Nuclear Instruments and Methods in Physics Research B 232 (2005) 98–107
www.elsevier.com/locate/nimb
Energetic electronic processes on extraterrestrial surfaces
Raúl A. Baragiola
*
Laboratory for Atomic and Surface Physics, University of Virginia, Charlottesville, VA 22904, USA
Available online 27 April 2005
Abstract
Astronomical bodies without atmospheres are subject to irradiation with energetic photons and particles. The resulting surface alterations are evident in the optical reflectance spectra discernible through remote optical sensing from
Earth. Here I review the most important processes initiated by electronic excitations and ionizations by ions. These
include electronic desorption or sputtering, chemical reactions, and charge transfer. It is argued that dissociative electron attachment and Auger initiated desorption play a role in desorption of ice mantles on grains in dense interstellar
molecular clouds. The paper describes the essentials of laboratory simulations and points out to opportunities for
atomic collision physicists to contribute to important astrophysical and planetary problems, including the evolution
of galactic matter and the possible extraterrestrial genesis of life.
2005 Elsevier B.V. All rights reserved.
PACS: 79.20.La; 82.50.Gw; 95.30.Wi; 98.58.Bz
Keywords: Desorption; Radiation chemistry; Interstellar dust; Sputtering
1. The problem of mass balance in dense interstellar
clouds
Let us start by addressing the balance of molecules in the gas and the dust grains in dark molecular clouds (MCs), one of the central unsolved
problems in astrophysics [1]. The problem consists
in the following. Due to the low temperature of
MCs (10–15 K) and high condensation coeffi*
Tel.: +1 434 982 2907.
E-mail address: [email protected]
cients, all gaseous species, with the exception of
H2 and He would be depleted by condensation
on grains, in times of 105 years for typical gas
densities [2]. However, no desorption mechanism
of those proposed so far can convincingly maintain the gas population observed, during typical
MC lifetimes of millions of years. The lack of
understanding is such that desorption rates have
been taken as adjustable parameters when modeling MCs [3,4]. In the following, I discuss the
desorption mechanisms that have been considered
so far and, in the face of their insufficiency,
0168-583X/$ - see front matter 2005 Elsevier B.V. All rights reserved.
doi:10.1016/j.nimb.2005.03.031
R.A. Baragiola / Nucl. Instr. and Meth. in Phys. Res. B 232 (2005) 98–107
propose additional mechanisms that may help
solve the problem.
1.1. Photodesorption by stellar Lyman-a photons
We have shown that this mechanism is effective
in limiting water ice mantles on grains in lowdensity diffuse clouds [5,6]. Its importance in denser molecular clouds is uncertain; although
Lyman-a photons do not significantly penetrate
dense clouds, they will affect grains if they circulate
between the inside and outside of the cloud, or if
the cloud is lumpy. Hard ultraviolet photons,
which may also be produced in collisions of cosmic
rays with gas phase H2 [7–9], can desorb ice mantles of grains inside the cloud by electronic excitations. However, this UV flux appears to be very
low from recent estimates [2,10].
1.2. Desorption by cosmic rays
These energetic particles can desorb by spot
heating (a misnomer) or by whole grain heating
and sublimation. Spot heating refers to the prompt,
non-thermal desorption of molecules around the
track of the particle and thus is independent of
grain size [11,12]. The estimated lifetime of condensed volatiles due to this mechanism is 4 MY
[13]. Thermal desorption induced by cosmic rays
is delayed and results from the temperature increase of the whole grain due to the deposited
energy [11,14]. But again here the desorption rate
is too low, with an estimate mean time to remove
volatiles of 8 MY for a 0.25 lm grain [11]. Moreover, recent more accurate molecular dynamics calculations [15] yield desorption rates of volatiles by
cosmic ray that are an order of magnitude smaller
than the earlier values given above, and thus even
longer lifetimes.
