Latest from Enceladus

[StefanR]

I Introduction

An exciting period of exploration of the outer solar system is underway by spacecraft, by remarkably improved ground-based observations and by orbiting telescopes: International Ultraviolet Explorer (IUE) and Hubble Space Telescope (HST). These studies have revolutionized our understanding of the solar system revealing a kaleidoscope of unusual worlds. Because of the low surface temperatures, typically < 130K, ice is the 'rock' in the outer solar system. That is, excluding the four giant planets and Io, it is the structural and thermal properties of ice that determines the surface geology of many objects from the orbit of Jupiter outward (Burns and Matthews, 1986). Therefore, understanding the radiation chemistry of and desorption from ice or low-temperature hydrated minerals is critical. Other more volatile molecular species, such as N2, O2, CO, CO2, NH3, CH4, and SO2 form atmospheres and polar 'ices' or can cause the surface to be geologically active. Io, a moon of Jupiter, is an exception. Owing to its tidal interaction with Jupiter, Io is volcanically active and has lost its water and other light volatiles. Because of this, frozen SO2, a volcanic gas on earth, covers Io's surface (Burns and Matthews, 1986).
Since most small, outer solar system bodies, with the exception of Titan, have either no atmospheres or tenuous ones, their icy surfaces are exposed to the solar UV and to the local plasma causing desorption as well as physical and chemical alterations (Johnson, 1990; 1998). During the Voyager I tour of the outer solar system, W.L. Brown, L.J. Lanzerotti and colleagues at AT&T Bell Labs measured the ejection of molecules induced by energetic ion impact of ice. They discovered that the sputtering from low-temperature ices by fast, light ions is determined by the electronic excitations produced in the ice, rather than by knock-on collisions (Brown et al., 1978) and, hence, is an electronically-stimulated-desorption process. This exciting discovery opened a new field of study. Below the relevance of desorption to a few outer solar system bodies is described; for extended descriptions see Johnson (1990; 1996; 1998).
II Desorption from Solar System Bodies
The samples collected during the Apollo missions show the lunar surface is modified by the impacting solar-wind ions (~ 1 keV/u H+ and He++) and by energetic solar particles (Taylor, 1982). This aspect of planetary physics has recently been revived by the observation of Na and K 'atmospheres' around Mercury and the Moon (Potter and Morgan, 1985; 1988). Such atmospheres are produced by stimulated-desorption (the ions, electrons and UV photons) of these atoms from the rocky surfaces (Madey et al. 1998). The sodium atmosphere has been seen to extend to ~ 5 lunar radii from the moon's surface (Flynn and Mendillo, 1993), providing an impressive manifestation of desorption.

http://www.thunderbolts.info/forum/phpBB3/viewtopic.php?f=4&t=677&start=30

Sodium salts in E-ring ice grains from an ocean below
the surface of Enceladus
F. Postberg1,2, S. Kempf2,3, J. Schmidt4, N. Brilliantov5,6, A. Beinsen7, B. Abel7,8, U. Buck9 & R. Srama2

Saturn’s moon Enceladus emits plumes of water vapour and ice particles from fractures near its south pole1–5, suggesting the possibility of a
subsurface ocean5–7. These plume particles are the dominant source of Saturn’s E ring7,8. A previous in situ analysis9 of these particles
concluded that the minor organic or siliceous components, identified in many ice grains, could be evidence for interaction between Enceladus’ rocky
core and liquid water9,10. It was not clear, however, whether the liquid is still present today or whether it has frozen. Here we report the identification
of a population of E-ring grains that are rich in sodium salts ( 0.5–2% by mass), which can arise only if the plumes originate from liquid water.
The abundance of various salt components in these particles, as well as the inferred basic pH, exhibit a compelling similarity to the predicted
composition of a subsurface Enceladus ocean in contact with its rock core11. The plume vapour is expected to be free of atomic sodium. Thus, the
absence of sodium from optical spectra12 is in good agreement with our results. In the E ring the upper limit for spectroscopy12 is insufficiently
sensitive to detect the concentrations we found.

