Cassini Is On The Trail Of A Runaway Mystery

Source:

http://www.sciencedaily.com/releases/2007/10/071008145004.htm

Science Daily — NASA scientists are on the trail of Iapetus' mysterious dark side, which seems to be home to a bizarre "runaway" process that is transporting vaporized water ice from the dark areas to the white areas of the Saturnian moon.

This "thermal segregation" model may explain many details of the moon's strange and dramatically two-toned appearance, which have been revealed exquisitely in images collected during a recent close flyby of Iapetus by NASA's Cassini spacecraft.
Infrared observations from the flyby confirm that the dark material is warm enough (approximately minus 230 degrees Fahrenheit or 127 Kelvin) for very slow release of water vapor from water ice, and this process is probably a major factor in determining the distinct brightness boundaries.
"The side of Iapetus that faces forward in its orbit around Saturn is being darkened by some mysterious process," said John Spencer, Cassini scientist with the composite infrared spectrometer team from the Southwest Research Institute, Boulder, Colo.
Using multiple instruments on Cassini, scientists are piecing together a complex story to explain the bright and dark faces of Iapetus. But yet to be fully understood is where the dark material is coming from. Is it native or from outside the moon? It has long been hypothesized that this material did not originate from within Iapetus, but instead was derived from other moons orbiting at a much greater distance from Saturn in a direction opposite to Iapetus.
Scientists are now converging on the notion that the darkening process in fact began in this manner, and that thermal effects subsequently enhanced the contrast to what we see today.
"It's interesting to ponder that a more than 30-year-old idea might still help explain the brightness difference on Iapetus," said Tilmann Denk, Cassini imaging scientist at the Free University in Berlin, Germany. "Dusty material spiraling in from outer moons hits Iapetus head-on, and causes the forward-facing side of Iapetus to look different than the rest of the moon."
Once the leading side is even slightly dark, thermal segregation can proceed rapidly. A dark surface will absorb more sunlight and warm up, explains Spencer, so the water ice on the surface evaporates. The water vapor then condenses on the nearest cold spot, which could be Iapetus's poles, and possibly bright, icy areas at lower latitudes on the side of the moon facing in the opposite direction of its orbit. So the dark stuff loses its surface ice and gets darker, and the bright stuff accumulates ice and gets brighter, in a runaway process.
Scientists say the result is that there are virtually no shades of gray on Iapetus. There is only white and very dark.
Ultraviolet data also show a non-ice component in the bright, white regions of Iapetus. Spectroscopic analysis will reveal whether the composition of the material on the dark hemisphere is the same as the dark material that is present within the bright terrain.
"The ultraviolet data tell us a lot about where the water ice is and where the non-water ice stuff is. At first glance, the two populations do not appear to be present in the pattern we expected, which is very interesting," said Amanda Hendrix, Cassini scientist on the ultraviolet imaging spectrograph team at NASA's Jet Propulsion Laboratory, Pasadena, Calif.
Because of the presence of very small craters that excavate the bright ice beneath, scientists also believe that the dark material is thin, a result consistent with previous Cassini radar results. But some local areas may be thicker. The dark material seems to lie on top of the bright region, consistent with the idea that it is a residual left behind by the sublimated water ice.
Some other mysteries are coming together. There are more data on the signature mountain ridge that gives Iapetus its "walnut" appearance. In some places it appears subdued. One big question that remains is why it does not go all the way around.
Was it partially destroyed after it formed, or did it never extend all the way around the moon? Scientists have ruled out that it is a youthful feature because it is pitted with craters, indicating it is old. And the ridge looks too solid and competent to be the result of an equatorial ring around the moon collapsing onto its surface. The ring theory cannot explain features that look like tectonic structures in the new high resolution images.
Over the next few months, scientists hope to learn more about Iapetus' mysteries.
New Iapetus images, temperature maps and other visuals on Iapetus are available at: http://saturn.jpl.nasa.gov/ and http://www.nasa.gov/cassini .
The Cassini-Huygens mission is a cooperative project of NASA, the European Space Agency and the Italian Space Agency. JPL, a division of the California Institute of Technology in Pasadena, manages the Cassini mission for NASA's Science Mission Directorate, Washington, D.C.
Note: This story has been adapted from material provided by NASA/Jet Propulsion Laboratory.

Fausto Intilla

www.oloscience.com

Cassini Flies By Saturn's Walnut-Shaped Moon Iapetus

Source:

http://www.sciencedaily.com/releases/2007/09/070911212807.htm

Science Daily — Cassini completed its closest flyby of the odd moon Iapetus on Sept. 10, 2007. The spacecraft flew about 1,640 kilometers (1,000 miles) from Iapetus' surface and is returning amazing views of the bizarre moon.

