|Studies Offer New Glimpse of Melting Under Antarctic Glaciers|
Fast Facts:Two new studies by researchers at NASA and the University of California, Irvine (UCI), detect the fastest ongoing rates of glacier retreat ever observed in West Antarctica and offer an unprecedented direct view of intense ice melting from the floating undersides of glaciers. The results highlight how the interaction between ocean conditions and the bedrock beneath a glacier can influence the glacier's evolution, with implications for understanding future ice loss from Antarctica and global sea level rise.
› Related studies of three West Antarctic glaciers have measured the intense melting on their floating undersides and how fast the glaciers are coming unstuck from bedrock.
› The fastest-changing glacier of the three (Smith Glacier) is melting nearly six times as fast as a previous estimate for this region, losing up to 230 feet in ice thickness each year.
› Smith's fast retreat and thinning are likely related to the shape of its underlying bedrock. The other two glaciers studied are on differently shaped beds and are retreating more slowly.
The two studies examined three neighboring glaciers in West Antarctica that are melting and retreating at different rates. Smith, Pope and Kohler glaciers flow into the Dotson and Crosson ice shelves in the Amundsen Sea Embayment in West Antarctica, the part of the continent with the largest loss of ice mass.
A study led by Bernd Scheuchl of UCI, published in the journal Geophysical Research Letters on Aug. 28, used radar measurements from the European Space Agency's Sentinel-1 satellite and data from the earlier ERS-1 and ERS-2 satellites to look at changes in the glaciers' grounding lines -- the boundary where a glacier loses contact with bedrock and begins to float on the ocean. The grounding line is important because nearly all glacier melting takes place on the underside of the glacier's floating portion, called the ice shelf. If a glacier loses mass from enhanced melting, it may start floating farther inland from its former grounding line, just as a boat stuck on a sandbar may be able to float again if a heavy cargo is removed. This is called grounding line retreat.
Scheuchl's team found a rapid retreat of Smith Glacier's grounding line of 1.24 miles (2 kilometers) per year since 1996. Pope retreated more slowly at 0.31 mile (0.5 kilometer) per year since 1996. Kohler, which had retreated at a slower pace, actually readvanced a total of 1.24 miles (2 kilometers) since 2011.
These differences motivated Ala Khazendar of NASA's Jet Propulsion Laboratory, Pasadena, California -- a coauthor of Scheuchl's study -- to measure the ice losses at the bottoms of the glaciers, which he suspected might be underlying the changes in their grounding lines. Khazendar's study, published Oct. 25 in the journal Nature Communications, used measurements of changes in the thickness and height of the ice from radar and laser altimetry instruments flown by NASA'sOperation IceBridge and earlier NASA airborne campaigns. Radar waves penetrate glaciers all the way to their base, allowing direct measurements of how the bottom profiles of the three glaciers at their grounding lines changed between 2002 and 2014. Laser signals reflect off the surface, so for the floating ice shelves, laser measurements of changes in surface elevation can be used to infer changes in ice thickness.
Previous studies using other techniques estimated the average melting rates at the bottom of Dotson and Crosson ice shelves to be about 40 feet per year (12 meters per year). Khazendar and his team, using their direct radar measurements, found stunning rates of ice loss from the glaciers' undersides on the ocean sides of their grounding lines. The fastest-melting glacier, Smith, lost between 984 and 1,607 feet (300 and 490 meters) in thickness from 2002 to 2009 near its grounding line, or up to 230 feet per year (70 meters per year). Those years encompass a period when rapid increases in mass loss were observed around the Amundsen Sea region. The regional scale of the loss made scientists strongly suspect that an increase in the influx of ocean heat beneath the ice shelves must have taken place. "Our observations provide a crucial piece of evidence to support that suspicion, as they directly reveal the intensity of ice melting at the bottom of the glaciers during that period," Khazendar said.
"If I had been using data from only one instrument, I wouldn't have believed what I was looking at, because the thinning was so large," Khazendar added. However, the two IceBridge instruments, which use different observational techniques, both measured the same rapid ice loss.
Khazendar said Smith's fast retreat and thinning are likely related to the shape of the underlying bedrock over which it was retreating between 1996 and 2014, which sloped downward toward the continental interior, and oceanic conditions in the cavity beneath the glacier. As the grounding line retreated, warm and dense ocean water could reach the newly uncovered deeper parts of the cavity beneath the ice shelf, causing more melting. As a result, "More sections of the glacier become thinner and float, meaning that the grounding line continues retreating, and so on," he said. The retreat of Smith might slow down as its grounding line has now reached bedrock that rises farther inland of the 2014 grounding line.
Pope and Kohler, by contrast, are on bedrock that slopes upward toward the interior.
The question remains whether other glaciers in West Antarctica will behave more like Smith Glacier or more like Pope and Kohler. Many glaciers in this sector of Antarctica are on beds that deepen farther inland, like Smith's. However, Khazendar and Scheuchl said researchers need more information on the shape of the bedrock and seafloor beneath the ice, as well as more data on ocean circulation and temperatures, to be able to better project how much ice these glaciers will contribute to the ocean in a changing climate.
