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Section I. SpuItering
of ices
ASTROPHYSICAL
IMPLICATIONS
L.J. LANZEROTTI
and W.L. BROWN
OF ICE SPUTTERING
A T&T Bell L.uhorcrrone.~s,Murray Hdl. New Jersyv 07974. C’SA
Volatiles
Saturn)
condensed
arc important
particle environments
on surfaces and condensed as major constituent,
constituents
such as the solar wind and planetary
icy obJects. producing physical and chemical alterations.
ob~ccts,
using laboratory-based
of astronomical
of the solar system and, likely. the interstellar
and spacecraft-measured.
medium.
bodies (e.g.. comets or the satellites of
In most regions of space the charged
magnetospheres can interact directly with the ice-covered surfaces and the
Many of the recent studies of the effects of charged particles on solar system
data are summarized
1. Introduction
The solar system is filled with photons from the sun
and the stars, and with charged particles (plasma populations),
primarily
electrons
and protons,
in the solar
wind, from solar flare-produced
cosmic rays, from
cosmic rays in the galaxy, and in the magnetospheres
of
many of the planets. If the surfaces of planets and
planetary satellites are not shielded by atmospheres
or
magnetic fields, these particles can impact the surfaces,
physically eroding and chemically changing them. Other,
smaller objects in the solar system such as asteroids,
minor planets, comets and grains are generally not
shielded and are thus subject to the full irradiation of
the particle environments
in which they are found.
Stimulated
particularly
by considerations
that are
important
for understanding
lunar materials obtained
during the Apollo missions to the moon, a number of
investigations
have reported the effects of radiation on
the surfaces of planetary objects or their satellites e.g.
refs. [l-3]. In the same era, Kimura [4] proposed that
cosmic rays could modify and limit the growth of grains
in interstellar
space and thus affect their properties.
This work does not seem to have been followed up in
any detail. except in very general considerations
of the
growth. destruction, and stability of grain mantles, e.g.,
ref. 151. Recently the first evidence has been reported of
cosmic ray tracks in interplanetary
grains, incident on
Earth and collected at high altitudes [6].
The constituent
materials of the outer solar system
are vastly different than those of bodies orbiting within
a few astronomical
units of the sun (one astronomical
unit (AU) is defined as the Sun-Earth
distance).
In
addition to the giant gaseous planets Jupiter (- 5 AU)
here.
and Saturn (- 10 AU), with densities near one, the
outer reaches of the solar system consist of planetary
satellites made up largely of condensed volatiles, water
in the case of several of Jupiter’s satellites and Saturn’s
rings, and possibly water and ammonia mixtures in the
case of some of the Saturnian satellites. The temperatures continue to drop for objects more distant from the
sun and volatiles such as methane can be condensed on
the rings and moons of Uranus, and probably on the
surface of the planet Pluto and its satellite Charon.
Characteristic
temperatures
in the solar system are
shown in fig. 1 171. The solid line is the temperature
profile expected from simple black-body radiation equilibrium on a rapidly rotating object with an albedo
(reflectance
coefficient)
equal to zero 181. The solid
point for each planet indicates the effective average
temperature
of the respective planet using measured
albedos. On the right-hand scale are indicated the melting points for various molecular species found in the
solar system and the effective average of temperatures
(broken lines) of bodies composed of the indicated ices
exposed to solar photon irradiation [9]. It is clear that
although close to the sun the icy objects will lose mass
rapidly through sublimation,
in the outer solar system
the condensed-gas
frosts will be stable.
In addition to the icy satellites of the planets, comets
and ice grains are constituents
of the outer regions of
the solar system and, in particular, of the regions beyond the orbit of Pluto (aphelion
- 50 AU). Halley’s
comet is actually a quite nearby resident of the sun,
with an aphelion distance of - 35 AU, just beyond
Neptune. Virtually all of the nonperiodic comets which
enter the solar system are believed to originate in a large
“cloud” of icy bodies residing at distances up to - 5 x
1. SPUTTERING
OF ICES
L.J.
