Nuclear Instruments and Methods in Physics Research B 193 (2002) 720–726
www.elsevier.com/locate/nimb
Atomic collisions in solids: Astronomical applications
R.A. Baragiola *, C.L. Atteberry 1, C.A. Dukes, M. Fam
a, B.D. Teolis
Laboratory for Atomic and Surface Physics, Engineering Physics, University of Virginia, Thornton Hall, Charlottesville,
VA 22904-4238, USA
Received in revised form 30 September 2001
Abstract
Airless bodies in space are subject to irradiation with energetic atomic particles, which generate atmospheres by
sputtering and alter the surface composition. Astronomical observations with telescopes and space probes continuously
provide new data that require new laboratory experiments for their interpretation. Many of these experiments also serve
to expand the current frontier of atomic collisions in solids by discovering previously unknown phenomena. Some of the
experimental techniques used in these experiments could find applications in other areas of atomic collisions in solids. We
present results from our current experimental research program on sputtering and surface modification of ices and
minerals and point out opportunities for research in this area. Ó 2002 Elsevier Science B.V. All rights reserved.
1. Introduction
This is a progress report on recent work from
our laboratory on atomic collisions with astronomical surfaces to which we have added key
references to work done elsewhere. Statements of
some unsolved problems in this area are intended
to stimulate the interest of other researchers in
atomic collisions in solids. We start by pointing
out that the solar system is a natural laboratory
for atomic collisions in solids. The harsh space
environment is populated by energetic ions, electrons and photons that impact the surface of any
body not protected by a relatively thick atmo-
*
Corresponding author. Tel.: +1-804-982-2907; fax: +1-804924-1353.
E-mail address: [email protected] (R.A. Baragiola).
1
Present address: Department of Physics, USAF Academy,
CO 80840, USA.
sphere, like most satellites, asteroids, comets,
Mercury, Pluto and spacecraft. Radiation from
the Sun that can alter materials consists of
UV photons, especially Lyman-a (10.2 eV), the
1 keV/amu solar wind, and occasional solar
flares. Fluxes decay with the square of the distance
R to the Sun; near Earth (R ¼ 1 AU) they are, on
average, 4 1011 Ly-a/cm2 and 2 108 ions
(electrons)/cm2 . Magnetospheric ion fluxes around
Jupiter (R ¼ 5:2 AU) and Saturn (R ¼ 9:54 AU)
are more intense and mostly Hþ , oxygen and sulfur ions; their energy distribution has a broad peak
at 10–100 keV and a thermal component. Indication of atomic collision processes comes from the
optical reflectance (sunlight reflected from the
surface), optical emission from ionospheres and,
for the Moon, from laboratory analysis of actual
surface rocks. Johnson’s book [1] is an invaluable
resource, describing the radiation environment,
observations, and modeling of atomic collisions
on planetary surfaces up till 1989. Many new
0168-583X/02/$ - see front matter Ó 2002 Elsevier Science B.V. All rights reserved.
PII: S 0 1 6 8 - 5 8 3 X ( 0 2 ) 0 0 8 9 3 - 5
R.A. Baragiola et al. / Nucl. Instr. and Meth. in Phys. Res. B 193 (2002) 720–726
discoveries appeared since then from improved
telescopes and new space probes.
Laboratory simulations can mimic the low
pressures of the tenuous atmospheres but not astronomical time scales, thus extrapolations are
needed. Lifetimes of a planetary satellite surface
between significant micrometeorite impacts may
be 100–1000 years. Ingredients to consider in the
extrapolation are the relative fluxes of incoming
radiation and atmospheric gases, of sublimation
from surfaces, and the rates of chemical reactions,
diffusion, segregation, phase transformations, etc.
Unfortunately, very little is known about those
processes and about the actual porosity, roughness and detailed composition of surfaces. Thus,
experiments aim at the basic understanding of
physical processes needed to model a large number
of possible situations.
Below we report different experiments designed
to understand how atomic collisions change the
surface region of icy satellites, and the rocky
surfaces of the Moon, Mercury and asteroids.
