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Article Not peer-reviewed version
Influence of the Surface
Temperature Evolution over
Organic and Inorganic
Compounds on Iapetus
Katherine Villavicencio-Valero , Emilio Ramírez-Juidias * , Antonio Madueño-Luna ,
José Miguel Madueño Luna , Miguel Calixto López Gordillo
Posted Date: 5 July 2023
doi: 10.20944/preprints202307.0349.v1
Keywords: energy budget; albedo dichotomy; surface temperature; organic material; diffusion
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Article
Influence of the Surface Temperature Evolution over
Organic and Inorganic Compounds on Iapetus
Katherine Villavicencio Valero1, Emilio Ramírez-Juidias2*, Antonio Madueño-Luna3, José Mi-
guel Madueño-Luna2 and Miguel Calixto López-Gordillo2
1 Department of Geology and Engineering, University of Chieti-Pescara, Pescara, Italy
2 Graphic Engineering Department, University of Seville, 41092 Seville, Spain
3 Department of Aerospace Engineering and Fluid Mechanics, University of Seville, 41092 Seville, Spain
* Correspondence: erjuidias@us.es
Abstract: Iapetus, a Saturn moon, shows the most differentiated albedo dichotomy of the Solar Sys-
tem. The dark leading side has a lower albedo than the bright trailing side. Spectral data on the
visible light reveal the existence of two types of materials on the surface. The darkening in the lead-
ing side is thought to be originated by the presence of organic material and carbonaceous com-
pounds on surface, while the trailing side is covered by water ice due to migration processes from
the dark side. On airless bodies like Iapetus, the surface escape speed is greater than the speed of
water molecules, resulting in the retention of a H2O atmosphere that allows some species to get
diffused through it. Here, there were performed simulations of the evolution of the surface temper-
ature for each of the two hemispheres of Iapetus. The results showed a slow yet steady increment
of temperatures for both sides, with a steeper slope for the dark hemisphere. It was also simulated
how much energy budget can be accumulated in both sides and its consequences. Finally, we cal-
culated the diffusion coefficients for ammonia, methane, and water ice. The results let us infer how
these compounds could evolve over time.
Keywords: energy budget; albedo dichotomy; surface temperature; organic material; diffusion
1. Introduction
Iapetus has two different types of terrains on its surface. One is the dark material, concentrated
in the leading side, possibly composed by carbonaceous elements and organic material, that are
thought to come from Phoebe, Titan, or Hyperion [1,2]. The other one is the bright material that, in
the trailing side, shows the spectral signature of water ice, which could have been originated from
the migration of water as a result of temperature variations in both hemispheres [3]. The leading
hemisphere presents a lower albedo than the trailing side, a fact that might have two possible expla-
nations; the first one, through endogenic processes like flooding of magma, which evidence is proved
by the existence of in-filled craters, and the second one due to the accretion of exogenous particles,
where previous studies reveal the existence of dark material [1,4,5]. These divergences in the hemi-
spheres make Iapetus the celestial body with the most notorious albedo dichotomy of the solar sys-
tem. The dark hemisphere called Cassini Regio has a low albedo of 0.04 and the bright one an albedo
of 0.39 [6]. The dark side is warmer than the bright side, facilitating the sublimation and volatile
migration of compounds that could form and retain a fast exosphere [7].
Spectral data taken by the Visible and Infrared Mapping Spectrometer (VIMS) on board of the
Cassini Mission detected a prominent presence of CO2 in the dark side [8]. On the other hand, [2]
reported the concentration of some polycyclic aromatic hydrocarbon (PAH) molecules in a region of
low albedo and an association of CH2 aliphatic hydrocarbons. There were also found concentrations
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of water ice placed in the dark regions far away from the apex, which are consistent with the thermal
segregation of water ice at the darkest and warmest latitudes [9,10]. This might be a hint of an em-
placement of dark material from an exogenous source. In this regard, [1] proposed a thermodynamic
internal origin for the organic and carbonaceous material on the surface. Volatile compounds like
ammonia have been also detected in the trailing hemisphere [7].
Dynamical dust models, like the one proposed by [11], suggest that deposition regions are either
formed from the Phoebe’s dust ring, or this material could have been transported onto the moon from
interplanetary dust [12]. Volatile frosts tend to create high albedos and prolong residence times, the
sublimation of such volatile compounds and water molecules acts as a powerful coolant in the pro-
cess of producing transient exospheres located at the impact places, where the pressure decreases
symmetrically with the distance to the impact zone. During the formation of the fast exosphere,
around 23% of created water molecules escape to space. Moreover, in the bright hemisphere, the
sublimation of ice molecules is just possible during diurnal cycles [7].