[url]zolotov.faculty.asu.edu/publ/Postberg-E-ring-salts-Nature.pdf[/url]

Growls from the Tiger Stripes: the Latest on Enceladus
Jennifer G. Winters

4. Tiger Stripes
As stated above, the ’tiger stripes’ were the first indication of interesting activity in
the south pole. These are four roughly parallel fractures, each about 130 km long and 300
m deep, flanked by ridges 100 m high. As the radius of Endeladus is only 250 km,
the fact that these features are so sizeable is remarkable. It is thought that these features
were formed by the upwelling of low density material (diapirism) as a result of tidal heating
(Nimmo & Pappalardo 2006). Figure 1 (from Porco et al. 2006) shows these fractures at
increasing magnifications in composites from Cassini’s NAC (Narrow Angle Camera). The
false blue-green color of B and C indicates the relatively coarse-grained ice particles that lie
within and just along the tiger stripes.
The temperature in the area of the tiger stripes is much higher than that of the surrounding
areas (114-157 K vs. 74-81 K) (Porco et al. 2006), due to a still unknown
geothermal source (see H 4.1 for further discussion), but it has been determined that the
high temperatures originate in or under the tiger stripes.
4.1. Geothermal Activity
Many have puzzled over the source of the abnormally high thermal activity occurring
at the tiger stripes. 4-8 GW of energy is being released from this area, known as the south
polar thermal anomaly. Radiogenic sources (those that release heat from radioactive decay
in an assumed differentiated chondritic rocky core) can only contribute 0.32 GW of this heat.
Other suggestions include shear heating from tidal forces (Nimmo & Pappalardo et al. 2007;
Hurford et al. 2007) and influences from the near-resonance with Dione (another of Saturn’s
moons) (Squyres et al. 1983). While these appear to be the most widely accepted causes
for the high energy being observed, Tobie et al. (2008) have proposed that friction due to
low viscosity in the boundary between the overlying ice layer and a possible subsurface sea
could generate enough energy to explain this thermal activity.
Abramov & Spencer (2009) observe that over the 16 month period between Cassini
flybys, the thermal emission varied by less than 15%, so it seems to be staying fairly constant
at this time. But Tobie et al.(2008) note that if there is liquid at depth, it is impossible to
sustain it over long timescales if the heat output is this great. Based on this, they propose
that the thermal emission rate at this time is abnormally high. And as if it is not difficult
enough to explain the extra heat, they have pointed out that this 4-8 GW of observed
energy does not include radiation that is outside the wavelength detection limits of the
CIRS (Composite Infrared Spectrometer) (7-1000 μm) or any heat flow from regions other
than the south polar area, so the total energy is probably even more than that observed.
This issue will remain a mystery until more conclusive data is received.
The location of this south pole anomaly (SPT) is also perplexing. Why are there tiger
stripes only at the south pole and not at both poles? Tobie et al. (2008) and Collins &
Goodman (2007) suggest that a negative gravity anomaly created by a subsurface sea, which
is created by the thermal output, will tend to reorient the satellite’s rotation axis so that the
hotspot is always at the south pole. This idea is a modification of one proposed by Nimmo
& Pappalardo (2006).
Figure 2 (from Tobie et al. 2008) shows a schematic of their model of the geothermal
processes occurring at the south pole, including a silicate core, a layer of liquid water, a
layer of warm ice, and then the overlying cold ice surface. Also illustrated are the vents and
plumes and the ’negative gravity anomaly’ due to the accumulation of melt.

http://www.chara.gsu.edu/~thenry/PLANETS/paper.winters.pdf

[StefanR]

Interesting overview of reasons of certain hypotheses pertaining to outgassing from oceans on Enceladus, fun and a bit saddening to read as there seems to be something overlooked along the way. It stands in sharp contrast with the ideas in the in the text following this one.

Surface, Subsurface and Atmosphere Exchanges
on the Satellites of the Outer Solar System
G. Tobie · B. Giese · T.A. Hurford · R.M. Lopes ·
F. Nimmo · F. Postberg · K.D. Retherford · J. Schmidt ·
J.R. Spencer · T. Tokano · E.P. Turtle

