than our sun.

"I'm really intrigued by the interaction between stars and the interstellar environment," Redfield said. "These kinds of interactions are happening around most stars, and it is a dynamic process."

The heliosphere is compressed when the sun moves through dense material, but it expands back out when the star passes through low-density matter. This expansion and contraction is caused by the interaction between the outward pressure of the stellar wind, composed of a stream of charged particles, and the pressure of the interstellar material surrounding the star.

The Hubble Space Telescope is a project of international cooperation between NASA and the European Space Agency. NASA's Goddard Space Flight Center in Greenbelt, Maryland, manages the telescope. The Space Telescope Science Institute (STScI) in Baltimore, Maryland, conducts Hubble science operations. STScI is operated for NASA by the Association of Universities for Research in Astronomy in Washington, D.C. The Voyagers were built by the Jet Propulsion Laboratory (JPL), which continues to operate both spacecraft. JPL is a division of Caltech.

Science and Information / Understanding blended galaxies
« on: January 15, 2017, 08:04:50 PM »
In roughly four billion years, the Milky Way will be no more.

Indeed, our home galaxy is on course to collide and unite with the Andromeda Galaxy, at present some two million light years away.

Of course, we don't notice that the two galaxies are drawing closer together.

"To the human perspective, our galaxy doesn't appear to be changing," says University of Iowa astrophysicist Hai Fu, "but in the history of the universe, it is changing all the time."

Galaxies have been merging for most of the universe's 13-billion-year history, and scientists have been observing these mergers for some time. What they don't fully understand is how mergers occur.

Fu, an assistant professor in physics and astronomy, aims to clarify the phenomenon by observing supermassive black holes (with a mass of about one billion suns), which are at the center of most galaxies. Astrophysicists believe large galaxies grow by devouring smaller ones. In such cases, the black holes of both are expected to orbit each other and eventually merge. Fu and his team won a three-year, $405,011 grant from the National Science Foundation to find and characterize these celestial events.

"What we're trying to see is the late stages of merging galaxies, when two galaxies are so close together they unleash tidal forces of energy, kind of like the pulsing tidal forces caused when the sun and moon line up with Earth but much, much more intense," he says.

Fu will scan a large chunk of the night sky -- imagine the moon multiplied 1,200 across the sky and you'll have a sense of the size -- to find evidence of black holes' accretion, or mass-gathering.

"Pairs of galaxies with accreting black holes are rare and difficult to find," Fu says, "and that's why we need such a large area to survey."

Black holes aren't always accreting. But those that are resemble someone on an eating binge. Accreting black holes hungrily absorb material around them. Slowly, as they munch on more and more cosmic food, they pull their host galaxies closer together.

"They're no longer on a diet," Fu says.

All that eating unleashes a torrent of energy, intense bursts of light called quasars that are so bright they nearly obscure the galaxies themselves. Those quasars should be easy to observe, even at great distances, but most of the light they produce is actually extinguished by the dust brewed up in the merging activity.

Thankfully, supermassive black holes also emit radio waves, and those emissions "come to the rescue because they don't get extinguished by the dust," Fu says.

Fu and his team will examine radio-emission maps captured by the Very Large Array, one of the world's premier astronomical radio observatories, located in New Mexico and operated by the National Radio Astronomy Observatory, an NSF facility. The group will confirm its findings through optical observations at the W.M. Keck Observatory, located on Mauna Kea, a dormant volcano in Hawaii.

The NSF grant also will fund the student-led building of an "augmented reality sandbox" to demonstrate gravity's influence in the universe, such as on the orbits of planets, the accretion disk around a black hole or neutron star, and the complex orbits of stars in elliptically shaped galaxies.

Nine undergraduates have so far been involved in the project; they divided into teams to write the software programming, build the sandbox (with actual sand), and create an app for Android tablets.

The sandbox will be used in astronomy classes, physics demonstrations for K-12 students in the greater Iowa City area, and exhibitions at the UI Museum of Natural History and the UI Mobile Museum.

The sandbox is expected to be complete by the end of the spring 2017 semester.

"It is quite impressive," Fu says. "The students may not necessarily like taking exams, but they work really well in teams."

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Science and Information / Meteorites reveal lasting drought on Mars
« on: January 15, 2017, 08:01:40 PM »
The lack of liquid water on the surface of Mars today has been demonstrated by new evidence in the form of meteorites on the Red Planet examined by an international team of planetary scientists.

In a study led by the University of Stirling, an international team of researchers has found the lack of rust on the meteorites indicates that Mars is incredibly dry, and has been that way for millions of years.

The discovery, published in Nature Communications, provides vital insight into the planet's current environment and shows how difficult it would be for life to exist on Mars today.

Mars is a primary target in the search for life outside Earth, and liquid water is the most important pre-requisite for life.