1.3. Explosive desorption in exothermic reactions
during spike heating of photolyzed ice
This proposed mechanism [11,16–19] requires
the buildup of concentrations of a few percent of
radical species by photolysis with background
UV light [20,21] and, in addition, some heating
process that activates chemical reactions involving
99
the stored radicals. This needed heating could be
achieved by grain–grain collisions (an original proposal later abandoned) or by a passing cosmic ray.
This proposal is based on few experiments on specific gas mixtures (H2O, CO, NH3, and CH4) and
many assumptions, several of which have been
questioned [2]. The time to obtain >1% radical
concentration is large if one considers that a fluence of a few 1018 photons/cm2 were needed in
the experiments of dÕHendecourt et al. [16] to
reach radical concentrations at the percent level
required for explosion of the ice when heated.
The time to achieve this dose in a MC, with the
normally assumed background UV photon flux
of 4 · 103/cm2/s driven by cosmic rays [7] is again
very large, of the order of 30 MY. However, the
fluence dependence was not studied, and it is likely
that lower fluences (and therefore times) may suffice to build the required radical concentration.
For instance, saturation of CO2 synthesis in CO
occurs at a fluence of 4 · 1017 Lyman-a/cm2
[22], and similar fluences also apply for CO production from CO2 [23]. The explosive desorption
mechanism is likely highly selective; for instance,
we have seen desorption spikes when heating
photolyzed carbon dioxide [23], but not photolyzed water ice.
1.4. Desorption resulting from reactions involving
incident free radicals
Impinging radical atoms, like H, O, C, N,
produced by UV photons and cosmic rays in
gas-phase dissociation collisions [19], can initiate
exothermic reactions that result in desorption.
This mechanism has been proposed for the formation of H2 by recombination of incoming H (by far
the most abundant radical), making the assumption that at least 1% of the 4.5 eV released in
H-H recombination on a grain surface goes into
desorption of a molecule in an adjacent site
[2,24]. However, no experimental evidence supports this model and it is likely that the probability
of the process was overestimated. From what is
known of molecular dynamics simulations in the
solid state, the H2 molecule would cool fast by
multiphonon relaxation involving a large number
of atoms in the grain.
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1.5. Desorption by dissociative electron attachment
1.6. Auger desorption by neutralization of slow ions
Electrons from gas-phase ionization in the MC
by passing cosmic rays will degrade in energy very
slowly once they fall below 10 eV (so-called subexcitation electrons) and join other low energy
electrons from photoionization of gas molecules
and photoelectron emission from grains. These
slow electrons may collide with grains and be
trapped, and in fact this is one of the processes
often mentioned when discussing possible dust
charging. Electrons of a few eV interact most efficiently with molecular ices through the process of
dissociative electron attachment (DEA), symbolized by AB + e ! AB* ! A + B. That is, an
electron (e) attaches to a molecule AB to produce
a transient excited anion AB* in a repulsive potential, which then dissociates into the molecular
fragments A and B. Since DEA has not been considered before for IS grains, I discuss it further
here.
A competitive channel to DEA in the gas
phase is the decay of the repulsive state back
to the ground state by emitting the electron
(autodetachment), with the only effect remaining
being vibrational excitation in the final state of
AB and a lower energy electron. When the molecule dissociates, the excess kinetic energy gained
by the fragments can result in direct desorption.
So far, experiments such as those by Sanche and
co-workers [25–28] have measured desorbed negative ions because the more abundant neutral
species could not be detected. If DEA occurs in
the bulk, the energized fragment will collide with
adjacent molecules and start a collision cascade, such as in the case of sputtering. If this
collision cascade is close to the surface – as is
the case for thin ice mantles – the result can
be desorption of a surface molecule, most likely
uncharged.
Dissociative electron attachment appears to
occur in the most common ices that are relevant
in astrophysics, including CO, O2 [25], and amorphous H2O ice [27]. With cross sections of the
order of r 1017 cm2 [28], the probability of a
DEA event will be unity in a typical 100-monolayer ice mantle impacted by these low energy
electrons.
