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Organic Input to Titan’s Subsurface Ocean
Through Impact Cratering
Catherine Neish,
1
Michael J. Malaska,
2
Christophe Sotin,
3
Rosaly M.C. Lopes,
2
Conor A. Nixon,
4
Antonin Affholder,
5
Audrey Chatain,
6
Charles Cockell,
7
Kendra K. Farnsworth,
8
Peter M. Higgins,
9
Kelly E. Miller,
10
and Krista M. Soderlund
11
Abstract
Titan has an organic-rich atmosphere and surface with a subsurface liquid water ocean that may represent a
habitable environment. In this work, we determined the amount of organic material that can be delivered from
Titan’s surface to its ocean through impact cratering. We assumed that Titan’s craters produce impact melt
deposits composed of liquid water that can founder in its lower-density ice crust and estimated the amount of
organic molecules that could be incorporated into these melt lenses. We used known yields for HCN and Titan
haze hydrolysis to determine the amount of glycine produced in the melt lenses and found a range of possible
flux rates of glycine from the surface to the subsurface ocean. These ranged from 0 to 10
11
mol/Gyr for HCN
hydrolysis and from 0 to 10
14
mol/Gyr for haze hydrolysis. These fluxes suggest an upper limit for biomass pro-
ductivity of *10
3
kgC/year from a glycine fermentation metabolism. This upper limit is significantly less than
recent estimates of the hypothetical biomass production supported by Enceladus’s subsurface ocean. Unless
biologically available compounds can be sourced from Titan’s interior, or be delivered from the surface by
other mechanisms, our calculations suggest that even the most organic-rich ocean world in the Solar System
may not be able to support a large biosphere. Key Words: Titan—Habitability—Impact processes. Astrobiology
24, xxx–xxx.
1. Introduction
Titan has all the ingredients needed to be a habitable
world. Organic compounds are produced in its methane-
rich atmosphere through photochemical processes (Ho
¨rst,
2017) and eventually settle to the surface to produce thick
deposits of organic plains, dunes, and lakes over a water ice
rich crust ( Janssen et al., 2016; Malaska et al., 2020).
Liquid water is abundant in Titan’s interior, forming a
subsurface ocean under a 40 to 170 km thick ice crust layer
(Baland et al., 2014). A significant contribution of organic
compounds may have leached from Titan’s rocky core into
the ocean (Miller et al., 2020), but unique contributions
from the surface may be key to achieving a theoretically
habitable ocean by providing novel inputs and a continu-
ous chemical energy source. However, transporting organic
1
Department of Earth Sciences, The University of Western Ontario, London, Ontario, Canada.
2
Jet Propulsion Laboratory, California Institute of Technology, Pasadena, California, USA.
3
Laboratoire de Plane
´tologie et Ge
´osciences, Nantes Universite
´, Univ Angers, Le Mans Universite
´, CNRS, UMR 6112, Nantes, France.
4
Planetary Systems Laboratory, NASA Goddard Space Flight Center, Greenbelt, Maryland, USA.
5
Department of Ecology and Evolutionary Biology, The University of Arizona, Tucson, Arizona, USA.
6
Departamento de Fı
´sica Aplicada, Escuela de Ingenierı
´a de Bilbao, Universidad del Paı
´s Vasco/Euskal Herriko Unibertsitatea (UPV/
EHU), Bilbao, Spain.
7
UK Centre for Astrobiology, School of Physics and Astronomy, University of Edinburgh, Edinburgh, United Kingdom.
8
NASA Postdoctoral Program Fellow, NASA Goddard Space Flight Center, Greenbelt, Maryland, USA.
9
Department of Earth Sciences, University of Toronto, Toronto, Ontario, Canada.
10
Southwest Research Institute, San Antonio, Texas, USA.
11
Institute for Geophysics, Jackson School of Geosciences, The University of Texas at Austin, Austin, Texas, USA.
ASTROBIOLOGY
Volume 24, Number 2, 2024
ªMary Ann Liebert, Inc.
DOI: 10.1089/ast.2023.0055
1
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compounds from the surface to the interior ocean is difficult
given their large vertical separation. There is little evidence
for the extensional tectonics needed to move material from
the surface to the brittle-ductile boundary (Cook-Hallett
et al., 2015; Walker et al., 2021), where convection could
move it to the ocean.
The largest impact crater on Titan, Menrva (with a
diameter of *400 km), may have directly breached the ice-
ocean interface (Cro
´sta et al., 2021), but it seems unlikely
that this occurred for any of Titan’s smaller and more
numerous impacts (Wakita et al., 2023). The lack of organic
input from the surface through breaching impacts would
necessarily limit the ocean’s long-term habitability. Orga-
nics and water may mingle on the surface in impact melts
and cryovolcanic flows (Neish et al., 2018), but these liquid
oases are short-lived (at most hundreds to thousands of
years) (Thompson and Sagan, 1992; O’Brien et al., 2005;
Hedgepeth et al., 2022) and thus unlikely to support a long-
term habitable environment.
How then might significant amounts of organic material
move from Titan’s surface to its interior ocean to maintain a
habitable environment? Recent work on Europa (Carnahan
et al., 2022) suggests that smaller impacts may be capable of
transporting material to the subsurface ocean through foun-
dering of denser impact-derived melt in the ice shell over long
timescales (thousands of years). Carnahan et al. (2022) found
that, when the depth of the transient cavity exceeds half of
the conductive ice shell thickness, a large fraction (40–90%) of
the melt will eventually drain into the subsurface ocean.
Titan’s conductive ice shell thickness is thought to be
15 km thick if a layer of methane clathrates is present, but up
to 42 km thick if it is composed of pure water ice (Kalousova
´
and Sotin, 2020). Wakita et al. (2023) found that a 4 km
impactor into a 10 km thick methane clathrate layer produces
a transient cavity that is *20 km deep and a final crater that
is *90 km in diameter. This would pierce the conductive ice
shell if it contains clathrates, but would not be deep enough
to cause impact melt foundering in a pure water ice shell. On
Titan, we would therefore need craters greater than *90 km
in diameter to transport melt to a subsurface ocean in the
pure water ice case, while smaller craters would be capable
of transporting melt in the methane clathrate case.
In this work, we determine the amount of impact melt and
organic materials that can be transported to Titan’s interior
ocean through the foundering mechanism described by
Carnahan et al. (2022). We consider two plausible cratering
rates, as determined by Artemieva and Lunine (2005) and
Korycansky and Zahnle (2005), to estimate the number of
impacts that could founder in Titan’s ice shell. Next, we
examine two plausible interior structures, one that includes
an upper methane clathrate layer and one that is composed
of pure water ice to constrain the range of crater sizes that
could deliver surface material to Titan’s subsurface ocean.
