# The Influence of Temperature and Photobleaching on Irradiated Sodium
Chloride at Europa-like Conditions

William T.P. Denman Division of Geological and Planetary Sciences, California
Institute of Technology, Pasadena, CA 91125, USA  Samantha K. Trumbo Division
of Geological and Planetary Sciences, California Institute of Technology,
Pasadena, CA 91125, USA  Cornell Center for Astrophysics and Planetary
Science, Cornell University, Ithaca, NY 14853, USA  Michael E. Brown Division
of Geological and Planetary Sciences, California Institute of Technology,
Pasadena, CA 91125, USA

###### Abstract

Europa’s leading hemisphere chaos regions have a spectral feature at 450 nm
that has been attributed to absorption by crystal defects in irradiated sodium
chloride, known as F-centers. Some discrepancies exist between the laboratory
data of irradiated sodium chloride and the observations, including a $\sim$10
nm shift in central wavelength of the F-center band and the lack of the
prominent 720 nm absorption on Europa from M-centers, which result from the
coalescence of pairs of F-centers. Here, we perform irradiation experiments on
sodium chloride in an attempt to understand these discrepancies. We show that
careful control of the temperature of the sample at a temperature of 120 K
yields F-centers with an absorption wavelength comparable to that of Europa.
In addition, we measure the effect of photobleaching – the destruction of
F-centers by photons – and show that at the energetic particle and photon flux
on Europa, an equilibrium will be reached where only a modest F-center
absorption develops. The density of F-centers never reaches high enough values
for the creation of secondary M-centers. Our experiments predict that
F-centers grow during the night on Europa in the absence of photobleaching and
then partially decay during the daytime. We show observations from the Hubble
Space Telescope consistent with this prediction. All observations of the 450
nm F-center on Europa are now consistent with laboratory measurements of
sodium chloride, confirming the presence of this salt on Europa.

Galilean satellites (627), Europa (2189), Planetary surfaces (2113), Surface
composition (2115)

††journal: PSJ

##  1 Introduction

Jupiter’s moon Europa has a young, icy surface which undergoes high energy
radiation bombardment and covers a salty liquid water ocean (Kivelson, 2000;
Paranicas et al., 2001, 2002). The salts in the ocean presumably derive from
the interaction between the liquid water and the silicate seafloor (Kargel et
al., 2000), and the composition of the salts offer insight into the manner of
this interaction. While no direct access to the ocean is currently possible,
Europa’s geologically young chaos regions have likely been resurfaced by
material ultimately derived from the interior ocean and thus could provide
information on its composition (Carr et al., 1998; Glein & Shock, 2010;
Schmidt et al., 2011; Soderlund et al., 2014). On the trailing side of Europa
these chaos terrains are bombarded by sulfur plasma, confusing the
interpretation of their initial composition; the leading side chaos terrains
are thus the most likely to retain signatures of the original composition.

Trumbo et al. (2019) recently reported Hubble Space Telescope (HST)
observations that showed the existence of an absorption feature at 450 nm in
Europa’s leading-side chaos terrains and proposed that this feature is due to
sodium chloride salts. While salts are generally indistinct in the visible
wavelength range, irradiation of alkali halides, in particular, produce
multiple defects and dislocations within the crystal structure, giving rise to
distinct “color center” absorptions. For NaCl, the most prominent color center
is the F-center, occurring at approximately 450 nm (Mador et al., 1954).

F-centers occur when radiation ejects a chlorine atom from its location in the
crystal lattice and an electron fills the hole. More complicated color centers
also form, including M-center absorption at 720 nm, which appear when the
density of F-centers is sufficiently high that pairs begin to coalesce. This
F-center coalescence can eventually lead to macroscopic sodium colloids which
broadly absorb in the 580 nm region. F-centers can be destroyed by absorption
of photons within the 400-500 nm band of the absorption feature in a process
called photobleaching, which excites the electron into the conduction band
(Herman & Wallis, 1955).