Exospheres could act as a limit between the surface and the adjacent environment, which com-
position is based on a combination of gases released from the surface through various processes like
thermal release or vaporization [13]. Those molecules emitted from the surface are ejected through
trajectories until they make collisions again with the surface, altering the chemistry of the material
present on the surface, and modifying the optical surface properties [14,15]. Here we assume that the
sublimation of volatiles like ammonia, water ice, and methane contributes to the formation of an H2O
exosphere on airless bodies like Iapetus [7,13], and the sublimation of water ice is two orders of mag-
nitude higher on the dark side than in the bright one [16]. We also assume that the bright hemi-
sphere is fully covered by pure water ice due to its migration from the dark side [3].
Studying the evolution of the surface temperature of Iapetus, and the influence of the energy
budget, is crucial because it could help us to improve our understanding of the evolution over time
of certain organic, and inorganic, compounds when they are present on airless bodies. Here, we con-
sider a time window (1 Gyrs) that is wide enough to observe a variation in the temperature. Then we
simulate how much energy budget can be concentrated on each hemisphere, considering different
albedos [6], during the same time window. This has the purpose to figure out whether this contribu-
tion could be of help with the dispersion of species into the atmosphere on the moon [7,17]. Then, we
calculate the diffusion coefficients for ammonia, methane, and water ice, based on the increment of
the temperature obtained by the simulation. The aim of this study is to understand how these organic,
and inorganic, compounds evolve in extreme environments, like the surface of Iapetus.
2. Materials and Methods
In this study, we use the Navier-Stokes Equations [18,19] to simulate the evolution of the surface
temperature in a two-dimensional cartesian geometry for 1 Gyrs of model time. As is known, accord-
ing to [19], the system of flows between the atmosphere, the soil and the ice is highly interactive, and
introduces positive feedback. For example, if the ice on the bright side of Iapetus starts to melt, the
ground it leaves bare absorbs solar radiation, so it gets hotter and more ice melts, which warms the
atmosphere, which acts on the ice again.
The conservation of the equations for mass, momentum, and energy are solved on a staggered
grid for a compressible fluid with an infinitive Prandtl number (a full description of these equations
can be found, in e.g., in [20]). We fixed an initial surface temperature of 90 K for the dark side and
130 K for the bright side of Iapetus. Different albedos were also considered for each hemisphere, 0.04
and 0.39 respectively [6,11,21].
For the energy budget, we considered the effect of the eccentricity of the orbit of Iapetus around
the Sun, the influence of emissivity and heat capacity of Saturn, the contribution of Saturn radiation
and the internal heating from Iapetus. This study also takes into account the diurnal cycles in both
hemispheres. For albedos, we selected the values described above [6]. In order to obtain the energy
budget, we computed the net radiation, the ground heat flux, the sensible heat flux, and the latent
heat flux (see Eq. (1)).
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(1)
The diffusion coefficient can be estimated from the empirical relation of diffusion of species (H2O
vapor, NH3 and CH4) in air proposed by [22], where they simplified the two theoretical equations of
the kinetic theory of gases, derived by Stefan and Maxwell [23]. In our calculation, we modified the
original empirical relation as:
(2)
Where 2·10-12 is a coefficient that we calculated assuming a column of homogeneous dark mate-
rial overlaying ice of 1 cm [24], the collision diameter of the species in angstroms, the turbulence
effect, the average atmospheric temperature at 110 K and the integral collision. We used a data mining
process, through the application of modified genetic algorithms [25] to calculate the concentration
(g/kg) of each species present in the atmosphere. It is necessary to highlight that data mining tech-
niques uses multidimensional rotation, translation, reflection and transformation along with random
tuple (ordered immutable set of elements “atmospheric species“ of the same or different types) shuf-
fling and randomized expansion.
In base, the modified genetic algorithms used [25] consist in a parallel procedure that works as
follows. The big data set that needs to be categorized is stored within a central storage unit. When
the execution begins, individual sections are handled by separate mapping tasks. Each mapping task
initially accesses the training dataset and proceeds to train the classifier. Subsequently, the trained
classification model is employed to categorize the extensive dataset. The repeated training process
for each mapping task should have minimal impact on computational performance since the training
data is relatively small in comparison to the significant dataset, which is responsible for the majority
of the processing time.
