Cassini Shows Why Jet Streams Cross-Cut Saturn

Image above: A particularly strong jet stream churns through Saturn's northern hemisphere in this false-color view from NASA's Cassini spacecraft. Image credit: NASA/JPL-Caltech/SSI.

Turbulent jet streams, regions where winds blow faster than in other places, churn east and west across Saturn. Scientists have been trying to understand for years the mechanism that drives these wavy structures in Saturn's atmosphere and the source from which the jets derive their energy. In a new study appearing in the June edition of the journal Icarus, scientists used images collected over several years by NASA's Cassini spacecraft to discover that the heat from within the planet powers the jet streams. Condensation of water from Saturn's internal heating led to temperature differences in the atmosphere. The temperature differences created eddies, or disturbances that move air back and forth at the same latitude, and those eddies, in turn, accelerated the jet streams like rotating gears driving a conveyor belt. A competing theory had assumed that the energy for the temperature differences came from the sun. That is how it works in the Earth's atmosphere. "We know the atmospheres of planets such as Saturn and Jupiter can get their energy from only two places: the sun or the internal heating. The challenge has been coming up with ways to use the data so that we can tell the difference," said Tony Del Genio of NASA's Goddard Institute for Space Studies, N.Y., the lead author of the paper and a member of the Cassini imaging team. The new study was possible in part because Cassini has been in orbit around Saturn long enough to obtain the large number of observations required to see subtle patterns emerge from the day-to-day variations in weather. "Understanding what drives the meteorology on Saturn, and in general on gaseous planets, has been one of our cardinal goals since the inception of the Cassini mission," said Carolyn Porco, imaging team lead, based at the Space Science Institute, Boulder, Colo. "It is very gratifying to see that we're finally coming to understand those atmospheric processes that make Earth similar to, and also different from, other planets." Rather than having a thin atmosphere and solid-and-liquid surface like Earth, Saturn is a gas giant whose deep atmosphere is layered with multiple cloud decks at high altitudes. A series of jet streams slice across the face of Saturn visible to the human eye and also at altitudes detectable to the near-infrared filters of Cassini's cameras. While most blow eastward, some blow westward. Jet streams occur on Saturn in places where the temperature varies significantly from one latitude to another. Thanks to the filters on Cassini's cameras, which can see near-infrared light reflected to space, scientists now have observed the Saturn jet stream process for the first time at two different, low altitudes. One filtered view shows the upper part of the troposphere, a high layer of the atmosphere where Cassini sees thick, high-altitude hazes and where heating by the sun is strong. Views through another filter capture images deeper down, at the tops of ammonia ice clouds, where solar heating is weak but closer to where weather originates. This is where water condenses and makes clouds and rain. In the new study, which is a follow-up to results published in 2007, the authors used automated cloud tracking software to analyze the movements and speeds of clouds seen in hundreds of Cassini images from 2005 through 2012. "With our improved tracking algorithm, we've been able to extract nearly 120,000 wind vectors from 560 images, giving us an unprecedented picture of Saturn's wind flow at two independent altitudes on a global scale," said co-author and imaging team associate John Barbara, also at the Goddard Institute for Space Studies. The team's findings provide an observational test for existing

This figure (above) examines a particularly strong jet stream and the eddies that drive it through the atmosphere of Saturn's northern hemisphere. Image credit: NASA/JPL-Caltech/SSI.

models that scientists use to study the mechanisms that power the jet streams. By seeing for the first time how these eddies accelerate the jet streams at two different altitudes, scientists found the eddies were weak at the higher altitudes where previous researchers had found that most of the sun's heating occurs. The eddies were stronger deeper in the atmosphere. Thus, the authors could discount heating from the sun and infer instead that the internal heat of the planet is ultimately driving the acceleration of the jet streams, not the sun. The mechanism that best matched the observations would involve internal heat from the planet stirring up water vapor from Saturn's interior. That water vapor condenses in some places as air rises and releases heat as it makes clouds and rain. This heat provides the energy to create the eddies that drive the jet streams. The condensation of water was not actually observed; most of that process occurs at lower altitudes not visible to Cassini. But the condensation in mid-latitude storms does happen on both Saturn and Earth. Storms on Earth – the low- and high-pressure centers on weather maps – are driven mainly by the sun's heating and do not mainly occur because of the condensation of water, Del Genio said. On Saturn, the condensation heating is the main driver of the storms, and the sun's heating is not important. Images of one of the strongest jet streams and a figure from the paper can be found at http://www.nasa.gov/cassini ,http://saturn.jpl.nasa.gov and http://ciclops.org . The Cassini-Huygens mission is a cooperative project of NASA, the European Space Agency and the Italian Space Agency. The Jet Propulsion Laboratory, a division of the California Institute of Technology in Pasadena, manages the Cassini-Huygens mission for NASA's Science Mission Directorate, Washington. The Cassini orbiter and its two onboard cameras were designed, developed and assembled at JPL. The imaging team is based at the Space Science Institute in Boulder, Colo. Images (mentioned), Credits: NASA's Goddard Space Flight Center / Bill Steigerwald / Nancy Neal Jones / Space Science Institute / Joe Mason / JPL / Jia-Rui C. Cook.Orbiter.ch Space News: Source: Orbiter Chspace

Read More........→September 16, 2012

Nasa's Sampex Mission: A Space Weather Warrior

Image above: An artist's rendition of the Solar, Anomalous, and Magnetospheric Particle Explorer or SAMPEX. Credit: NASA.