Experiments performed in order to understand the possibility of NaCl color
centers on Europa (Hand & Carlson, 2015; Poston et al., 2017; Hibbitts et al.,
2019) suggested that irradiation at conditions similar to those on Europa
would form both F-centers and M-centers. The HST data, however, did not
completely match the laboratory data. In the laboratory, the central
wavelength of the F-center was measured to be 460 nm, rather than the
$\sim$450 nm seen on Europa, and the HST data showed no hint of the predicted
absorption near the 720 nm M-center. Trumbo et al. (2019) noted that when the
irradiation of the NaCl stopped in the Poston et al. (2017) experiments, the
F-center wavelength quickly shifted blueward as the absorption rapidly
decayed. They hypothesized that irradiation at the lower flux level seen on
Europa could lead to the shorter wavelength feature. They further hypothesized
that the lack of M-centers on Europa is the result of solar photobleaching
which would destroy F-centers at nearly the same rate as their creation
through irradiation. This destruction would lead to an equilibrium with a low
number of F-centers that would not have a high enough density for M-centers to
form.

Here, we perform experiments to test these hypotheses. Our goal is to
determine if the differences in appearance between the HST spectra and the
laboratory data can be reconciled with the interpretation that the $\sim$450
nm absorption feature seen on Europa is indeed due to irradiated NaCl.

##  2 Method

Our experiments were designed to measure the spectral changes in sodium
chloride from electron and photon bombardment at temperatures similar to
Europa. For each experiment, fresh hydrated sodium chloride powder, USP
(CAS-7647-14-5) was pressed into a layer of indium foil inside of a copper
sample cup. Excess salt was brushed off to ensure good thermal coupling
between the remaining grains and the sample cup. The sample cup was then
loaded onto a cold finger inside of a Kimball Physics ultrahigh vacuum
chamber. The chamber was pumped down to $~{}10^{-8}$ torr using an Agilent ID3
backing pump and TwisTorr 84 molecular turbo pump. The cold finger was cooled
with a Janis ST-400 liquid nitrogen cryostat. Once the desired temperature was
reached, the grains were irradiated with 10 keV electrons from a Kimball
Physics EGG301 electron gun. The current is measured with a Kimball Physics
Faraday cup mounted on a linear actuator that can be inserted into the
electron beam path. Electron currents used in the different experiments and
their relationship to the approximate energy flux on Europa are shown in Table
1. We obtained spectra of the samples from 300-1000 nm by illuminating the
sample via an external lens fed by a fiber optic connected to both a
stabilized deuterium and a stabilized tungsten-halogen lamp from Thorlabs. We
illuminated the sample at 45 degrees from the surface normal and collected the
diffuse reflection 90 degrees from the specular direction and 45 degrees from
the surface normal. Since NaCl is nearly featureless through the spectral
range, unirradiated salt at the desired temperature was used as a spectral
reference for the our samples.

Table 1: Experiments

| current density | electron flux | 450 nm irradiance | dose rate | temperature
---|---|---|---|---|---
| (nA/cm2) | (electrons/cm2/s) | (mW/m2/nm) | (eV/16amu/yr) | (K)
Low | $12$ | $8\times 10^{10}$ | $6.0$ | $2.5\times 10^{3}$ | $120$
Low | $12$ | $8\times 10^{10}$ | $-$ | $2.5\times 10^{3}$ | $120$
$-$ | $48$ | $3\times 10^{11}$ | $6.0$ | $1\times 10^{4}$ | $120$
Medium | $505$ | $3\times 10^{12}$ | $6.0$ | $1\times 10^{5}$ | $120$
Medium | $505$ | $3\times 10^{12}$ | $-$ | $1\times 10^{5}$ | $120$
Medium | $505$ | $3\times 10^{12}$ | $6.0$ | $1\times 10^{5}$ |  $120$(no indium foil)
Medium | $505$ | $3\times 10^{12}$ | $-$ | $1\times 10^{5}$ | $200$
High | $3410$ | $2\times 10^{13}$ | $6.0$ | $7\times 10^{5}$ | $120$
High | $3410$ | $2\times 10^{13}$ | $-$ | $7\times 10^{5}$ | $120$
Europa | n/a | n/a | $78$ | $10$ | $120$

Note. — The energy deposited is for the top 1.2 $\mu$m micron of the sample.
For the experiment without indium foil the temperature of the salt is unknown.
The irradiance of our spectral lamp is estimated at 450 nm. For Europa, the
photon flux and temperature are given for equatorial noon.