4.2 Eruption Processes, Plume-Surface Interactions and Deposits
Io, Enceladus and Triton are the only bodies on which eruptions have been observed.
Plumes on Io, Enceladus, and Triton provide insights into their sub-surface volatiles and
processes. These plume activities are responsible for the generation of transient and tenuous
atmospheres around these moons and for deposits that give Io and Triton their distinctive
appearance, and subtly affect features on Enceladus. Europa and Titan are two other moons
on which eruptive processes are likely to have occurred in the very recent past, but direct evidence
is still lacking. The presence of methane and argon on Titan suggests that outgassing
and hence eruptions have occurred during much of its history. On Europa, several surface
features suggest that effusive processes have recently occurred.
4.2.2 Enceladus
The Enceladus plume emerges from discrete sources (Spitale and Porco 2007) located on a
system of cracks in the ice crust (Spencer et al. 2006). Although a certain variability of the
plume activity is expected from the response of the cracks to tidal stresses (Hurford et al.
2007), the plume appears surprisingly steady over the time it has been observed. Similar
gas densities are derived from stellar plume occultations recorded by the Cassini Ultraviolet
Imaging Spectrograph (UVIS) with an increase by a factor of 1.7 from 2005 (Hansen et al.
2006) (1.5 × 1016 molecules per cm2 for a line of sight passing at 15 km altitude over
the south pole) to 2007 (Hansen et al. 2008). Moreover, unchanged individual dust jets are
observed in images taken over more than two years (Spitale and Porco 2007), which further
supports the idea that the plume sources remain active over long periods.
Such a continuous plume activity requires a steady mechanism for the production of
gas and grains. Explosive and self-limiting processes appear implausible (Brilliantov et al.
2008). This excludes geyser-like processes, with a build up of pressure, suddenly released
in an eruption, and it rules out the so-called cold faithful model (Porco et al. 2006). Also the
decomposition of clathrates (Kieffer et al. 2006) seems difficult to reconcile with the steady
production of gas and grains, although this idea is attractive since it offers an explanation
for the observed abundance of roughly 10% of volatile gases (CO2, N2, CO) in the plume
gas (Waite et al. 2006).
Alternative scenarios for the production of gas are direct evaporation either from liquid
water (Schmidt et al. 2008; Postberg et al. 2009) or warm ice (Nimmo et al. 2007) at depth
under the ice crust. In these models the gas flows through cracks in the ice to surface. At the
site of evaporation the gas is in near thermal equilibrium with the water and/or ice. When the
vapour is accelerated it expands and cools, in accordance with the laws of thermodynamics,
and thus becomes super-saturated, and ice grains may condense from the vapour.
When a new crack is opened in the ice crust both models (sublimation from liquid or
warm ice) lead to a nearly steady state after a transition period, in which the gas slowly
heats the ice in the vicinity of the crack. Initially, the vapour will condense entirely at depth
on the cold crack walls and by latent heat of condensation the ice will gradually warm
up. Eventually, the process approaches a steady state when the heat flow in the ice is in
equilibrium with the radiative loss at the surface and the heat supplied by advection by
the gas from depth. This transition to the steady state may take hundreds of years for a
crack of several kilometers depth (Ingersoll and Pankine 2010). During this period narrow
cracks might even be sealed by the condensing vapour before the gas flow reaches vacuum.
Systems of nearby cracks may act together heating the ice in their vicinity. In steady state
condensation at the walls will be mostly limited to the region close to the surface, since
there the temperature gradient, and thus the heat flow, is largest. In this way a conduit should
naturally develop its narrowest point near the surface.
Averaging over more complex channel profiles, Schmidt et al. (2008) obtained a differential
particle size distribution for the condensed grains which has a slope corresponding
to an exponent of −4 for grains in the size range of one micron but globally falling off
steeper than a power law. Also, the distribution exhibits a local minimum for submicron
sized grains. This distribution is consistent with the narrow E ring size distribution derived
from pre Cassini data (Showalter 1991; Nicholson et al. 1996) and Cassini CDA data (Kempf
et al. 2008).
From the structure of the dust plume seen in images it was early concluded that most of
the grains must fall back to the surface, ejected at a mean speed of 120 m/s (Porco et al.
2006), which is smaller than Enceladus’ escape speed of 240 m/s. The plume model of
Schmidt et al. (2008) gives a mean grain speed of 100 m/s, which is practically the same
number. The velocity distribution was recently constrained (Hedman et al. 2009) from a detailed
analysis and modeling of spectral slopes of the plume obtained from data taken by
the Cassini Visual and Infrared Mapping Spectrometer (VIMS). The authors find clear evidence
that large particles (around 3 micron radius) are ejected at low velocities, practically
all falling back to the surface and populating the lower parts of the plume, while smaller
particles are systematically faster.
Altogether, the low speed of the ejected grains appears surprising, since large gas speeds
on the order of 500 m/s were inferred from Cassini UVIS data (Hansen et al. 2006;