374
10
-0.1
I
Lmrerotti
I
et al. / Astrophysical
I
1
DISTANCE
I
10
FROM
SUN
implications
-
of icesputtering
H2
50
(AU)
Fig. 1. Temperature
in K as a function of distance from the sun in astronomical
units (AU; 1 AU = average Sun-Earth
distance).
Solid points: effective average temperature
of the indicated planets. Solid line: black body temperature
on a rapidly rotating body
with zero albedo. Dashed and dashed-dotted
lines: effective average temperature
(determined by equilibrium between solar photon
irradiation
and sublimation) of an object composed of the indicated ice. Melting points for several ices indicated on right-hand axis.
Adapted from [7].
104AU from the sun (the Oort cloud of comets). These
objects, some lOI to 1013 in number, are believed to
surround the solar system. Individual members can occasionally be perturbed by the gravitational
force of a
passing star, causing the icy object to enter the inner
solar system where its ices can be sublimed.
The plasma environments
in the solar system have a
wide range of particle energies and intensities.
For
example, the solar wind, continually flowing out from
the sun with a density of a few particles per cubic
centimeter at the orbit of Earth, is composed of electrons and ions (primarily protons with a few percent
helium) with energies of approximately
1 keV. Eruptions on the sun produce sporadic outbursts of solar
cosmic rays with energies from tens of thousands to tens
of millions of electron volts. The magnetosphere
particle
populations
(fluences and compositions)
vary widely
from planet to planet and have spatial and temporal
dependencies
at each planet. The magnetosphere
of
Jupiter contains intense fluxes of keV-energy sulfur and
oxygen ions, while Saturn has a population of nitrogen
and oxygen ions. The Pioneer and Voyager spacecraft
missions to Jupiter, Saturn, and the outer solar system
have been crucial in providing the necessary data without
which it would have been impossible
to assess the
effects of particle impacts on the icy bodies in the outer
solar system.
Summarized briefly below are a number of the studies that have reported on the effects of charged particles
on icy bodies in the solar system. As there has been
little work in relating specific laboratory experiments to
extra-solar
system astrophysical
problems,
the concentration
here is on the solar system. Details of the
work can be found in the listed references. References
primarily
to laboratory
experiments
and data not
specifically addressing astophysical problems are largely
omitted. Several recent reviews of various aspects of this
work include those of Brown et al. [lo], Lanzerotti et al.
[ll], and Cheng et al. [12].
2. Applications
A number of problems related to solar system astrophysics require consideration
of the effects of charged
particles on condensed
volatiles. A molecule or atom
eroded from a surface in space by a charged particle can
suffer a variety of fates. The sketches in fig. 2 illustrate
some of these, with the resultant effect depending upon
the physical characteristics
(size, density, gravitational
attraction) of the body (ice grain or planetary satellite,
for example) and the mechanism(s)
of the sputtering
process [lo]. An eroded particle can escape directly or
can strike the surface again (fig. 2a), depending upon
the gravitational attraction of the body and the sputtered
particle energy. If the body has a sufficient atmosphere
(as some planetary satellites do), the outgoing sputtered
particle will be stopped by atmospheric
collisions, dif-
L.J. Lanrerotti et al. / Astrophysrcal implications of ice sputtering
375
cbl
(a)
Fig. 2. Schematic illustration of possible fates of material eroded from a surface, depending upon gravitational
attraction and external
environment
of the object. (a) An eroded atom or molecule can escape or enter into a ballistic trajectory, depending on its ejection
energy and the gravitational
attraction of the object. In the case of an interplanetary
or interstellar grain, the incident particle may
pass completely
through and the gravitational
attraction
on the ejected species will be negligible. (b) An atmosphere
around a
planetary satellite can produce scattering and re-impact of the ejected species on the surface. (c) Ionization by solar or stellar photons
(hv) of an ejected species in the presence of a externally-imposed
magnetic field can cause a loss of the species. In practice. various
combinations
of these three conditions can exist in most environments.
tary or interplanetary
magnetic field). Some relevant
physical parameters of some representative
solar system
objects are listed in table 1.
