Saturn's moon - Rhea

[nick c]

The latest data from Cassini's flyby measurements at Saturn’s moon Rhea reveal a tenuous oxygen–carbon dioxide atmosphere. As B.D.Teolis has put it in the latest edition of Science:

...The presence of CO2 suggests radiolysis reactions between surface oxidants and organics, or sputtering and/or outgassing of CO2 endogenic to Rhea’s ice. Observations of outflowing positive and negative ions give evidence for pickup ionization as a major atmospheric loss mechanism...

Sadly to say in the corresponding NASA-PR the possible cause "sputtering" turns to "complex chemistry", so again trying to avoid the E-word.

This although could turn out to be another confirmation of the EU-predictions, if we remember Thornhill's theorizing about the origin of CO2 on Venus at:
http://www.holoscience.com/news.php?article=9aqt6cz5

..."When performing comparisons, we must allow for the fact that the Venusian atmosphere is being modified continually by electric discharge activity on the surface of that planet. It has increased the carbon dioxide content of the Venusian atmosphere at the expense of nitrogen and water vapor. Scientists think that most of Venus' water must have split into hydrogen and oxygen and all the hydrogen was lost to space. But if so, where is the oxygen that was left behind? The four Pioneer probe craft didn't find it in the atmosphere. The answer is that it has combined with carbon monoxide to form a heavy atmosphere of carbon dioxide. The process I envisage is this:

"Venus probably began with an atmosphere more like Titan's and the Earth's, where nitrogen dominates, and with more water. It suggests that Saturn must have considerable nitrogen at depth in its atmosphere. The icy rings and satellites of Saturn and abundant water on Earth also point to water on Saturn. On the Venusian surface, nitrogen molecules are converted to carbon monoxide molecules by a catalytic surface nuclear reaction in the presence of red-hot iron. The brilliant French chemist, Louis Kervran, when investigating carbon monoxide poisoning of welders, discovered this surprising nuclear transformation. The carbon monoxide reacts at the hot surface of Venus with water vapor to form carbon dioxide and hydrogen. It is a well-known industrial process. The hydrogen produced escapes from Venus...

This can turn out to be interesting...

FS3

[StefanR]

Rhea’s oxygen and carbon dioxide atmosphere
Saturn’s second-largest moon, Rhea, has a tenuous atmosphere of its own, as recently reported in the journal Science. To pinpoint the source near the surface, planetary group researchers helped trace ions seen at the spacecraft but originating in the atmosphere, to their source near Rhea’s surface. The positive and negative ‘pickup’ ions follow a curved path in Saturn’s magnetic field – and the path depends on the polarity. (Teolis et al., Science, December 2010).

http://www.ucl.ac.uk/mssl/news/mssl-news/earlier-news/saturns-moons

[StefanR]

Cassini catches Saturn moons in paintball fight


Hemispheric color differences on Saturn's moon Rhea are apparent in this false-color view from NASA's Cassini spacecraft. This image shows the side of the moon that always faces the planet

On Tethys, Dione and Rhea, darker, rust-colored, reddish hues paint the entire trailing hemisphere, or the side that faces backward in the orbit around Saturn. The reddish hues are thought to be caused by tiny particle strikes from circulating plasma, a gas-like state of matter so hot that atoms split into an ion and an electron, in Saturn's magnetic environment. Tiny, iron-rich "nanoparticles" may also be involved, based on earlier analyses by the Cassini visual and infrared mapping spectrometer team.


These three views of Saturn’s moon Rhea were made from data obtained by NASA’s Cassini spacecraft, enhanced to show colorful splotches and bands on the icy moon’s surface. Image credit: NASA/JPL/SSI/LPI

Enceladean spray also appears to splatter the parts of Tethys, Dione and Rhea that run into the spray head-on in their orbits around Saturn. But scientists are still puzzling over why the Enceladean frost on the leading hemisphere of these moons bears a coral-colored, rather than bluish, tint.