We then determine the amount of organics that can be
incorporated into the melt deposits in these craters. Finally,
we discuss the implications of this flux of organics to the
habitability of Titan’s subsurface ocean.
2. Impact Cratering Rates on Titan
Two different crater production rates on Titan are dis-
cussed at length by Neish and Lorenz (2012). They were
derived from the work of Korycansky and Zahnle (2005)
(referred to as KZ05) and Artemieva and Lunine (2005)
(referred to as AL05). Both papers used the same cratering
rate from Zahnle et al. (2003) (case A in their text) but
different time dependencies and crater scaling laws.
There has been more recent work on impactor populations
and dynamics in the outer Solar System in the past 20 years
(Greenstreet et al., 2019; Di Sisto and Rossignoli, 2020), but
Kirchoff et al. (2022) argued that the changes to the imp-
actor rate do not alter the derived model ages beyond the
uncertainties inherent in the calculations. We therefore con-
tinue to use the impactor rates provided by Zahnle et al.
(2003). The case A cratering rate presented by Zahnle et al.
(2003) estimated the impact rates in the outer Solar System
by extrapolating the impact rate on Jupiter by ecliptic
comets. It inferred the distribution of projectiles smaller
than 20 km from the distribution of impact craters on Europa
and Ganymede as opposed to Triton (which was denoted
case B in their text), and the distribution of projectiles
>50 km from the distribution of Kuiper Belt objects. The
main difference between case A and case B is in the number
of small projectiles, with case A assuming fewer small
projectiles compared to case B. We focus on case A in the
present study, since that was the impactor population used
by AL05. However, we note that the differences in cratering
rates are minimal for the size ranges considered here, that is,
craters larger than 20 km in diameter (Zahnle et al., 2003).
Although the two papers used the same present-day imp-
act rate, _
NdðÞ, KZ05 assumed the impact rate was constant
with time (Equation 1), while AL05 assumed the impact
rate had decreased over time with a 1/time dependence
(Equation 2).
N1d,tðÞ¼
Z
T
Tt
_
NdðÞds¼_
NdðÞt(1)
N2d,tðÞ¼
Z
T
Tt
_
NdðÞ
T
sds¼_
NdðÞTln T
Tt
(2)
Here, d=projectile diameter, t=the age of the surface,
and AL05 used T=4.56 Gyr, the age of the solar system.
The other main difference in the two papers is the crater
scaling law used. Specifically, the two papers used (1) dif-
ferent projectile densities (0.5 g/cm
3
vs. 1.0 g/cm
3
), (2) dif-
ferent scaling relationships for the size of the transient
crater, D
s
(d) (based on dry sand vs. water targets), and (3) a
different scaling exponent for complex craters, x(0.13 from
McKinnon et al., 1991 vs. 0.176 from Croft, 1985). The
final scaling laws are given by Equation 3 (KZ05) and
Equation 4 (AL05):
Ds¼1:02 v2
g
0:217 qi
qt
0:32
d0:783 cos hðÞ
0:333 (3)
Ds¼1:36 v2
g
0:22 qi
qt
0:31
d0:78 (4)
Here, D
s
=transient crater diameter (cm), v=impact veloc-
ity (cm/s), g=gravity (cm/s
2
), q
i
=impactor density (g/cm
3
),
2 NEISH ET AL.
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q
t
=target density (g/cm
3
), d=projectile diameter (cm), and
y=impact angle (degrees relative to a horizontal surface).
When viewed side by side, the two scaling laws appear
similar. However, despite the minimal differences between
the two equations, the predicted crater diameters differ by a
factor of *2, with the data presented by KZ05 representing
a minimum crater diameter, and the data presented by AL05
representing a maximum crater diameter (Neish and Lorenz,
2012).
In effect, AL05 assumes Titan’s crater population was
created by more common small impactors (and hence is
younger), while KZ05 assumes Titan’s crater population
was created by less common large impactors (and hence is
older). We plotted the cumulative crater counts for the two
sets of assumptions in Fig. 1. Here, we focus on craters with
diameters >20 km, because Titan’s atmosphere tends to dis-
rupt impactors that form smaller craters (Korycansky and
Zahnle, 2005). When using these crater production rates and
translating them to apparent surface age, Neish and Lorenz
(2012) and Hedgepeth et al. (2020) determined that the
observed crater distribution on Titan fits best to a 200-Myr-
old surface using the AL05 crater production rate and a
1-Gyr-old surface using the KZ05 crater production rate.
3. Melt Volumes Delivered to the Ocean
Next, we determine the amount of melt that is generated
in these impacts over Titan’s 4.5-billion-year history. From
this, we can determine how much organic material can be
transported to Titan’s subsurface ocean through the foun-
dering of melt in Titan’s ice shell. We estimate the amount
of melt produced in a Titan crater by using the results of
Artemieva and Lunine (2005) and Artemieva and Lunine
(2003) for a pure water ice crust (with 20% porosity) and
Wakita et al. (2023) for a methane clathrate crust (with no
porosity). Using the shock physics code iSALE, Artemieva
and Lunine (2003) found that a 2 km impactor striking an
icy crust at 45produced a *25 km diameter impact crater
with *60 km
3
of melt that remains within the crater, and
Artemieva and Lunine (2005) found that a 10 km impactor
striking an icy crust produced a *150 km diameter crater
with a total melt volume of *10,000 km
3
. Wakita et al.
(2023) used the same shock physics code and found incipient
melting of 400 km
3
and complete melting of 200 km
3
in a
Selk-sized crater (*90 km) formed by a 4 km diameter
impactor in a 5 to 15 km thick clathrate layer over water ice.
We scaled the amount of melt predicted by Artemieva
and Lunine (2005) and Artemieva and Lunine (2003) and
Wakita et al. (2023) to different crater sizes based on rela-
tionships developed for terrestrial craters by Grieve and
Cintala (1992). Lorenz (2000) expressed these results as
V
melt
fR
c
3.27
, where V
melt
is the melt volume in km
3
, and R
c
is the crater radius in kilometer. To fit the data given above,
the scaling for craters impacting into porous water ice
surfaces is:
Vmelt, ice ¼0:01Rc3:27 (5)
To fit the data for impacts into a non-porous methane
clathrate crust, we use a coefficient that is a factor of 10
smaller:
Vmelt, clath ¼0:001Rc3:27 (6)
These expressions are given to the first order of magni-
tude; additional iSALE modeling at a variety of crater diam-
eters is needed to constrain the coefficients further. We note
that the observed difference in melt production in the two
models is likely due to the difference in porosity (20% vs.
0%), not the difference in composition (water ice vs.
clathrate) (Wu
¨nnemann et al., 2008). Multiplying these
values by the number of craters produced in each diameter
range, we get a cumulative melt volume for all the craters
on Titan. For the methane clathrate scenario, we restrict
ourselves to a diameter range of 20 to 500 km, as this rep-
resents the range of craters observed on Titan that are not
disrupted by the atmosphere (Artemieva and Lunine, 2003;
Hedgepeth et al., 2020).
