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Article https://doi.org/10.1038/s41467-022-35187-4
Liquid water on cold exo-Earths via basal
melting of ice sheets
Lujendra Ojha
1
, Bryce Troncone
1
,JacobBuffo
2
, Baptiste Journaux
3
&
George McDonald
4
Liquid water is a critical component of habitability. However, the produc-
tion and stability of surficial liquid water can be challenging on planets
outside the Habitable Zone and devoid of adequate greenhouse warming.
On such cold, icy exo-Earths, basal melting of regional/global ice sheets by
geothermal heat provides an alternative means of forming liquid water.
Here, we model the thermophysical evolution of ice sheets to ascertain the
geophysical conditions that allow liquid water to be produced and main-
tained at temperatures above the pressure-controlled freezing point of
water ice on exo-Earths. We show that even with a modest, Moon-like
geothermal heat flow, subglacial oceans of liquid water can form at the base
of and within the ice sheets on exo-Earths. Furthermore, subglacial oceans
may persist on exo-Earths for a prolonged period due to the billion-year
half-lives of heat-producing elements responsible for geothermal heat.
These subglacial oceans, often in contact with the planet’scrustand
shielded from the high energy radiation of their parent star by thick ice
layers, may provide habitable conditions for an extended period.
The habitability potential of planets in the circumstellar habitable zone
(CHZ) of M-dwarf stars are of great interest because M-dwarf stars make
up 75% of the population of stars in the galaxy, and >40% of M-dwarfs
stars are expected to harbor Earth-sized planets (exo-Earths) in their
CHZ1,2. However, key differences between the stellar and planetary
environments between M-dwarf stars and the more luminous Sun-like
stars have led to a long-standing debate around the habitability of
M-dwarf orbiting exo-Earths3.Inparticular,thehigherfractionofstellar
luminosity emitted in the X-rays and ultraviolet (UV) by M-dwarfs,
relative to sun-like stars4,aswellastheirexhibitionofmoreflaring than
sun-like stars5, presents challenges for the surface habitability of these
planets. High surface radiation levels, which can present significant
challenges for the stability of biological molecules, can be expected
from the tendency of these planets to be depleted in X-ray/UV shielding
ozone6,whileflaring can lead to periodic, orders of magnitude higher
surface X-ray fluxes7. Although such radiation only penetrates about
20 cm of liquid water8, this radiation still poses a challenge for near-
surface aquatic environments where ocean circulation will periodically
expose water from below this depth to the surface.
Even if the harsh effects of the M-dwarf stellar environment were
absent, a significant fraction of the M-dwarf orbiting exo-Earths would
still require substantial greenhouse warming for liquid water to be
stable on the surface, given their relatively low equilibrium tempera-
ture (T
eq
)(Fig.1;Table1). However, the efficacy of greenhouse
warming depends on various factors such as albedo, cloud cover,
greenhouse gas species, and their residence time in the atmosphere;
parameters that are not well constrained for most Earth-sized
exoplanets9,. Another notable, common feature of these planets is
tidal locking, possibly leading to an eyeball-like climate state, where
most of the planet is frozen, with the exception of the substellar point,
where liquid water may exist10. Over time, even the liquid water at the
substellar point may completely freeze due to sea-ice drift, and the
planet may resemble large icy moons of the solar system10.Insuch
cold, icy, rocky planets, basal melting may provide an alternative
Received: 17 June 2022
Accepted: 22 November 2022
Check for updates
1
Department of Earth and Planetary Sciences, Rutgers University, Piscataway, NJ, USA.
2
Thayer School of Engineering, Dartmouth College, Hanover, NH, USA.
3
Department of Earth and Space Science, Universityof Washington,Seattle, WA,USA.
4
Department of Earth Sciences, University of Oregon, Eugene, OR, USA.
e-mail: luju.ojha@rutgers.edu
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means of forming liquid water in a subsurface environment shielded
from high-energy radiation (Fig. 2). In terrestrial glacial studies, the
term basal melting is used to describe any situation where the local
geothermal heat flux, as well as any frictionalheat produced by glacial
sliding, is sufficient to raise the temperature at the base of an icesheet
to its melting point11. Basal melting is responsible for the formation of
subglacial liquid water lakes in various areas of Earth12,suchastheWest
Antarctic Ice Sheet13,Greenland
14–16, and possibly the Canadian
Arctic17,18. The Earth may have been globally glaciated at least three
times19–21. During these periods, geothermal heat flow from the Earth’s
interior would have inhibited the deep ocean from completely freez-
ing. As a result, only the shallow upper-extent of the ocean would have
frozen19. Liquid water would have existed under the ice shell, and life
could have survived through the snowball glaciations22.Similarly,basal
melting of thick ice deposits during the Noachian era [>4 Ga] has been
proposed as a potential solution to reconciling fluvial feature genera-
tion with the faint young sun paradox on Mars23–26. Although much
debated27–30,, basal melting may also be responsible for the formation
of a putative subglacial lake in the south pole of Mars today31–35,where
the mean annual surface temperature lies around 165 K. As such, basal
melting may play an equally important role in the habitability of cold,
icy exo-Earths (Fig. 2).