Dr Christian Schröder, Lecturer in Environmental Science and Planetary Exploration at the University of Stirling and Science Team Collaborator for the Mars Exploration Rover Opportunity mission, said:

"Evidence shows that more than 3 billion years ago Mars was wet and habitable. However, this latest research reaffirms just how dry the environment is today. For life to exist in the areas we investigated, it would need to find pockets far beneath the surface, located away from the dryness and radiation present on the ground."

A study published last year, which used data from the Curiosity Rover investigating Gale crater on Mars, suggested that very salty liquid water might be able to condense in the top layers of Martian soil overnight.

"But, as our data show, this moisture is much less than the moisture present even in the driest places on Earth," explains Dr Schröder.

Using data from the Mars Exploration Rover Opportunity, the scientists examined a cluster of meteorites at Meridiani Planum -- a plain just south of the planet's equator and at a similar latitude to Gale crater.

Dr Schröder and his team have for the first time calculated a chemical weathering rate for Mars, in this case how long it takes for rust to form from the metallic iron present in meteorites.

This chemical weathering process depends on the presence of water. It takes at least 10 and possibly up to 10,000 times longer on Mars to reach the same levels of rust formation than in the driest deserts on Earth and points to the present-day extreme aridity that has persisted on Mars for millions of years.

High above the surface, Earth's magnetic field constantly deflects incoming supersonic particles from the sun. These particles are disturbed in regions just outside of Earth's magnetic field -- and some are reflected into a turbulent region called the foreshock. New observations from NASA's THEMIS mission show that this turbulent region can accelerate electrons up to speeds approaching the speed of light. Such extremely fast particles have been observed in near-Earth space and many other places in the universe, but the mechanisms that accelerate them have not yet been concretely understood.

The new results provide the first steps towards an answer, while opening up more questions. The research finds electrons can be accelerated to extremely high speeds in a region farther from Earth than previously thought possible -- leading to new inquiries about what causes the acceleration. These findings may change the accepted theories on how electrons can be accelerated not only in shocks near Earth, but also throughout the universe. Having a better understanding of how particles are energized will help scientists and engineers better equip spacecraft and astronauts to deal with these particles, which can cause equipment to malfunction and affect space travelers.

"This affects pretty much every field that deals with high-energy particles, from studies of cosmic rays to solar flares and coronal mass ejections, which have the potential to damage satellites and affect astronauts on expeditions to Mars," said Lynn Wilson, lead author of the paper on these results at NASA's Goddard Space Flight Center in Greenbelt, Maryland.

The results, published in Physical Review Letters on Nov. 14, 2016, describe how such particles may get accelerated in specific regions just beyond Earth's magnetic field. Typically, a particle streaming toward Earth first encounters a boundary region known as the bow shock, which forms a protective barrier between the sun and Earth. The magnetic field in the bow shock slows the particles, causing most to be deflected away from Earth, though some are reflected back towards the sun. These reflected particles form a region of electrons and ions called the foreshock region.

Some of those particles in the foreshock region are highly energetic, fast moving electrons and ions. Historically, scientists have thought one way these particles get to such high energies is by bouncing back and forth across the bow shock, gaining a little extra energy from each collision. However, the new observations suggest the particles can also gain energy through electromagnetic activity in the foreshock region itself.

The observations that led to this discovery were taken from one of the THEMIS -- short for Time History of Events and Macroscale Interactions during Substorms -- mission satellites. The five THEMIS satellites circled Earth to study how the planet's magnetosphere captured and released solar wind energy, in order to understand what initiates the geomagnetic substorms that cause aurora. The THEMIS orbits took the spacecraft across the foreshock boundary regions. The primary THEMIS mission concluded successfully in 2010 and now two of the satellites collect data in orbit around the moon.

Operating between the sun and Earth, the spacecraft found electrons accelerated to extremely high energies. The accelerated observations lasted less than a minute, but were much higher than the average energy of particles in the region, and much higher than can be explained by collisions alone. Simultaneous observations from the Wind and STEREO spacecraft showed no solar radio bursts or interplanetary shocks, so the high-energy electrons did not originate from solar activity.

"This is a puzzling case because we're seeing energetic electrons where we don't think they should be, and no model fits them," said David Sibeck, co-author and THEMIS project scientist at NASA Goddard. "There is a gap in our knowledge, something basic is missing."

The electrons also could not have originated from the bow shock, as had been previously thought. If the electrons were accelerated in the bow shock, they would have a preferred movement direction and location -- in line with the magnetic field and moving away from the bow shock in a small, specific region. However, the observed electrons were moving in all directions, not just along magnetic field lines. Additionally, the bow shock can only produce energies at roughly one tenth of the observed electrons' energies. Instead, the cause of the electrons' acceleration was found to be within the foreshock region itself.

"It seems to suggest that incredibly small scale things are doing this because the large scale stuff can't explain it," Wilson said.