In this mechanism, also not considered before
for desorption of interstellar ices, an ion captures
a valence electron from a condensed molecule,
with the energy released in exciting another
valence electron (an Auger process). Energetically,
it is favored that the two final holes remain in different molecules. The Coulomb repulsion energy
between the two ions, acting before the holes can
drift away, can transform into kinetic energy and
result in desorption. For this process to occur,
the recombination energy of the incoming ion
must exceed the energy of the two holes in the lattice. Such a condition will exist for He+ impacting
most condensed gases, and for H+ on some ices
with sufficiently small band gap. Less abundant
multiply charged ions can also be expected to produce Auger desorption, with a higher probability
per ion impact.
2. Radiation synthesis of complex molecules
Ice-coated interstellar grains are thought to be
the matter from which comets form during the
gravitational collapse of a part of a dense molecular cloud – the presolar nebula – into the protoplanetary disk. This is suggested by the content of
volatiles gases in comets [29–34]. Interstellar
grains, including their ice mantles are processed
by gas-surface reactions, radiation (ultraviolet
photons and fast ions) [35–40] which result in
the synthesis of complex molecules, including prebiotic species [41], that may have had a role in the
formation of life on Earth. Such molecules are
identified on interstellar ice mantles from the
analysis of absorption bands in infrared spectra,
most notably lately from the Infrared Space
Observatory (ISO) [42], based on laboratory simulations that include the essential aspects of
the problem. These simulations have normally involved the growth of interstellar ice mantle analogs (thin films of pure ices or ice mixtures),
irradiation with UV photons or energetic particles and characterization by infrared spectroscopy. Comparison of the infrared spectra of the
processed ice samples and astronomical spectra
R.A. Baragiola / Nucl. Instr. and Meth. in Phys. Res. B 232 (2005) 98–107
then serve to identify the physical state of particular ices and obtain abundances which serve to
test hypotheses and constrain models [32,41,43–
46].
A specific question in this area is the observed
abundance of condensed carbon dioxide, which is
greater than expected from the known concentration in the gas phase. Carbon dioxide can be synthesized by radiolysis of solid carbon monoxide,
but recent measurements using Lyman-a photons
and 200 keV protons suggest that radiolysis is
insufficient to produce the observed CO2/CO
abundance ratios in interstellar ices [22]. It is possible that an additional process, found in our laboratory, can account for the discrepancy. This is
the synthesis of CO2 when carbon grains coated
with water ice are irradiated by 100 keV ions [47].
Another mechanism for molecular synthesis can
occur on grain surfaces by impinging radical
atoms created in dissociation collisions by cosmic
rays and UV photons [19]. Reactions between radical atoms and surfaces will occur except at low
temperatures in cases where there is an activation
barrier; thus their study is important to provide
input to models of gas–grain chemistry [48]. There
have been interesting experimental studies of
reactions involving radicals on analogs of astrophysical surfaces [49–53]; few of those are quantitative. In a significant study Vidali and co-workers
[54] searched for the reaction O (thermal) + CO
(ice) ! CO2. They detected no CO2 formation at
5 K, consistent with the existence of a predicted
activation barrier [55]. However, they observed
the release of CO2 when heating a frozen
CO + O ice to 160 K indicating that the reaction
occurred while heating.
3. Radiation processing of solar system ices
The tenuous atmospheres around the icy satellites in the outer solar system allow solar ultraviolet photons and energetic charged particles from
the solar wind and planetary magnetospheres to
impact on the icy surfaces, causing sputtering
and radiolysis. The sputtered species may escape
the gravitational pull or may contribute to the formation of an atmosphere, adding to any existing
101
contribution of sublimation [56–58] or meteorite
impact ejection. The incoming radiation also generates radiation products in the atmosphere that
in turn interact with the surface through solid-state
chemical processes. Therefore, the state of the surface of the icy bodies is determined by a competition of radiation damage and erosion due to
energetic particles, photons, meteorite impact,
and sublimation, by thermal diffusion, by interactions with the atmosphere, and by a variety of
radiation enhanced chemical processes. These
include formation of radicals, synthesis of new
molecules, creation of optically active centers,
phase transitions, release of trapped gases, and
surface roughening [56,59–62]. Disentangling the
interrelated phenomena requires laboratory studies of individual processes and their mutual effects,
to expand our knowledge and constrain parameters in theoretical models.