Tian et al. 2007). If grains condense in the gas, one would expect them to have the same
velocity. If the grains are formed by some other mechanism, then the gas must be sufficiently
dense to accelerate them. Also in this case the grain will rapidly reach gas speed,
again in contradiction to the observations.
There are in principle two ways to understand the slow velocity of the dust compared to
the gas. One possibility (Schmidt et al. 2008) is that frequent collisions of grains with the
vent walls repeatedly decelerate the grains relative to the gas. Such collisions are in practice
unavoidable in a realistic channel, i.e. one that is not perfectly straight, since the streamlines
of the gas and the trajectories will differ. Owing to their smaller speeds, large grains populate
the plume at lower altitudes. This model reproduces the particle densities measured by CDA
(Schmidt et al. 2008), the brightness of the plume seen in images, and the gas production rate
inferred by INMS and UVIS. Another possibility to obtain in principle slow grains and fast
gas is that the gas and dust decouple only in the uppermost, funnel shaped, part of a channel.
In this region the gas is further accelerated by the pressure drop from the channel to vacuum,
while it simultaneously dilutes by large factors. If this mechanism works quantitatively,
simulating the observed particle speed-size distribution (Hedman et al. 2009) remains to be
verified.
The composition of ejected material addresses the question of whether liquid water or
warm ice is Enceladus’ primary plume source (Zolotov 2007). If the jets originate from
sublimating ice, trapped gases would be the major non-water compound, whereas minerals
leached from the large Enceladian rock core should be present in a possible subsurface
ocean. In the latter case Na+ and Cl− ions, are expected to be the most abundant non-water
species, followed by bicarbonate (HCO−
3 ) and K+ (Zolotov 2007). During the slow downward
freezing expected after formation of icy planetary bodies, alkali salts always stay in the
liquid phase and the ice crust remains practically salt free. On one hand the abundant detection
of CO2, CH4, N2 and/or CO in the plume vapour (Waite et al. 2006) suggest the presence
of clathrate hydrates and gave rise to the proposition that the decomposition of such ices is
the actual plume driver (Kieffer et al. 2006). On the other hand the in situ measurements of
Cassini CDA show alkali metals in 93% of mass spectra from E ring ice grains (Postberg
et al. 2009). The E ring can be considered as a storage ring for previously ejected plume
grains. Whereas most of these grains show only traces of sodium and potassium (Na/H2O
≈ 10−7), about 6% exhibit a salt content increased by several orders of magnitude (Na/H2O
≈ 10−2–10−3). Particles with intermediate concentrations are rare (<3%).
The Na-rich CDA spectra imply 0.05–0.2M/kg NaCl and 0.02–0.1M/kg NaHCO3 yielding
an alkaline pH of 8.5–9 in a liquid with the ice grains composition. This is in compelling
similarity with the model calculations (Zolotov 2007). However, the inferred Na/K ratio of
100–300 is about 10 times higher than expected from the solar abundance. The presence
of salts in concentration as found in Na-rich E ring grains is difficult to reconcile with ice
sublimation (Nimmo et al. 2007) or clathrate decomposition as the main plume producing
process (Kieffer et al. 2006). The observed composition strongly favors an origin from water
that is or has been in contact with the rocky core.
The non-water molecules found in the plume vapour (Waite et al. 2006) suggest clathrate
decomposition as a parallel process. Although probably not relevant as a plume driver, gas
release from clathrates might take place at ice/water interfaces of the source or in ice layers
above such a reservoir. However, a “Soda Ocean” rich in bicarbonates could also work as a
source of CO2. Hydrothermal processes (Matson et al. 2007) might also be considered for
the production of N2, CH4, and other organic compounds. Postberg et al. (2009) introduce
a scenario in which Na-rich and Na-poor grain populations are both produced from liquid
plume sources (Fig. 8): Na-rich ice grains are formed by freezing of aerosols created by ascending
gas bubbles (e.g. from CO2) or other turbulent processes in the liquid. The Na-poor
grains (which represent the main E ring population) are suggested to stem from nucleation
of the supersaturated vapour (Fig. 8B) as suggested by Schmidt et al. (2008). Trace amounts
of NaCl molecules (present in low concentration in the liquid Zolotov 2007) can traverse
from liquid solution into the gas phase. Due to its high energy of solution, a significant
Na escape via the plume gases is not expected and in agreement with the non detection by
Cassini instruments and Earth bound spectroscopy (Schneider et al. 2009).
One might consider the possibility of parallel venting mechanisms from a liquid and from
warm ice. The first would produce the Na-rich grains, the latter the majority of non-water
plume gases and Na-poor grains. However, the formation of Na-rich particles requires a
liquid reservoir, so evaporation above such a liquid is probably the most plausible driver for
Enceladus’ plumes in general.
From heat flow arguments it can be shown that the liquid/gas interface below Enceladus’
surface must be orders of magnitude larger than the vent cross sections, otherwise implausibly
large temperature gradients would be necessary to balance the loss due to latent heat
(Postberg et al. 2009). Therefore large vapour chambers which narrow to the vent channels
are required above a liquid plume source.