High-energy particles have been observed in the foreshock region for more than 50 years, but until now, no one had seen the high-energy electrons originate from within the foreshock region. This is partially due to the short timescale on which the electrons are accelerated, as previous observations had averaged over several minutes, which may have hidden any event. THEMIS gathers observations much more quickly, making it uniquely able to see the particles.

Next, the researchers intend to gather more observations from THEMIS to determine the specific mechanism behind the electrons' acceleration.

Science and Information / Great valley found on Mercury
« on: January 15, 2017, 08:00:57 PM »
Scientists have discovered a new large valley on Mercury that may be the first evidence of buckling of the planet's outer silicate shell in response to global contraction. The researchers discovered the valley using a new high-resolution topographic map of part of Mercury's southern hemisphere created by stereo images from NASA's MESSENGER spacecraft. The findings were reported in a new study published in Geophysical Research Letters, a journal of the American Geophysical Union.

The most likely explanation for Mercury's Great Valley is buckling of the planet's lithosphere -- its crust and upper mantle -- in response to global contraction, according to the study's authors. Earth's lithosphere is broken up into many tectonic plates, but Mercury's lithosphere consists of just one plate. Cooling of Mercury's interior caused the planet's single plate to contract and bend. Where contractional forces are greatest, crustal rocks are thrust upward while an emerging valley floor sags downward.

"There are examples of lithospheric buckling on Earth involving both oceanic and continental plates, but this may be the first evidence of lithospheric buckling on Mercury," said Thomas R. Watters, senior scientist at the Center for Earth and Planetary Studies at the Smithsonian's National Air and Space Museum in Washington, D.C., and lead author of the new study.

The valley is about 400 kilometers (250 miles) wide with its floor as much as 3 kilometers (2 miles) below the surrounding terrain. The valley is more than 1,000 kilometers (600 miles) long and extends into the Rembrandt basin, one of the largest and youngest impact basins on Mercury.

The valley is bound by two large fault scarps -- steps on the planet's surface where one side of a fault has moved vertically with respect to the other. Mercury's contraction caused the fault scarps bounding the Great Valley to become so large they essentially became cliffs. The elevation of the valley floor is far below the terrain surrounding the mountainous faults scarps, which suggests the valley floor was lowered by the same mechanism that formed the scarps themselves, according to the study authors.

"Unlike Earth's Great Rift Valley in East Africa, Mercury's Great Valley is not caused by the pulling apart of lithospheric plates due to plate tectonics; it is the result of the global contraction of a shrinking one-plate planet," Watters said. "Even though you might expect lithospheric buckling on a one-plate planet that is contracting, it is still a surprise when you find that it's formed a great valley that includes the largest fault scarp and one of the largest impact basins on Mercury."

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Sputnik Planitia, a 1,000-kilometer-wide basin within the iconic heart-shaped region observed on Pluto's surface, could be in its present location because accumulation of ice made the dwarf planet roll over, creating cracks and tensions in the crust that point toward the presence of a subsurface ocean.

Published in the Nov. 17 issue of Nature, these are the conclusions of research by James Keane, a doctoral student at the University of Arizona's Lunar and Planetary Laboratory, and his adviser, assistant professor Isamu Matsuyama. They propose evidence of frozen nitrogen pileup throwing the entire planet off kilter, much like a spinning top with a wad of gum stuck to it, in a process called true polar wander.

"There are two ways to change the spin of a planet," Keane said. "The first -- and the one we're all most familiar with -- is a change in the planet's obliquity, where the spin axis of the planet is reorienting with respect to the rest of the solar system. The second way is through true polar wander, where the spin axis remains fixed with respect to the rest of the solar system, but the planet reorients beneath it."

Planets like to spin in such a way that minimizes energy. In short, this means that planets like to reorient to place any extra mass closer to the equator -- and any mass deficits closer to the pole. For example, if a giant volcano were to grow on Los Angeles, Earth would reorient itself to place L.A. on the equator.

To understand polar wander on Pluto, one first has to realize that unlike Earth, whose spin axis is only slightly tilted so that the regions around the equator receive the most sunlight, Pluto is like a spinning top lying on its side. Therefore, the planet's poles get the most sunlight. Depending on the season, it's either one or the other, while Pluto's equatorial regions are extremely cold, all the time.

Because Pluto is almost 40 times farther from the sun than we are, it takes the little ball of rock and ice 248 Earth-years to complete one of its own years. At Pluto's lower latitudes near the equator, temperatures are almost as cold as minus 400 degrees Fahrenheit -- cold enough to turn nitrogen into a frozen solid.

Over the course of a Pluto year, nitrogen and other exotic gases condense on the permanently shadowed regions, and eventually, as Pluto goes around the sun, those frozen gases heat up, become gaseous again and re-condense on the other side of the planet, resulting in seasonal "snowfall" on Sputnik Planitia.

