Rainy days on Saturn

[jjohnson]

Apparently the astronomers at the ESA's Herschel team aren't reading back issues of Astronomy and Astrophysics. I'm referring to the May 6, 1999, issue, and the article by Valery Dikarev titled Dynamics of Particles in Saturn's E Ring: Effects of Charge Variations and the Plasma Drag Force.

The E-ring, of course, is also known as Saturn's plasma torus, and articles on its mass-loading rate and plasma composition have been around for a while, with the widely held view that Enceladus is ejecting water (vapor, grains, icy chunks - you name it - it's actually ionized (charged ) water components.) This ejected plasma form of water constitutes an ionosphere around Enceladus, plus the fraction that reaches escape velocity typically enters the E-ring. The E-ring actually IS visible in when back-lit - just look at NASA's Cassini gallery for photos. It might appear larger or more spread out to Herschel's IR cameras, but the plasma ring itself is hardly a secret that they just managed to uncover.

The interesting part is how this water gets to Saturn. The more recent April 2011 release of the measurement of a highly energetic "flux tube" (electric current, in EU and electricians' terminology) connecting the polar region of Enceladus with a co-moving spot (visible in UV) on Saturn's northern polar auroral area is the vehicle. This is a current of charged particles, which likely will be found (my prediction) to be largely composed of the above water-originated radicals from Enceladus.

Not all currents are composed of electrons, of course - in plasma both positive and negative charged particles exist simultaneously, and probably in pretty close to equal numbers. All you need for current to flow is an electric field and charged particles. Enceladus's is a charged body. Its motion through Saturn's magnetic field - it's within the magnetosphere in the E-ring - creates the voltage or electrical field that pushes charged particles along magnetic "field lines" between Enceladus and Saturn. The return current hasn't been found or measured yet. It might be via an equatorial current, but more likely would be similar to Jupiter - a south polar exit back to the thermally "hot" Tiger Stripes area around Enceladus's south pole, where surface ice is removed by electric machining or sputtering as the ring current enters the moon.

This is the second measured flux tube between a ringed gas giant and a moon traveling in a plasma torus created by electrical interactions between a moon and its planet. (The well-studied Jupiter-Io connection being the other, along with two other moons there which also have auroral footprints.) There are two more ringed gas giants to go. There are strong indications that Uranus has plasma regions in which [some of] its moons orbit, with Miranda exhibiting "nitrogen volcanoes or geysers" and distant Neptune may, too. Further exploration of those bodies may yield similar results, in that case.

[Sparky]

Enceladus is ejecting water (vapor, grains, icy chunks - you name it - it's actually ionized (charged ) water components.)

"nitrogen volcanoes or geysers"

Jim, great post!

In outer space water would always be in ice form, wouldn't it? Would glow mode plasma be warm enough to melt ice in space.?
Will water with a few missing electrons have different characteristics, boiling point, freeze point, etc.?

What would account for the Nitrogen?

It would be interesting to see the raw spectral data for this ice and nitrogen ejections...

thanks

[jjohnson]

I do not know if water would be in the form of ice in space or not. If it were a mass of water, and "flash-frozen" in space, relatively quickly, say, it or smaller icy grains might continue to exist, but as the pressure in space is extremely low, I wonder if there still wouldn't be sublimation - solid phase directly to gas phase - and the cloud would dissipate eventually. I don't know of a direct experiment having been done in this regard, say, on the ISS, but haven't looked it up, either. The presence of ionizing radiation and solar wind ions impacting the ice could also play a role in breaking down grains into distinct molecules, and possibly even ionizing the water molecules.

THere is a fairly good article on Triton on Wikipedia. The surface is seen to be largely nitrogen ice, as at 38K nitrogen can't exist in the liquid state. The dark smears photographed by Voyageur 2 are alleged to be nitrogen laced with dust being expelled from the surface by cryovulcanism and subsurface heating of some sort, possibly (in their minds) tidal heating, which was also trotted out as to why Io and Enceladus expel material. I do not know if the planetary geologists think that it is liquid or gaseous nitrogen in the "volcanoes" - that would depend on the local temperature in which it is changed out of the solid state, I presume.