Interplanetary grains. The solar wind and solar energetic particles can dominate sublimation, and thus be
the principal determinant
of the lifetime of icy (H,O)
fuse in the atmosphere,
and ultimately be recondensed
on the surface (fig. 2b). A molecule or atom eroded
from a body can be ionized by solar or stellar photons
or by an external astrophysical plasma (fig. 2c), with the
ionized
species probably escaping, especially if there is
an externally imposed magnetic field (such as a plane-
Table 1
Escape energies
and velocities
Object
Jovian satellites
10
Europa
Ganymede
Callisto
Saturnian satellites
Mimas
Encelades
Tethys
Dione
Rhea
Titan
A-ring object
Moon
Earth
from the surfaces
Radius
Density
(km)
(g/cm’
of various
)
solar system bodies.
&m/z’
)
1820
1500
2640
2500
3.5
3.5
2.0
1.6
178
147
147
114
195
250
525
560
765
2570
- O.M)l
1738
6378
1.2
1
1.1
1.4
1.3
1.9
6.6
7
15
22
28
136
- 2.6x 1O-5
164
980
-1
3.34
5.5
Escape
energy
(evfamu)
0.035
0.023
0.040
0.029
0.00013
0.0002
0.0008
0.0013
0.0023
0.036
- 2.6X10_‘5
0.029
0.64
Escape
velocity
(m/s)
2580
2080
2790
2390
150
190
400
500
660
2640
- 7.2~10-~
2400
11200
I. SPUTTERING
OF ICES
376
L.J. Lanzerotti et al. /Astrophysical implications of ice sputtering
grains, for distances beyond - 2 AU [13-151. For the
erosion of large surface areas in interplanetary
space,
the solar wind dominates the erosion of H,O ice surfaces
- 5 AU and the erosion of CO, ice surfaces
beyond
beyond - 20 AU [14]. The particles could also act as a
means of fusing aggregates of grain constituents
[16].
Comets and icy bodies. The icy conglomerate
model
of a comet nucleus [17] is consistent with the body of
observations
to date, although water ice in the solid
phase may only recently have been identified [18]. There
may be a number of icy bodies the size of a comet
(nucleus a few to several tens of km in diameter) that
reside in stable elliptical orbits with large ellipticities in
the outer solar system. Even Halley’s, in a highly elliptical orbit with aphelion - 35 AU, spends - 75% of its
- 76 year orbital period beyond the orbit of Uranus
(- 20 AU). The amount of matter lost from solar wind
erosion is small, though non-negligible,
- 10m5 g/cm’
of H,O or - 2 X lop4 g/cm* of CO, per orbit for an
object in a near-circular
orbit at - 50 AU [14]. Of
possibly greater importance
is the modification
of the
surface layers of the comet due to particle impact on the
ice mixtures [14,19-241. Since the particle fluxes are
unknown beyond the orbit of Pluto, it is not possible to
reliably estimate the irradiation history of comets in the
Oort cloud. It is also very difficult to reliably determine
the effects of interstellar ions on the mantles of interstellar grains, matter which may be important in comet
formation [25]. It may well be that in some instances the
processing of such mantles by particles should be considered together with the important
UV irradiation
processes [25].
Earth’s Moon. Only in the polar regions of the moon
is the temperature
low enough for frozen volatiles to
exist. The redistribution
of any frozen volatiles from the
polar regions [26] by sputtering by the solar wind and
by Earth’s magnetosphere
particles effectively eliminates
the possibility of surface water ice on that body [27].
Jupiter: lo. The intensely volcanic satellite 10 [28]
has been a subject of much study since the discovery of
the sodium emissions accompanying
IO in its orbit [29].