"Each time Pluto goes around the sun, a bit of nitrogen accumulates in the heart," Keane said. "And once enough ice has piled up, maybe a hundred meters thick, it starts to overwhelm the planet's shape, which dictates the planet's orientation. And if you have an excess of mass in one spot on the planet, it wants to go to the equator. Eventually, over millions of years, it will drag the whole planet over."

In a sense, Pluto is a (dwarf) planet whose shape and position in space are controlled by its weather.

"I think this idea of a whole planet being dragged around by the cycling of volatiles is not something many people had really thought about before," Keane said.

The two researchers used observations made during New Horizons' flyby and combined them with computer models that allowed them to take a surface feature such as Sputnik Planitia, shift it around on the planet's surface and see what that does to the planet's spin axis. And sure enough, in the models, the geographic location of Sputnik Planitia ended up suspiciously close to where one would expect it to be.

If Sputnik Planitia were a large positive mass anomaly -- perhaps due to loading of nitrogen ice -- it would naturally migrate to Pluto's tidal axis with regard to Charon, Pluto's largest moon, as it approaches a minimum energy state, according to Keane and Matsuyama. In other words, the massive accumulation of ice would end up where it causes the least wobble in Pluto's spin axis.

This phenomenon of polar wander is something that was discovered with Earth's moon and with Mars, as well, but in those cases it happened in the distant past, billions of years ago.

"On Pluto, those processes are currently active," Keane said. "Its entire geology -- glaciers, mountains, valleys -- seems to be linked to volatile processes. That's different from most other planets and moons in our solar system."

And not only that, the simulations and calculations also predicted that the accumulation of frozen volatiles in Pluto's heart would cause cracks and faults in the planet's surface in the exact same locations where New Horizons saw them.

The presence of tectonic faults on Pluto hint at the existence of a subsurface ocean at some point in Pluto's history, Keane explained.

"It's like freezing ice cubes," he said. "As the water turns to ice, it expands. On a planetary scale, this process breaks the surface around the planet and creates the faults we see today."

The paper is published alongside a report by Francis Nimmo of the University of California, Santa Cruz, and colleagues, who also consider the implications for Pluto's apparent reorientation. The authors of that paper agree with the idea that tidal forces could explain the current location of Sputnik Planitia, but in order for their model to work, a subsurface ocean would have to be present on Pluto today.

Both publications underscore the notion of a surprisingly active Pluto.

"Before New Horizons, people usually only thought of volatiles in terms of a thin frost veneer, a surface effect that might change the color, or affect local or regional geology," Keane said. "That the movement of volatiles and shifting ice around a planet could have a dramatic, planet-moving effect is not something anyone would have predicted."

Co-authors on the research paper are Shunichi Kamata of the Creative Research Institution, Hokkaido University, Sapporo, Japan, and Jordan Steckloff of Purdue University in West Lafayette, Indiana, and the Planetary Science Institute in Tucson, Arizona.

A liquid ocean lying deep beneath Pluto's frozen surface is the best explanation for features revealed by NASA's New Horizons spacecraft, according to a new analysis. The idea that Pluto has a subsurface ocean is not new, but the study provides the most detailed investigation yet of its likely role in the evolution of key features such as the vast, low-lying plain known as Sputnik Planitia (formerly Sputnik Planum).

Sputnik Planitia, which forms one side of the famous heart-shaped feature seen in the first New Horizons images, is suspiciously well aligned with Pluto's tidal axis. The likelihood that this is just a coincidence is only 5 percent, so the alignment suggests that extra mass in that location interacted with tidal forces between Pluto and its moon Charon to reorient Pluto, putting Sputnik Planitia directly opposite the side facing Charon. But a deep basin seems unlikely to provide the extra mass needed to cause that kind of reorientation.

"It's a big, elliptical hole in the ground, so the extra weight must be hiding somewhere beneath the surface. And an ocean is a natural way to get that," said Francis Nimmo, professor of Earth and planetary sciences at UC Santa Cruz and first author of a paper on the new findings published November 16 in Nature. Another paper in the same issue, led by James Keane at the University of Arizona, also argues for reorientation and points to fractures on Pluto as evidence that this happened.

Impact basin

Like other large basins in the solar system, Sputnik Planitia was most likely created by the impact of a giant meteorite, which would have blasted away a huge amount of Pluto's icy crust. With a subsurface ocean, the response to this would be an upwelling of water pushing up against the thinned and weakened crust of ice. At equilibrium, because water is denser than ice, that would still leave a fairly deep basin with a thin crust of ice over the upwelled mass of water.

"At that point, there is no extra mass at Sputnik Planitia," Nimmo explained. "What happens then is the ice shell gets cold and strong, and the basin fills with nitrogen ice. That nitrogen represents the excess mass."

Nimmo and his colleages also considered whether the extra mass could be provided by just a deep crater filled with nitrogen ice, with no upwelling of a subsurface ocean. But their calculations showed that this would require an implausibly deep layer of nitrogen, more than 25 miles (40 kilometers) thick. They found that a nitrogen layer about 4 miles (7 km) thick above a subsurface ocean provides enough mass to create a "positive gravity anomaly" consistent with the observations.