The experiments are done in ultrahigh vacuum,
using mass spectrometry, optical spectroscopy
(0.1–1 lm), and X-ray photoelectron spectroscopy
(XPS). Details on the different experimental
methods have been published [2–4]. Mineral surfaces are produced by fracturing while thin ice
films are grown by vapor deposition onto a cooled
microbalance. We measure sputtering yields with
the microbalance and from the flux of sputtered
species with a mass spectrometer (MS). This instrument is also used to measure the gas desorbed
from irradiated films while they are heated using
a linear temperature ramp. Irradiation fluxes are
insufficient to significantly heat the samples. Our
techniques complement studies using infrared
spectroscopy [5,6] and incident electrons [7–9] and
UV light [10–12]. Here we give representative references, usually to the most recent work, which
can lead to all relevant references that we have no
space to cite.
2. Atomic collisions with ices
Sputtering causes surface erosion by ejecting
molecules that contribute to the local atmosphere
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around the astronomical object. Knowledge of the
velocity distribution of sputtered particles is limited but important because it determines the fraction of particles that can escape gravity; those that
do not escape return to the surface (perhaps tens
of km away) and contribute transiently or permanently to the atmosphere. Planetary surfaces
are very porous due to micrometeorite bombardment. Porosity alters sputtering since, e.g. atoms
sputtered from the walls of a pore may redeposit
[13]. Due to varying microscopic and macroscopic
topography a range of impact angles are important; in addition, ion fluxes depend strongly on
latitude and longitude; thus sputtering effects and
local atmospheres are highly inhomogeneous. Irradiation alters the chemical composition of many
materials; exceptions are solid H2 O, O2 and N2 ,
which approximately maintain their stoichiometry
during irradiation.
Atmosphere generation by sputtering dominates over sublimation in the icy satellites of the
giant planets, due to large fluxes of magnetospheric ions [14] and low temperatures. Additional
sputtering occurs due to solar UV [10]. Examples
include the recently detected atmospheres of oxygen at Europa [15] and Ganymede [16], and hydrogen at Ganymede [17]. Atmospheres created by
sputtering likely occur over other icy satellites, the
icy rings of Saturn, and comets. Although sputtering data exist, especially for MeV light ions [18],
many questions appear when trying to model ion
irradiation effects, as in our study of the production of atmospheres around the icy satellites of
Jupiter and Saturn [14].
2.1. Sputtering of water ice
Water ice, the main condensed gas on the icy
satellites (except for Io) sputters more efficiently by
electronic excitations (electronic sputtering) than
from the typical recoil sputtering prevalent in refractory materials. Fig. 1 summarizes measurements done by others and us (adapted from
[19]), which are dominated by electronic sputtering [18,20] (recoil sputtering is apparent in the
low energy ‘plateaus’). Largely unknown are the
sputtering yields of mixed ices, an important concern since optical remote sensing has revealed that
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Fig. 1. Sputtering yield of water ice versus energy/amu for
different singly charged ions [19].
that O2 formed in the ice cannot be trapped permanently; it diffuses out at Ganymede’s reported
temperatures [23]. Transient trapping of O2 in ice
can be made by co-depositing O2 and water in a
film. When warmed above 70 K, the absorption
bands become those of liquid oxygen, and different
from those observed on Ganymede [24]. Our explanation is that very cold regions exist on Ganymede, made of segregated, bright ice patches,
which are not visible to the Galileo infrared radiometer. Details of the findings and discussion of
the model were published recently [25]. Further
studies are needed on other materials (like silicates) that can trap O2 .
Related to this problem, we have measured the
synthesis of O2 molecules in water ice by 100 keV
Arþ to simulate irradiation of the Jovian satellites
(Arþ has not been observed but should behave
similarly to the abundant Sþ ions, an important
sputtering source). Fig. 2 shows that the temperature dependence of O2 emission is very strong
compared to that of H2 O (the data were taken
after saturation of the fluence dependence [26]).
Improving on earlier observations for MeV ions
[26,27], we measured the relative efficiencies of our
other volatile components (e.g. CO2 , SO2 , O2 , O3 ,
H2 O2 , SO4 H2 ) exist on the icy satellites [14] either
segregated, trapped in inclusions, or dissolved in
the ice. Optical reflectance samples a depth more
than two orders of magnitude larger than that
responsible for sputtering; and therefore is not
usually useful to characterize the surface, where
the composition depends on a still unclear combination of sputtering, diffusion, sublimation, recondensation and molecular synthesis.