http://www.sciences.univ-nantes.fr/lpgnantes/lpg/fichiers/tobie-g/PUBLICATION/Tobie_et_al_10.pdf

[StefanR]

Unlike the previous text, this one is I think a recommended read. Gives a nice oversight of ideas
which do deserve to be given more attention, as the authors ask for.
There seems to be some light coming through the cracks in the fabric of dull science.

Space Weathering Impact on Solar System Surfaces and Mission Science*
John F. Cooper?, Richard E. Hartle, Edward C. Sittler Jr., Rosemary M. Killen
NASA Goddard Space Flight Center, Greenbelt, MD, 20771, USA
Steven J. Sturner
CRESST, University of Maryland College Park, College Park, Maryland, USA
Chris Paranicas, Matthew E. Hill, Abigail M. Rymer
Applied Physics Laboratory, Johns Hopkins University, Laurel, MD, USA
Paul D. Cooper
George Mason University, Fairfax, VA, USA
Dan Pascu
U.S. Naval Observatory, Washington, DC, USA (retired)
Robert E. Johnson, Timothy A. Cassidy
University of Virginia, Charlottesville, VA, USA
Thomas M. Orlando
Georgia Institute of Technology, Atlanta, GA, USA
Kurt D. Retherford
Southwest Research Institute, San Antonio, TX, USA
Nathan A. Schwadron
Boston University, Boston, MA, USA
Ralf I. Kaiser
University of Hawaii at Manoa, Honolulu, HI, USA
François Leblanc
LATMOS, Institut Pierre Simon Laplace, Université Versailles Saint Quentin/CNRS, Verrièresle-
Buisson, France
Louis J. Lanzerotti
New Jersey Institute of Technology, Newark, NJ, USA
Claudia J. Alexander, Henry B. Garrett, Amanda R. Hendrix
NASA Jet Propulsion Laboratory, Pasadena, CA, USA
Wing H. Ip
Institutes of Astronomy and Space Sciences, National Central University, Taiwan
*Full references & other docs at http://nssdcftp.gsfc.nasa.gov/miscellan ... weathering
? John F. Cooper, Code 672, NASA GSFC; Phone: 301-286-1193; John.F.Cooper@nasa.gov.
Submitted to Planetary Science Decadal Survey, National Research Council, Sept. 11, 2009

Abstract

Space weathering is the collection of physical processes acting to erode and chemically modify
planetary surfaces directly exposed to space environments of planetary magnetospheres, the
heliosphere, and the local interstellar environment of the solar system. Space weathering affects
the physical and optical properties of the surfaces of planetary bodies, so understanding its
specifics is critical for interpreting surface data from remote and landed measurements. For full
coverage of space environmental measurements, we recommend expanded interdisciplinary
cooperation between NASA’s Planetary Science and Heliophysics divisions. To grow the field
in the next decade and maximize impact on mission studies, we suggest a balanced mixture of
laboratory measurements, modeling, and theoretical investigations in support of all missions.