These three enhanced-color views of an equatorial region on Saturn's moon Rhea were made from data obtained by NASA's Cassini spacecraft. Image credit: NASA/JPL/SSI/LPI

Schenk and colleagues also found a unique chain of bluish splotches along the equator of Rhea that re-open the question of whether Rhea ever had a ring around it. The splotches do not seem related to Enceladus, but rather appear where fresh, bluish ice has been exposed on older crater rims. Though Cassini imaging scientists recently reported that they did not see evidence in Cassini images of a ring around Rhea, the authors of this paper suggest the crash of orbiting material, perhaps a ring, to the surface of Rhea in the not-too-distant past could explain the bluish splotches.

http://www.nasa.gov/mission_pages/cassini/whycassini/cassini20101007.html

[StefanR]

The Dust Halo of Saturn’s Largest Icy Moon, Rhea

Saturn’s moon Rhea had been considered massive enough to retain a thin, externally generated
atmosphere capable of locally affecting Saturn’s magnetosphere. The Cassini spacecraft’s in situ
observations reveal that energetic electrons are depleted in the moon’s vicinity. The absence of a
substantial exosphere implies that Rhea’s magnetospheric interaction region, rather than being
exclusively induced by sputtered gas and its products, likely contains solid material that can absorb
magnetospheric particles. Combined observations from several instruments suggest that this
material is in the form of grains and boulders up to several decimetres in size and orbits Rhea as an
equatorial debris disk. Within this disk may reside denser, discrete rings or arcs of material.

http://www-ssc.igpp.ucla.edu/personnel/russell/papers/dust_halo.pdf

Rhea’s Atmosphere
http://www.thunderbolts.info/wp/2011/11/16/rheas-atmosphere/

[StefanR]