Figure 1: Spectra of irradiated sodium chloride at 120K with our high
irradiation dosage and no photobleaching. M-center growth is observed after
F-center saturation.

For our series of experiments the electron flux and illumination conditions
were varied. In our initial experiments, we simply tested the system to ensure
that we could create and measure F- and M-centers. Our initial electron flux,
as measured in the Faraday cup, was $2\times 10^{13}$ electrons cm-2 s-1.
Since the penetration depth of 10 KeV electrons is around 1.2$\mu$m (Hand &
Carlson, 2015) such a flux gives the equivalent of $7\times 10^{5}$ eV (16
amu)-1 deposited each year into the NaCl. The energy deposited on the apex of
Europa’s leading hemisphere is estimated to be 10 eV (16 amu yr)-1 (Nordheim
et al., 2018). Our initial experiments are at a dosage rate approximately
70000 times that of Europa. The true dosage rate on Europa is both poorly
determined and variable, so this ratio is only an approximation. Additional
experiments were run at fluxes 7 times lower and nearly 300 times lower than
our initial experiment. These three settings are referred to as high, medium
and low flux. It should be noted that even the low flux is $\sim 250$ higher
than that of Europa. Figure 1 shows the results of our first experiment at
high flux. F-centers are quickly created and become saturated within
approximately 30 minutes. As the density of F-centers increases, coalescence
of these defects begins to form M-centers, which begin to absorb at 720 nm by
30 minutes. By the end of the experiment both F- and M-center absorptions are
prominent.

In some experiments, we test the effects of photobleaching by leaving our
spectral lamp illuminated during the irradiation. While we have no means of
measuring the absolute flux density of our spectral lamp inside of the chamber
at 450 nm, we estimate this parameter by using the spectral curve and total
output provided by the manufacturer as well as the illumination spot size on
the sample. We find the irradiance of the lamp at 450 nm to be 6 mW m-2 nm-1.
This irradiance is approximately $8\%$ of the flux density at Europa from
solar illumination.

##  3 Experiments

The visible spectrum of the leading side chaos regions on Europa differs from
laboratory spectra of irradiated NaCl in two important ways. First, the Europa
feature appears at $\sim$450 nm, rather than the 460 nm wavelength found for
NaCl F-centers in the laboratory (Poston et al., 2017; Hand & Carlson, 2015).
Second, the laboratory spectra all show the formation of an M-center at $\sim
720$ nm, which the HST data strongly rule out. We perform a set of experiments
to explore these phenomena.

Figure 2: Irradiation at 120K with systematic warming of the sample. This
shows the redward shift of the F-center with temperature. The central
wavelengths from 120K to 280K are 451.2 nm, 453.1 nm, 454.1 nm, 456.1 nm,
457.6 nm, 459.0 nm, 460.5 nm, 461.9 nm, 463.4 nm. The minimum values were
found by doing a parabolic fit to the F-center range (400-500 nm) and then
finding the wavelength minimum of the parabola.

###  3.1 Wavelength Shift

Trumbo et al. (2019) noted that, in experiments by Poston et al. (2017), the
F-center spectral feature quickly decayed and shifted blueward when
irradiation ceased. They hypothesised that the higher irradiation flux for the
laboratory spectra caused the mismatch in wavelength between the Europa and
the laboratory spectra. Our first set of experiments were designed to test
that hypothesis.

We first attempted to reproduce some of the key results of Poston et al.
(2017). We irradiated a NaCl sample at a temperature of 120 K at our medium
flux level, which is approximately 6 times that of the initial Poston et al.
(2017) irradiation, with no photobleaching. This flux is approximately
$10^{4}$ times the flux level at Europa. We irradiated for 2 hours,
periodically obtaining spectra, to measure the growth of both the F-center and
the M-centers. This 2 hour irradiation is equivalent to 2.3 years on Europa.
We observed a central wavelength for the F-center of 451 nm, which is
significantly shifted from the Poston et al. (2017) results and much closer to
the spectra from Europa (Figure 2), even though our flux level was 6 times
higher. At the end of the irradiation sequence, the samples were left in the
dark at 120K overnight. In contrast to Poston et al. (2017), we observed no
change in the spectrum after ceasing irradiation. We then systematically
warmed the sample, allowing the sample to stabilize at each new temperature,
and obtained a spectrum at each, in order to examine the effects of
temperature. As noted by Schwartz et al. (2008), the wavelength of the
F-center systematically shifts redward with increasing temperature. As well as
shifting, the depth of the F-center decreases with increasing temperature. By
240 K the peak of the F-center absorption is at 460 nm, and at 260 K,
M-centers begin to spontaneously grow, as the higher temperatures allows the
F-centers to mobilize and begin to coalesce (Figure 2).