There have been several experiments that show
the presence of trapped radicals in radiolyzed or
photolyzed condensed gases. Here we summarize
the results for water ice, the most fundamental
of those substances. Dissociation in the solid state
often leads to immediate reformation of the molecules, since the dissociation fragments suffer collisions with surrounding molecules and cannot
escape. The consequence of this phenomenon,
called the cage effect, is a substantially smaller
yield of radiation products in solids as compared
to the gas phase [63]. The early stages of radiation chemistry studies used gamma ray irradiation and electron spin resonance analysis,
showing formation and trapping of isolated H,
O and OH radicals at low temperatures. Such
radicals become mobile at 100 to 120 K, and
react to form stable molecular products such as
H2O2, O2, and HO2. Energetic ions, on the other
hand, can produce a high density of radicals in
their track, which can recombine immediately.
This causes a higher yield of molecular products
as compared with the case of gamma rays or fast
electrons.
Of the molecular products, O2 is particularly
important because solid oxygen was detected on
Ganymede, a satellite of Jupiter. This finding has
been very perplexing, because at the reported
high diurnal temperatures in Ganymede the vapor
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pressure of O2 would exceed the atmospheric pressure by several orders of magnitude. The interpretation of the astronomical observations has been
controversial. Our experiments show that the
quantity of O2 generated by radiolysis in water is
orders of magnitude smaller than assumed [64–
66] but suggest that atmospheric oxygen can condense on cold patches of the surface during the
night and evaporate slowly during the day.
Whenever O2 is present in a radiation experiment, the synthesis of ozone is expected. Several
experiments were performed in ozone synthesis in
condensed gases to understand observations by
the Hubble space telescope of ozone on several
icy satellites. We quantified the ozone column density by measuring the depth of the Hartley band in
reflectance spectroscopy, during irradiation with
100 keV protons, finding that these ions are two
orders of magnitude more efficient in producing
ozone that previously thought [67] We could synthesize ozone not only with ices containing O2,
but also irradiating CO2 and H2O2. No ozone
was found irradiating pure water ice, within the
detection sensitivity of our optical and mass spectrometer techniques. Ozone was also detected by
mass spectrometry during thermal desorption of
the irradiated ices. An interesting finding has been
that that ozone formation doubles the sputtering
yield of solid O2 [68].
In our experiments of photodesorption of
water ice using Lyman-a radiation [5,6] and
100 keV protons [69], we observed that evaporation of the photolyzed ice released not only water,
as expected, but also HO2 and H2O2 molecules.
The hydrogen peroxide molecule is important because it was identified in the Galilean satellite
Europa through the absorption of solar infrared
and ultraviolet light [70]. This observation led to
new studies where H2O2 synthesis was detected
using infrared spectroscopy of ice during irradiated with protons and heavy ions [71–73]. All
these results not only explain the presence of
H2O2 in the Galilean satellites but also lead to
the prediction of even larger concentrations of this
molecule in colder icy satellites (such as those
around Saturn, currently being explored by the
Cassini probe), and in the ice mantles of interstellar grains.
4. The prospects of extraterrestrial life
Finding extraterrestrial life would have profound implications for humanity, and it is therefore natural that the possibility drives strong
scientific curiosity. Potential habitats for life in
our solar system beyond Earth include the icy Galilean satellites, especially Europa, which is covered
with water ice and which, according to several
pieces of evidence, may have a warmer subsurface
ocean, hundreds of kilometers below an icy crust.