All the data were successfully recorded on the spacecraft. Twenty-one minutes into the first post-flyby data downlink, the spacecraft went into a precautionary condition called safe mode. The cause has been determined to be a solid state power switch that was tripped due to a galactic cosmic ray hit.
While in safe mode, the spacecraft turns off all unnecessary activities and transmits only essential engineering telemetry at a low data rate, while it awaits commands from Earth.
Tuesday morning, Sept. 11, commands were sent to the spacecraft to resume high rate science and engineering data playback. The project expects all data on the spacecraft will be returned to Earth during downlinks on Tuesday and Wednesday, with no impact on the Iapetus science data return beyond a brief delay.
Due to the safing event, the sequence executing on the spacecraft was halted, and Cassini's instruments will not be turned back on for three or four days. The last time Cassini was in safe mode was over four years ago.
Note: This story has been adapted from a news release issued by National Aeronautics And Space Administration.

Fausto Intilla

www.oloscience.com

Saturn's Skewed Ring Current

Source:

http://www.sciencedaily.com/releases/2007/08/070824130101.htm

Science Daily — Images taken by Cassini’s Magnetospheric Imaging Instrument (MIMI) show that Saturn’s ring current is a warped disc that balloons out of the equatorial plane on the planet’s dayside and remains a thin disk that rises above the plane at larger distances on the nightside.

Dr Stamatios "Tom" Krimigis, the Principal Investigator for the instrument, who is presenting images at the European Planetary Science Congress in Potsdam on Thursday 23rd August, said, "Ring currents surround planets sort of like the brim of a hat.
Uniquely in Saturn’s case, that brim has been crushed at the front and tipped up at the back, so it’s pretty bent out of shape!"
The presence of a ring current around Saturn was first suggested in the early 1980s following magnetic anomalies observed by the Pioneer 11 and Voyager 1 and 2 spacecraft. Ring currents are also found around Earth and Jupiter. They are caused when plasma becomes trapped between mirror points on magnetic field lines, similar to the Van Allen radiation belts surrounding Earth, and gradually drifts around the planet. The aggregate motion of all of the hot ions distributed around the equator generates an electric current.
On Saturn, the source of the plasma is material from the rings and gas vented by geysers on the moon Enceladus, which is subsequently ionized and accelerated. The MIMI images show that the ring current occupies a region of the equatorial plane between 540 000 kilometres and 1 080 000 kilometres from the centre of Saturn. They also show that Saturn’s ring current is persistently asymmetric (unlike Earth’s), and that the asymmetry rotates nearly rigidly with Saturn.
MIMI, which was developed by an international team led by the Johns Hopkins University Applied Physics Laboratory (APL), Laurel, Maryland, has three distinct sensors that allow it to "visualize the invisible" and show the plasma and radiation belts in Saturn’s environment in an image. The MIMI instrument includes an Ion and Neutral Camera developed by APL, a spectrometer built by the University of Maryland, and a low energy particle detector developed by the Max-Planck-Institut fuer Sonnensystemforschung and a number of co-investigator institutions including CESR in Toulouse.
Ring currents
A ring current at Earth was proposed by Chapman and Ferraro in the 1930s to explain the decrease in the equatorial magnetic field during geomagnetic storms; the detailed nature in terms of composition and energy content was not determined until the mid-eighties by the Active Magnetospheric Particle Tracer Explorers (AMPTE) mission, a joint programme by the US, Germany and the UK. The Principal Investigator for the AMPTE mission was Dr Krimigis.
In the case of Earth, ionospheric sources dominate the formation of the ring current.
Jupiter’s ring current was first measured partially by Voyager. The Galileo mission to Jupiter added considerably to the understanding of Jupiter’s ring current, showing thatat Jupiter, Io’s volcanoes provide the gas that is subsequently ionized and accelerated
Note: This story has been adapted from a news release issued by European Planetology Network.

Fausto Intilla

www.oloscience.com

Astronomers Get First Look At Uranus's Rings As They Swing Edge-on To Earth

Source:

http://www.sciencedaily.com/releases/2007/08/070823141039.htm

Science Daily — As the rings of Uranus swing edge-on to Earth - a short-lived view we get only once every 42 years - astronomers observing the event are getting an unprecedented, glare-free view of the rings and the fine dust that permeates them.