The difference between these experiments and the results of Poston et al.
(2017) are striking. We obtain a significantly shorter wavelength for the
F-center, and we see no spontaneous spectral change post-irradiation until we
purposely raise the temperature. We hypothesize that these differences are due
to a lack of adequate thermal coupling between the salt grains and the cold
finger in the Poston et al. (2017) experiments. The angular grains have little
contact with each other or with the cold finger and are exposed to thermal
emission from the walls of the experimental chamber. These grains could be at
a significantly higher temperature than the copper sample cup and the cold
finger. By embedding the samples in indium foil, as was done in the present
experiments, the thermal coupling is greatly increased. To test this theory,
we irradiated salts in the same manner as before, but without pressing the
sample into indium foil. We found that the central wavelength shifted to 460
nm, and the growth of both the F-center and M-center occurred on much faster
time scales when compared to spectra at 120K (Figure 3). As a final test, we
irradiated indium-pressed thermally coupled salt grains at 200K. For these
spectra the central wavelength was observed at 458 nm. There was no
discernible shift in the central wavelength post irradiation but a slight
growth in the M-center was observed post irradiation. The peak of the
absorption for these 200K experiments was blueward of both the Poston et al.
(2017) spectra as well as our experiments without pressing the salt into the
indium foil (Figure 3). Based on these experiments, we hypothesis that the
grains in the Poston et al. (2017) experiments were not well coupled to the
cold finger. We believe they were approximately $\sim$ 240K, based on the
central wavelength and the growth of the M-center. This higher temperature
would naturally lead to the redward shift of the F-center and the post-
irradiation spectral changes observed in those experiments but not observed in
our thermally coupled 120 K experiments. We conclude that the shift in the
F-center between Europa and laboratory data is not due to the increased
irradiation in the laboratory, but rather from the inadvertently elevated
sample temperature in the previous laboratory data. For thermally coupled
samples at 120K, the central wavelength of 451 nm is within the uncertainty of
the positional measurement of the feature on Europa. The position we observed
also coincides with the central wavelength previously measured for these
temperatures (Mador et al., 1954; Schwartz et al., 2008).

Figure 3: Spectra of NaCl irradiated at medium irradiation for 15 minutes.
Significant differences are seen in samples with indium foil at 120K, and
200K, and without indium foil. F-center growth is significantly faster for the
200 K and no-indium foil samples, while the F-center wavelength is also
shifted redward. For the no-indium foil sample, M-center growth can already be
seen. We conclude that the sample sitting on a 120 K cold finger but without
indium foil to thermally couple it is even warmer than 200 K.

###  3.2 M-Center Formation

The second critical difference between the laboratory and HST data is the
strong growth in the laboratory of the M-center absorption feature located at
720 nm compared to its non-detection from HST. Trumbo et al. (2019)
hypothesized that photobleaching, the destruction of F-center color centers by
photons, could suppress the formation of F-centers sufficiently that
M-centers, which require a certain density of F-centers in order to coalesce,
might never form. To test the magnitude of this photobleaching effect, we
performed two sets of experiments. The first was a series of experiments with
and without photobleaching at three different irradiating levels. For
photobleaching we use the same lamp as is used for spectral reflection. Our
lamp provides roughly 8% of the photon flux density in the 400 to 500 nm
wavelength span of the F-center compared to noon at Europa’s equator. At all
irradiation levels, photobleaching slowed the growth of the F-centers, as can
be seen in the comparison between photobleached and non-photobleached spectra
with otherwise identical conditions in Figure 4. To additionally examine this
effect, we measured the mean absorption depth of the F-center from 425-475 nm
as a function of time for each of our irradiation experiments. As seen in
Figure 5, the average absorption depth of the F-center is dependent on both
electron flux and whether or not there is photobleaching. The growth rate of
the F-centers, without photobleaching, is approximately linear until
saturation occurs and the formation of M-centers begins. This linear region is
poorly resolved in the medium and high irradiation experiments, but if we take
the 5.6% absorption growth in the first 20 minutes of the low irradiation
experiment to be typical of the quasi-linear growth regime, we find a growth
rate of 17% hr-1. Scaling by the irradiation flux gives consistent results for
the medium and high irradiation levels. We thus assume that for small F-center
absorption depths, the growth of the absorption depth, $A$, can be described
as a simple linear growth, $dA/dt=aF$. Here, $F$ is the irradiation flux in
units of eV $(16{\rm amu}{\rm yr})^{-1}$, where the flux at Europa is
$\sim$10, and $a$ is a constant, here found to be $6.8\times 10^{-5}$ in the
same units as $F$ and with $t$ in hours. At the flux of Europa, a $\sim$7%
absorption feature would grow in a little more than one orbital period in the
absence of competing effects.