The feasibility of basal melting depends on the thermal and phase
evolution of an ice sheet which is primarily governed by a planet’s
surface temperature and geothermal heat. Other factors, such as sur-
face gravity and hydrosphere thickness, can also play a major role in
the feasibility of basal melting, given the pressure-dependent melting
temperature of water ice (Fig. 2). Notably, if the pressure in the
hydrosphere is in excess of 0.2 GPa, dense high-pressure ice poly-
morphs (II, III, V, and VI) can form at the bottom trapping the inter-
glacial ocean from access to vital solutes/nutrients limiting the
interglacial ocean’s habitability potential. Basal melting, if present,
would enable water-rock interaction and the formation of buoyant
aqueous solutions that can rise through the high-pressure ice
layer to feed the interglacial ocean enabling potential life-sustaining
conditions (Fig. 2).
Fig. 1 | Estimatedequilibrium temperature (T
eq
) and mass (relative to Earth) of
various M-dwarf orbiting exo-Earths with a high Earth Similarity Index value.
The colors of the plot show the estimated surface gravity, and the dashed line
marks the temperature of 273K. The feasibility of basal melting onexoplanets with
T
eq
less than 273K is considered in thisstudy. Source dataare provided as a Source
Data file.
Table 1 | M-dwarf orbiting, potentially habitable planets with T
eq
<273K
Exoplanet T
eq
(K) Surface gravity (m s−2)M
Earth
R
Earth
GEL of H
2
O* (km) Age of the star (Gyr)Reference
Reference Earth 255 9.81 1 1 4.5 4.6
1Proxima Cen b 257 10.6 1.27 1.08 6.36 4:8+1:1
1:4
79
2 GJ 1061 d 247 12.14 1.64 1.15 7.24 7 +0:5
0:5
80
3 TRAPPIST-1 e 258 7.98 0.69 0.92 4.76 7:6+2:2
2:2
59
4 Kepler-442 b 263 12.68 2.36 1.35 7.56 2:9+8:1
0:2
81
5 GJ 667 C f 249 11.82 2.54 1.45 7.05 3:5+1:5
1:5
82
6 TRAPPIST-1 f 225 9.41 1.04 1.04 5.61 7:6+2:2
2:2
59
7 Teegarden’s Star c 225 10.04 1.11 1.04 5.99 7+3
3
83
8 Kepler-1229 b 217 12.68 2.54 1.4 7.56 1:2+0:7
0:3
84
9 Kepler-186 f 212 12.23 1.71 1.17 7.29 4:0+0:6
0:6
81
10 GJ 667 C e 213 11.82 2.54 1.45 7.05 3:5+1:5
1:5
82
11 TRAPPIST-1 g 204 10.12 1.32 1.13 6.03 7:6+2:2
2:2
59
12 LHS 1140 b 235 23.22 6.38 1.64 13.85 5 +2:4
0
85
*Global Equivalent Layer (GEL) of H2O assuming Earth like WMF 0.05%.
Article https://doi.org/10.1038/s41467-022-35187-4
Nature Communications | (2022) 13:7521 2
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Numerous M-dwarf orbiting exo-Earths with a high Earth Simi-
larity Index (ESI) value, a multiparameter index that compares attri-
butes such as mass, radius, and temperature of an exoplanet to Earth,
have been discovered in the last decade36,(Fig. 1;Table1). These exo-
Earths’position within the CHZ of their parent stars also suggest that
they could be water-rich planets. For example, the exo-Earth Proxima
Centauri B orbits the star nearest to Earth. While the planet’swater
content is unknown, a wide range of internal structure models for
Proxima Centauri B suggests that it could be a water-rich planet (up to
50% by mass)37. However, the stability of liquid water on the surface of
Proxima Centauri B is hard to reconcile with the relatively low T
eq
of
257 K38. A recent study used an atmospheric general circulation model
(GCM) coupled with a dynamic ocean GCM to show that even with an
atmosphere consisting of 10,000 parts per million volume (ppmv)
carbon dioxide (CO
2
)and2000ppmvmethane(CH
4
), only a portion of
the planet may be able to sustain liquid water38. Thus, if Proxima
Centauri B is a water-rich planet, as its internal structure model
suggests37,asignificant fraction of the water may exist in the form of
solid ice, where basal melting may be feasible.