"We tried to think of other ways to get a positive gravity anomaly, and none of them look as likely as a subsurface ocean," Nimmo said.

Moon anomalies

Coauthor Douglas Hamilton of the University of Maryland came up with the reorientation hypothesis, and Nimmo developed the subsurface ocean scenario. The scenario is analogous to what occurred on the moon, where positive gravity anomalies have been accurately measured for several large impact basins. Instead of a subsurface ocean, however, the dense mantle material beneath the moon's crust pushed up against the thinned crust of the impact basins. Lava flows then flooded the basins, adding the extra mass. On icy Pluto, the basin filled with frozen nitrogen.

"There's plenty of nitrogen in Pluto's atmosphere, and either it preferentially freezes out in this low basin, or it freezes out in the high areas surrounding the basin and flows down as glaciers," Nimmo said. The images from New Horizons do show what appear to be nitrogen glaciers flowing out of mountainous terrain around Sputnik Planitia.

As for the subsurface ocean, Nimmo said he suspects it is mostly water with some kind of antifreeze in it, probably ammonia. The slow refreezing of the ocean would put stress on the icy shell, causing fractures consistent with features seen in the New Horizons images.

There are other large objects in the Kuiper belt that are similar to Pluto in size and density, and Nimmo said they probably also have subsurface oceans. "When we look at these other objects, they may be equally interesting, not just frozen snowballs," he said.

In addition to Nimmo and Hamilton, the coauthors include researchers at six other institutions and the New Horizons Geology, Geophysics and Imaging Team led by NASA scientist Jeffrey Moore. This research is supported by NASA.

Frozen beneath a region of cracked and pitted plains on Mars lies about as much water as what's in Lake Superior, largest of the Great Lakes, researchers using NASA's Mars Reconnaissance Orbiter have determined.

Scientists examined part of Mars' Utopia Planitia region, in the mid-northern latitudes, with the orbiter's ground-penetrating Shallow Radar (SHARAD) instrument. Analyses of data from more than 600 overhead passes with the onboard radar instrument reveal a deposit more extensive in area than the state of New Mexico. The deposit ranges in thickness from about 260 feet (80 meters) to about 560 feet (170 meters), with a composition that's 50 to 85 percent water ice, mixed with dust or larger rocky particles.

At the latitude of this deposit -- about halfway from the equator to the pole -- water ice cannot persist on the surface of Mars today. It sublimes into water vapor in the planet's thin, dry atmosphere. The Utopia deposit is shielded from the atmosphere by a soil covering estimated to be about 3 to 33 feet (1 to 10 meters) thick.

"This deposit probably formed as snowfall accumulating into an ice sheet mixed with dust during a period in Mars history when the planet's axis was more tilted than it is today," said Cassie Stuurman of the Institute for Geophysics at the University of Texas, Austin. She is the lead author of a report in the journal Geophysical Research Letters.

Mars today, with an axial tilt of 25 degrees, accumulates large amounts of water ice at the poles. In cycles lasting about 120,000 years, the tilt varies to nearly twice that much, heating the poles and driving ice to middle latitudes. Climate modeling and previous findings of buried, mid-latitude ice indicate that frozen water accumulates away from the poles during high-tilt periods.

Martian Water as a Future Resource

The name Utopia Planitia translates loosely as the "plains of paradise." The newly surveyed ice deposit spans latitudes from 39 to 49 degrees within the plains. It represents less than one percent of all known water ice on Mars, but it more than doubles the volume of thick, buried ice sheets known in the northern plains. Ice deposits close to the surface are being considered as a resource for astronauts.

"This deposit is probably more accessible than most water ice on Mars, because it is at a relatively low latitude and it lies in a flat, smooth area where landing a spacecraft would be easier than at some of the other areas with buried ice," said Jack Holt of the University of Texas, a co-author of the Utopia paper who is a SHARAD co-investigator and has previously used radar to study Martian ice in buried glaciers and the polar caps.

The Utopian water is all frozen now. If there were a melted layer -- which would be significant for the possibility of life on Mars -- it would have been evident in the radar scans. However, some melting can't be ruled out during different climate conditions when the planet's axis was more tilted. "Where water ice has been around for a long time, we just don't know whether there could have been enough liquid water at some point for supporting microbial life," Holt said.

Utopia Planitia is a basin with a diameter of about 2,050 miles (3,300 kilometers), resulting from a major impact early in Mars' history and subsequently filled. NASA sent the Viking 2 Lander to a site near the center of Utopia in 1976. The portion examined by Stuurman and colleagues lies southwest of that long-silent lander.