The exciting possibility that such an ocean might
contain life brings the question of the source of
energy required for life, at the depths below the
surface where photosynthesis is not possible.
Chyba [74] has proposed that energy may be provided by the relatively intense flux of energetic
charged particles that strike EuropaÕs surface [58]
(1012 keV/cm2 s). Such radiation could liberate
oxygen from oxygen-bearing species like water,
carbon dioxide, and sulfur dioxide, which are
known to exist in the surface ice. This oxygen
and other radiolytic oxidants like hydrogen peroxide or ozone, would in turn release energy in
reactions with hydrocarbon compounds. Since
the penetration of radiation is limited to depths
of typically microns and at most a few centimeters,
a mechanism is required to transport the energy
kilometers deep to reach the ocean, where it could
fuel bacterial life. Such transport mechanism could
be sporadic surface cracking and melting of the
surface ice, created by tidal stresses, tectonic activity or meteorite impact. Ample evidence of surface
cracking is contained in high quality images
obtained by the Galileo spacecraft. Indirect evidence of a salty ocean comes from the observation
that the infrared reflectance spectra of Europa is
very similar to that of hydrated salts [75] and from
the magnetic response of the satellite to JupiterÕs
magnetic field.
Thus, the subject of radical storage in ice as a
biological fuel merits investigation in the laboratory. Although this phenomenon is fully expected
from existing knowledge of radiation biology and
chemistry, there are two critical pieces of information that are lacking for application to planetary
ices. The first is the quantification of the amount
of species trapped, especially for the case of light
R.A. Baragiola / Nucl. Instr. and Meth. in Phys. Res. B 232 (2005) 98–107
and heavy ions in the 10–200 keV energy range,
typical of EuropaÕs magnetosphere (except for
photolysis below 40 K [17,16,76] and the case of
H2O2 formation, mentioned above, there is little
published data for ice between 70 and 130 K, typical of Europa). The second piece missing is a body
of quantitative data on reactions of trapped radicals and energy release in ice, particularly in conditions of temperature excursions. The complexity of
the problem has been reviewed recently [77].
5. Charge transfer between ions and grains
The ion/neutral ratio in molecular clouds is
known trough high resolution spectroscopy with
radio telescopes. The relative ionic abundances of
different species in various environments suggest
that neutralization in ion-grain collisions is important in the overall charge balance. Resonant neutralization by a valence band electron is likely for
protons, O+ and N+, and Auger neutralization
for He+ incident on grains coated with molecular
ices. Ions of lower ionization potential may not
be able to neutralize on grains. It is also possible
that some electronegative atoms, like O and C
may capture electrons from some surfaces to
become negative ions, if their velocity is high enough. Grains can also charge positively by electron
emission when hit by a hard ultraviolet photon or
a cosmic ray, and they can charge negatively by
capturing sub-excitation electrons. In the latter
case, they will present a larger cross section for
incoming positive ions than the geometrical value.
The nature of each type of charge on the grain surface and the mobility of the charge across the grain
have not been ascertained yet. Analysis of charging is still in its infancy; the literature is divided
in the applications to the interstellar medium
[78,79] and planetary plasmas [80] but a unified
treatment of the mechanisms should be possible.
6. Notes on laboratory simulations
Since one purpose of this article is to stimulate
experimental research on atomic collisions of
importance to astrophysics, it is pertinent to give
103
information on experimental techniques which
are used to study electronic processes in laboratory
analogues of interstellar and solar-system ices. Our
experimental arrangement has been described in
several publications [6,64,81]. Standard ultrahigh
vacuum conditions (1010 Torr) are used to simulate the space environment and to prevent condensation of unwanted gases on the samples.