Magnetic portraits of Tethys and Rhea

Sub-magnetosonic and super-magnetosonic plasma interactions with inert moons share many similarities. In both cases, a long wake devoid of plasma is formed downstream of the moon which is eventually filled by plasma drawn into the wake region by expansion fans. The field strength is strengthened in the central wake but is depressed in the expansion fan region surrounding the central wake. However, there are many differences in the two types of interactions.
The main key difference is in the extent of the ion depleted wake in the field-aligned direction. For a super-magnetosonic interaction, just behind the moon, the ion absorption wake radius is nearly identical to that of the moon while in a sub-magnetosonic interaction, the wake is elongated appreciably in the direction of the magnetic field.
Another key difference that distinguishes the two interactions is that in a sub-magnetosonic flow the expansion fan in the field-aligned direction recedes to large distances from the moon whereas it begins at the boundary of the moon in a supermagnetosonic interaction.We would also like to remark about the absence of any i nternal field possessed by these moons. The closest approach distance for Tethys was 3.83 RT and for Rhea it was 1.67 RRH. We believe that in both of these data sets, any dipolar field at a level of 1 nT would have been easily detectable. This puts an upper bound of 56 nT for the surface field strength for any internal field from Tethys and 5 nT for Rhea.
We also did not detect any measurable induction field from these moons. This is expected because in Saturn’s magnetosphere, because of the axisymmetric nature of Saturn’s field, there is no strong driving signal at the rotation period of Saturn. As a global conductor induces a dipolar response from a time-varying uniform field, induction studies from flyby data are inadequate to infer the extremely weak dipolar response that would be generated by a global conductor such as a subsurface ocean.
Our observations of the magnetic field interactions also rule out any appreciable mass-loading of Saturn’s magnetosphere by Tethys and Rhea. In contrast, near Enceladus, modeling studies of the plasma sputtering environment and the magnetic field signatures show that between 1.5 and 3 kg/s of the neutral material (mostly water molecules) is picked-up by the magnetosphere of Saturn in the immediate vicinity of Enceladus (Burger et al., 2007; Khurana et al., 2007). The rest of the neutral gas and dust spread around the orbit of Enceladus to
ultimately form the neutral cloud and the E ring of Saturn.
Fig. 10 compares the plasma interaction of a mass-loading (or a highly conducting) moon with that of a mass-absorbing moon in a downstream plane normal to the background flow. When a moon mass-loads the flow, the field lines bulge out to avoid the plasma slow-down region (Fig. 10, left). If the wake is filled with newly picked-up plasma from the moon’s vicinity, the bulge persists into the wake and a field minimum is observed at the center of the wake (for example, in Io’s wake). However, for an inert moon, the field lines are drawn into the wake to compensate for the reduction of plasma pressure there, enhancing the local field strength (Fig. 10, right). Fig. 10 shows that the expected signatures for the By component are exactly opposite in sign to each other for the two situations. Our observations of By are consistent with an inert interaction and rule out any appreciable mass-loading. Our data therefore show that sputtering of material from an icy satellite’s surface is inadequate in Saturn’s magnetosphere to generate an appreciable atmosphere from which plasma mass-loading can take place. However, we would like to remark that upstream flybys of Tethys and Rhea are desirable to fully characterize the interactions of these icy satellites with Saturn’s magnetosphere. The upstream flybys would be especially useful for characterizing the very small amount of sputtered material picked-up from the surfaces of these moons.
Recently, Saur and Strobel (2005) explored the generation of atmospheres by plasma sputtering at the five large inner icy moons of Saturn. Their study concluded that other than Rhea, the gravities of the icy moons were too feeble to hold any appreciable sputter generated atmospheres. For Rhea, Saur and Strobel Strobel estimated that it had the potential to hold a thin atmosphere with a column density approaching 6×1017 particles/m2. They further estimated that the height-integrated Pedersen conductivity in the ionosphere of Rhea would be ∼30 S, much greater
than the local Alfvén conductance of ∼7.6 S. Such a large conductivity would result in a strong field line draping type
Alfvénic interaction producing a δb/B0 of 30%. Our observations rule out such an interaction and are at variance with their modeling results for Rhea.
Finally, we would like to remark on the possibility of strong wave–particle interactions in the wakes of inert moons. The moon/plasma interaction described in Fig. 9 creates phase space holes over a large range of velocities and distances. Electrostatic and electromagnetic waves would be expected to be generated that smooth out these phase space holes and isotropize the plasma distribution function once again.

http://www-spc.igpp.ucla.edu/personnel/russell/papers/magnetic_portraits.pdf

[StefanR]

The Moon Rings That Never Were

They would have been the first rings ever observed around a moon: three narrow bands of icy debris encircling Saturn's second-largest satellite, Rhea. Space physicists announced their existence in March 2008. But a more definitive search finds that they simply aren't there. And that raises the question of what exactly the first team saw.
The original ring detection was based on plasma measurements the Cassini spacecraft made while orbiting Saturn in 2005. In a 2008 paper in Science, space physicist Geraint Jones of University College London and colleagues described how some unseen solid material around Rhea, which is less than half the size of our moon, seemed to be absorbing energetic electrons that are trapped in Saturn's magnetosphere. That absorption appeared to be creating electron shadows, much as a ring creates a light shadow when it passes in front of a star. At Rhea, the stunningly symmetrical electron shadows—three on either side of the moon—implied three narrow ringlets embedded in a broad disk of icy debris in the moon's equatorial plane. That, said Jones at the time, was "the only reasonable explanation we've been able to come up with."
But Rhea's supposed rings bothered ring specialists from the start. Such rings weren't likely to form because just the right sort of grazing impact on Rhea would be required, they said. And the rings would be quickly destroyed by the tug of Saturn's gravity and worn down by eroding small impacts.