Use of the Italian-built SHARAD instrument for examining part of Utopia Planitia was prompted by Gordon Osinski at Western University in Ontario, Canada, a co-author of the study. For many years, he and other researchers have been intrigued by ground-surface patterns there such as polygonal cracking and rimless pits called scalloped depressions -- "like someone took an ice-cream scoop to the ground," said Stuurman, who started this project while a student at Western.

Clue from Canada

In the Canadian Arctic, similar landforms are indicative of ground ice, Osinski noted, "but there was an outstanding question as to whether any ice was still present at the Martian Utopia or whether it had been lost over the millions of years since the formation of these polygons and depressions."

The large volume of ice detected with SHARAD advances understanding about Mars' history and identifies a possible resource for future use.

"It's important to expand what we know about the distribution and quantity of Martian water," said Mars Reconnaissance Orbiter Deputy Project Scientist Leslie Tamppari, of NASA's Jet Propulsion Laboratory, Pasadena, California. "We know early Mars had enough liquid water on the surface for rivers and lakes. Where did it go? Much of it left the planet from the top of the atmosphere. Other missions have been examining that process. But there's also a large quantity that is now underground ice, and we want to keep learning more about that."

Joe Levy of the University of Texas, a co-author of the new study, said, "The ice deposits in Utopia Planitia aren't just an exploration resource, they're also one of the most accessible climate change records on Mars. We don't understand fully why ice has built up in some areas of the Martian surface and not in others. Sampling and using this ice with a future mission could help keep astronauts alive, while also helping them unlock the secrets of Martian ice ages."

SHARAD is one of six science instruments on the Mars Reconnaissance Orbiter, which began its prime science phase 10 years ago this month. The mission's longevity is enabling studies of features and active processes all around Mars, from subsurface to upper atmosphere. The Italian Space Agency provided the SHARAD instrument and Sapienza University of Rome leads its operations. The Planetary Science Institute, based in Tucson, Arizona, leads U.S. involvement in SHARAD. JPL, a division of Caltech in Pasadena, manages the orbiter mission for NASA's Science Mission Directorate in Washington. Lockheed Martin Space Systems of Denver built the spacecraft and supports its operations.

A research team led by University of Minnesota School of Physics and Astronomy Professor Yong-Zhong Qian uses new models and evidence from meteorites to show that a low-mass supernova triggered the formation of our solar system.

The findings are published in the most recent issue of Nature Communications.

About 4.6 billion years ago, a cloud of gas and dust that eventually formed our solar system was disturbed. The ensuing gravitational collapse formed the proto-Sun with a surrounding disc where the planets were born. A supernova -- a star exploding at the end of its life-cycle -- would have enough energy to compress such a gas cloud. Yet there was no conclusive evidence to support this theory. In addition, the nature of the triggering supernova remained elusive.

Qian and his collaborators decided to focus on short-lived nuclei present in the early solar system. Due to their short lifetimes, these nuclei could only have come from the triggering supernova. Their abundances in the early solar system have been inferred from their decay products in meteorites. As the debris from the formation of the solar system, meteorites are comparable to the leftover bricks and mortar in a construction site. They tell us what the solar system is made of and in particular, what short-lived nuclei the triggering supernova provided.

"This is the forensic evidence we need to help us explain how the solar system was formed," Qian said. "It points to a low-mass supernova as the trigger."

Qian is an expert on the formation of nuclei in supernovae. His previous research has focused on the various mechanisms by which this occurs in supernovae of different masses. His team includes the lead author of the paper, Projjwal Banerjee, who is a former Ph.D. student and postdoctoral research associate, and longtime collaborators Alexander Heger of Monash University, Australia, and Wick Haxton of the University of California, Berkeley. Qian and Banerjee realized that previous efforts in studying the formation of the solar system were focused on a high-mass supernova trigger, which would have left behind a set of nuclear fingerprints that are not present in the meteoric record.

Qian and his collaborators decided to test whether a low-mass supernova, about 12 times heavier than our sun, could explain the meteoritic record. They began their research by examining Beryllium-10, a short-lived nucleus that has 4 protons (hence the fourth element in the periodic table) and 6 neutrons, weighing 10 mass units. This nucleus is widely distributed in meteorites.

In fact the ubiquity of Beryllium-10 was something of a mystery in and of itself. Many researchers had theorized that spallation -- a process where high-energy particles strip away protons or neutrons from a nucleus to form new nuclei -- by cosmic rays was responsible for the Beryllium-10 found in meteorites. Qian said that this hypothesis involves many uncertain inputs and presumes that Beryllium-10 cannot be made in supernovae.

Using new models of supernovae, Qian and his collaborators have shown that Beryllium-10 can be produced by neutrino spallation in supernovae of both low and high masses. However, only a low-mass supernova triggering the formation of the solar system is consistent with the overall meteoritic record.

"The findings in this paper have opened up a whole new direction in our research," Qian said. "In addition to explaining the abundance of Beryllium-10, this low-mass supernova model would also explain the short-lived nuclei Calcium-41, Palladium-107, and a few others found in meteorites. What it cannot explain must then be attributed to other sources that require detailed study."