Ice films with thickness from sub-monolayers to
several mm can be grown by condensing high purity gases or gas mixtures on a quartz crystal microbalance (QCM) cooled to the desired temperature
using closed-cycle He refrigerators or liquid He. In
the QCM technique, changes in the mass of thin
deposits cause a change in the resonant frequency
of the quartz crystal, driven at its natural frequency. The stability of the microbalance is equivalent to a mass change rate per unit area of
5 · 1011 H2O mol/cm2 s, or about 0.0005 monolayers/s. Different pure and mixed gases are directed
onto the target through a uniform gas doser, consisting in a 25 mm diameter array of microcapillaries, 50 lm diameter, and 500 lm long). The
porosity of the condensed gas, which might affect
the stability of trapped species, can be varied over
a wide range by changing the angle of incidence of
the molecules onto the surface [82]. When using
gas mixtures, the composition of the gases in the
manifold is adjusted to take into account the
mass-dependent conductance of the gas doser,
and the actual composition of the films is measured by a mass spectrometer (residual gas analyzer or RGA) during thermal desorption. A
technique we often use is isotopic labeling (i.e.
water with 18O, carbon monoxide with 13C) to
reduce or eliminate interference from residual gases
in the mass spectrometer or infrared measurements.
The RGA is also used to monitor the composition of the desorbed flux during irradiation. The
instrument is calibrated using sublimation fluxes
from ices deposited on the QCM, which can measure the mass loss absolutely. It is convenient to
install the RGA in a separately pumped ultrahigh
vacuum chamber, with a cryogenic shield around
the ion source, to reduce interference by background gas. The RGA is also useful to determine
which molecules were synthesized in the ices while
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warming them up. It is important to note that the
information in this case is not just what happens
during irradiation, but may also include the effect
of reactions during warming up the ice.
Condensed gas samples can be characterized
using standard surface analysis techniques. For
instance we have shown the usefulness of electron
energy loss spectroscopy (EELS) in monitoring
trapped radicals during irradiation [73], in conditions where X-ray photoelectron and Auger spectroscopies give little information. EELS is closely
related to optical absorption spectra but the information depth is limited, thus it is most useful to
study surface processes. Optical techniques give
integral information of composition inside thin
films. For this purpose we measure optical reflectance using an ultraviolet-visible spectrometer
(0.1–0.8 lm) and a Fourier Transform infrared
spectrometer (FTIR) over a wide range (0.5–
20 lm) that allows identification of molecules by
their light absorption at particular vibrational frequencies. We obtain the density of the ice films by
measuring the index of refraction over a range of
wavelengths [82]. Since the QCM measures the column (mass) density of the films, we have the ability
to determine absolute optical constants, for
instance the infrared absorbance of a species of
interest, using the FTIR.
Different techniques are needed to study analogues of non-icy extraterrestrial materials, such
as silicates. An example is given in our work on
the formation of metallic iron in the mineral olivine under simulated irradiation by the solar wind
[83]. Perhaps unexpectedly, experiments on mineral samples are often harder than those with condensed gases. There are several reasons for this. In
most cases, these natural samples have multiple
components and are inhomogeneous, leading to
questions of reproducibility. Sample preparation
is not straight forward, since there is the need to
avoid contamination from exposure to the atmosphere. We prefer to prepare samples by fracture
done just before the experiments. Removal of contaminants by sputtering is undesirable, since it
often produces changes in the sample composition.
Annealing of ion beam damage by heating is often
undesirable since heating may cause compositional
changes and alter the crystallinity of the sample.
Preparation by fracture is not without problems,
since materials tend to fracture along planes of
reduced bonding, and this may imply a concentration of impurities larger than average. Fracture
also produces rough surfaces, with the possibility
that protrusions shadow depressions during irradiation and/or analysis.
Since all the surfaces of interest are electrical
insulators, an important concern is electrostatic
charging during irradiation. For thin ice films, this
is not an issue, since hole conductivity is sufficient
to drain most of the charge deposited by the ion
beam and carried away by secondary electrons.