For the water ice scenario, we restrict ourselves to a diam-
eter range of 90 to 500 km, since smaller craters will not
produce transient craters large enough to puncture the con-
ductive ice shell [Note: As described in the Introduction, we
need craters that produce transient cavities deeper than
half of the conductive ice thickness to transport melt to the
interior ocean. Wakita et al. (2023) found that a 90 km
diameter crater will produce a transient cavity half the depth
FIG. 1. Cumulative crater counts for two reasonable crater production rates for Titan. The blue line represents the number
of craters expected to form over the last 200 Myr, the black line represents the number of craters expected to form over the
last 1 Gyr, and the red line represents the number of craters expected to form over 4.5 Gyr (the age of the solar system).
Color images are available online.
ORGANIC INPUT TO TITAN’S SUBSURFACE OCEAN 3
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of the conductive ice shell in a water ice crust.] For the
calculations, we assume 65% of the melt reaches the ice-
ocean interface, the average amount predicted by Carnahan
et al. (2022). These results are given in Fig. 2 for the four
cases presented in Table 1. Note that we have used the
results of Carnahan et al. (2022) as a good first-order app-
roximation to the foundering of melt lenses in Titan’s
conductive ice shell, despite the focus on Europa in their
work. Carnahan et al. (2022) assumed conductive ice shells
that ranged in thickness from 10 to 40 km, similar to the
thickness of Titan’s conductive ice layer in the case of
methane clathrates (*10 km) and pure water ice (*40 km).
However, there are a number of differences between Europa
and Titan besides the thicker ice shell, which should be
addressed with additional modeling work. The main one is
the likely presence of a layer of methane clathrates, which
are stable under Titan’s surface conditions. An ongoing
study (Kalousova
´et al., 2023) uses the temperature, melt,
and methane clathrate distribution present just after impact
to model the propagation of impact melt down to the ocean.
This model uses starting conditions from recent impact
simulations that result in crater characteristics consistent
with those of Selk crater on Titan (Wakita et al., 2023). This
preliminary work finds that the values presented for Europa
in the work of Carnahan et al. (2022) likely represent an
upper bound for Titan. In other words, the amount of melt
being transported to Titan’s ocean may be much smaller
than assumed in this work because of melt freezing during
the transfer. Thus, the values presented here represent an
upper limit on the amount of organic molecules that can be
transported to Titan’s ocean.
There are obvious differences between the total melt vol-
ume estimated for the two different crater production rates
(KZ05 vs. AL05) and the two different materials (porous
water ice vs. solid clathrate) when summed over the age of
the Solar System (Fig. 3). The most melt is produced in
the porous water-ice case, assuming the AL05 cratering
rate, while the least melt is produced in the solid meth-
ane clathrate case, assuming the KZ05 cratering rate. When
summed over all crater sizes (Table 1), the difference
between these two end members is three orders of magni-
tude, from 2 ·10
6
km
3
of melt in the KZ05 solid clathrates
FIG. 2. Cumulative melt volume produced at different crater diameters for the four cases considered in this work. The
colored lines represent different timeframes for cratering (red, 4.5 Gyr; black, 1 Gyr; blue, 0.2 Gyr). Color images are
available online.
Table 1. List of the Four End Members Considered
in This Work
Case
number
Cratering
rate
Composition
of ice shell
Crater diameters
considered, km
1 KZ05 Porous water ice 90–500
2 AL05 Porous water ice 90–500
3 KZ05 Solid methane
clathrate over
water ice
20–500
4 AL05 Solid methane
clathrate over
water ice
20–500
AL05 =Artemieva and Lunine (2005); KZ05 =Korycansky and
Zahnle (2005).
4 NEISH ET AL.
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case (case 3) to 1 ·10
9
km
3
of melt in the AL05 porous
water ice case (case 2) over 4.5 Gyr (Fig. 3).
4. Organic Components Delivered to the Ocean
Next, we determine the amount of organic material that is
incorporated into the melt. This will determine the amount of
carbon that can be transported to Titan’s subsurface ocean,
where it will be available for use by any putative biosphere
there. Artemieva and Lunine (2005) and Artemieva and
Lunine (2003) modeled the incorporation of organic material
into melt deposits on Titan. They found a significant fraction
(*10%) of the ejected material will be lightly shocked and
deposited back inside the crater, mixing with the impact
melt there (Note: This is only true for oblique impacts; in
vertical impacts, all of the ejecta material will be deposited
outside the crater. However, purely vertical impacts are rare,
and the most common impact occurs at 45.). They estimate
the total volume of organics in the crater to be:
Vorg ¼0:3d2horg (7)
where dis the diameter of the projectile and h
org
is the
thickness of the organic-rich layer. We assume the entirety
of this material ends up in the melt pool.
Thus, to determine the amount of organics being incor-
porated into Titan’s melt pools, we must estimate the thick-
ness of the various organic deposits on its surface (h
org
).
Malaska et al. (2020) summarized the present-day thickness
of organics in different regions on Titan and demonstrated
that organic plains and labyrinths are an important reservoir
of solid organics. Recent remapping by Williams et al.
(2023) has provided new surface area values for the terrain
units covering Titan’s surface (Table 2). These terrains
include (1) bright plains, (2) dark plains, (3) dunes, (4)
mountains, (5) labyrinths, (6) lakes, and (7) craters. Of these
terrains, the craters and mountains are assumed to be ice-
rich and devoid of organic molecules, due to their relatively
low emissivity (Lopes et al., 2020). We therefore need to
estimate the thickness of organics in the remaining five
terrains. We use the maximum reported thickness values
reported by Malaska et al. (2020) for our work (Table 2).
This will provide us with the most optimistic estimate for
organic transport to Titan’s interior. To estimate the thick-
ness of organic materials in the dune units, we use the
maximum values provided by Rodriguez et al. (2014). For
the bright and dark plains, we use the maximum thickness
derived from the work of Lopes et al. (2016). For the lab-
yrinths, we use the thickness derived by Malaska et al.
(2020). Lastly, we assume that the lakes are down warped
and flooded plains, so the thickness of the plains also rep-
resents the buried thickness of organics under the lakes. We
use the depth values provided in Table 2 as inputs for h
org
in
Equation 7 to determine the amount of organics incorpo-
rated into individual melt pools. We then multiply these
values by the total number of craters formed over the age of
the solar system to get a value for the organics transported to
Titan’s subsurface ocean through impact events as a func-
tion of crater diameter (Fig. 4).