Scheuchl's study is titled "Grounding Line Retreat of Pope, Smith, and Kohler Glaciers, West Antarctica, Measured with Sentinel-1a Radar Interferometry Data." It was published in Geophysical Research Letters. Khazendar's paper, titled "Rapid Submarine Ice Melting in the Grounding Zones of Ice Shelves in West Antarctica," was published in Nature Communications.
NASA collects data from space, air, land and sea to increase our understanding of our home planet, improve lives and safeguard our future. NASA develops new ways to observe and study Earth's interconnected natural systems with long-term data records. The agency freely shares this unique knowledge and works with institutions around the world to gain new insights into how our planet is changing.
quinta-feira, 27 de outubro de 2016
sexta-feira, 7 de outubro de 2016
|BRASIL-EJA OS ANUNCIOS-OBRIGADO.|
Hubble Detects Giant 'Cannonballs' Shooting from StarGreat balls of fire! NASA's Hubble Space Telescope has detected superhot blobs of gas, each twice as massive as the planet Mars, being ejected near a dying star. The plasma balls are zooming so fast through space it would take only 30 minutes for them to travel from Earth to the moon. This stellar "cannon fire" has continued once every 8.5 years for at least the past 400 years, astronomers estimate.
The fireballs present a puzzle to astronomers, because the ejected material could not have been shot out by the host star, called V Hydrae. The star is a bloated red giant, residing 1,200 light-years away, which has probably shed at least half of its mass into space during its death throes. Red giants are dying stars in the late stages of life that are exhausting the nuclear fuel that makes them shine. They have expanded in size and are shedding their outer layers into space.
The current best explanation suggests the plasma balls were launched by an unseen companion star. According to this theory, the companion would have to be in an elliptical orbit that carries it close to the red giant's puffed-up atmosphere every 8.5 years. As the companion enters the bloated star's outer atmosphere, it gobbles up material. This material then settles into a disk around the companion, and serves as the launching pad for blobs of plasma, which travel at roughly a half-million miles per hour.
This star system could be the archetype to explain a dazzling variety of glowing shapes uncovered by Hubble that are seen around dying stars, called planetary nebulae, researchers say. A planetary nebula is an expanding shell of glowing gas expelled by a star late in its life.
"We knew this object had a high-speed outflow from previous data, but this is the first time we are seeing this process in action," said Raghvendra Sahai of NASA's Jet Propulsion Laboratory in Pasadena, California, lead author of the study. "We suggest that these gaseous blobs produced during this late phase of a star's life help make the structures seen in planetary nebulae."
Hubble observations over the past two decades have revealed an enormous complexity and diversity of structure in planetary nebulae. The telescope's high resolution captured knots of material in the glowing gas clouds surrounding the dying stars. Astronomers speculated that these knots were actually jets ejected by disks of material around companion stars that were not visible in the Hubble images. Most stars in our Milky Way galaxy are members of binary systems. But the details of how these jets were produced remained a mystery.
"We want to identify the process that causes these amazing transformations from a puffed-up red giant to a beautiful, glowing planetary nebula," Sahai said. "These dramatic changes occur over roughly 200 to 1,000 years, which is the blink of an eye in cosmic time."
Sahai's team used Hubble's Space Telescope Imaging Spectrograph (STIS) to conduct observations of V Hydrae and its surrounding region over an 11-year period, first from 2002 to 2004, and then from 2011 to 2013. Spectroscopy decodes light from an object, revealing information on its velocity, temperature, location and motion.
The data showed a string of monstrous, superhot blobs, each with a temperature of more than 17,000 degrees Fahrenheit (9,400 degrees Celsius) -- almost twice as hot as the surface of the sun. The researchers compiled a detailed map of the blobs' locations, allowing them to trace the first behemoth clumps back to 1986. "The observations show the blobs moving over time," Sahai said. "The STIS data show blobs that have just been ejected, blobs that have moved a little farther away, and blobs that are even farther away." STIS detected the giant structures as far away as 37 billion miles (60 million kilometers) away from V Hydrae, more than eight times farther away than the Kuiper Belt of icy debris at the edge of our solar system is from the sun.
The blobs expand and cool as they move farther away, and are then not detectable in visible light. But observations taken at longer, sub-millimeter wavelengths in 2004, by the Submillimeter Array in Hawaii, revealed fuzzy, knotty structures that may be blobs launched 400 years ago, the researchers said.
Based on the observations, Sahai and his colleagues Mark Morris of the University of California, Los Angeles, and Samantha Scibelli of the State University of New York at Stony Brook developed a model of a companion star with an accretion disk to explain the ejection process.