Soon after, Matson et al. [30] proposed
a sputtering
hypothesis to remove sodium from the surface. Much
subsequent
work has been devoted to studies of the
energetic particle erosion of SO, frost layers on colder
portions of the satellite [31,32] in order to understand
the complicated plasma torus discovered by the Voyager
spacecraft [33]. The considerations
for formation of the
torus and of various atmosphere
conditions around 10
have become very detailed, and are reviewed in Cheng
et al. [12].
Jupiter: Europa, Ganymede, and Cal&to. These three
Galilean satellites of Jupiter are all water-ice covered
and, indeed, water is a major ice. There have been a
number of considerations
of the effects of particle
bombardment
of these bodies [34-391, using particle
fluxes measured in the magnetosphere
of the planet by
the Voyager spacecraft.
Recent measurements
of the
velocity distributions
of the ejected species imply that,
because of the graviational attraction of these Galilean
satellites,
less loss will occur than was previously
thought, with more redistribution
and production
of
thin “atmospheres”
[39].
Saturn’s satellites and rings. Considerations
similar to
those of Jupiter’s satellites have been applied to Saturn’s,
again using particle fluxes measured in the magnetosphere by Voyagers 1 and 2. Escape of eroded species is
more likely because of the reduced gravitational attraction of these satellites (table 1, ref. [40]), with the eroded
water products capable of forming a heavy ion (oxygen)
torus in the inner magnetosphere
[16,41]. It has been
proposed that ammonia ice, suggested to be important
in the internal dynamics of, for example, Enceladus
[42,43], is not observed by Voyager remote sensing
instruments
because of the preferential erosion of NH,
compared to H,O [44].
The magnetosphere
particles decrease in intensity at
the edge of the major A ring, so that there do not
appear to be significant effects by the particles on the
principal rings of Saturn. However, galactic cosmic rays
do strike these ring particles, producing nuclear interactions and high energy protons and electrons from the
decay of produced neutrons [45]. The ions in the magnetosphere can interact with the tenuous E ring (in the
region of the satellites Enceladus
and Tethys). The
interactions,
together possibly with micrometeroid
impacts (whose fluxes are very uncertain), predict a very
short (on a cosmic scale) lifetime of - lo*-lo4
years
for this ring if it is not replenished [12,46].
Uranus and Pluto. It has been proposed, from laboratory-based results, that particle impacts on CH, ice in
the rings of Uranus would produce a loss of hydrogen
and a polymerization
of the residue, making them dark,
as observed [47]. These considerations
have been extended to discussions
of methane ice on the Uranian
satellites [12,48] and on Pluto [49]. Solar wind and
magnetosphere
particles could also produce polymerization of organic ices and materials on other satellites in
the solar system, such as Iapetus (around Saturn), which
has an unusual large, dark surface feature. However, no
quantitative
discussions
have yet been published.
A
proposal has been made that solar UV could produce
the Iapetus feature by polymerization
[50].
3. Summary
The astrophysical
implications
of charged particle
irradiation of ices in the solar system and the interstellar
medium can be very extensive. Considerations
of particle-induced
sputtering
and modification
of surfaces
must be taken into account in order to obtain a com-
L.J. Lanrerotti et al. / Astrophysical
plete physical understanding
of observed phenomena in
many regions of space. Studies of the solar system
present
the happy possibility
that observations
and
measurements
of all of the relevant factors - surface
characteristics,
particle fluxes, etc. - can be made in
order to ultimately arrive at firm physical understanding.
This summary is extracted from an invited talk presented at the 5th International
Workshop on Inelastic
Ion-Surface
Collisions, Gold Canyon Ranch, Arizona.
The work at the University of Virginia was supported in
part by the NSF Astronomy Division, grant No. AST82-00477 and the NASA Geophysics/Geochemistry
Division, grant NAGW-186.
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I. SPUTTERING
OF ICES