This calculation assumes that the majority of Titan’s
organics were deposited early in its history, and the rate of
deposition has been minimal since then. This is likely an
overestimate, as Titan probably had a methane-rich atmo-
sphere that produced complex organics for a significant
fraction of its history (Tobie et al., 2006), and impacts early
in Titan’s history may therefore have been into a thinner
layer of surface organic material than more recent impacts.
To take this into account, we also calculate the amount of
organics transported to Titan’s interior assuming a steady
state deposition of solid organics over time. To determine
the deposition rate, we note that Table 2 suggests that
the total amount of solid organic material on Titan is 2.0 ·
10
6
km
3
. If this amount of organic material were spread
uniformly over Titan’s entire surface area (83.3 ·10
6
km
2
),
there would be an average global depth of 25 m. This
translates to an organic production rate of 5 m of solid organ-
ics per gigayear over Titan’s 4.5 Gyr history.
In the early organic deposition scenario, the highest vol-
ume of organic material is generated in craters with a
diameter of *40 km for the KZ05 case and *100 km for
the AL05 case. The volume peaks at these intermediate
diameters for the following reason: although the total vol-
ume of organics per crater is less for a smaller crater (given
the squared dependence on impactor size in Equation 7),
there is a larger number of small craters to contribute to the
FIG. 3. The total melt volume produced over the age of
the solar system that can be transported to the subsurface
ocean. The results are given for the four end members
considered in this work, for craters with diameters between
20–500 km (solid clathrates) and 90–500 km (porous water
ice). Color images are available online.
Table 2. Estimated Thickness of Solid Organics
for Various Terrain Units on Titan
Terrain unit
Surface
area, %
Maximum
thickness, km
a
Total volume
of organics, km
3
Bright and dark
plains
66.7 0.025 1.4 ·10
6
Dune areas 17.3 0.03 4.3 ·10
5
Mountains 12 Minimal Minimal
Labyrinths 2 0.121 1.9 ·10
5
Lakes (includes
mare and
empty lakes)
b
1.4 0.025 2.9 ·10
4
Craters 0.6 Minimal Minimal
a
These values are taken from Malaska et al. (2020).
b
Lakes of all types are assumed to have the same amount of
deposits as plains. They are treated as flooded plains; we do not
consider liquid hydrocarbons in our calculations.
ORGANIC INPUT TO TITAN’S SUBSURFACE OCEAN 5
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overall volume (Indeed, Menrva, D=400 km, is the only
crater with a diameter larger than 150 km observed on Titan.).
We can sum up the values for all terrain types on Titan,
normalizing by the percent surface area (pSA) covered by
that terrain, to get an estimate for the total volume (V) of
organic material incorporated into Titan’s impact melt
(Equation 8).
Vorg D,tðÞ¼ND,tðÞVorg, plains DðÞpSAplains
þND,tðÞVorg, dunes DðÞpSAdunes
þND,tðÞVorg, lab DðÞpSAlab
þND,tðÞVorg, lakes DðÞpSAlakes
(8)
By integrating over all relevant diameters, we get the total
volume of organics that could be delivered to Titan’s ocean
(Table 3). These values range from 3.6 ·10
3
km
3
for case 1
(KZ05 porous water ice) to 3.9 ·10
4
km
3
for case 4 (AL05
solid clathrate).
In the linear deposition over time scenario, we defined a
linearly increasing thickness of organics (Equation 9) and
used it to calculate the volume of organics expressed in
Equation 7 as a function of time. We then multiplied the
volume by the differential of the cratering rate and inte-
grated over time (Equation 10). We multiplied the entire
equation by the fraction of Titan’s surface covered by
plains, labyrinths, dunes, and lakes (87.3%), ignoring the
minimal organic contribution from the craters and moun-
tains. As we see in Fig. 5, the differences between the two
situations are minimal, except in the 4.5 Gyr case. This is
because by 1 Gyr ago, the majority of Titan’s solid organics
had already been deposited on its surface in this scenario;
the differences are only apparent very early in Titan’s
history.
horg t
ðÞ
¼5m
Gyr
t(9)
Vorg, total DðÞ¼
Z
t2
t1
dN D,tðÞ
dt Vorg D,horg tðÞ
0:873 dt
(10)
By integrating over all relevant diameters, we get the
total volume of organics that could be delivered to Titan’s
ocean in this scenario (Table 3). These values range from
1.6 ·10
3
km
3
for case 1 to 7.9 ·10
3
km
3
for case 4. This is a
factor of a few less than calculated in the early deposition
scenario (specifically, a factor of two less for the KZ05
cases and a factor of five less in the AL05 cases).
From the calculated volumes in the two scenarios, we
determined the flux of organics from Titan’s surface to its
interior ocean. We integrated Equation 10 over 0.1 Gyr
timescales (e.g., 0.0–0.1, 0.1–0.2, 0.2–0.3 Gyr, etc.) and all
relevant diameters (20–500 km for the clathrate cases, 90–
500 km for the water ice cases). We converted these values
into a flux of organics (km
3
/Gyr) over time. The results are
shown in Fig. 6. As expected, the values in the early depo-
sition scenario stay constant over time in the KZ05 cases
(since the volume of organics is constant over time, as is the
impactor rate in the KZ05 case). The values for the AL05
cases decrease with time, as the assumed cratering rate
decreases with a 1/tdependence. The flux through the ice
shell ranges from *10
3
km
3
/Gyr for the KZ05 cases,
to *10
3
–10
5
km
3
/Gyr in the AL05 cases. In the linear de-
position scenario, the flux through the ice shell gradually
increases in the KZ05 cases, as solid organics build up on
Titan’s surface, increasing from *10 km
3
/Gyr early in Ti-
tan’s history to *10
3
km
3
/Gyr. The values for the AL05
cases remain constant at *10
3
km
3
/Gyr over time, as the 1/t
dependence in cratering rate is balanced by the linear
deposition of organics.
FIG. 4. Organic volume incorporated into melt lenses for different terrain types on Titan over the age of the solar system.
This calculation assumes the majority of Titan’s solid organics were deposited early in its history. Color images are
available online.
Table 3. Total Volume of Organics Incorporated
into Titan’s Impact Melt Deposits Over 4.5 Gyr
Case Volume, km
3
Early organic deposition
1: KZ water ice 3.6 ·10
3
2: AL water ice 3.2 ·10
4
3: KZ clathrate 5.2 ·10
3
4: AL clathrate 3.9 ·10
4
Linear deposition over time
1: KZ water ice 1.6 ·10
3
2: AL water ice 6.5 ·10
3
3: KZ clathrate 2.3 ·10
3
4: AL clathrate 7.9 ·10
3
6 NEISH ET AL.
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5. Concentration of Organic Molecules in the Ocean
If melts generated in large impacts on Titan can founder
in its ice shell, and deliver surface organics to its subsurface
ocean, this would provide an input of potential organic
nutrients to any putative life there. However, is this amount
of organics substantial enough to support a biosphere? We
first consider the overall concentration of organic molecules
in Titan’s subsurface ocean, noting that the size of this
ocean is not well constrained.