To calculate the concentration (g/kg) of each species present in the atmosphere, we identified the
spectral signatures detected in the bands between 0.35 μm to 1.07 μm from the data taken by the
VIMS instrument on board of the Cassini Mission. Then, we calculate the total number of moles per
mole of air for each compound and multiply it by their molecular weight. In this way, we obtain the
mass in grams for each species. Eq. (2) considers the molar fraction which is calculated by dividing
the number of moles by the total number of moles of all species present in the air. In our case, we
measured the sum of the moles of water ice, ammonia, carbon dioxide, and methane. For Iapetus, we
found a total weight of one mol of air of around 17.48 g/mol. The atmosphere thickness model was
assumed to play a dominant role in heat distribution. We also included the concentrations of moles
in one mole per air of some compounds detected on the surface of Iapetus such as CO2, CH, and NH
[26], in order to obtain the total molecular weight of species in the air. The results of these calculations
are displayed in Table 1.
Table 1. Concentration of species present in the atmosphere of Iapetus. Previous researches assume
that the formation of an exosphere on Iapetus is mainly composed of H2O [7]. In this study, there were
included other species detected in the bands between 0.35 μm to 1.07 μm by the VIMS instrument on
board of the Cassini Mission. The concentration of species was calculated by applying genetic algo-
rithms [25]. Water ice and ammonia are the main compounds present in the atmosphere.
Atmospheric
composition
Concentration
(g/kg)
Presence among
other species (%)
Presence in
the air (%)
Mass
(g)
Molar
fraction
(mol/g)
Air
100
H2O
8.82
50.49
49
8.82
0.49
NH3
8.33
47.68
49
8.33
0.49
CH4
0.33
1.83
2
0.32
0.02
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Total
17.47
100
100
17.47
1
3. Results
We first describe, in section 3.1, the evolution of the surface temperature. In this model, the evo-
lution of the temperature is treated as a transfer energy model. In section 3.2, we describe the out-
going thermal radiation and the incoming stellar radiation of the energy budget for both hemi-
spheres. In this model, we consider both contributions, the one with the fluxes that can be retained
on the surface and the ground heat flux that tends to escape into space. Finally, in section 3.3, we
describe the diffusion coefficients found for water ice, ammonia, and methane.
3.1. Surface temperature evolution
Figure 1 illustrates the changes in surface temperature for the bright and dark hemispheres of
Iapetus, considering that both the atmosphere and surface are in a state of chemical and phase equi-
librium. The atmospheric structure is constructed from the bottom to the top, influenced by the in-
teraction between the surface and atmosphere. The simulation shows that, for both sides of the moon,
the temperature slightly and steadily increases from the onset up to the end of the model time (1
Gyrs). We initiated the temperature in the dark hemisphere to 90 K. The maximum temperature
reached in this hemisphere is 128 K, which is marginally above the seasonal maximum temperature
of 130 K conditioned by the apparent surface thermal inertia, as reported by [24], where the effects of
the maximum temperature were neglected. Volatile compounds like the segregation of ammonia on
the surface, and the fast vaporization of water ice due to the migration to the bright side, could influ-
ence the increment of this temperature [27,28]. Moreover, the increase in the temperature in our
model is enough to represent a significant change in the surface thermal inertia on the dark side of
Iapetus [21,29–31]. The dark hemisphere absorbs approximately 58% more solar radiation than the
bright one [24,27]. Because of this percent of absorption, it is possible to infer that the thermal segre-
gation of some organic, and inorganic, compounds on this side could play an important role in the
surface alteration process [24].
Figure 1. Surface temperature evolution for the dark and bright hemispheres of Iapetus. The initial
temperature for the leading side, represented with a blue line, starts at 90 K and evolves until almost
130 K during the model time. The trailing side, depicted with a red line, starts at 130 K and grows to
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147 K. There is not a substantial increment of temperature for both sides, but the increment in the
leading side is greater, possibly due to the low albedo of 0.04 and its composition.