Ring specialist Matthew Tiscareno of Cornell University was one of the skeptics. So in late 2008 and early 2009, he and three colleagues took a closer look around Rhea using the visible-light camera on board Cassini. The camera had views of Rhea backlit by the sun—which would light up dust inevitably associated with any rings—as well as other views with the sun behind the camera, when objects up to boulder size would be most easily detected. But nothing showed up. The Cassini search was good enough to detect several orders of magnitude less material than needed to account for the observed electron shadows, the researchers say. "We are now ruling out the possibility that [the electron shadows] are due to rings of solid material," Tiscareno says.
Jones's team concedes. "We can't argue with the conclusions of Tiscareno and colleagues," Jones says. But if not rings, then what? "What we saw is clearly real," he says. "It's probably due to interactions between Rhea and the surrounding magnetosphere." Nothing like the Rhea electron shadows has been seen elsewhere, so Jones doesn't have a clue yet what those interactions might be. Fortunately, Cassini has more Rhea flybys planned during its recently extended mission.

http://news.sciencemag.org/2010/06/moon-rings-never-were

[StefanR]

Intense plasma wave emissions associated with Saturn’s moon Rhea

Measurements by the Cassini spacecraft during a close flyby of Saturn’s moon Rhea on March 2, 2010, show the presence of intense plasma waves in the magnetic flux tube connected to the surface of the moon. Three types of waves were observed,
(1) bursty electrostatic waves near the electron plasma frequency,
(2) intense whistler‐mode emissions below one half of the electron cyclotron frequency, and
(3) broadband electrostatic waves at frequencies well below the ion plasma frequency.
The waves near the electron plasma frequency are believed to be driven by a low energy (∼35 eV) electron
beam accelerated away from Rhea. Their bursty structure is believed to be due to a nonlinear process similar to the threewave interaction that occurs for Langmuir waves in the solar wind. The whistler‐mode emissions are propagating toward Rhea and are shown to be generated by the loss‐cone anisotropy (at parallel cyclotron resonance energies around 230 eV) caused by absorption of electrons at the surface of the moon. Scattering by these whistler‐mode waves may be able to explain previously reported depletions of energetic electrons in the vicinity of the moon. The low‐frequency waves may play a role in nonlinear three‐wave interactions with the bursty electrostatic waves.

http://www-pw.physics.uiowa.edu/~dag/publications/2011_IntensePlasmaWaveEmissionsAssociatedWithSaturnsMoonRhea_grl.pdf

[StefanR]

Cassini Measures Rhea’s Gravity

Originally, Rhea was believed to have been big enough that gravity had allowed the denser rock to separate from the surrounding ice and sink to its core. However, data from previous Cassini flybys showed that might not be the case. The problem was that a measure of the moon’s gravity distribution, its moment of inertia, was too high. The value calculated by Cassini flybys would mean that the moon hadn’t differentiated at all. That number has been refined somewhat lower, suggesting that Rhea has at least partially differentiated, but the exact value of the moment of inertia has yet to be determined.
Previously, the moment of inertia had been calculated by simply measuring the change in Cassini’s orbit caused by the flyby. However, the large spread in the moment of inertia derived from previous encounters meant that the more sensitive Radio Science Subsystem (RSS) needed to be used. The RSS is designed to transmit radio waves of known frequency back towards Earth. Because the original frequency is known to scientists here on Earth, it allows for very precise determinations of velocity changes caused by slight changes in gravitational fields as the probe moves around a body. These precise measurements allow for a more refined measure of the moment of inertia, which will allow us to finally settle the question of whether or not the moon has differentiated.


The close-up images reveal a long, narrow fracture where a portion the surface has faulted and sank relative to its surroundings. The fracture is similar to scarps found on the Moon and Mercury, where cooling has caused the crust to contract. The scarp imaged by Cassini appears to be fairly young, as it offsets fresher-looking craters. The exact age of the scarp will probably never be determined, it does indicate that Rhea is still contracting as it cools.

http://astronomyaggregator.com/exploration/cassini-measures-rheas-gravity/