Although observations of Titan by the Cassini-Huygens
mission strongly suggest the presence of a liquid water
ocean beneath an ice shell, the thickness of both layers
remains unknown (Sotin et al., 2021). Even if the values for
Titan’s average surface radius, mass, and moment of inertia
(MoI) were well known, there are different solutions for
Titan’s interior structure (composition, layer thicknesses,
and densities) that would reproduce the observed data. Sotin
et al. (2021) present two models of Titan’s interior structure
for consideration. One assumes convection in the outer ice
shell and an MoI of 0.341, while the other assumes con-
duction in the outer ice shell and an MoI of 0.330. These
give ocean thicknesses of 250 and 502 km, respectively. We
use these two ocean thickness values as end members when
calculating the ocean volume. The volume of Titan’s ocean
is given by the mass of Titan’s ocean divided by its density.
The two end members presented in Sotin et al. (2021) have a
mass of 1.92 ·10
22
and 4.04 ·10
22
kg and densities of 1122
and 1219 kg/m
3
, respectively. This results in volumes of
1.7 ·10
10
and 3.3 ·10
10
km
3
, which represents a volume 12
to 25 Earth oceans in size.
To determine the concentration of various molecules
delivered to Titan’s ocean, we must assume a specific com-
position for the solid organic material on Titan’s surface.
As a starting point, we use the flux of solid organics
delivered to Titan’s surface from the atmosphere as deter-
mined from the photochemical model of Krasnopolsky
(2009). Table 4 reports the top nine solid organic products
delivered to the surface in that model (All remaining prod-
ucts contribute <1% of the total volume produced.). We
focus specifically on two products that are known to produce
biological molecules when hydrolyzed: HCN and haze
particles. According to the Krasnopolsky (2009) model, 5%
of the total volume of solid organics is HCN while 74% is
Titan aerosols. Note that this model ignores any shock
synthesis of organic compounds by meteoroids in Titan’s
atmosphere. At present, such a process only accounts for
1% of the production rate of organic compounds formed by
photochemistry (Flowers and Chyba, 2023). However, if
impact fluxes were higher in the primitive solar system, this
process could have enhanced the deposition of solid organ-
ics on Titan’s surface early in its history.
FIG. 5. Cumulative organic volume incorporated into melt lenses for different time frames in Titan’s history, assuming
either an early deposition or linear deposition of organics scenario. Color images are available online.
FIG. 6. Flux of organics from Titan’s surface to its interior ocean as a function of time for all four cases considered in this
work. On the left, we show the flux assuming an early deposition of solid organics on Titan. On the right, we show the flux
assuming a linear deposition of solid organics over time. Note that this does not apply at t=0, where no organics have been
deposited on Titan’s surface, since Equation 10 is indefinite at that point. Color images are available online.
ORGANIC INPUT TO TITAN’S SUBSURFACE OCEAN 7
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5.1. Organic delivery to the ocean: HCN
HCN is a common product of Titan’s atmospheric photo-
chemistry (Ho
¨rst, 2017) and is known to react with water at
high concentrations to produce molecules of biological inter-
est (Ferris et al., 1978). HCN is also known to be an elemental
subunit in the formation of Titan tholins (i.e., Titan haze an-
alogues produced in the laboratory) (Maillard et al., 2020).
Crystalline HCN at 120K has a density of 1.037 –0.005 g/cm
3
(Gerakines et al., 2022) and a molar mass of 27.03 g/mol. If
5% of the organic inventory of solid organics was composed
of HCN, b etween 3 ·10
15
and 7 ·10
16
mols could be deliv-
ered to Titan’s ocean in descending plumes (Table 5). The
range of concentrations expected in Titan’s ocean could thus
rangefromalowerestimateof9·10
-8
Mto an upper esti-
mate of 4 ·10
-6
M. These concentrations are much too low to
produce molecules of biological interest; at concentrations
<0.01 M, HCN mostly hydrolyzes to produce the biologically
less interesting formamide and formic acid. Concentrations
>0.1 Mare needed to form HCN oligomers, which can fur-
ther hydrolyze to produce all three major classes of nitrogen-
containing biomolecules, that is, purines, pyrimidines, and
amino acids (Ferris et al., 1978).
If local concentrations in the melt pools exceed 0.1 M,
however, the yields of biomolecules can be significant.
Ferris et al. (1978) found that a 1 L solution of 0.1 MHCN
held at room temperature for 1 year yielded sufficient
oligomers to produce 7.5 mmol of adenine, 130 mmol of 4,5-
dihydroxypyrimidine, and 160 mmol of glycine (produced
after the oligomers had been hydrolyzed at 110C for 24 h).
This amounts to a yield of 0.1% glycine per mole of HCN
at a pH of 8.5. Reactions undertaken at lower temperatures
produced similar results.
Miyakawa et al. (2002) prepared a solution of NH
4
CN
from 0.15 MHCN and 0.1 MNH
3
and froze the solution at
-78C for 27 years. After that time, they hydrolyzed one
sample at 100C for 24 h in 6 MHCl, and a second sample at
140C for 3 days in pH 8, 0.01 Mphosphate. A third sample
was not hydrolyzed. Adenine, guanine, and orotic acid were
detected in all three samples (though with smaller yields
in the non-hydrolyzed sample). Eight other pyrimidines and
purines were also detected in at least one of the samples.
Glycine was not studied in this work, so further research
into low-temperature hydrolysis of HCN is warranted.
Converting the volume of organics present in each melt lens
to moles of HCN, and dividing by the volume of water pres-
ent, we can plot the bulk concentrations of different melt
deposits (Fig. 7). We find that the melt lenses achieve a bulk
concentration >0.1 Min only one case. These concentrations
only are possible for (1) small impacts (since the ratio of
organics to melt remains high), (2) into non-porous clath-
rates (since the volume of melt produced is lower), and
(3) into labyrinth terrain (since the thickness of organics is
high). As these melt lenses descend into the lithosphere and
begin to freeze, their HCN concentration will increase by a
factor of a few (Hedgepeth et al., 2022). Thus, the values
presented in Fig. 7 may increase by two to three before
eventually freezing or being delivered to the ocean.
Using the yields provided by Ferris et al. (1978), we esti-
mate the flux of glycine to Titan’s ocean through hydrolysis
of HCN. We take the bulk concentrations of HCN provided
in Fig. 7 and multiply them by a factor of 3 (to account for
the increase in concentration as the melt lens freezes) to
determine which cases support hydrolysis of HCN in the
freezing melt pools. Only one case has bulk concentrations
sufficient to produce glycine and transport it to Titan’s
interior ocean: Case 3 (KZ05 solid clathrates), when the imp-
actor strikes the labyrinths for crater diameters up to 153 km.