However, Ohmic or Joule heating, similar to the processes at work on Io and Enceladus, and similar to induction heating in a metal foundry, are never brought up in the press as a potential cause of the heat, even though high amperage currents (those pesky flux tubes) have been observed on those two moons, and the amount of heating is 10X on Enceladus what they had predicted based on their tidal theories. Go figure. I have to run; will try to post a couple of interesting Neptune references here later.

[jjohnson]

To find references to use in discussions like this, my process is simple. I'll Google a name or a process or something and look for papers on the subject. If it is a subject about which I don't know much, I'll look for Wiki information, and especially look at Wiki's references and read those if they appear applicable. I like papers from NASA and NRAO scientists, and similar agencies in other countries, most of which are posted in my native English, fortunately. I avoid pay walls and have come to know which sites always have a payment to read beyond the Abstract, and which ones occasionally let some papers go for free. Also, sometimes you can find a PDF of a paper (that is for sale by some journal or collection) that the author herself has posted on her University site, for example - free and clear. And of course there is arXiv, increasingly used as pre-press postings by scientists for reviews (in a limited but useful set of subject areas), with a final version on arXiv and the accepted final paper in a peer-reviewed journal or other pay wall.

Another route is to get images of the subject - my browser is Google Chrome, but I imagine others do much the same - click on Images and look at a sometimes motley collection that the computer thinks relates to your key word(s). Don't be surprised if you put in Triton and get a rock band photo, or a logo, or someone's vacation picture in with useful images of the moon - you just have to parse through the noise. Then, if you have an interesting picture, click on its source, which may take you straight to an article or a set of class notes by an instructor or something really useful that you might not otherwise have known about or stumbled over. Well, we all have our browsing methods. You learn to read real fast!

On Triton and the linkage with Neptune, there are many papers concerned with the plasma environment observed by Voyager 2 as she passed through the neighborhood. Some good photos as well. Here's one abstract:

Plasma Temperature Profiles in the Magnetosphere of Neptune
Aharon Eviatar
Also at Center for Space Physics, Boston University, Boston, Massachusetts.
Vytenis M. Vasyliūnas
Max-Planck-Institut für Aeronomie, Katlenburg-Lindau, Germany
John D. Richardson
Center for Space Physics, Massachusetts Institute of Technology, Cambridge

It is generally agreed that plasma in the magnetosphere of Neptune is produced primarily in the vicinity of the orbit of Triton, near and beyond 14 RN , and is then transported inwards toward Neptune. The observed temperature profiles of protons and nitrogen ions, however, show an initial decrease with decreasing distance from Neptune, the expected increase from adiabatic compression not setting in until distances below 8 RN have been reached. We propose that cooling produced by collisions with atoms of a neutral hydrogen cloud that extends inward to 8 RNmay explain this behavior, at least for protons. Inclusion of inelastic scattering in addition to the more commonly invoked charge exchange makes cooling sufficiently effective so that the observed temperatures may be accounted for with plausible values of neutral hydrogen density.
Received 2 September 1994; accepted 30 March 1995

Here's another one, a little longer. Note that there are quite often useful concepts in the abstracts, even if you can't get the whole paper, which is always the best if possible. In these two abstracts, note the plasma physics terms, including ions, magnetic field data, magnetic-field aligned, anti-planetward streaming ions and electrons (i.e. electric currents!) and so on. Conclusions are also summarized, also useful in knowing what the authors are presenting for critical examination. You can "get the drift" in good abstracts.