Space Environments

Vast expanse of the space weathering environment interacting with solar system bodies is
illustrated in Figure 1 by logarithmic horizontal scale of radial distance from the Sun to α-
Centauri. Principal sources of energy for space weathering of planetary surfaces are UV photons,
solar wind plasma, and energetic particles from the Sun, within a few hundred AU, and external
sources of plasma and energetic particles entering into the solar system from the local interstellar
environment. Beyond the realm of terrestrial planets and the asteroid belts, the solar influence
significantly wanes with the decline in density of the expanding solar wind plasma and
magnitude of the frozen-in magnetic field, while the interstellar influence progressively grows
through interaction of interstellar neutral winds with solar ultraviolet radiation and the solar wind
plasma. Across the heliospheric boundary region near 100 AU, now being explored by the two
Voyager spacecraft, occurs a transition from supersonic (400 – 800 km/s) plasma flows of solar
coronal expansion, the solar wind, to 26 km/s inward flow of the interstellar wind. This plasma
contains both a bulk flow and thermal components associated with typical ion energies up to a
few keV, and energetic components extending to far higher energies, ultimately to the full range
of galactic cosmic ray ions easily penetrating into the heliosphere at GeV energies and higher.
The Sun contributes the innermost source of energetic particles in association with solar flare
and coronal mass ejection (CME) events, the interstellar environment contributes the outermost
sources, and the dynamics of the expanding and variable solar wind provide additional energy to
MeV energies within the heliosphere. The solar wind termination shock, the supersonic-subsonic
flow boundary crossed by both Voyagers (Stone et al., 2005, 2008), and/or the heliosheath region
beyond this shock out to the heliopause, the contact boundary with interstellar plasma, may
further accelerate plasma particles into the energetic range but this is not yet established by the
Voyager measurements. Neither spacecraft detected local particle acceleration at the respective
crossings, although the bulk of acceleration may be occurring elsewhere along the shock
boundary (McComas and Schwadron, 2006). Other possibilities are that the heliosheath ions are
energized by turbulent or reconnecting magnetic fields in the heliosheath, or that these ions
originate instead from penetrating interstellar ions (Cooper et al., 2006; Cooper, 2008). As solar
activity increases and then again declines within the next decade from the current minimum, the
continuing Voyager measurements, supplemented by direct energetic neutral atom measurements
of boundary region emissions by the Interstellar Boundary Explorer (IBEX) in earth orbit, are
expected to resolve origin of the heliosheath ions and to locate the heliopause. What is already
clear is that the termination shock boundary marks the transition from dominance of some space
weathering effects, e.g. erosive sputtering, by the supersonic plasma flow to a broader range of
effects from plasma and energetic particles at higher energies to the cosmic ray regime.
Within these expanding near-solar to heliospheric to local interstellar space environments we
find the objects of primary interest to planetary science: the terrestrial planets, the asteroid belt,
the gas and ice giant planets, comets, the Kuiper Belt, and finally the Oort Cloud. Aside from the
first known member of the Kuiper Belt, Pluto, now officially designated as an ice dwarf planet,
our direct knowledge of Kuiper Belt Objects (KBO) began with the first discovery in 1992, then
followed to date by about a thousand other discoveries of such objects, including a few classified
as members of the inner Oort Cloud. Presumably there are thousands more of similarly
detectable size waiting to be discovered, and far more at smaller sizes. Looking back towards the
Sun, there are also thousands of known asteroidal bodies, including Near Earth Objects
potentially of concern for future Earth impacts, and as a remote possibility the first members of
the fabled Vulcanoid Belt that might be found via increasing sensitivity of near-solar
observations. At the smallest scales there are interplanetary dust grains, the source of the
zodiacal light, extending down in size to nanometers or less (e.g., molecular clusters) and
thought to arise from impact surface weathering of small bodies and from comet outgassing.
The red and white stars of Figure 1 denote the distinctly different space environments of solar
system bodies with and without internally generated magnetic fields. Except for the highest
energy cosmic rays and their atmospheric interaction products, the direct effects of space
weathering do not extend to the solid surfaces of Venus, Earth, and Mars. While the planetary
magnetospheres (red stars) substantially deflect interplanetary plasmas and energetic particles
away from the atmospheres and underlying surfaces, even an ionospheric (white star) interaction
arising from ionization of a thin atmosphere, or surface-bound exosphere, can significantly
impede or totally inhibit access of space plasma to otherwise exposed surfaces. The surface of
Mars is notably oxidized by solar ultraviolet irradiation and to a lesser extent from high energy
(> 100 MeV) cosmic rays and solar energetic particles, while medium-energy (> 1 MeV)
energetic ions can easily penetrate Pluto’s microbar-pressure atmosphere to the surface. On the
other hand, the acceleration of charged particles within the planetary magnetospheres, and
magnetic pickup of exospheric ions, provides additional and potentially more dominant energy
sources for space weathering of surfaces exposed to those environments.