The situation is different when studying mineral
samples, such as natural rocks, which cannot be
easily prepared with small thickness. In addition,
since these natural surfaces are compositionally
complex, holes produced by irradiation have low
mobility or tend to trap efficiently, leading to a
high concentration of charges on the surface and
in the bulk. The electric field due to this charge distribution can produce large surface potentials,
even on mm thick samples, that can deflect energetic ion beams. We have observed the total reflection of a 40 keV Ga+ focused ion beam from an
olivine sample a few mm thick. For meaningful
results, charging must be eliminated as much as
possible. The standard method for charge compensation is to spray the samples with electrons from a
‘‘flood gun’’, consisting of a thermoionic emitter
(heated W filament) biased at a small negative
voltage (<1 V) with respect to ground. When the
sample charges positive as a result of ion bombardment, it attracts electrons from the flood
gun until the charge is cancelled. This procedure
does not work for electron irradiation, where the
surfaces may charge negatively (for energies below
a few tens of eV and above a few keV). In this case,
it is possible to neutralize the charge with low
energy ions from a specialized ion source. Another
solution to the charging problem that works for
some, but not all applications, consists in pressing
a grounded, highly transparent metal mesh, with
small openings, against the sample surface. This
decreases the leakage distance for the surface
charges and eventually limits, through field emission, the maximum surface potential that can build
up during irradiations.
R.A. Baragiola / Nucl. Instr. and Meth. in Phys. Res. B 232 (2005) 98–107
6.1. How accurate are laboratory simulations?
The basic need is that the selection of material
and irradiation energies match the astronomical
situation to be simulated. Only for the Moon
and interplanetary dust we have actual samples
that we can use. In other cases we must infer what
the surface materials are from the interpretation of
their optical reflectance and our knowledge of
likely surface processes, both of which are uncertain. In addition, a central question is to know
what variables are important for the simulation
since laboratory time scales are necessarily orders
of magnitude shorter than those relevant in most
astronomical problems. In order to accelerate simulations, particle and photon fluxes are used that
are much larger than those in space. For instance,
one hour of irradiation in the laboratory may correspond from thousands to millions of years in
space. The first consideration is always to ensure
that fluxes are below those that produce significant
heating in the samples. This is tested by varying
the irradiation flux, in some of our experiments
over several orders of magnitude, and verifying
linear behavior. Although flux effects may exist
(they are known, for instance, in radiation assisted
diffusion in some materials) in practice we have
not found them under our experimental conditions
were significant heating is avoided. Similarly, films
of condensed gases are always deposited at rates
low enough that the effects of the release of condensation energy is negligible, as determined by
the independence of results on deposition rate.
7. Outlook
Having presented a succinct description of the
astronomical scenario and some unique laboratory
issues, I end up by pointing to some topics that
need to be researched, with the goal of motivating
new experiments. In the area of interstellar processes, there is the need to measure desorption
cross sections in a wide variety of condensed gas
solids, mainly for Lyman-a radiation and heavy
ions simulating cosmic rays. In such experiments,
it would be very useful to determine what species
are synthesized, and how their population is
105
affected by thermal excursions, looking specifically
for explosive desorption. Another area in need of
quantitative experiments is the investigation of
the outcome of reactions of thermal ions, radical
atoms, and electrons with condensed gases, silicates and carbon grains.
Two main goals can be identified for such
experiments. One is the solution of the problem
of balance of molecules between the gas and solid
phases, enunciated at the beginning of this paper.
The other is to understand the limits in the generation of complex organic molecules that, carried
by comets, could have triggered life on Earth.
These limits appear because radiation can both
make complex molecules from simple ones, and
destroy complex molecules. In fact, life as we
know it, cannot exist in the presence of large
amounts of radiation. In this context, it would
be interesting to know the conditions necessary
for a particular microorganism to survive space
travel.
Acknowledgements
The author is grateful to the Okayama University of Science for travel support. The research is
supported by the NASA program on Origins of
the Solar System.
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