For the bright side, the initial temperature was set at 130 K. Figure 1a shows the growth of this
temperature up to 147 K, at the end of the simulation. Here, the temperature increases with a lower
slope compared to the dark side one. This could be a consequence of the high albedo of 0.38, which
makes its surface reflect more light [29–31] or, in other words, absorb less energy from light. The
condensed water is highly reflective [32], and it is present on the bright side due to the migration
from the dark hemisphere [3]. On the bright side, which is less emissive than the dark one, the diffuse
scattering of water ice particles takes place [31]. Analysis of polarization in the bright side performed
by [33], shows that this effect is deeper on surface, making it more sensitive to a diversity of scattering
parameters like particle size, shape, surface texture, and reflectance of objects, thus making it difficult
to interpret their differences. However, it is still possible to infer the diffusion of methane ice and
water ice because the surface of Trans Neptunian Objects is dominated by these two compounds [34].
3.2. Energy budget
Figure 2a shows the outgoing thermal radiation and incoming stellar radiation, on the dark hem-
isphere of Iapetus, that has an albedo of 0.04 [6], over a time window of 1 Gyrs. At the very beginning
of the simulation, due to changes in frost on the surface, the outgoing thermal radiation drops from
roughly 10 W m-2 to 2 W m-2; then it bounces back to 2.8 W m-2 and, after some fluctuations between
2.7 W m-2 and 3.1 W m-2, it stabilizes to this value. On the other hand, Figure 2b displays the outgoing
thermal radiation and incoming stellar radiation for the bright side, with an albedo of 0.39 [6]. Here,
there were obtained values between 25 W m-2 and 29 W m-2 for the first million years, then it tends to
be stable at 26 W m-2 of model time. For both hemispheres, the incoming stellar radiation is around
0.2 W m-2 (although slightly higher in the dark hemisphere), and it does not change over time. There
is a notable difference of outgoing thermal radiation between the two hemispheres. The bright side
is covered by water ice that, being reflective, is an obstacle for absorbing energy.
Previous analysis of the energy balance on Iapetus [24] shows that the thermal inertia for the
dark terrain ranges between 11 J m-2 K-1 s-1/2 and 14.8 J m-2 K-1 s-1/2, while for the bright terrains it ranges
between 15 J m-2 K-1 s-1/2 and 25 J m-2 K-1 s-1/2. These values allow some species, like water ice and carbon
dioxide, which are assumed to be present in both sides, to have a slow mass rate, through both inter-
molecular and pore interactions. Our models show, in fact, that an airless surface with a low tem-
perature cannot retain too much energy on surface; nonetheless, the small accumulation observed in
the simulations (between 0.2 W m-2 and 2.8 W m-2) could provoke the volatilization of ammonia and
methane, allowing for the diffusion in the atmosphere [7,24].
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Figure 2. The outgoing thermal radiation and the incoming stellar radiation are displayed for the dark
and bright hemispheres in Figure 2a and 2b respectively. In the leading hemisphere, the outgoing
radiation is constant at 2.8 W m-2, while in the trailing side it remains constant at 26 W m-2. This
discrepancy could be a consequence of the different values of their albedos. Low temperatures and
high albedo in the bright side do not allow for the retaining of flux radiation in the atmosphere, that
is why, the outgoing thermal radiation is higher. In both sides, the incoming radiation is weak, with
a roughly value of 0.2 W m-2.
3.3. Diffusion coefficients of ammonia, methane and water ice
All the diffusion coefficients obtained in this work, for organic and inorganic compounds that
grow at atmospheric temperatures ranging from 90 K to 130 K, are displayed in figure 3. In these
simulations, ammonia, methane, and water ice are assumed not to be homogeneously distributed on
the surface.
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The diffusion coefficients obtained for methane vary between 2.7·10-8 cm2 s-1 at 90 K, and 5.6·10-8
cm2 s-1 at 130 K. Hence, for the temperatures considered, they are always bigger than the coefficients
for the other two compounds. This can be explained due to the increment of temperature found in
the dark hemisphere, and possibly because of the high concentration of organic material on the sur-
face [1,35]. In this sense, [36] report diffusion coefficients for methane of 2.5·10-13 cm2 s-1 at 50 K, which
values differ from the ones obtained in this study since those experiments were performed in a range
between 30 K to 50 K. We could expect a high diffusion of methane if the atmosphere of Iapetus were
similar to the one of Pluto, where the interaction between methane and cosmic rays produces C2H2.
This interaction also warms the atmosphere, bringing the methane to high altitudes and generating
a greenhouse effect, starting a virtuous circle [37,38].