Qian said the group would like to examine the remaining mysteries surrounding short-lived nuclei found in meteorites. The first step, however is to further corroborate their theory by looking at Lithium-7 and Boron-11 that are produced along with Beryllium-10 by neutrino spallation in supernovae. Qian said they may examine this in a future paper and urged researchers studying meteorites look at the correlations among these three nuclei with precise measurements.

A thrilling ride is about to begin for NASA's Cassini spacecraft. Engineers have been pumping up the spacecraft's orbit around Saturn this year to increase its tilt with respect to the planet's equator and rings. And on Nov. 30, following a gravitational nudge from Saturn's moon Titan, Cassini will enter the first phase of the mission's dramatic endgame.

Launched in 1997, Cassini has been touring the Saturn system since arriving there in 2004 for an up-close study of the planet, its rings and moons. During its journey, Cassini has made numerous dramatic discoveries, including a global ocean within Enceladus and liquid methane seas on Titan.

Between Nov. 30 and April 22, Cassini will circle high over and under the poles of Saturn, diving every seven days -- a total of 20 times -- through the unexplored region at the outer edge of the main rings.

"We're calling this phase of the mission Cassini's Ring-Grazing Orbits, because we'll be skimming past the outer edge of the rings," said Linda Spilker, Cassini project scientist at NASA's Jet Propulsion Laboratory, Pasadena, California. "In addition, we have two instruments that can sample particles and gases as we cross the ringplane, so in a sense Cassini is also 'grazing' on the rings."

On many of these passes, Cassini's instruments will attempt to directly sample ring particles and molecules of faint gases that are found close to the rings. During the first two orbits, the spacecraft will pass directly through an extremely faint ring produced by tiny meteors striking the two small moons Janus and Epimetheus. Ring crossings in March and April will send the spacecraft through the dusty outer reaches of the F ring.

"Even though we're flying closer to the F ring than we ever have, we'll still be more than 4,850 miles (7,800 kilometers) distant. There's very little concern over dust hazard at that range," said Earl Maize, Cassini project manager at JPL.

The F ring marks the outer boundary of the main ring system; Saturn has several other, much fainter rings that lie farther from the planet. The F ring is complex and constantly changing: Cassini images have shown structures like bright streamers, wispy filaments and dark channels that appear and develop over mere hours. The ring is also quite narrow -- only about 500 miles (800 kilometers) wide. At its core is a denser region about 30 miles (50 kilometers) wide.

So Many Sights to See

Cassini's ring-grazing orbits offer unprecedented opportunities to observe the menagerie of small moons that orbit in or near the edges of the rings, including best-ever looks at the moons Pandora, Atlas, Pan and Daphnis.

Grazing the edges of the rings also will provide some of the closest-ever studies of the outer portions of Saturn's main rings (the A, B and F rings). Some of Cassini's views will have a level of detail not seen since the spacecraft glided just above them during its arrival in 2004. The mission will begin imaging the rings in December along their entire width, resolving details smaller than 0.6 mile (1 kilometer) per pixel and building up Cassini's highest-quality complete scan of the rings' intricate structure.

The mission will continue investigating small-scale features in the A ring called "propellers," which reveal the presence of unseen moonlets. Because of their airplane propeller-like shapes, scientists have given some of the more persistent features informal names inspired by famous aviators, including "Earhart." Observing propellers at high resolution will likely reveal new details about their origin and structure.

And in March, while coasting through Saturn's shadow, Cassini will observe the rings backlit by the sun, in the hope of catching clouds of dust ejected by meteor impacts.

Preparing for the Finale

During these orbits, Cassini will pass as close as about 56,000 miles (90,000 kilometers) above Saturn's cloud tops. But even with all their exciting science, these orbits are merely a prelude to the planet-grazing passes that lie ahead. In April 2017, the spacecraft will begin its Grand Finale phase.

After nearly 20 years in space, the mission is drawing near its end because the spacecraft is running low on fuel. The Cassini team carefully designed the finale to conduct an extraordinary science investigation before sending the spacecraft into Saturn to protect its potentially habitable moons.

During its grand finale, Cassini will pass as close as 1,012 miles (1,628 kilometers) above the clouds as it dives repeatedly through the narrow gap between Saturn and its rings, before making its mission-ending plunge into the planet's atmosphere on Sept. 15. But before the spacecraft can leap over the rings to begin its finale, some preparatory work remains.

To begin with, Cassini is scheduled to perform a brief burn of its main engine during the first super-close approach to the rings on Dec. 4. This maneuver is important for fine-tuning the orbit and setting the correct course to enable the remainder of the mission.

"This will be the 183rd and last currently planned firing of our main engine. Although we could still decide to use the engine again, the plan is to complete the remaining maneuvers using thrusters," said Maize.