2.2. Molecular synthesis in water ice
Solid O2 was detected on Ganymede from its
absorption bands in the red [21], which prompted
the question: how can O2 exist at the reported high
temperatures where the vapor pressure would exceed the atmospheric pressure by many orders of
magnitude? There has been a controversy in the
literature on the explanation of this puzzle. Johnson and Jesser have proposed that O2 is formed
inside the ice by radiolysis, and trapped in bubbles
or inclusions [22]. Our experiments show instead
Fig. 2. Total and partial sputtering yields of water for 100 keV
Arþ versus temperature. (- - -): yield calculated from the MS
contributions, normalized to the yield measured by the microbalance at 17 K, taking into account that the relative contribution of O2 and H2 O is according to their mass ratio (36=20)
using H18
2 O.
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MS for O2 and H2 O, and have avoided problems
related to direct line of sight detection (noise induced by scattered projectiles and the dissimilar
velocities of sputtered molecules).
Some of the radiolytic O2 is transiently stored in
the solid, but eventually leaves by diffusion. We
found that 0.52 O2 are produced and trapped in
the ice by each 200 keV Hþ (G 2:6 104 per
100 eV of deposited electronic energy) [28]. The
experiments showed also the synthesis of smaller
quantities of HO2 and H2 O2 . Detailed studies of
the temperature dependence of H2 O2 synthesis in
ice by fast Hþ were done recently by infrared
spectroscopy [29] to understand evidence of this
molecule on the surface of Europa, a satellite of
Jupiter.
723
spectroscopy or mass spectrometry during thermal
desorption of irradiated films. We quantify
O3 production from the depth of the Hartley absorption band in ultraviolet reflectance spectra.
The shape of the band can account for part of the
ultraviolet absorption seen on Ganymede, Dione
and Rhea – the differences may be due to sulfur
compounds formed by implantation of magnetospheric sulfur. Microscopic modeling of the fluence dependence of band depth (Fig. 3) shows that
fast Hþ synthesize O3 orders of magnitude more
efficiently than previously thought [33]. Additional
studies of O3 synthesis in solid O2 are reported
elsewhere in this volume [34].
3. Atomic collisions in minerals
2.3. Ozone synthesis in ices containing oxygen
Ozone has been detected on the surface of
Ganymede [30], and the Saturnian satellites Rhea
and Dione [31]; a possible origin is radiolysis of
solid oxygen condensed from the atmosphere. We
found that Hþ irradiation of condensed O2 , CO2
and H2 O2 leads to the formation of ozone [32]
(Fig. 3). In contrast, we found no detectable
O3 synthesis in water ice using either optical
Fig. 3. Absorbance of Hartley O3 band produced by 100 keV
Hþ on different ices at 20 K (5 K for O2 and the 1:1 H2 O:O2
mixture).
We are interested in the question of the source
of ordinary chondritic meteorites, the most abundant on Earth. Probable meteoritic parent bodies
are S(IV)-type asteroids that, however, show
spectral reflectance strikingly different from that of
chondritic meteorites. This may result from prolonged irradiation by solar wind ions, visible and
UV radiation, and micrometeoritic bombardment.
We made the first in situ UHV quantitative study
of chemical changes of olivine due to irradiation
with 1 keV protons and 4 keV helium ions using
XPS [35] and found that the primary chemical
effect is reduction of iron to the metallic form. In a
meteorite this iron will reoxidize instantaneously
when entering the Earth’s atmosphere. The effect
of iron reduction on the spectral reflectance of
asteroids remains to be demonstrated. Fig. 4
shows the change in composition of labradorite (a
plagioclase feldspar found in Moon basalt) versus
fluence of 4 keV Heþ . Unlike olivine, the surface of
labradorite is very stable, only the Na concentration is seen to decrease significantly while the C
shows a peculiar behavior. These differences indicate the difficulty of extrapolating data from one
mineral to another.
Impact desorption of alkalis from surface minerals is thought to contribute to exospheres of Na
and K at Mercury and the Moon, a topic of great
current interest [13,36–40]. Recent studies show
that Na is photodesorbed from evaporated oxides
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perature ranges appropriate to the Moon and
Mercury.