We found diffusion coefficients for water ice varying between 9.4·10-10 cm2 s-1 at 90 K and 1.9·10-
9 cm2 s-1 at 130 K, values that are lower than the ones for methane. These low diffusion coefficients
can be explained by taking into account the transportation of water isotopes through atom diffusion
in dry ice in a matrix of interconnected porosity [23]. Experimental studies show that some species
can be diffused through water ice backgrounds, like water vapor with coefficients of 0.2·10-5 cm2 s-1
[39], or amorphous ices with coefficients of 4.2·10-4 cm2 s-1 at 130 K [40]. On Iapetus, it could be ex-
pected that species like ammonia or methane can be diffused with small coefficients due to the low
temperatures on surface [23,41].
The diffusion coefficients found for ammonia ice vary between 1.8·10-9 cm2 s-1 at 90 K and 3.9·10-
9 cm2 s-1 at 130 K. Previous experiments on diffusion of ammonia in hemihydrate environments, reveal
a diffusion coefficient of 4.3·10-10 cm2 s-1 at 140 K [42]. Other experiments show ammonia adsorption
bands in combination with ice particles at 120 K [43]. In this way, [44] also report peaks of ammonia
adsorption in ice at 120 K. On Iapetus, the emergence of regions where ammonia can diffuse, after
reaching 113 K, could contribute to the sublimation and volatilization of elements along the surface
[7,24].
4. Discussion
Our study of the diffusion of organic and inorganic material detected in the bright and dark
hemispheres of Iapetus relies on understanding how they could evolve in airless bodies, starting with
initial surface temperatures of 90 K and 130 K. On a slow rotator, like Iapetus, maximum tempera-
tures are dependent on Sun-Saturn distance, albedo, and the available solar heat flux [21,24]. The
simulations performed for this work show a steady increase from the initial temperatures, reaching
at the end the maximum temperatures for the dark and the bright hemisphere of 130 K and 147 K
respectively. During the 1 Gyrs of model time, the increment of these temperatures was not enough
to provoke significant changes on the surface, but the growth for the dark hemisphere was bigger,
due to its composition, low albedo, and high flux absorption [21,35,45]. Organic compounds in the
dark hemisphere are spread all along it, across a layer that probably was formed by internal thermo-
chemistry between methane and HCN compounds, that transformed them into organic material [1].
There had been experiments conducted by [46] that show how methane is thought to be transformed
into organic material due to ion bombardment on Phoebe; the same process, mutatis mutandis, could
happen on Iapetus.
In that study, it is suggested that the constant ion bombardment on Phoebe converts methane
into yellow material, then into brown material, and finally into black material, originating the dark-
ening on the surface [47,48]. The time scale required to do this conversion is 107 years [46]. In another
vein, [2] proposed that the growth of organic material on Iapetus is a uniform process, with an accre-
tion rate of 9·10-10 g cm-2 year-1, where the time required to do the carbonization is 2·106 years, lower
than the carbonization time of 107 years for Phoebe. It could be possible to infer that the diffusion
coefficients obtained for methane could provoke a constant evolution of organic material into carbo-
naceous compounds on Iapetus, with a small accretion rate during 1 Gyrs.
Generally, low temperatures represent a barrier in the diffusion of species through the atmos-
phere [23]. Water TDS curves have demonstrated that the presence of ammonia molecules changes
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the water desorption behavior [44]. At an atomic level, the interaction between ammonia molecules
and icy surfaces generates reactions of conversion from ice into ammonia [44]. This phenomenon
occurs because water molecules directly bonded with the surface are replaced by ammonia molecules
during heating processes at 113 K. This could be an explanation of why the ranges of diffusion coef-
ficients obtained for ammonia and water ice are almost similar.
Experimental results of ammonia diffusion in amorphous ice for temperatures ranging between
35 K to 140 K [40], show diffusion coefficients of 1.1·10−11 cm2 s−1 and 1.9·10−11 cm2 s−1 at 120K, which
are almost similar to the ones obtained in our simulations. The diffusion timescale in that analysis
corresponds to 102 s to 106 s. If on Iapetus we would like to observe how ammonia and water ice could
evolve through time, we should expect billions of years for the sublimation of water ice and one
billion years for its evaporation at 110 K [49,50].
Since ammonia and methane are the most abundant non-aqueous volatiles in the outer Solar
System, it can be expected an increase of them over time (Murchie and Head), possibly throughout
the internal geodynamic activity when they, in combination with water ice, reach the melting point
and contribute to rising their abundance on the surface. But this analysis is beyond the scope of this
study. Another possible way to extend this work is through an investigation of the effects of a possible
global warming caused by an increment of methane and the concentration of carbon dioxide [51] on
the surface of Iapetus.