To further prepare, Cassini will observe Saturn's atmosphere during the ring-grazing phase of the mission to more precisely determine how far it extends above the planet. Scientists have observed Saturn's outermost atmosphere to expand and contract slightly with the seasons since Cassini's arrival. Given this variability, the forthcoming data will be important for helping mission engineers determine how close they can safely fly the spacecraft.

For details about Cassini's Ring-Grazing Orbits, including timing, closest approach distances and highlights, visit:

The Cassini-Huygens mission is a cooperative project of NASA, ESA (European Space Agency) and the Italian Space Agency. NASA's Jet Propulsion Laboratory, a division of Caltech in Pasadena, manages the mission for NASA's Science Mission Directorate, Washington. JPL designed, developed and assembled the Cassini orbiter.

More information about Cassini:

Pluto's "icy heart" is a bright, two-lobed feature on its surface that has attracted researchers ever since its discovery by the NASA New Horizons team in 2015. Of particular interest is the heart's western lobe, informally named Sputnik Planitia, a deep basin containing three kinds of ices -- frozen nitrogen, methane and carbon monoxide -- and appearing opposite Charon, Pluto's tidally locked moon. Sputnik Planitia's unique attributes have spurred a number of scenarios for its formation, all of which identify the feature as an impact basin, a depression created by a smaller body striking Pluto at extremely high speed.

A new study led by Douglas Hamilton, professor of astronomy at the University of Maryland, instead suggests that Sputnik Planitia formed early in Pluto's history and that its attributes are inevitable consequences of evolutionary processes. The study was published in the journal Nature on December 1, 2016.

"The main difference between my model and others is that I suggest that the ice cap formed early, when Pluto was still spinning quickly, and that the basin formed later and not from an impact," said Hamilton, who is lead author of the paper. "The ice cap provides a slight asymmetry that either locks toward or away from Charon when Pluto's spin slows to match the orbital motion of the moon."

Using a model he developed, Hamilton found that the initial location of Sputnik Planitia could be explained by Pluto's unusual climate and its spin axis, which is tilted by 120 degrees. For comparison, Earth's tilt is 23.5 degrees. Modeling the dwarf planet's temperatures showed that when averaged over Pluto's 248-year orbit, the 30 degrees north and south latitudes emerged as the coldest places on the dwarf planet, far colder than either pole. Ice would have naturally formed around these latitudes, including at the center of Sputnik Planitia, which is located at 25 degrees north latitude.

Hamilton's model also showed that a small ice deposit naturally attracts more ices by reflecting away solar light and heat. Temperatures remain low, which attracts more ice and keeps the temperature low, and the cycle repeats. This positive feedback phenomenon, called the runaway albedo effect, would eventually lead to a single dominating ice cap, like the one observed on Pluto. However, Pluto's basin is significantly larger than the volume of ice it contains today, suggesting that Pluto's heart has been slowly losing mass over time, almost as if it was wasting away.

Even so, the single ice cap represents an enormous weight on Pluto's surface, enough to shift the dwarf planet's center of mass. Pluto's rotation slowed gradually due to gravitational forces from Charon, just as Earth is slowly losing spin under similar forces from its moon. However, because Charon is so large and so close to Pluto, the process led to Pluto locking one face toward its moon in just a few million years. The large mass of Sputnik Planitia would have had a 50 percent chance of either facing Charon directly or turning as far away from the moon as possible.

"It is like a Vegas slot machine with just two states, and Sputnik Planitia ended up in the latter position, centered at 175 degrees longitude," said Hamilton.

It would also be easy for the accumulated ice to create its own basin, simply by pushing down, according to Hamilton.

"Pluto's big heart weighs heavily on the small planet, leading inevitably to depression," said Hamilton, noting that the same phenomenon happens on Earth: the Greenland Ice Sheet created a basin and pushed down the crust that it rests upon.

While Hamilton's model can explain both the latitude and longitude of Sputnik Planitia, as well as the fact that the ices exist in a basin, several other models were also presented in the December 1, 2016 issue of the journal Nature.

In one of those papers, UC Santa Cruz Professor of Earth and Planetary Sciences Francis Nimmo, Hamilton and their co-authors modeled how Sputnik Planitia may have formed if its basin was caused by an impact, such as the one that created Charon. Their results showed that the basin may have formed after Pluto slowed its rotation, migrating only slightly to its present location. If this late formation scenario proves correct, the properties of Sputnik Planitia may hint at the presence of a subsurface ocean on Pluto.

"Either model is viable under the right conditions," said Hamilton. "While we cannot conclude definitively that there is an ocean under Pluto's icy shell, we also cannot state that there is not one."

Although Pluto was stripped of its status as a planet, an ice cap is a surprisingly Earth-like property. In fact, Pluto is only the third body -- Earth and Mars being the others -- known to possess an ice cap. The ices of Sputnik Planitia may therefore offer hints relevant to more familiar ices here on Earth.

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