The rings were discovered in 1977, so this is the first opportunity astronomers have had to observe a Uranus ring crossing and perhaps to discover a new moon or two.
While the Keck II telescope and the Hubble Space Telescope have been looking at the planet for years in anticipation of this event, ground-based telescopes in Chile and southern California have targeted the planet during the actual ring crossing.
Based on the Keck observations, a team of astronomers led by Imke de Pater of University of California, Berkeley, reports Thursday, Aug. 23 in Science Express, the online edition of Science magazine, that the rings of micron-sized dust have changed significantly since the Voyager 2 spacecraft photographed the Uranus system 21 years ago. She will discuss the results during a talk August 23 at the European Planetary Science Congress 2007 meeting in Potsdam, Germany.
The inner rings are much more prominent than expected, revealing material in otherwise empty regions of the system of rings.
"People tend to think of the rings as unchanging, but our observations show that not to be the case," said de Pater, a UC Berkeley professor of astronomy. "There are a lot of forces acting on small dust grains, so it is not that crazy to find that the arrangement of rings has changed."
Using the near infrared camera (NIRC2) and adaptive optics on the Keck II telescope on May 28, the team took striking images of the nearly edge-on ring appearing as a bright line bisecting a dim Uranus, which appears dark in the infrared. The observations were conducted during an engineering run by Marcos van Dam, adaptive optics scientist at the W. M. Keck Observatory, after the installation of a new wavefront sensor.
"The improvements to the adaptive optics systems allowed us to capture unbelievably crisp images of Uranus; it was as if the Keck telescope was orbiting in space," said van Dam.
On Aug. 14, the Hubble Space Telescope also imaged the planet very near the moment when the rings were perfectly aligned with Earth, showing similar features but also including some recently-discovered outer rings. The image was released today by the Space Telescope Science Institute.
"The outermost ring is not visible in our infrared images," said de Pater's co-author, Heidi B. Hammel of the Space Science Institute in Boulder, Colo. "This ring is very blue, and therefore harder to see in the infrared. We may detect it when the rings are fully edge-on and when we can observe it for several hours."
With further analysis of the Hubble data, astronomer Mark Showalter of the SETI Institute hopes to detect some of the small moons, and perhaps some not seen before, that shepherd the debris into distinct rings.
"Two little satellites called Cordelia and Ophelia straddle the brightest ring, the epsilon ring, and keep it in place, but people have always assumed there must be a bunch more of these satellites that are confining the nine other narrow rings," Showalter said. "This is the unique viewing geometry that only comes along once in 42 years, when we have a chance of imaging these tiny satellites, because normally they are lost in the glare of the rings. Now, the rings are essentially invisible."
Astronomers at the Very Large Telescope (VLT) in Chile, run by the European Southern Observatory (ESO), and at the Palomar Observatory in southern California operated by the California Institute of Technology, also observed Uranus during the current crossing.
"The VLT took data at the precise moment when the rings were edge-on to Earth," said de Pater, who worked with two team members observing in Chile: Daphne Stam of the Technical University of Delft in the Netherlands and Markus Hartung of ESO. Meanwhile, astronomers Philip Nicholson of Cornell University in New York and Keith Matthews of Caltech observed from atop Mt. Palomar.
Until Voyager flew by in January 1986, the rings were only known from the way they temporarily blocked the light of stars passing behind Uranus. Earth-based images have been too blurry until recently, with the advent of Keck adaptive optics and the Advanced Camera for Surveys of the Hubble telescope. Nevertheless, when the sunlit side of Uranus's rings are in full view of Earth, the densely-packed rings reflect so much light that their glare completely dominates the fainter glow from micron-sized dust.
Earth's orbit around the sun permits three opportunities to view the rings edge-on: Uranus made its first ring crossing as seen from Earth on May 3, it made its second crossing on Aug. 16, and will cross for the third and last time on Feb 20, 2008. Though the last ring crossing relative to Earth will be hidden behind the sun, most of Earth's premier telescopes, including Keck, Hubble, VLT and Palomar, plan to focus on the planet again in the days following Dec. 7.
"December 7 is the Uranian equinox, when the rings are perfectly edge-on to the sun, and after that, there is a brief period again when we will view the dark side of the rings, before they become illuminated again for another 42 years," Hammel said.
The advantage of observations at a ring-plane crossing is that it becomes possible to look at the rings from the shadowed side. From that vantage, the normally-bright outer rings grow fainter because their centimeter- to meter-sized rocks obscure one another, while the dim inner rings get brighter as their material merges into a thin band along the line of sight.
The dust belts that Voyager saw differ radically from today's dust distribution, according to coauthors de Pater and Showalter. Most interesting is a broad, inner ring called zeta, whose position today is several thousand kilometers farther from the planet than when it was discovered by Voyager.
"The ring may have moved, or it may be an entirely new ring," noted Showalter.
Similar, dramatic changes in dust distribution have also been observed recently in Saturn's and Neptune's rings. This is not surprising, because gravity keeps the larger ring particles in orbit, but other smaller forces can nudge the tiny dust grains around, de Pater said. These forces include pressure from sunlight, drag produced as the dust plows through ionized plasma around Uranus, and even drag from the planet's magnetic field.
"Impacts into the larger bodies in the system also could knock dust off and create new rings," de Pater said.
"With further observations, the time scales over which these variations occur should provide new insight into the physical processes at work," the authors concluded.
Support for the Keck observations came from the National Science Foundation, the National Aeronautics and Space Administration, the Keck Observatory and the UC Santa Cruz Technology Center for Adaptive Optics.
Note: This story has been adapted from a news release issued by University of California - Berkeley.