The internal structure models of several other exo-Earths suggest
that they may be water-rich planets. For example, the internal structure
model of the TRAPPIST-1 planetary systems suggest that they could
potentiallybemorewater-richthanEarth
39–41, although with notable
uncertainty. Despite the strong far-UV photolysis of water (H
2
O) and
large H
2
escape rates expected from M-dwarf orbiting exo-Earths,
models suggest that TRAPPIST-1 planets may have retained a sig-
nificant amount of water42. While liquid water may be stable on the
surface of TRAPPIST- 1 e with atmospheric H
2
O alone, the other two
planets (TRAPPIST-1 f, g) require greenhouse gases such as CO
2
and a
thick atmosphere to sustain surface liquid water43. LHS 1140 B is a
super Earth with an estimated mass 6 times than of Earth’s. A recent
work44 utilized the internal structure model developed by45 to indicate
that the water mass fraction (WMF) on LHS 1140 b could be as much as
4% (80 times more than Earth), leading to an average global ocean of
779 ± 650 km. That study’s1%confidence level corresponds to a
WMF = 0.007, still about 1.5 times more water than Earth44.The
transmission spectrum of LHS 1140 b from the Hubble Space Tele-
scope also shows tentative evidence of water, but future observations
are needed to confirm this putative detection46. The exo-Earth GJ 1061
d receives a similar amount of energy as Earth receives from the Sun
and may be a water-rich planet as well47.GJ667CeandGJ667Cflie
also within the CHZ of its host star and may be water-rich; however,
they may still require a thicker atmosphere than Earth and greenhouse
gases like CO
2
and CH
4
for the liquid water to be stable on their
surface48. Kepler-442 b is a potentially water-rich exo-Earth with a high
ESI value and potential for habitability49. Both Kepler 1229 b and 186 f
have extremely low T
eq
and thus would require a thick atmosphere
with green houses gases to sustain liquid water50. In the absence of a
thick atmosphere, a significant fraction of water on these two exo-
Earths may exist in the form of ice. While there are significant uncer-
tainties regarding the possible presence and the volume of the
hydrosphere on these planets, even with an Earth-like WMF of 0.05%,
these bodies could contain kilometers-thickglobal ice sheets (Table1),
where basal melting may not only be possible but alsoplay a significant
role in habitability.
In this work, we model the thermophysical evolution of ice sheets
of various thicknesses and demonstrate that basal melting is likely
prevalent on M-dwarf orbiting exo-Earths, even with modest, Moon-
like geothermal heat flow. We show that thick subglacial oceans of
liquid water can form and persist at the base of and within the ice
sheets on exo-Earths for a prolonged period. Our findings suggest that
exo-Earths resembling the snowball Earth or the icy moons of Jupiter
and Saturn may be common in the Milky-way galaxy.
Results
We take a conservative approach and assume that the surface tem-
perature (T
s
) equals the estimated T
eq
(Fig. 1) for all exo-Earths con-
sidered in this study. Depth-dependent initial profiles of ice phases,
density, specific heat, thermal conductivity, and melting temperature
are estimated self-consistently (Fig. 3) and coupled with the thermal
evolution model23 to explore the feasibility of basal melting, while
accounting for the time-varying ice phase evolution of the thick ice cap
(see Methods subsection thermal evolution of ice sheets). An example
of a thermal profile and the time-dependent phase evolution of a 2-km
thick ice sheet on Proxima Centauri B, assuming a T
s
of 257 K and a
basal heat flux of 30 mW m−2, is shown in Fig. 3. In this scenario, basal
melting occurs within a few hundred thousand years post-deposition
of the ice resulting in approximately 800 m thick liquid water ocean at
the ice-crust interface. The exo-Earths considered here have a wide
range ofsurface temperature and gravity. Similarly, the hydrosphere, if
present, on these bodies may encompass a wide range of thicknesses.
Thus, we vary these parameters and run the thermophysical evolution
models (Fig. 3) to ascertain their effect on the feasibility of basal
melting.
Ice Ih
Ocean
Ice V
Ice VI
Basal
Melting
Rocky interior
Atmosphere?