No glycine is produced in case 1, 2, and 4, as the concen-
trations in the melt ponds that would be delivered to the ocean
are not high enough. We then convert the flux of organics,
Table 4. Expected Fraction of Solid Organics on Titan’s Surface, Based on the Photochemical
Model of Krasnopolsky (2009)
Species
Flux to
surface,
g/(cm
2
$Gyr)
Density of
species,
g/cm
3
Thickness of
organics,
cm/Gyr Volume, % Weight, %
Source of
density estimate
C
x
H
y
N (haze) 1800 1.343 1340 35.1 38.7 He et al. (2017) (tholins)
C
x
H
y
(haze) 1780 1.343 1325 34.7 38.3 He et al. (2017) (tholins)
C
2
H
2
439 0.76 578 15.1 9.4 Hudson et al. (2014)
Ion (haze) 230 1.343 171 4.5 4.9 He et al. (2017) (tholins)
HCN 218 1.037 210 5.5 4.7 Gerakines et al. (2022) (crystalline)
C
4
H
6
48 0.831 58 1.5 1.0 Cornet et al. (2015)
C
2
H
3
CN 45 1.046 43 1.1 1.0 Cornet et al. (2015)
HC
3
N 34 1.075 32 0.8 0.7 Khanna (2005)
CH
3
CN 27 0.77 35 0.9 0.6 Gerakines et al. (2022)
All others 32.4 — 30 0.8 0.7 —
Table 5. Specific Compounds Incorporated into
Titan’s Impact Melt Deposits Over 4.5 Gyr
Case
Total organic
volume, km
3
Moles
HCN
a
Mass
haze, g
b
Early organic deposition
1: KZ water ice 3.6 ·10
3
6.8 ·10
15
3.5 ·10
18
2: AL water ice 3.2 ·10
4
6.2 ·10
16
3.2 ·10
19
3: KZ clathrate 5.2 ·10
3
1.0 ·10
16
5.2 ·10
18
4: AL clathrate 3.9 ·10
4
7.5 ·10
16
3.9 ·10
19
Linear deposition over time
1: KZ water ice 1.6 ·10
3
3.0 ·10
15
1.5 ·10
18
2: AL water ice 6.5 ·10
3
1.2 ·10
16
6.4 ·10
18
3: KZ clathrate 2.3 ·10
3
4.4 ·10
15
2.3 ·10
18
4: AL clathrate 7.9 ·10
3
1.5 ·10
16
7.8 ·10
18
a
This calculation assumes that 5% of the solid organics are in the
form of HCN.
b
This calculation assumes that 74% of the solid organics are in
the form of haze.
8 NEISH ET AL.
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F
organics
(km
3
/Gyr), calculated in Section 4 into a flux of
glycine, F
glycine
(mol/Gyr), to Titan’s ocean (Equation 11).
Fglycine ¼mol HCN 0:1%
¼Forganics 0:05 qHCN
MHCN
1015 10 3(11)
We calculate the total moles of HCN by multiplying the
total organic volume by 5% (using a factor of 10
15
to con-
vert from km
3
to cm
3
), then converting to moles by taking
the ratio of the density of HCN (r
HCN
, g/cm
3
) to molar mass
(M
HCN
, g/mol). Finally, we multiply by the 0.1% yield seen
in Ferris et al. (1978). In this case, the present-day flux of
glycine to Titan’s ocean is 1 ·10
11
mol/Gyr for the early
deposition scenario; not enough organics build up in the
linear deposition scenario to produce sufficient concentra-
tions to produce glycine.
These calculations all assume that chemistry is occurring
within the bulk melt pool. At the microscale, it is likely that
much higher concentrations can occur. Observations of
microbrine pockets (Barletta and Roe, 2012; Barletta et al.,
2012) have shown that exclusion of non-water materials in
the ice can enhance concentrations of dissolved materials to
several orders of magnitude. This could be enough to cause
a local enhancement well above the bulk concentrations in
Fig. 7. This would effectively make each micropocket a
miniature chemical reactor, possibly converting HCN into
the molecules and at the yields observed by Ferris et al.
(1978). Melt deposits may also remain concentrated near the
ice-ocean interface. At the local scale beneath a descending
plume, any draining fluid will emerge at the ice-ocean int-
erface as a negatively buoyant jet with a diameter that is
approximately half of its height (Turner, 1966).
The penetration depth depends on the jet injection speed
and width of the drainage outlet (Pantzlaff and Lueptow,
1999). Modeling of the drainage process through the ice is
thus necessary to estimate the spatial extent of any HCN-
concentrated fluid that might stay near the ice-ocean inter-
face. Its depth is particularly important because it will be the
primary control on whether biologically relevant HCN
concentrations (and potential reaction products) will persist
below the impact structure or be efficiently mixed into the
bulk ocean, serving as one potential way to assess the pot-
ential for mini-biospheres underneath impact structures.
HCN is less dense than water at standard temperature and
pressure, however, and will likely be buoyant compared to the
bulk ocean water (Hedgepeth et al., 2022). If this holds true at
the higher pressures in Titan’s interior, HCN may become
concentrated near the ice-ocean interface, allowing for pre-
biotic reactions to occur. If the total amount of HCN were
floating buoyantly at the top of the ocean and distributed
globally, it would range in thickness from *1mm to a few
tens of mm considering the end-member scenarios (Table 5).
We must, however, consider the possibility that turbulent
ocean dynamics will erode this buoyant layer. As a first
approximation, we can compare the thickness estimates
above to that of the thermal boundary layer, where heat is
transferred through conduction as opposed to convection.
The viscous boundary layer thickness could alternatively
be considered, but scaling estimates show that it is likely
thicker than the thermal boundary layer, motivating our focus
on the latter. If the HCN-enriched layer exceeds this distance,
FIG. 7. Bulk concentration of HCN in Titan’s foundering melt pools, assuming 5% of Titan’s solid organics are composed
of HCN. Concentrations of HCN >0.1 Mare known to produce biomolecules such as purines and amino acids. Color images
are available online.
ORGANIC INPUT TO TITAN’S SUBSURFACE OCEAN 9
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it is more likely to mix with the underlying ocean, reducing
its concentration. The thickness of the thermal boundary
layer (d) relative to the ocean depth (H) can be estimated as
d=H¼0:56Nu1(Gastine et al., 2015, 2016), where the
Nusselt number Nu is the ratio of total to conductive heat
transfer and depends on the thermal Rayleigh number (Ra,
ratio of thermal buoyancy to diffusion) and the Ekman
number (E, ratio of viscous to Coriolis forces). Using scaling
relations and thermodynamic properties from Soderlund
(2019), estimates of the thermal boundary layer thickness
range from a few millimeters (assuming a strongly rotation-
ally constrained Nu ¼0:60 Ra3=2E2scaling law) up to *10
centimeters (assuming a non-rotating Nu ¼0:07 Ra1=3scal-
ing law), noting that the thickness is much more sensitive to
the assumed scaling law than the physical properties of
Titan’s ocean (e.g., ocean thickness, heat flux, etc.).