5. Conclusions
In this manuscript, we have presented the results of our research on the evolution of the surface
temperature of Iapetus, with the goal to understand the link between the surface temperature evolu-
tion and the distribution of organic, and inorganic, compounds on the surfaces of airless bodies. We
assumed a solid planetary surface that is in a state of chemical and phase equilibrium with the nearby
atmospheric layer close to the crust. This equilibrium ensures that the deposition and outgassing
rates of all the considered species are in balance. The model employs a step-by-step approach, starting
from the bottom layer, to identify the condensates that become thermally stable in each subsequent
layer. We then gradually eliminate the elements present in these condensates until chemical and
phase equilibrium is once again achieved.
Our model could offer initial understanding regarding the anticipated arrangement of cloud
layers in the atmospheres of Iapetus. This arrangement depends on factors such as surface tempera-
ture, surface pressure, and crust composition. These cloud condensation sequences in equilibrium
serve as a foundation for future exploration in kinetic models that study cloud formation. However,
our paper does not encompass models that consider boundary fluxes at the atmospheric boundaries,
atmospheric escape rates, or non-equilibrium processes like photochemistry. These aspects extend
beyond the scope of our current study.
To correctly simulate the temperature evolution over organic, and Inorganic, compounds on
Iapetus, it was decided to analyze the visible spectral signature obtained by the VIMS instrument (see
Figure 3a and 3b). In this regard, Iapetus spectra, with absorption bands annotated with identifica-
tions derived in [26], were used, along with Digital Elevation Models (DEM) generated from ImageJ
of both Iapetus hemispheres, (see Figure 4a and 4b), in order to carry out the simulation.
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Figure 3. The first image (a) displays the spectra of Iapetus, indicating absorption bands that have
been identified according to [26]. In contrast, the second image (b) presents a range of representative
spectral variations observed in the VIMS data during the Rev 49 fly-by. It's worth noting that the
position of the blue peak around 0.5 µm varies depending on the reflectance level and the amount of
dark material present. Although the phase angle for the ORSHIRES001 sequence changed from ap-
proximately 127 at 13:30 to 12.6 at 14:42 and then to 30 at 17:11, the average phase angle for these
spectra is around 20, except for the bottom spectrum. The bottom spectrum was obtained earlier and
has been scaled by a factor of 2.0x to compensate for the higher solar incidence and phase angle,
allowing for comparison with the spectra at a lower phase angle (https://s100.copyright.com/Cus-
tomerAdmin/PrintableOrder.jsp?appSource=cccAdmin&orderID=501824388).
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Figure 4. DEM of both bright and dark side of Iapetus obtained by authors from ImageJ free soft-
ware.
Finally, it is worth mentioning that this research study is the basis of others focused on Iapetus,
in which, from the DEMs of the icy moon, it is possible to infer the surface erosion processes caused
by the temporal evolution of temperatures in both hemispheres.
Author Contributions: Conceptualization, KVV, E.R.J., A.M.-L, J.M.M.L and M.C.L.G.; methodology, KVV,
E.R.J., A.M.-L, J.M.M.L and M.C.L.G.; formal analysis, KVV, E.R.J., A.M.-L, J.M.M.L and M.C.L.G.; investigation,
KVV, E.R.J., A.M.-L, J.M.M.L and M.C.L.G.; resources, KVV, E.R.J., A.M.-L, J.M.M.L and M.C.L.G.; writing—
original draft preparation, KVV, E.R.J., A.M.-L, J.M.M.L and M.C.L.G.; writing—review and editing, KVV, E.R.J.,
A.M.-L, J.M.M.L and M.C.L.G.; supervision, KVV, E.R.J., A.M.-L, J.M.M.L and M.C.L.G. All authors have read
and agreed to the published version of the manuscript.
Funding: This research received no external funding.
Data Availability Statement: The data used to support the findings of this study can be made available by the
corresponding author upon request. The images in Figure 3 have been used with permission from the publisher,
as shown in the following link: https://s100.copyright.com/CustomerAdmin/Printable-
Order.jsp?appSource=cccAdmin&orderID=501824388.
Acknowledgments: Authors are grateful for the disinterested collaboration of the Knowledge-Based Company
RSV3 Remote Sensing S.L.
Conflicts of Interest: The authors declare no conflict of interests.
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