Hydrosphere
Geothermal
heat ux
(a)
Buoyant melt
migration
(b)
Fig. 2 | Schematic of a basal melting model for icy exo-Earths. a Due to the high
surface gravity of super-Earths, ice sheets may undergo numerous phase trans-
formations. Liquid water may form within the ice layers and at the base via basal
melting with sufficient geothermal heat. If high-pressure ices are present, melt-
water will be buoyant and migrate upward,feeding the mainocean. The red arrows
show geothermal heat input from the planet’s rocky interior. bPure water phase
diagram from the SeaFreeze representation32 illustrating the variety of phases
possible in a thick exo-Earth ice sheet. Density differences between the ice phases
lead to a divergence from a linear relationship between pressureand ice-thickness.
Article https://doi.org/10.1038/s41467-022-35187-4
Nature Communications | (2022) 13:7521 3
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Effect of surface temperature, gravity, and ice-thickness on the
feasibility of basal melting
The feasibility of basal melting is strongly dependent on T
s
,with
relatively low heat flow required for basal melting on planets with high
T
s
(Fig. 4). For example, given the relatively high T
s
of Proxima Centauri
B, TRAPPIST-1 e, and Kepler 442 b (260 K) (Ta ble 1), even a 1-km
thick ice sheet would undergo basal melting with a heat flow of
20–40 mW m−2(Fig. 4). In contrast, heat flow in excess of 175 mW m−2
would be required for a 1-km ice sheet on Trappist 1 g (T
s
=204) to
undergo basal melting (Fig. 4;Fig.5a, b). Basal melting is also more
likely to occur on planets with thicker ice sheets and higher surface
gravity because the melting temperature of water-ice initially decrea-
ses with depth due to the pressure-reduced melting point of ice Ih
(Fig. 2; Supplementary Fig. 1). Given the considerable uncertainty
associated with the volume of the possible hydrosphere on these
planets, we first consider a conservative scenario in which the WMF on
exo-Earths is substantially lower than that of Earth and model the
thermophysical evolution of 1–4 km thick icesheets (Fig. 4). Heat flow
required for basal melting generally decreases with increasing ice
thickness (Fig. 4). For example, while heat flow in excess of 30 mW m−2
is needed for basalmelting of 1-km thick ice on Kepler 442 b (Fig. 4), a
4-km ice sheet on the same planet would undergo basal melting with a
modest Moon-like heat flow of 10 mW m−2(Fig. 4; Fig. 5c, d). If the exo-
Earths considered here were to contain Earth-like WMF, the global
equivalent layer (GEL) of hydrosphere on these bodies would lie
between 5 and 14 km (Table 1). The general trend of the feasibility of
basal melting with surface temperature, surface gravity, and ice
thickness doesnot change whenwe consider thicker ice-sheets(Fig. 6);
however, the total heat flow required for basal melting decreases
notably (Fig. 6; Supplementary Fig. 2).
Basal melting on super-Earths
If exo-Earths were to contain much thicker ice sheets, as proposed
for LHS 1140 b, the basal pressure and the melting temperature
could drastically increase. For example, at the base of a 75 km thick
ice sheet on LHS 1140 b which has a surface gravity of 23 m s−2, basal
pressure would approach 2300 MPa, and basal temperatures
higher than 350 K would be required to generate meltwater at the
bottom (Fig. 2). However, liquid water generated at the rock/high-
pressureicemantleboundarywillbebuoyantandcanquickly
migrate upward feeding the upper ocean below the ice Ih crust, as
showninpreviousstudyonhighpressureicemantleoficy
moons51,52 and in Fig. 2. Considering the lower end T
s
estimate of
235 K for LHS 1140 b9,heatflow above 30 mW m−2can result in basal
melting within 70 km ice sheets (Fig. 7). If the higher end T
s
esti-
mate of 265 K is considered9,heatflow of 5 mW m−2is sufficient for
melting within the upper tens of km of the ice sheet (Fig. 7). Given
the uncertainty in the water content of LHS 1140 b, we explore the
feasibility of basal melting of ice sheets of a wide range of thick-
nesses (Fig. 7). On a super-Earth like LHS 1140 b, ice sheets in the
10–40 km thickness range are most likely to undergo basal melting
due to the pressurized ice at the base, which has a reduced melting
point (Fig. 7).