Figure 4: Spectrum of NaCl irradiated at low irradiation for 40 minutes with
and without photobleaching. The central wavelength is the same for both
spectra.  Figure 5: The average fractional absorption depth of the F-center
from 425-475 nm vs time of irradiation for our three irradiation levels, with
and without photobleaching. In all cases, photobleaching slowed the growth of
the F-centers, though the lamp is too weak to completely balance even the
lowest electron irradiation level.

It is difficult to simultaneously match the photon and electron flux at Europa
with our experimental setup. Even our lowest level of irradiation is $\sim$250
times that experienced at Europa, while our photobleaching photon flux density
is $\sim$13 times less. Moreover, every time we take a spectrum we are
photobleaching for the $\sim$1 minute duration of the spectral exposures. Thus
even the unbleached spectra have actually had some amount of photobleaching.
For these reasons, it is difficult to extrapolate the results of Fig. 5 to
estimate the effect at Europa.

To attempt to obtain a better estimate of the effect of photobleaching on
Europa, we performed one additional experiment. We irradiated a sample at four
times our low irradiation flux for 10 minutes, leading to an F-center with a
fractional absorption depth of about 20%, approximately three times that seen
on Europa. We than ceased irradiation and began photobleaching. The F-center
absorption decayed rapidly for the first few hours (Fig. 6). By the time it
reached values more comparable to those seen on Europa, the decay slowed, and
we fit this section of the curve to the theoretically derived and
experimentally verified equation for photobleaching of lightly irradiated
NaCl, $dA/dt=-bIA^{2}$, where $A$ is the fractional absorption depth of the
F-center, $I$ is the intensity of the light, and $b$ is a constant. (Herman &
Wallis, 1955). We find a value of 0.035 for the product $bI$ for our lamp,
which scales to 0.46 for the noon time equatorial solar flux at Europa.

We used this photobleaching equation and an assumed linear growth rate of
F-centers with irradiation to estimate an equilibrium value of F-center
absorption depth. We model the F-center growth rate as $dA/dt=aF-bIA^{2}$. In
steady-state and assuming continuous photobleaching at the average equatorial
day/night irradiance, this equation predicts an absorption depth of 6.8% for
Europa. This calculated value is remarkably similar to the depth seen in the
HST data. Crucially, such an equilibrium value for the F-center is much lower
than that seen to be required for formation of M-centers. In reality,
photobleaching is not constant on Europa, but ranges from zero during the
night to a maximum at noon. Incorporating this changing photobleaching rate
into a simple numerical model which accounts for nighttime and sinusoidally
varying illumination in the daytime suggests that, in equilibrium, the
F-center absorption depth would grow to 8.7% overnight, decay to 5.6% by the
end of the day, and regrow overnight (Figure 7).

Intriguingly, the HST data on Europa show evidence for this precise
photobleaching effect. While Trumbo et al. (2019) report only average values
for the F-center strength across the disk, we note that a portion of Tara
Regio was observed both in the morning and in the afternoon. Reproducing their
data reduction, but examining overlapping spectra separately, we find that the
morning absorption depth is indeed deeper than that of the afternoon. These
absorption depths are well fit by our simple time-dependent mode. Given the
crudeness of the modeling and the difficulty of extrapolating laboratory
conditions to those at Europa, we believe that the extremely close agreement
between the precise values of the model and of the data must be mainly
coincidental. Nonetheless, we find the fact that the relative change in the
F-center absorption depth from HST agrees with the model to be an encouraging
sign that our extrapolations to Europa’s surface conditions and crude modeling
are capturing the real behaviour at Europa.