Since Titan’s ocean is expected to lie between these end-
member heat transfer scenarios (Soderlund, 2019) and no
clear Nu(Ra,E) scaling exists for the transitional convective
regime, we assume an intermediate thickness on the order of
1 cm. In this case, the HCN-enriched fluid would likely
remain within the boundary layer and be less prone to mixing
with the bulk ocean fluid. If the HCN remains near the ice-
ocean interface, its concentration can be significant. The two
ocean models presented in Sotin et al. (2021) suggest an
ocean depth between 36 and 112 km. If we assume the
thermal boundary layer ranges in thickness between 10mm
and 10 cm (see text above), the volume of water in the res-
ulting spherical shell ranges from 1.6 ·10
11
to 1.6 ·10
13
L.
The values of HCN that accumulate in the ocean over the age
of the solar system range from 3.0·10
15
to 7.5 ·10
16
mol
for the different scenarios considered in this work (Table 5).
Thus, we could have concentrations well in excess of the
HCN:H
2
O eutectic, which forms at T=250K with an HCN
concentration of *22 M(Coates and Hartshorne, 1931). La-
boratory results show that HCN polymerization proceeds at a
reasonable rate (0.1% yield of HCN-tetramer in 3 days) even at
this low temperature (Sanchez et al., 1966). Thus, if HCN re-
mains within the thermal boundary layer, it could produce
molecules of biological interest like amino acids. However, it
should be noted that there could be many CN
-
ions still present
in solution, and these are known to be toxic to terrestrial life.
This is due to their high affinity for metal ions, which are crucial
co-factors in biological catalysts (Luque-Almagro et al., 2018).
On Earth, this toxicity can be mitigated by specific evolutionary
innovations, such as cyanide dehydratase and cyanide hy-
dratase enzymes, which detoxify cyanides (Thuku et al., 2009),
so the presence of high concentrations of CN
-
ions is not in
principle an insurmountable barrier to life. Nonetheless, they
could render an environment uninhabitable to any organism
without such adaptations. It is also important to remember that
cyanide is but one component of the organic mixture expected
to be produced in Titan’s atmosphere. Small aromatic mole-
cules such as benzene and its derivatives could also shape the
habitability of Titan’s oceans.
5.2. Organic delivery to the ocean: Haze
Titan haze analogues (i.e., ‘‘tholins’’) are also known to
hydrolyze to produce molecules of biological interest, such
as glycine (Khare et al., 1986; Neish et al., 2010; Ramı
´rez
et al., 2010; Poch et al., 2012; Cleaves et al., 2014; Brasse
´
et al., 2017). The yield of glycine in these experiments
ranges from 0 to 0.7 wt % of the starting tholin weight
(Khare et al., 1986; Ramı
´rez et al., 2010; Poch et al., 2012;
Brasse
´et al., 2017) and is dependent on the experimental
parameters. For example, the most glycine was produced at
warmer temperatures (e.g., 277K in Ramı
´rez et al., 2010,
279K in Poch et al., 2012), with the inclusion of ammonia in
the solvent (e.g., 12 wt % in Ramı
´rez et al. (2010), 25% in
Poch et al. (2012)). To determine the flux of glycine, F
glycine
(mol/Gyr), to the ocean due to tholin hydrolysis, we multi-
plythefluxoforganics,F
organics
(km
3
/Gyr), given in Fig. 6 by
74% to get the total volume of haze materials (Equation 12).
We convert this to a mass using the density (q
tholins
,g/
cm
3
) given in Table 4, and a factor of 10
15
to convert from
km
3
to cm
3
. Finally, we assume an upper limit of 1 wt %
from Ramı
´rez et al. (2010) for the yield of glycine from
tholin hydrolysis and convert it to moles of glycine using its
molar mass (M
glycine
, mol/g). The present-day flux of gly-
cine to Titan’s ocean is 1–3 ·10
14
mol/Gyr, although it may
be larger or smaller earlier in Titan’s history depending on
the deposition scenario (Fig. 8).
Fglycine ¼mhaze 1%
Mglycine
¼Forganics 0:74 qhaze
Mglycine
1015 102
(12)
6. Implications for Biology
The hydrolysis of both HCN and haze aerosols produces
glycine, which can be transported to Titan’s ocean. The flux of
glycine is on the order of 10
11
to 10
14
mol/Gyr in the most
optimistic scenarios. Could this flux sustain any lifeforms that
might be present in Titan’s ocean? One candidate organism to
consider for Titan’s ocean is amino-acid anaerobic fermenters
that acquire energy through the Stickland reaction (Stickland,
1934; Nisman, 1954). In these reactions, amino acids serve
both as electron donor and acceptor, removing the need for
another electron acceptor. Amino-acid anaerobic fermenta-
tion might have been among the first heterotrophic carbon and
energy pathways to emerge on Earth and are found in diverse
environments ranging from wastewater mud to hydrothermal
vents (Scho
¨nheit et al., 2016). Stickland (1934) fermentations
are also hypothesized to have powered the metabolism of the
prebiotic RNA-world and be connected to the origin of
the genetic code (de Vladar, 2012). However, we note that
our calculations suggest only 7.5 to 7500 kg of glycine is de-
livered to Titan’s ocean each year. How much biomass pro-
duction could this support in a Titanian ocean? The best-case
metabolic scenario expected of amino acid fermenting mi-
crobes on Earth is approximately six amino acids consumed
for each one assimilated into biomass (Orsi et al., 2020).
If protein synthesis is comparable in energetic cost to the
synthesis of other biomacromolecules (Amend et al., 2013),
this sets an upper bound on biomass generation within the
entire Titan ocean at 1250 kg/year. If we assume that most of
this mass is composed of carbon (an overestimate, since
biomolecules also contain elements like hydrogen, nitrogen,
oxygen, etc.), we have an upper limit on the productivity
of *10
3
kgC/year. This is three orders of magnitude less than
the estimated productivity of Enceladus’ ocean, *10
6
kgC/
year (Affholder et al., 2022). Note that Affholder et al. (2022)
estimated the productivity of Enceladus’s hydrothermal vents
10 NEISH ET AL.
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using an ecological thermodynamic model, which assumed a
methanogenic biosphere using CO
2
and H
2
as its energy
source. This is different than the fermentation-based bio-
sphere we assumed for Titan. However, methanogenesis
is almost always associated with fermentation in Earth’s
ecosystems, because fermentation produces H
2
and CO
2
.