Thickness and the stability of basal melt
All else being equal, the thickness of the basal melt increases with heat
flow and time. For example, in Supplementary Fig. 3, we show the
thermophysical evolution of a 10-km thick ice sheet on Proxima Cen-
tauri B for two different heat flow va lues (5 and 10 m W m−2). In both
cases, basal melting occurs within a few hundred thousand years post-
deposition of the ice, and the thickness of basal melt increases until the
system achieves thermal equilibrium. However, the basal melt result-
ing from a heat flow of 10 mW m−2is notably thicker than basal melt
from a heat flow of 5 mW m−2. Supplementary Fig. 4 shows similar
results for GJ 1061 D. For computational efficiency and to demonstrate
the feasibility of basal melting, we have limited the thermophysical
evolution of ice for 5 million years. This time frame is adequate for the
thermal equilibration of ice sheets and to ascertain whether a given
heat flow is sufficient for basal melting. To assess the stability of basal
melt over timescales relevant to the genesis of life on Earth (0.5–1Gyr),
we model the thermophysical evolution of a 1-km ice sheet on Proxima
Centauri B for a billion years (Fig. 8). Over a billion years, geothermal
heat flowcanbeexpectedtodeclinegiventhebillionyearshalf-lifeof
most heat producing elements. Thus, we consider two scenarios: an
Fig. 3 | Steady state initial profiles of various thermophysical parameters,
temperature, and ice phases as a function of depth within a thick ice sheet on
Proxima Centauri B assuming a surface temperature of 257 K. a An example of
initialsteady-state profiles of temperature (a),specificheat(b), density (c), and ice-
phase (d) as a function of depth in a 2 km thick ice sheet on exo-Earth Proxima
CentauriB assuminga surface temperature of257 K. The temperature is assumed to
increasewith depth in an adiabatic fashion. Theseinitial profiles are coupled with a
thermal evolution model to explore the feasibility of basal melting and the time-
varying ice phase evolution of the thick ice cap. eTemperature distribution as a
function of depth and time on Proxima Centauri B assuming a 2 km thick ice sheet,
T
s
of 257 K, and heat flow of 30 mW m−2.fIce phase evolution as a function of depth
and time on Proxima Centauri B assuming a 2km thick ice sheet, T
s
of 257 K, and
heat flow of 30 mW m−2.
Article https://doi.org/10.1038/s41467-022-35187-4
Nature Communications | (2022) 13:7521 4
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extreme heat loss scenario in which the heat flow on Proxima Centauri
B exponentially declines from the current Earth-like heat flow of
60 mW m−2to the current Mars-like heat flow of 30 mW m−2within a
billion years. In this rapid heat loss scenario basal melt would only be
stable for approximately 750 million years. In the second, moderate
heat loss scenario, the same amount of heat flow declines over a
4-billion-year period. As shown in Fig. 8, basalmelt in an exo-Earthwith
moderate heat loss can persist for a geologically prolonged period (>3
Gyrs). Supplementary Figs. 5 and 6 show similar results for the coldest
exo-Earthinourstudy,TRAPPIST-1g.Theefficiency of heat loss on
exo-Earths and its impact on basal melting is further assessed in the
Discussion section.
Fig. 5 | Temperature and ice phases as a function of depth within a thick ice
sheet on TRAPPIST 1 g and Kepler 442b. a Temperature distributionas a function
of depth and time on TRAPPIST 1 g for a 1 km thick ice sheet, T
s
of 200 K, and heat
flow of 175 mW m−2.bIce phase evolution as a function of depth and time on
TRAPPIST 1 g over a 5-million-year time frame. cTemperature distribution as a
function of depth and time on Kepler 442 b for a 4 km thick ice sheet, T
s
of 260 K,
and heat flow of 10 mW m−2.dIce phase evolution as a function of depth and time
on Kepler 442 b over a 5-million-year time frame.
Fig. 4 | Heat flowrequired for basal melting as a function of surface gravity and
temperature for ice sheets 1–4 km in thickness. a Heat flow required for basal
meltingas a function of surface gravity and surface temperaturefor a 1-km thick ice
sheet. The white dots (the numbers correspond to the index number of planets in
Table 1) showthe approximate surface gravity and surface temperature of the exo-
Earths considered in the study. b–dSame as (a), but for 2–4km thick ice sheets.
eA box and whisker diagram showing the heat flow distribution in the continental
and the oceanic regions of the Earth. The box’s lower and upperextent corresponds
to the 25th and 75th percentile of the heat flow values, and the center corresponds
to the median heat flow values.
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Discussion
The primary goal of this paper is to demonstrate the relative ease by
which basal melting may be attainable on M-dwarf orbiting exo-Earths.
While there are notable uncertainties about the presence and the
volume of hydrospheres on these bodies, if even a handful of poten-
tially habitable exo-Earths discovered so far (or in the future) were to
contain thick (>few km) hydrospheres, then liquid water via basal
melting may be present on those bodies with relatively modest heat
flow. Our heat flow constraint required for basal melting, under the
assumption that the estimated T
eq
is equal to the T
s
, is an overestimate.