Space Weathering Effects
[.....]

Laboratory Measurements
[.....]

Recommendations

Support comprehensive specification of space weathering environments through expansion of
cooperation between NASA heliophysics and planetary science divisions on placement of
environmental radiation instrumentation on appropriate missions, compilation of data and
semiempirical models from measurements, and on predictive models for each environment.
Rationale: In the heliophysics community there is the concept of the Heliophysics Global Observatory
(HGO), the collective fleet of operational heliophysics missions, that should be expanded for
interdisciplinary applications to include planetary missions. Heliophysics support for space environment
modeling, e.g. the Earth-Moon-Mars Radiation Environment Module (Schwadron et al., 2007) and earlier
(e.g., NASA GSFC, JPL) models for solar and cosmic ray energetic particle modeling in the terrestrial
planet domain, can be usefully applied to planetary interaction applications. Similarly, missions and data
models for planetary interactions can also support investigation and modeling of interplanetary
environments. HGO data virtual observatory approaches could be applied to planetary missions.

Encourage community-wide and interdisciplinary investigations of universal space weathering
processes through balanced mixture of initiatives on mission instrument data analysis,
laboratory measurements, computational modeling, and relevant theoretical investigations.
Rationale: The space environment is universal in the sense of connecting all the planetary environments,
and so it most efficient to approach space weathering processes from the universal perspective, e.g. that
similar processes act everywhere and the effects differ only in the relative energy deposition rates and
compositional impacts of each process in different locations. Process investigations must be wellgrounded
in measurements for different environments, in broad-spectrum approaches to laboratory
simulations, and best available inputs from theory and high performance computing.

Enable development of plasma ion, energetic particle, and neutral composition spectrometers
for in-situ analyses to characterize elemental and isotopic range of interconnected planetary
surface, atmospheric, ionosopheric, magnetospheric, and heliospheric environments.
Rationale: Our knowledge of composition in these environments beyond the Earth is limited to some
major species with little or no information on the full range of elemental and isotopic composition that is
critical to determination of origins, evolutionary processes, and astrobiological potential. Sample return is
too expensive for general application, advanced in-situ analysis capabilities being required for one-way
missions to most non-terrestrial destinations of the solar system. There is also strong coupling of
composition for these connected environments and this coupling should be considered in weighting the
relative priorities of measurements in each environment.

Provide facilities for more realistic laboratory science and engineering simulations of
planetary surface environments under simultaneous influence of extreme limits on pressure,
temperature, radiation, composition, physical structure, and endogenic or impact activity.
Rationale: There are no truly flat surfaces, particularly when viewed at the microscopic level of most
space weathering processes, and multiple energy sources are typically operating on affected surfaces. The
sensible and accessible surfaces have impact regolith layers extending to meters in depth and likely with
high porosity under conditions of reduced gravity. Multi-phase interactions of ice, grains, and volatiles in
irradiated bulk surface samples need much further investigation with appropriate facilities. Engineering
simulation facilities require development to support realistic and extreme environment testing for future
orbital and landed missions to irradiated icy bodies such as at Europa, Ganymede, Enceladus, and Triton

http://nssdcftp.gsfc.nasa.gov/miscellaneous/space_weathering/

[jjohnson]

Don't forget the April 20th announcement from the Cassini team that they had discovered and measured a very strong electric current (aka "flux tube with an axial electron beam") that is correlated with a co-revolving northern auroral footprint, seen in UV light. They did not write up their estimate for the current strength (amperage), but the very similar one found connecting Io with Jupiter's northern and southern auroral ovals was estimated at 2 trillion amps. The team also did not mention that there might be southern hot spot on Saturn similar to Jupiter's, nor that if found it may represent the return side of the electric circuit circulating through Enceladus's polar regions.

In particular, the "anomalous" or "tidal" heating in the tiger stripes area centered around Enceladus's south pole, where geysers (ion and electron mass ejections) are observed, may simply be local Joule or ohmic heating of the crustal material there where the current flows into the moon. It is not simply local heating that is ejection or "mass loading" Enceladus's ionosphere and plasma torus with electrons,water ions and ionized salt molecules at the rate of approximately 100 kg/s. c.o.n.n.e.c.t the dots!