The magnetosphere of Neptune: Hot plasmas and energetic particles
B. H. Mauk
Applied Physics Laboratory, The Johns Hopkins University, Laurel, Maryland
E. P. Keath
Applied Physics Laboratory, The Johns Hopkins University, Laurel, Maryland
M. Kane
Applied Physics Laboratory, The Johns Hopkins University, Laurel, Maryland
S. M. Krimigis
Applied Physics Laboratory, The Johns Hopkins University, Laurel, Maryland
A. F. Cheng
Applied Physics Laboratory, The Johns Hopkins University, Laurel, Maryland
M. H. Acuña
Goddard Space Flight Center, Greenbelt, Maryland
T. P. Armstrong
University of Kansas, Lawrence, Kansas
N. F. Ness
Bartol Research Institute, University of Delaware, Newark, Delaware

Abstract
A comprehensive overview is provided of the hot plasmas and energetic particles (>20 keV) observed in the vicinity of Neptune by the low energy charged particle (LECP) experiment on the Voyager 2 spacecraft. The LECP data are ordered with respect to magnetic field data and models derived from the Voyager magnetometer experiment. The findings include the following: (1) Weakly enhanced ion and electron fluxes were observed at the position of the subsolar bow shock. (2) Magnetic-field-aligned, antiplanetward streaming ions and electrons were sporadically observed within the inbound (subsolar) and outbound (tail flank) magnetosheaths, and within the unique ‘‘pole-on’’ cusp region encountered during the inbound trajectory. (3) Tangential ion streaming was observed at the positions of both the inbound (dawnward streaming) and outbound (tailward streaming) magnetopauses. (4) A distinct ‘‘trans-Triton’’ ion population outside the minimum L shell of Triton is characterized by large angular anisotropies that show that heavy ions (presumably N+) are a likely constituent. This population is at least partially corotating with Neptune out to at least L=27 RN and is also characterized at times by cigar-shaped (field-aligned) pick angle distributions, possibly indicative of an interaction with a neutral torus. (5) Within the middle magnetospheric regions (inside Triton), pitch angle distributions have well-developed trapped or ‘‘pancake’’ shapes. Also, in contrast to Uranus, flux profiles show no evidence of substorm-generated azimuthal asymmetries. (6) Triton (and/or Triton-generated neutral gas) controls the outer bounds of the hot plasmas and energetic particles, although the mechanism of that control is nuclear. Also, there are clear charged particle signatures of satellite 1989N1 and of ring 1989N3R. However, the large number of calculated critical L shell positions associated with all of the rings and satellites renders impractical at this time the unique determination of causal relationships between the many observed particle signatures and known material bodies.
(7) Concerning the bulk (integral) and spectral parameters of the hot plasmas, if it is assumed that the trans-Triton population is dominated by N+, the plasma β parameter reaches ∼1 within the near-planet magnetotail (L∼28 RN; in conjunction with a magnetic field depression ‘‘tail event’’), having only reached ∼0.2 in the more planetward regions. Integral electron energy intensities are such tht the more localized Neptune UV aurora can be explained if loss cone intensities are ≲1% of trapped intensities. In contrast to the Uranian magnetosphere, the lower-energy electron distributions appear generally to be at least as well characterized by hot Maxwellian distributions (kT=10 to 30 keV) as by power law distributions inside L ∼20RN, a characteristic generally exhibited at the other planets by the ions. At Neptune the ions have kT=12 to 100 keV, and kT is strongly correlated with position relative to Triton’sL shell. (8) Within the Neptunian magnetotail, planetward, magnetic-field-aligned streaming of ions and electrons is observed within the distant (∼67 RN) plasma sheet and within a closer region thought to be a detached or striated portion of the plasma sheet population. Within the near-planet magnetotail (L∼28 RN), where the spacecraft crossed from the plasma sheet to the tail lobes, cigarlike electron distributions are observed, suggestive of shell-splitting/magnetopause-sweeping effects. Consistent with the middle magnetospheric observations, and in sharp contrast to the Uranian magnetotail, the Neptunian magnetotail shows no evidence of substorm processes. ©American Geophysical Union 1991
Index Terms: 2756 Magnetospheric Physics: Planetary magnetospheres; 2740 Magnetospheric Physics: Magnetospheric configuration and dynamics; 2732 Magnetospheric Physics: Magnetosphere interactions with satellites and rings; 2724 Magnetospheric Physics: Magnetopause, cusp, and boundary layers.