The presence of greenhouse gases on any of these exo-Earths will raise
the T
s
, and therefore the heat flow required for basal melting would be
notably lower than the estimates presented here.
While reasonable constraints on T
s
of exo-Earths can be placed
based on the estimated T
eq
, heat flow on exo-Earths is entirely
unconstrained. The main source of long-term heat in a planet after
the initial stage of accretion and differentiation is the radiogenic
decay of isotopes of heat-producing elements with billion-year half-
lives such as 40K, 232Th, 235U, and 238U (i.e., geothermal heat). For
example, approximately 80% of the Earth’s present-day surface heat
flow can be attributed to the decay of radioactive isotopes53. Radio-
genic heat production as a function of age for cosmochemically
Earth-like exoplanets suggests that exo-Earths similar in age to Earth
should have a comparable heat production rate (H)54. However, there
is a considerable degree of variations and uncertainties associated
with the age of the M-dwarf systems considered in this study
(Table 1). A comparison of the H values required for basal melting on
the various exo-Earths to the age-dependent heat production values
of cosmochemically Earth-like exoplanets54,is shown in Fig. 9.
Despite the old age of some of the M-dwarf systems, the heat pro-
duction rates on these exo-Earths may be sufficient for basal melting
if they are cosmochemically Earth-like. The notable exceptions are
TRAPPIST-1 f and TRAPPIST-1 g, where basal melting of thin ice sheets
by geothermal heat alone may not be feasible given their old age
(hence lower radiogenic heat production) and their relatively low
surface temperature (also see Supplementary Fig. 2).
Acomparisonoftheheatflow required for basal melting on
various exo-Earths to the heat flow on moons and planets of our
solar system may also allow us to further contextualize whether
basal melting would be feasible on exo-Earths. The mean oceanic
and continental heat flow on Earth is 101 and 65 mW m−2, respec-
tively (Fig. 4e); however, in active hot spots like Yellowstone
and mid-oceanic ridges, the surface heat flow can exceed
1000 mW m−255. On the Moon, heat-flow was measured to be 21 ± 3
and 15 ± 2 mW m−2at Apollo 15 and 17 landing sites respectively56.No
direct measurements of the Martian surface heat flow are currently
available; however, indirect remote sensing and models indicate
heat flow up to 25 mW m−257,.Afirst-order comparison of the heat
flow required for basal melting on various exo-Earths to the heat
flow of Earth provides further corroboration that basal melting may
be entirely feasible on most exo-Earths with Earth like WMF (Fig. 9).
For thick (>3 km) ice sheets, the heat production rate on exo-Earths
can be ~50% that of Earth’s, and basal melting can still occur (Sup-
plementary Fig. 2). The heat production rate on Kepler 442 b can be
~5% that of Earth’s, and basal melting could still occur. The thin
horizontal lines in Fig. 9e show the range of heat flow for the Moon
and Mars. In some planets like Kepler 442 b, Proxima Centauri B,
and TRAPPIST-1e, even ice sheets that are 1-2 km thick may undergo
basal melting with Mars-and-Moon like heat flow (Supplementary
Fig. 2). Heat flows in excess of Earth’s continental and oceanic
average are only required to melt thin ice sheets on planets with
extremely low T
s
, such as TRAPPIST-1 g, GJ 667 C e, and Kepler-186 f
(Fig. 4; Fig. 6; Supplementary Fig. 2). It is conceivable that the exo-
Earths considered in this study are not cosmochemically Earth like
and the geothermal heat flow on those exo-Earths may not be suf-
ficient for basal melting. In such a scenario, tidal heating may pro-
vide an additional source of the heat on some exo-Earths around the
habitable zone of M-dwarf stars58. For example, the age of the
TRAPPIST-1 system is estimated to be 7.6 ± 2.2 Gyr;59 thus, if geo-
thermal heating has waned more than predicted by the age-
dependent heat production rate assumed here54, tidal heating
could be an additional source of heat for basal melting on the
TRAPPIST-1 system. On planets e and f of the TRAPPIST-1 system,
tidal heating is estimated to contribute heat flow between 160 and
180 mW m−260,. Thus, even if geothermal heating were to be negli-
gible on these bodies, basal melting could still occur via tidal
heating alone (Supplementary Fig. 2). However, for TRAPPIST-1 g,
the mean tidal heat flow estimate from N-body simulation is less
than 90 mW m−243,. Thus, ice sheets thinner than a few kilometers
are unlikely to undergo basal melting on TRAPPIST-1 g (Supple-
mentary Fig. 2).