NASA's very first small explorer, the Solar, Anomalous, and Magnetospheric Particle Explorer or SAMPEX, was launched July 3, 1992 to study the zoo of particles and cosmic rays surrounding Earth. Surviving much longer than its expected mission of three years and providing invaluable observations for those who study space weather, the SAMPEX mission is now almost over. In early November, the spacecraft's orbit will decay enough that it will re-enter Earth's atmosphere, burning up completely on re-entry. When SAMPEX launched, the sun was just finishing the peak of its 11-year solar cycle and beginning to move toward solar minimum. Scientists were eager to watch what happened in near-Earth space in those first few years, as eruptions on the sun shot out energy and solar material and eventually tapered down into a period of quiet. However, those same effects were also predicted to lead to the spacecraft's demise. As the sun once again ramped up to solar maximum around 2000, the sun's output would create enough atmospheric drag that SAMPEX was expected to tumble out of its stable orbit. Contrary to such predictions, SAMPEX is still in orbit having survived that maximum and continuing in orbit long enough to see the sun move toward another solar max, currently predicted for 2013. But time is running out. As the atmosphere near Earth heats and swells in response to the sun's activity, the expansion of the uppermost atmosphere has encased SAMPEX, slowing it down. Soon the 20-year-old spacecraft will succumb to the very space weather it has helped scientists to study. Some time at the end of 2012, the orbit of the five-by-three-foot craft will spiral far enough in that SAMPEX will re-enter Earth's atmosphere, burning up completely and disappearing forever. "SAMPEX was launched on a shoe string budget," says Shri Kanekal, a space weather scientist at NASA's Goddard Space Weather Center in Greenbelt, Md. who has been involved with SAMPEX research since its launch. "It was proposed as a minimum one-year mission with a goal of three years, but it lasted for an unexpectedly long time. It has provided 20 years of high quality data, used by nearly everyone who studies near-Earth space." In its two decades, SAMPEX provided one of the main sources of data on how the radiation environment around Earth changed over time, waxing and waning in response to incoming particles from the sun and galaxy.

Image above: SAMPEX data have provided some of the most useful observations of the Van Allen Belts -- two rings of radiation around Earth. This SAMPEX data shows the belts during what's known as the Halloween Storms in October 2003, a time when the radiation belts around Earth swelled so much that they merged into a single ring. Credit: NASA/Goddard Space Flight Center . 

SAMPEX confirmed earlier theories that cosmic rays streaming in from outer space were being trapped in Earth's own magnetic environment, the magnetosphere, and it helped pinpoint the location where they gathered in a belt around Earth. Another area of research has been to tease out the composition of various particle populations from high-speed and high-energy particles from the sun known as solar energetic particles, to the host of electrons in Earth's middle atmosphere. Also, SAMPEX has been one of our best eyes on the radiation belts – two giant donuts of radiation surrounding Earth that can affect satellites in orbit during their occasional bouts of swelling. Indeed, scientists are eager for SAMPEX data still, eking out the last weeks of observation time to compare with early data from the Radiation Belt Storm Probes (RBSP) mission that launched in August, 2012. When those who study the radiation belts realized how imminent was the demise of SAMPEX, they adjusted the schedule to turn on a SAMPEX-compatible instrument aboard RBSP, an instrument called Relativistic Electron Proton Telescope (REPT), earlier than planned. One of the space phenomena that SAMPEX has helped categorize is something called microbursts, an intense but short lived phase during which electrons drop out of the radiation belts. From its viewpoint under the radiation belts, SAMPEX can still record such microbursts. As part of RBSP, on the other hand, REPT can look at the electron population while traveling through the radiation belts proper. In combination, the data may help show what occurrences in the radiation belts correlate to the rain of electrons, the microbursts. "Since one of the main goals of RBSP is to understand why and how electrons rain down out of the radiation belts, this will be important science," says Kanekal. "It's made all the more impressive that we can do this kind of research despite the fact that SAMPEX's science mission officially ended in 2004." Although the spacecraft has remained in orbit, the official SAMPEX science mission ended in June 2004. New data remained available, however, thanks to The Aerospace Corporation of El Segundo, Calif., which continued to fund costs to download data, and to Bowie State University in Bowie, Md., which operated the spacecraft to maintain the download process as an educational tool for its students. Kanekal was also instrumental in getting a grant to process all the data from 2004 to 2012, so it will be usable by the science community. NASA's first small explorer had an impressive run, far outliving its planned three-year mission. It provided data crucial to understanding how the space around Earth responds to space weather from the sun and will continue to do so up until the moment it re-enters Earth's atmosphere, disappearing forever. NASA's SAMPEX Mission: http://science.nasa.gov/missions/sampex/, The SAMPEX Data Center: http://www.srl.caltech.edu/sampex/DataCenter/, Images (mentioned), Text, Credit: NASA Goddard Space Flight Center / Karen C. Fox., Greetings, Source: Orbiter.ch Space News

Read More........→November 14, 2012