For comparison, the productivity of hydrothermal vents on
Earth is on the order of 10
8
kgC/year, two orders of mag-
nitude larger than the value Affholder et al. (2022) estimated
for Enceladus, and the total productivity of Earth’s oceans
(which is supported by photosynthesis) is several orders of
magnitude higher than that. This value can be estimated by
taking the total marine biomass on Earth, *6·10
12
kgC
(Bar-On et al., 2018) and multiplying it by the mean biomass
turnover in the ocean water column, 8.5 year
-1
(i.e., on
average Earth’s marine biosphere ‘replaces itself’ 8.5 times
per year; Hoehler et al., 2023). This gives 5 ·10
13
kgC/year,
a number far in excess of the methanogenic or fermentative
biospheres hypothesized for Enceladus and Titan.
We acknowledge that these comparisons are flawed, how-
ever, since they assume a biosphere supported by metha-
nogenesis or photosynthesis rather than one based on
fermentation alone. Indeed, there may not be any good ana-
logue for a potential biosphere in Titan’s ocean. On Earth,
fermentation is a process that typically supports life by con-
suming materials generated by other species. Thus, the liter-
ature assessing the productivity of fermentative ecosystems on
Earth where fermenters are the primary producers and con-
sume abiotic organic material is limited if not non-existent.
These comparisons also do not consider the difference in size
between the three oceans (Titan, Enceladus, and Earth), which
could significantly affect biomass distribution and the acces-
sibility of any delivered energy sources. In addition, the
microbial population size that could be supported by such
gross biomass generation depends on a variety of other factors
that are beyond the scope of this discussion. For example,
extremely low energetical microbial maintenance needs or
mortality could allow a population to grow to a large size
(although slowly) despite a small biomass production rate.
Estimating those parameters requires assumptions of
microbial physiology and the value of several environmental
variables local to the candidate habitat such as temperature,
pH or pressure. In addition, sources of energy other than
glycine could be considered and yield higher biomass pro-
duction estimates. It should be noted, however, that glycine
is often one of the most abundant biomolecules produced in
relevant hydrolysis experiments (Cleaves et al., 2014), so it
should play an important role in any biochemistry on Titan.
As an exercise, we also consider the flux of glycine to
Titan’s ocean if the plains, lakes, and dune terrains were
underlain by a thicker organic layer than was estimated in
Malaska et al. (2020). For the purposes of this exercise, we
will assume that all of these terrains have the same thickness
as the labyrinth terrains; in essence, we are assuming
the labyrinths represent an ‘‘uplifted’’ average Titan. Using
these values, the maximum estimates for the glycine flux
increase by an order of magnitude, up to 5 ·10
12
mol/Gyr
for HCN hydrolysis (case 3 only) and 1 ·10
15
mol/Gyr for
tholin hydrolysis. This would increase the productivity by an
order of magnitude, to *10
4
kgC/year. This is still two
orders of magnitude less than the estimate for Enceladus.
Thus, Titan’s ocean becomes more habitable only if a very
thick layer of organics is present on its surface. Organics
may also be transported from the interior of Titan to its
subsurface ocean, and further work in this area is needed.
7. Conclusions
In this work, we constrained the amount of organic com-
pounds that can be delivered to Titan’s subsurface ocean
through the foundering of impact crater melt ponds. Using
two different impact cratering rates [from Korycansky and
Zahnle (2005) and Artemieva and Lunine (2003)] and two
end-member compositions for Titan’s conductive ice shell
(pure water ice and methane clathrate), we determined the
amount of impact melt that could be produced over time. We
estimated the concentration of organic compounds entrained
in those melt ponds and delivered to the ocean through the
foundering mechanism proposed by Carnahan et al. (2022).
Using the photochemical model of Krasnopolsky (2009),
we determined the proportion of organic compounds that
would be in the form of HCN and organic haze aerosols.
These compounds are both known to hydrolyze in aqueous
solutions to produce amino acids like glycine. Using the
assumptions described earlier, we found a range of possible
flux rates of glycine from the surface to the subsurface, from
zero to 10
11
mol/Gyr for HCN hydrolysis, and from zero to
10
14
mol/Gyr for haze hydrolysis. A flux of glycine could
feed an organism such as an amino acid anaerobic fer-
menter, but it is unlikely that the calculated fluxes are
FIG. 8. Flux of glycine to Titan’s interior ocean from the hydrolysis of haze particles, assuming a maximum yield of 1wt %.
Color images are available online.
ORGANIC INPUT TO TITAN’S SUBSURFACE OCEAN 11
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sufficient to maintain a detectable biosphere, unless the
thickness of organics on Titan’s surface is greater than
currently estimated, abundant biomolecules are available
from Titan’s rocky core, or surface biomolecules can be
delivered to the ocean by a process other than impact.
Our calculations suggest that despite Titan being the most
organic-rich ocean world in the Solar System, this does not
automatically imply an organic-rich and habitable ocean.
Author Disclosure Statement
No competing financial interests exist.
Funding Information
This research was supported by the International Space
Science Institute (ISSI) in Bern, through ISSI International
Team project #539 (The habitability of Titan’s subsurface
water ocean). Catherine Neish also recognizes support from an
NSERC Discovery grant. Charles Cockell acknowledges
support from the Science and Technology Facilities Council
(STFC), grant no. ST/V000586/1. Kelly E. Miller acknowl-
edges support from NASA grant 80NSSC19K0559. Kendra K.
Farnsworth was supported by an appointment to the NASA
Postdoctoral Program at NASA Goddard Space Flight Center,
administered by Oak Ridge Associated Universities under
contract with NASA. Krista M. Soderlund acknowledges
support by the NASA Astrobiology program grant Oceans
Across Space and Time (Grant No. 80NSSC18K1301). Rosaly
M.C. Lopes and Michael J. Malaska acknowledge support
from the NASA Astrobiology Institute through its JPL-led
team entitled Habitability of Hydrocarbon Worlds: Titan and
Beyond. Part of this work was carried out at the Jet Propulsion
Laboratory, California Institute of Technology, under contract
with NASA. Government sponsorship is acknowledged.
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Address correspondence to:
Catherine Neish
Department of Earth Sciences
The University of Western Ontario
London, ON N6A 5B7
Canada
E-mail: cneish@uwo.ca
Submitted 11 May 2023
Accepted 1 January 2024
Abbreviations Used
AL05 ¼Artemieva and Lunine (2005)
KZ05 ¼Korycansky and Zahnle (2005)
MoI ¼moment of inertia
pSA ¼percent surface area
ORGANIC INPUT TO TITAN’S SUBSURFACE OCEAN 13
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