Citation: Mauk, B. H., E. P. Keath, M. Kane, S. M. Krimigis, A. F. Cheng, M. H. Acuña, T. P. Armstrong, and N. F. Ness (1991), The magnetosphere of Neptune: Hot plasmas and energetic particles, J. Geophys. Res., 96(Supplement), 19,061–19,084.

I see a pattern here among the 4 ringed gas giants in our solar system. (Well, 3, although there is evidence that Uranus and Miranda may fall into the pattern as well.) A charged moon orbits in a plasma ring or torus, moving through the planet's magnetic field, within its magnetosphere, generating electric currents, already directly observed at the nearer two, Jupiter and Saturn. Cryovulcanism, possibly based on tidal heating, is observed. The ejecta happen to be of the same species of matter of which the surfaces of these moons are composed: Io and sulfur; Enceladus and water; Triton and nitrogen. The ejecta are found to be mostly charged particles — plasmas; a significant fraction of which have escape velocity and are entrained or "loaded" into the plasma torus or ring. The locations of the ejections or "geysers" are photographed and identified. Ohmic heating of the moon's crust or interior is not discussed as a possible mechanism. Electric discharge machining and etching by high amperage electric currents is not published as a potential machining and discharge process to explain the events or the morphologies of the surface near discharge regions.

Note that this pattern is not a theory - it is a small paradigm, or a little set of linked observations and mechanisms and speculations as to cause and effect. This is a part of causal reasoning, and qualitative physics. It could lead to a theory, or be incorporated into a wider theory, at some indefinite future time. It also could be shown to be false, but it is based on real measurements and papers and discussions and conclusions by real scientists in their papers. This is why references to others' papers in "good" sources are valuable here: it makes the arguments and ideas not only more plausible, in general, but harder to argue against. It is good practice for us to let each other know where we get our ideas in this way, because we can usually learn something from every new link we come across.

[Sparky]

jim, thanks for all the info.... you pointed out that water would sublimate to gas very quickly...of course...it's these spectral readings that are confusing me...conclusions are put forth that "water" is being seen in space...that doesn't make any sense to me...what are they really seeing?...could all isotopes of water show up as the same spectral line? or do they see a spike at H20 and ignore all the noise around it? the noise would be ions, and collectively, possibly more abundant than H2O...

my understanding is that there is no such thing as pure water in nature...it is a mixture of isotopes and other elements/compounds.

as for the nitrogen geysers, i guess IF a body was ejected by fissioning of a larger body, then it may have more of some elements than others...no two calves would be the same composition...

just thinking here....trying to keep my mind somewhat on the road....

thanks..

[jjohnson]

Sparky, I was just thinking out loud and speculating about what I thought might be forces influencing the outcome of releasing water into open space. I don't know for sure because I've never heard of that experiment being performed and observed. I don't know if different isotopes of atoms yield different spectrograms. Isotopes just have more or fewer neutrons in the nucleus than the standard or most abundant isotope, and I'm not sure how that difference would change emission or absorption wavelengths.

Regarding Triton and nitrogen, spectral analysis of its surface by Voyager 2 showed it was composed of nitrogen ice. Wiki notes that scientists think it has a rocky or rocky and metallic core, but I am not sure what that is based on. Anyway, there are at least 2 moons (and likely Miranda, too) which have solid surfaces whose constituents appear to be ionized by the action of high-amperage electric currents connecting the moon to its planet. Given the similar planetary magnetospheres and plasma regions ringing all 4 gas giants, it's my thought that currents and surface machining and ejection might be likely on all of them, after being observed so well on Io and Enceladus. It's my speculation.