Due to the billion-year half-lives of the heat-producing ele-
ments responsible for planetary geothermal heat, meltwater cre-
ated by basal melting may be sustained on exo-Earths for a
prolonged period (Fig. 8; Supplementary Fig. 5). However, the
longevity of basal melt relies on the cooling rate of the planet,
Fig. 6 | Heat flowrequired for basal melting as a function of surface gravity and
temperature for ice sheets 5–20 km in thickness. a Heat flow required for basal
melting as a function of surface gravity and surface temperature for a 5-km thick ice
sheet. The white dots (the numbers correspond to the index number of planets in
Table 1) showthe approximate surface gravity and surface temperature of the exo-
Earths considered in the study. b–dSame as (a), but for 10–20 km thick ice sheets.
Article https://doi.org/10.1038/s41467-022-35187-4
Nature Communications | (2022) 13:7521 6
Content courtesy of Springer Nature, terms of use apply. Rights reserved
which is controlled by the mechanism and efficiency of heat
transport from the interior of a planet to the surface53.Sincethe
cooling histories of exo-Earths are unknown, here, we assess the
longevity of basal melt by adopting an ad hoc cooling model in
which the heat flow declines exponentially over a specified period.
While the actual cooling history of the exo-Earths may be con-
siderably different than the model used here, the goal here is to
only investigate the first-order effect of planetary cooling on the
feasibility of basal melting. Even when we consider an accelerated
rate of planetary cooling, basal melt may be stable for a prolonged
period (Fig. 8; Supplementary Fig. 5; Supplementary Fig. 6).
The density of liquid water is higher than water-ice Ih; thus, a slon g
as the basal pressure does not exceed ~200 MPa, liquid water will
remain stable at the base of the ice sheets. The interaction of planetary
hydrospheres with silicate bedrock will also inevitably result in the
incorporation of other soluble minerals/chemicals that have the
potential for significantly lowering thefreezing point of pure water and
nutrients essential for sustaining habitable conditions61,62. However,
quantifying the role of solutes such as NaCl or NH
3
on the feasibility of
basal melting is currently not possible due to the lack of experimental
and theoretical data on these systems’thermodynamics at higher
pressures61,63. Nevertheless, limited available data suggest that these
solutes will have antifreeze effects on ice polymorphs of similar mag-
nitude at pressures up to 2 GPa64,65. Additionally, the inclusion of
solutes lowers the heat capacity of solutions with increasing
concentrations;61 thus, we can expect increased liquid stability for the
same heat flux when solutes are present. The results presented here
are based on pure water thermodynamics and thus represent a con-
servativescenariothatallowsliquidwatertoformandbestableinthe
hydrospheres of ice-rich exoplanets.
The subsurface world of these exo-Earths might resemble the
subsurface conditions found on Europa. In this class III habitat66,water
in subsurface oceans interacts with silicates. The ensuing water-rock
interactions at the crustal interface mayprovide a variety of chemicals
and energy that could play a role in the origin and sustenance of
putative life forms at the ocean floor, akin to those found at hydro-
thermal vents on Earth67. Despite the high pressures present at the
base of the ice sheets on super-Earths, it may not be a limiting habit-
ability agent as life on Earth has been observed at subduction forearc
with pressure exceeding 340 MPa68, and piezotolerant strains of
Ice Ih
Liquid
water
Ice II
Ice V
Ice VI
Ice III
Liquid
water
Ice Ih
Ice VI
(a)
(b)
(c)
(d)
0 10203040506070
Icesheet thickness (km)
0
20
40
60
80
100
Heat flow (mW m
-2
)
Heat flow required for Basal Melting on LHS 1140 B
Average Oceanic Heatflow
Average Continental Heatflow
Average Martian Heatflow
Average Lunar Heatflow
T
s
= 265 K
T
s
= 235 K
(e)
Fig. 7 | Heat flow required for basalmelting on super-Earths as a function of the
ice sheetthickness. a Temperature distribution as a functionof depth and time on
LHS 1140 b for a 70 km thickice sheet, T
s
of 235 K, and heat flow of 30 mW m−2.bIce
phase evolution as a function of depth and time on LHS 1140 b over a 10-million-
year time period. The melt water underneath the high-pressure ice phases quickly
migrates to shallower depths; however, the migration of buoyantliquid water is not
modeled here. cTemperature distribution as a function of depth and time on LHS
1140 b for a 70 km thick icesheet, T
s
of 265 K, and heatflow of 5 mW m−2.dIce phase
evolution as a function of depthand time on LHS 1140 b over a 10-million-year time
frame. eThe black and blue lines show the average geothermal heat flow (y-axis)
required for an ice sheet of certain thickness (x-axis) to undergo basal melting on
LHS 1140 b assuming a surface temperatureof 265 and 235 K. Theblack horizontal
lines show the average heat flow observed on planets in the solar system. The
presenceof pressurizedice phase Ih with reducedmelting pointat 10–40 km depth
enables basal melting with relatively low geothermal heat. Higher geothermal
heating is required to melt thicker ice sheets as the melting temperature increases
with ice thickness (see Fig. 2).