"This model provides the most plausible explanation because we know that the engines that produce jets are accretion disks," Sahai explained. "Red giants don't have accretion disks, but many most likely have companion stars, which presumably have lower masses because they are evolving more slowly. The model we propose can help explain the presence of bipolar planetary nebulae, the presence of knotty jet-like structures in many of these objects, and even multipolar planetary nebulae. We think this model has very wide applicability."
A surprise from the STIS observation was that the disk does not fire the monster clumps in exactly the same direction every 8.5 years. The direction flip-flops slightly, from side-to-side to back-and-forth, due to a possible wobble in the accretion disk. "This discovery was quite surprising, but it is very pleasing as well because it helped explain some other mysterious things that had been observed about this star by others," Sahai said.
Astronomers have noted that V Hydrae is obscured every 17 years, as if something is blocking its light. Sahai and his colleagues suggest that due to the back-and-forth wobble of the jet direction, the blobs alternate between passing behind and in front of V Hydrae. When a blob passes in front of V Hydrae, it shields the red giant from view.
"This accretion disk engine is very stable because it has been able to launch these structures for hundreds of years without falling apart," Sahai said. "In many of these systems, the gravitational attraction can cause the companion to actually spiral into the core of the red giant star. Eventually, though, the orbit of V Hydrae's companion will continue to decay because it is losing energy in this frictional interaction. However, we do not know the ultimate fate of this companion."
The team hopes to use Hubble to conduct further observations of the V Hydrae system, including the most recent blob ejected in 2011. The astronomers also plan to use the Atacama Large Millimeter/submillimeter Array (ALMA) in Chile to study blobs launched over the past few hundred years that are now too cool to be detected with Hubble.
The team's results appeared in the August 20, 2016, issue of The Astrophysical Journal.
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.
quinta-feira, 6 de outubro de 2016
|Study Predicts Next Global Dust Storm on MarsGlobal dust storms on Mars could soon become more predictable -- which would be a boon for future astronauts there -- if the next one follows a pattern suggested by those in the past.|
A published prediction, based on this pattern, points to Mars experiencing a global dust storm in the next few months. "Mars will reach the midpoint of its current dust storm season on October 29th of this year. Based on the historical pattern we found, we believe it is very likely that a global dust storm will begin within a few weeks or months of this date," James Shirley, a planetary scientist at NASA's Jet Propulsion Laboratory, Pasadena, California.
Local dust storms occur frequently on Mars. These localized storms occasionally grow or coalesce to form regional systems, particularly during the southern spring and summer, when Mars is closest to the sun. On rare occasions, regional storms produce a dust haze that encircles the planet and obscures surface features beneath. A few of these events may become truly global storms, such as one in 1971 that greeted the first spacecraft to orbit Mars, NASA's Mariner 9. Discerning a predictable pattern for which Martian years will have planet-encircling or global storms has been a challenge.
The most recent Martian global dust storm occurred in 2007, significantly diminishing solar power available to two NASA Mars rovers then active halfway around the planet from each other -- Spirit and Opportunity.
"The global dust storm in 2007 was the first major threat to the rovers since landing," said JPL's John Callas, project manager for Spirit and Opportunity. "We had to take special measures to enable their survival for several weeks with little sunlight to keep them powered. Each rover powered up only a few minutes each day, enough to warm them up, then shut down to the next day without even communicating with Earth. For many days during the worst of the storm, the rovers were completely on their own."
Dust storms also will present challenges for astronauts on the Red Planet. Although the force of the wind on Mars is not as strong as portrayed in an early scene in the movie "The Martian," dust lofted during storms could affect electronics and health, as well as the availability of solar energy.
The Red Planet has been observed shrouded by planet-encircling dust nine times since 1924, with the five most recent planetary storms detected in 1977, 1982, 1994, 2001 and 2007. The actual number of such events is no doubt higher. In some of the years when no orbiter was observing Mars up close, Mars was poorly positioned for Earth-based telescopic detection of dust storms during the Martian season when global storms are most likely.
Shirley's 2015 paper in the journal Icarus reported finding a pattern in the occurrence of global dust storms when he factored in a variable linked to the orbital motion of Mars. Other planets have an effect on the momentum of Mars as it orbits the solar system's center of gravity. This effect on momentum varies with a cycle time of about 2.2 years, which is longer than the time it takes Mars to complete each orbit: about 1.9 years. The relationship between these two cycles changes constantly. Shirley found that global dust storms tend to occur when the momentum is increasing during the first part of the dust storm season. None of the global dust storms in the historic record occurred in years when the momentum was decreasing during the first part of the dust storm season.
The paper noted that conditions in the current Mars dust-storm season are very similar to those for a number of years when global storms occurred in the past. Observations of the Martian atmosphere over the next few months will test whether the forecast is correct.
Researchers at Malin Space Science Systems, in San Diego, post Mars weather reports each weekbased on observations using the Mars Color Imager camera on NASA's Mars Reconnaissance Orbiter. A series of local southern-hemisphere storms in late August grew into a major regional dust storm in early September, but subsided by mid-month without becoming global. Researchers will be closely watching to see what happens with the next regional storm