Fausto Intilla

www.oloscience.com

Cassini Finds Possible Origin Of One Of Saturn's Rings

Source:

http://www.sciencedaily.com/releases/2007/08/070803113335.htm

Science Daily — Cassini scientists may have identified the source of one of Saturn's more mysterious rings. Saturn's G ring likely is produced by relatively large, icy particles that reside within a bright arc on the ring's inner edge. The particles are confined within the arc by gravitational effects from Saturn's moon Mimas.

Micrometeoroids collide with the particles, releasing smaller, dust-sized particles that brighten the arc. The plasma in the giant planet's magnetic field sweeps through this arc continually, dragging out the fine particles, which create the G ring.The finding is evidence of the complex interaction between Saturn's moons, rings and magnetosphere. Studying this interaction is one of Cassini's objectives. The study is in the Aug. 2 issue of the journal Science and was based on observations made by multiple Cassini instruments in 2004 and 2005. "Distant pictures from the cameras tell us where the arc is and how it moves, while plasma and dust measurements taken near the G ring tell us how much material is there," said Matthew Hedman, a Cassini imaging team associate at Cornell University in Ithaca, N.Y., and lead author on the Science paper. Saturn's rings are an enormous, complex structure, and their origin is a mystery. The rings are labeled in the order they were discovered. From the planet outward, they are D, C, B, A, F, G and E. The main rings -- A, B and C from edge-to-edge, would fit neatly in the distance between Earth and the moon. The most transparent rings are D -- interior to C -- and F, E and G, outside the main rings. Unlike Saturn's other dusty rings, such as the E and F rings, the G ring is not associated closely with moons that either could supply material directly to it -- as Enceladus does for the E ring -- or sculpt and perturb its ring particles -- as Prometheus and Pandora do for the F ring. The location of the G ring continued to defy explanation, until now. Cassini images show that the bright arc within the G ring extends one-sixth of the way around Saturn and is about 250 kilometers (155 miles) wide, much narrower than the full 5,955-kilometer width (3,700 miles) of the G ring. The arc has been observed several times since Cassini's 2004 arrival at the ringed planet and thus appears to be a long-lived feature. A gravitational disturbance caused by the moon Mimas exists near the arc. As part of their study, Hedman and colleagues conducted computer simulations that showed the gravitational disturbance of Mimas could indeed produce such a structure in Saturn's G ring. The only other places in the solar system where such disturbances are known to exist are in the ring arcs of Neptune. Cassini's magnetospheric imaging instrument detected depletions in charged particles near the arc in 2005. According to the scientists, unseen mass in the arc must be absorbing the particles. "The small dust grains that the Cassini camera sees are not enough to absorb energetic electrons," said Elias Roussos of the Max-Planck-Institute for Solar System Research, Germany, and member of the magnetospheric imaging team. "This tells us that a lot more mass is distributed within the arc." The researchers concluded that there is a population of larger, as-yet-unseen bodies hiding in the arc, ranging in size from that of peas to small boulders. The total mass of all these bodies is equivalent to that of an ice-rich, small moon that's about 100 meters wide (328 feet wide). Joe Burns, a co-author of the paper from Cornell University and a member of the imaging team, said, "We'll have a super opportunity to spot the G ring's source bodies when Cassini flies about 600 miles from the arc 18 months from now." The Cassini-Huygens mission is a cooperative project of NASA, the European Space Agency and the Italian Space Agency. The Jet Propulsion Laboratory, Pasadena, Calif., 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 the laboratory. The imaging team is based at the Space Science Institute, Boulder, Colo. The magnetospheric imaging instrument team is based at Johns Hopkins University, Laurel, Md.

Note: This story has been adapted from a news release issued by National Aeronautics And Space Administration.

Fausto Intilla

www.oloscience.com