Monday, June 11, 2012

Ice, Dust and Plasma. Go Figure.

Atention, class! Today the subject is plasma, often called the "Fourth State of Matter" (the other states being solid, liquid and gas). Plasma is similar to gas in which a certain portion of the particles are ionized. Heating a gas may ionize its molecules or atoms -- by reducing or increasing the number of electrons they have. The result is a plasma, which contains charged particles: positive ions and negative electrons or ions.

Still with me? Now, ionization can be induced by means other than heat. For example, a strong electromagnetic field. And the ionization is accompanied by the dissociation of molecular bonds, if they are present. The word "plasma" was first used in this way back in the 1920's, by American chemist and physicist Irving Langmuir (1881 - 1957), who worked to understand electric discharges. 

Okay. Now we must touch on dusty plasma, which is plasma containing nanometer (nm) or micrometer-sized particles suspended in it. Dust particles may be charged and the plasma and particles behave as a plasma, following electromagnetic laws for particles up to about 10 nm (or 100 nm if large charges are present). Dust particles may form larger particles resulting in "grain plasmas". Dusty plasmas can be found in industrial processing plasmas and space plasmas.

(cricket, cricket, cricket)

Wake up, people! This stuff is important. You must now apply this newfound knowledge to the icy world of Enceladus, the sixth-largest moon of Saturn. Composition-wise, Enceladus is 91 percent water vapor, 4 percent nitrogen, 3.2 percent carbon dioxide, and 1.7 percent methane.

Back in 2005 NASA’s Cassini mission first noted that Enceladus had geyser-like jets of icy particles rising from the crust. And now, recent findings from Cassini reveal that Enceladus is providing a special laboratory for watching unusual behavior of plasma. In these recent findings, some Cassini scientists think they have observed "dusty plasma," a condition that has been theorized but not previously observed near Enceladus. 

Data also show that the usual "heavy" and "light" species of charged particles in normal plasma are actually reversed near the plume spraying from the moon's south polar region. The findings are discussed in two recent papers in the Journal of Geophysical Research.

Ninety-nine percent of the matter in the universe is thought to exist as plasma, so scientists have been using Saturn as a site other than Earth to observe the behavior of this cloud of ions and electrons directly. Scientists want to study the way the sun sends energy into Saturn's plasma environment, since that jolt of energy drives processes such as weather and the behavior of magnetic field lines. They can use these data to understand how Saturn's plasma environment is similar to and different from that of Earth and other planets.

Enceladus is a major source of ionized material filling the huge magnetic bubble around Saturn. About 200 pounds (about 100 kilograms) of water vapor per second - about as much as an active comet - spray out from long cracks in the south polar region known as "tiger stripes." The ejected matter forms the Enceladus plume - a complex structure of icy grains and neutral gas that is mainly water vapor. The plume gets converted into charged particles interacting with the plasma that fills Saturn's magnetosphere. 

The nature of this unique gas-dust-plasma mixture has been revealed over the course of the mission with data from multiple instruments, including the Cassini plasma spectrometer, magnetometer, magnetospheric imaging instrument, and the radio and plasma wave science instrument. What scientists found most interesting is that the grains range continuously in size from small water clusters (a few water molecules) to thousandths of an inch (100 micrometers). They also saw that a large fraction of these grains trap electrons on their surface. Up to 90 percent of the electrons from the plume appear to be stuck on large, heavy grains. 

In this environment, Cassini has now seen positively charged ions become the small, "light" plasma species and the negatively charged grains become the "heavy" component. This is just the opposite of "normal" plasmas, where the negative electrons are thousands of times lighter than the positive ions.

In a paper published in the December issue of the journal, a team of Swedish and U.S. scientists on the Cassini mission examined radio and plasma wave science instrument observations from four flybys of Enceladus during 2008. They found a high plasma density (both ions and electrons) within the Enceladus plume region, although the electron densities are usually much lower than the ion densities in the plumes and in the E ring. The team concluded that dust particles a hundred millionth to a hundred thousandth of an inch (a nanometer to micrometer) in size are sweeping up the negatively charged electrons.  The mass of the observed "nanograins" ranges from a few hundred to a few tens of thousands of atomic mass units (proton masses), and must therefore contain tens to thousands of water molecules bound together. At least half of the negatively charged electrons are attached to the dust, and their interaction with the positively charged particles causes the ions to be decelerated. Because the dust is charged and behaves as part of the plasma cloud, this paper distinguishes this state of matter from dust that just happens to be in plasma.

In a dusty plasma, conditions are just right for the dust to also participate in the plasma's collective behavior. This increases the complexity of the plasma, changes its properties and produces totally new collective behavior. Dusty plasma are thought to exist in comet tails and dust rings around the sun, but scientists rarely have the opportunity to fly through the dusty plasma and directly measure its characteristics in place. And a separate analysis, based on data obtained by the Cassini plasma spectrometer, revealed the presence of nanograins having an electric charge corresponding to a single excess electron.

The nature of the Enceladus plume has been revealed over time due to the synergistic nature of the fields and particles instruments on Cassini, which has been orbiting in Saturn's magnetosphere since 2004. Following the original detection of the plume based on magnetometer measurements, team members found that the observed perturbation of Saturn's magnetic field required the presence of negatively charged dust grains in the plume. These findings were reported in the April and October 2011 issues of Journal of Geophysical Research Space Physics.  Previous data obtained by the ion and neutral mass spectrometer revealed the complex composition of the plume gas, and the cosmic dust analyzer revealed that the plume grains were rich in sodium salts. Because this scenario can only arise if the plume originated from liquid water, it provides compelling evidence for a subsurface ocean. 

Cassini will continue to study the complex nature of the plume region in the three planned additional flybys of Enceladus.  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 mission for NASA's Science Mission Directorate, Washington. More Cassini information is at www.nasa.gov/cassini and saturn.jpl.nasa.gov .

You may visit the Journal of Geophysical Research at this URL: www.agu.org/journals/jgr/


You may visit the Journal of Geophysical Research Space Physics at this URL: www.agu.org/journals/ja/

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