Figure 6: The average fractional absorption depth of the F-center from 425 to
475 nm with photobleaching post irradiation.  Figure 7: A simple numerical
model showing the F-center fractional absorption as a function of Europa’s 85
hour rotation period, including the effects of continuous irradiation on
Europa with solar flux dependent photobleaching during the daytime. The
F-center grows over the course of the night and begins decaying soon after
sunrise before recovering after sunset. The data points show the absorption
depths of two HST measurements of identical regions within Tara Regio taken at
different local times. The horizontal error bars show the geographic extent of
the observed region. The F-center decay predicted by the model is observed on
Europa.  Figure 8: HST observations of overlapping regions of Tara Regio in
the morning and in the afternoon. Dashed lines are second-order polynomial
continuum fits, which facilitate band-strength comparison. The continuum-
removed absorption bands are shown below. The observed decay in the F-center
absorption is consistent with the photobleaching decay predicted here.

##  4 Conclusions

We used new laboratory data with carefully controlled temperature and photon
illumination conditions to examine the discrepancy between the spectrum of the
Tara Regio region on Europa measured with HST and the spectrum of irradiated
NaCl found in previous laboratory work. We found that in our experiments
irradiated NaCl grains, which are thermally coupled to a 120 K cold finger by
pressing them into indium foil, develop F-center absorptions at 451 nm,
matching the spectral feature on Europa, rather than at the 460 nm wavelength
seen in previous experiments. Absorption at 460 nm is, however, seen when
samples are placed on the cold finger without pressing into the foil and when
thermally-coupled samples are heated to 240 K. We conclude that previous
experiments had poor thermal coupling between the cold finger and the angular
salt grains resting on a cold finger, resulting in a raised sample temperature
of approximately 240K. At the appropriate temperature, the band position of
laboratory-irradiated NaCl at 451 nm falls within the uncertainty of the band
position on Europa.

Previous laboratory data also predicted the presence of M-center absorption at
720 nm on Europa for typical irradiation doses expected, but the HST data put
strong upper limits on the existence of such an absorption. Here we found that
photobleaching – the destruction of F-centers by photons – can nearly
precisely balance the irradiation-induced creation of F-centers, leading to a
low equilibrium value for F-center density and a weak absorption strength,
similar to that seen on Europa. This low equilibrium density is well below the
density required for pairs of F-centers to begin to coalesce to form
M-centers, explaining the lack of this additional absorption on Europa.

Our model for F-center creation and destruction predicts that the absorption
will grow overnight in the absence of photobleaching and partially decay
during the day under solar illumination. We show that morning and afternoon
spectra of Tara Regio, when examined separately, show this effect at a similar
level as predicted.

These new laboratory experiments show that the visible spectrum of Tara Regio
is consistent with the presence of irradiated NaCl, including the wavelength
of the F-center feature, the lack of an M-center absorption at 720 nm, and the
change in the strength of the F-center feature over the course of the day. The
existence of NaCl in a chaos region on the leading hemisphere of Europa
strongly suggests that sodium and chlorine are dominant components of the
subsurface of Europa.

This research was supported by a grant number 668346 from the Simons
Foundation. The observations were made with the NASA/ESA Hubble Space
Telescope, obtained at the Space Telescope Science Institute, which is
operated by the Association of Universities for Research in Astronomy, Inc.,
under NASA contract NAS5-26555. These observations are associated with program
$\\#$14650\. This work is also associated with archival program $\\#$15789\.
This data was obtained from the Mikulski Archive for Space Telescopes (MAST).
This specific observations analyzed can be accessed
viahttps://doi.org/10.17909/t9-awev-6407 (catalog 10.17909/t9-awev-6407).
Support for program $\\#$15789 was provided by NASA through grants from the
Space Telescope Science Institute, which is operated by the Association of
Universities for Research in Astronomy, Inc., under NASA contract NAS5-26555.
S.K.T. is supported by the Heising-Simons Foundation through a 51 Pegasi b
postdoctoral fellowship.

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