Jim

[seasmith]

[seasmith]

"Snowfields" Revealed

Last year, scientists predicted that some of the material spewed out into space by the icy geysers at Enceladus's south pole would slowly fall down to certain areas on the surface. Detailed measurements by NASA's planetary probe Cassini, presented at the meeting, have now revealed the predicted snow fields, which measure up to 100 meters thick. They betray their presence by their conspicuous bluish color and by softening the outlines of buried craters and canyons (inset).

http://news.sciencemag.org/sciencenow/2 ... space.html


http://www.universetoday.com/85056/enceladus-and-saturn-are-linked-by-electromagnetic-currents/

[tholden]

http://news.yahoo.com/blogs/sideshow/mi ... 07321.html

"More than 90 jets of all sizes near Enceladus's south pole are spraying water vapor, icy particles, and organic compounds all over the place," says Carolyn Porco, an award-winning planetary scientist and leader of the Imaging Science team for NASA's Cassini spacecraft. "Cassini has flown several times now through this spray and has tasted it. And we have found that aside from water and organic material, there is salt in the icy particles. The salinity is the same as that of Earth's oceans."

[seasmith]

May 27th, 2012

Spaceprobe reveals details about charged 'nanograins' near Saturn moon Enceladus

In a new study, Hill and colleagues describe what they found in the data from Cassini: a new class of space particles — submicroscopic "nanograins" of electrically charged dust. Such particles are believed to exist throughout the universe, and this marks the first time researchers have measured and analyzed them.

"The nanograins are in a 'Goldilocks' size regime that no one's seen before: not too big and not too small to influence, and be influenced by, the plasma," Hill said. "That's one of the things that makes them interesting."
For instance, because of their size, nanograins are noticeably affected by both gravitational and electromagnetic forces. This contrasts sharply with both larger particles that are dominated by gravity and smaller charged particles that are dominated by electromagnetic forces.

... because they lie in a theoretically important but previously unobserved range where particles have an intermediate mass-to-charge ratio,"

http://www.nanowerk.com/news/newsid=253 ... oo%21+Mail

There have been three Cassini encounters with the south-pole eruptive plume of Enceladus for which the Cassini Plasma Spectrometer (CAPS) had viewing in the spacecraft ram direction. In each case, CAPS detected a cold dense population of heavy charged particles having mass-to-charge (m/q) ratios up to the maximum detectable by CAPS (∼104 amu/e). These particles are interpreted as singly charged nanometer-sized water-ice grains. Although they are detected with both negative and positive net charges, the former greatly outnumber the latter, at least in the m/q range accessible to CAPS. On the most distant available encounter (E3, March 2008) we derive a net (negative) charge density of up to ∼2600 e/cm3 for nanograins, far exceeding the ambient plasma number density, but less than the net (positive) charge density inferred from the RPWS Langmuir probe data during the same plume encounter. Comparison of the CAPS data from the three available encounters is consistent with the idea that the nanograins leave the surface vents largely uncharged, but become increasingly negatively charged by plasma electron impact as they move farther from the satellite. These nanograins provide a potentially potent source of magnetospheric plasma and E-ring material.

http://www.agu.org/pubs/crossref/2012/2 ... 7218.shtml

[Lloyd]

Enceladus Subsurface Ocean
* This corroborates Ted's last post.
http://phys.org/pdf257696769.pdf

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 in residence in Saturn's magnetosphere since 2004. Following the original detection of the plume based on magnetometer measurements, Sven Simon from the University of Cologne, Germany, and Hendrik Kriegel from the University of Braunschweig, Germany, 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.

* Enceladus' diameter is about 500 km, so it probably has no liquid water now, just frozen. But electrical forces that produce the geysers may melt the ice briefly before shooting the vapors into space. Ted said the salinity is the same as that of Earth's oceans. That may be good support for Cardona's and others' theory that Earth was a moon of Saturn a few thousand years ago.

[meemoe_uk]

And here's a pic of part of Enceladus' electric circuit, including Enceladus in the middle.

"Wispy fingers of bright, icy material" - I don't know if this is from NASA or the boston news reporter.
Or maybe glowing plasma? Note the twisting double filament structure.

more pics
http://www.boston.com/bigpicture/2008/1 ... close.html