In observations in space, scientists usually identify the spectral presence of OH- (hydroxyl) radicals as indicative of a water origin. A chemist will tell you, and maybe geologists, too, that other materials than water may also be able to form hydroxyl radicals, so they may not know for sure that they are observing water, but that's what they need to be seeing. You know, "dirty snowballs" and the like. Water masers are another phenomenon where hydroxyl presence is inferred to be water in origin by astronomers. It may be. But it's not an open and shut case, so far as I know.

No one has seen a moon being formed. Consequently, hypotheses abound as to how that goes. I have no idea. The thought arises that a moon with a preponderance of one surface material might have been formed at the radius in a z-pinch where materials with that range of ionization potentials are being condensed. Planets might work similarly, with a lot of silicates and rocky and metallic stuff. For Io, solid sulfur and sulfur compounds cover its surface. For colder moons, farther from the Sun, Enceladus and Triton have solid water ice and nitrogen ice surfaces, respectively. It is interesting that all three of those moons are in plasma rings, have high amperage currents connecting them to their planets, and eject ionized surface material into their ionospheres and plasma rings.

Voyager 2 passed close to the moon Miranda, at Uranus, but she is the smallest of the 5 major moons, and Uranus has a highly tilted plane of rotation, and a magnetic field that is tilted some 60 degrees of its rotation axis. Uranus and its moons are still a pretty sparsely studied system. We still have lots of things yet to unfold before us.

Jim

[Sparky]

Jim, thanks for ideas and directions i could take.....

this is a google search for water ice sublimation in low pressure...but it is a ppt, and i don't think i can view it....
http://www.google.com/url?q=http://www. ... 1lBL-r7eSA

found that N is one of the most abundant elements and is somewhat stable, so that would account for it being seen in gas form, i guess.

[Sparky]

Jim, during my poking around to learn about spectral lines for elements and isotopes, phases for elements, and the sidetracks during those investigations, i found another theoretical construct, metallic hydrogen. it is suppose to exist under large gravity conditions, ie., interior of saturn and jupiter...

so, a dwarf star like jupiter will not have fusion in it's interior, but instead have liquid hydrogen, which, like black holes, has not been demonstrated with earthly experiments. But it is theorized to be able to change phase states by magnetic fields . And being a good conductor, magnetic fields must be present. Oh, what a can of worms they have opened with metallic hydrogen...

I have found that each isotope has it's own spectral line.
have also found that magnetic fields affect spectral lines....

could not determine that sublimation of water at vacuum pressure and -455F is probable...need to find a phase graph that will show those extremes...

[jjohnson]

Sparky, e-mail me by clicking on the envelope to the right under my name and I'll see if I can send you a PDF or even a jpeg of the Powerpoint slide. It is just one big PP image, probably from a poster exhibit at a conference. I had no trouble opening it with a Mac.

Jim

[Sparky]

Jim. thanks for link.... http://www.lsbu.ac.uk/php-cgiwrap/water ... phase.html

whoa! no simple thing, water!

from diagram, i see that water in space is Ice...

Water has many solid phases (ices). There are sixteen or so crystalline phases (where the oxygen atoms are in fixed positions relative to each other but the hydrogen atoms may or may not be disordered but obeying the 'ice rules'j) and three amorphous (non-crystalline) phases. All the crystalline phases of ice involve the water molecules being hydrogen bonded to four neighboring water molecules (see [1300] for a recent review). In most cases the two hydrogen atoms are equivalent, with the water molecules retaining their symmetry, and they all obey the 'ice rules'j.The H-O-H angle in the ice phases is expected to be a little less than the tetrahedral angle (109.47°), at about 107°. The Clapeyron equationo for many ice phase changes has to be adapted due to water's negative expansion coefficient and anomalous change in entropy with volume