4. Electron emission and surface charging
Fig. 4. Surface composition of labradorite versus fluence of 4
keV Heþ (1 1017 ions/cm2 corresponds to 4400 years at the
asteroid belt). Oxygen (not shown) adds up to 100% composition.
below a certain threshold wavelength by an electron transfer mechanism [41]. We test whether
such processes occur also on more realistic planetary surfaces and for incident ions by irradiating
sub-monolayers of Na on olivine with 4 keV Heþ
ions, typical of the solar wind. The decrease of the
Na coverage with irradiation fluence follows the
expected exponential decay, from which we derive a desorption cross-section r ¼ 1:5 1015 cm2 ,
enormous compared to 2 1019 cm2 for desorption of Na from SiO2 by 40 eV electrons [41].
The exponential decay means that the sputtering
yield Y ¼ rNS depends on the surface concentration NS . Experiments with labradorite (Fig. 4),
which contains Na and K in the bulk show the
depletion of Na but not of K; the sputtering yield
of Na is quite lower than that of Na adsorbed on
olivine. Open questions are the time dependence of
replenishing Na from the bulk, under wide tem-
Most ices are electrical insulators and, therefore, charge when exposed to charged particles and
ionizing photons [42]. This charging can affect the
dynamical behavior of small grains in regions of
significant electromagnetic fields, like space plasmas. The amount of charge accumulated on a
grain depends on it properties, the balance between
fluxes of incoming and ejected charges, their energy distribution, and the electrical potential of the
grain. Detailed modeling reveals that the potential
of ice grains in Saturn’s E ring varies from negative
to positive as a function of distance from the planet
[43]. The accuracy of such models suffers from the
scarcity of data on electron emission from insulators by ions and electrons at energies below 100 eV.
Another situation occurs when the particle flux
and/or electromagnetic fields is not uniform (for
example part of the surface being in the shadow).
The induced electric fields resulting from inhomogeneous or differential charging will affect electron
emission and may cause dielectric breakdown.
These conditions are also relevant for insulating
surfaces in spacecraft, where the resulting breakdown can produce malfunction by spurious electrical noise, and also permanent damage [42,44].
The opportunity for studies is revealed by the
scarcity of quantitative data on surface charging
with ion beams. Questions include what is the
maximum charge density that a surface or bulk can
take [45], the nature of the charges, and the dependence of charging on insulator properties.
5. Remote planetary surface analysis
Most knowledge of planetary surfaces comes
from the analysis of the reflectance spectra, optical
emission of atmospheric species, and ionic abundances measured by space probes. The local ion
bombardment provided by solar wind and magnetospheric ions produces ion emission from the
R.A. Baragiola et al. / Nucl. Instr. and Meth. in Phys. Res. B 193 (2002) 720–726
surface, which can be analyzed by a MS on a
spacecraft, similar to the secondary-ion mass
spectrometry (SIMS) technique [13,46]. Space
based SIMS with the Cassini ion-mass spectrometer [47] on Saturn’s icy satellites, where ion escape is not hindered by atmospheric collisions,
should allow detection of minority species not
visible in optical reflectance spectra.
Another technique that might be useful to analyze surface composition is the possible luminescence of the night side of icy satellites caused by
irradiation with magnetospheric ions [48]. Ioninduced luminescence results from the decay of
radiative levels populated by electronic excitations.
In addition, thermoluminescence may result from
the decay of electron traps also excited by fast
particles. With a UHV setup similar to that used in
our studies of ion-induced luminescence of diamond [49] and solid argon [50], we sought luminescence of water ice surfaces during irradiation
with 100 keV Hþ . We found no detectable luminescence (<104 photons/Hþ ) except for a very
weak signal from sputtered excited OH [32]. A
spectrometer orbiting around an icy satellite,
however, may find luminescence on the night side
due to impurities, which would provide further
clues on surface composition. Luminescence measurements of mixed and impure ices are needed to
test this idea.
Acknowledgements
Our research is supported by NASA’s Office
of Space Science, NSF-Astronomy, and NASA’s
Cassini program under JPL contract 1210586.
BDT gratefully acknowledges IGERT fellowship
support under National Science Foundation
Grant # 9972790.
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