Article https://doi.org/10.1038/s41467-022-35187-4
Nature Communications | (2022) 13:7521 7
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bacteria on Earth can survive brief exposures up to 2000 MPa69.The
efficacy of geothermal heat sustained basal meltwater providing a
habitable niche70,isexemplified by the continual habitability on Earth
during the Neoproterozoic era (600–800 Ma) which was ma rk ed by
widespread glaciation20.
An isolated ocean between layers of low density, low pressure ice
and high density, high pressure ice may also form (Fig. 7), without
chemical exchange (i.e. nutrient flux) with the rocky core, which may
notbeadequateforhabitability
62. However, liquid aqueous solutions
generated by basal melting at the rock/high-pressure ice mantle
boundary will be buoyant and will permit the transportation of
important life sustaining solutes/nutrients from the rocky core to the
interglacial ocean. These bodies would classify as class IV habitat with
liquid water layers between two ice layers66, such as the internal
models of Ganymede and Callisto. In some scenarios, the melt is only
observed within the ice layer and not at the base of the ice sheet
(Fig. 7). Still, the absorptive properties of the frozen surface ice layer
would shield basal melt environments from X-ray, extreme, and far UV
radiation. Thus, basal melting provides a potentially habitable envir-
onment for cool, terrestrial planets orbiting M-dwarfs. It may also
provide an environment isolated from the active and variable radio-
active properties of M-dwarfs, which have long been a concern for the
habitability of these planets.
Methods
Thermal evolution of ice sheets
Underneath thickplanetary glaciers, the melting point of ice varies due
to both pressure and ice phase. Due to the estimated high surface
gravity on LHS 1140 b, Proxima Centauri B, and some of the planets in
the Trappist system, water-ice can experience extreme pressures and
temperatures at depth and evolve into high-pressure ice phases61.We
use the code Sea-Freeze (https://github.com/Bjournaux/SeaFreeze)71
in conjunction with a heat transport model to investigate the temporal
and depth-dependent evolution of the thermodynamic and elastic
Fig. 8 | Temperature and ice phases as a function of depth within a 1-km thick
ice sheet on Proxima Centauri B, assuming exponentially declining heat flow.
aTemperature distribution as a function of depthand time on Proxima Centauri B
for a 1 km thick ice sheet assuming T
s
of 257 K and variable heat flow over a billion
years. bIce phase evolution as a function of depth and time on Proxima Centauri B
over a bil lion years. cAn ad hoc exponential function that models heat loss on
Proxima Centauri B from 60 to 30 mW m−2over a billion years. dTemperature
distribution as a function of depth and time on Proxima Centauri B for a 1km thick
ice sheet assuming T
s
of 257 K and variable heat flow ove r 4b illion years. eIce phase
evolution as a function of depth and time on Proxima Centauri B over 4 billion
years. fAn ad-hoc exponential function that simulates heat loss on Proxima Cen-
tauri B from 60 to 30 mW m−2over 4 billion years.
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Nature Communications | (2022) 13:7521 8
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properties of water and ice polymorphs in the 0–2300 MPa and
220–500 K range.
Ice is compressive such that a large increase in pressure with
depth can lead to a significant adiabatic increase of temperature with
depth. The initial condition adiabatic temperature gradient imple-
mented in all our simulations is given by the following:
dT
dz
=αvgT
cp
ð1Þ
where α
v
,g,T,andc
p
are thermal expansivity, gravity, temperature,
and specific heat at constant pressure, respectively. An example of an
adiabatic temperature profile assuming a surface temperature of 235 K
and associated temperature-dependent thermal parameters is shown
in Fig. 3. Sea-Freeze is a thermodynamic model which predicts the
pressure and temperature dependent phase evolution of ice Ih and
high-pressure ice polymorphs as well as an array of their material
parameters71. Sea-Freeze does not model variations in thermal
conductivity; thus, we use average thermal conductivities (K
avg
)for
each ice phase as listed in Supplementary Table 1.
The duration of possible glaciation on these bodies is unknown,
but ice sheets on the night side of tidally locked planets may exist in a
near-equilibrium state with the atmosphere for an extended period72.
On Earth, widespread glaciation during the Neoproterozoic era
(snowball Earth) may have prevailed for over 200 million years20.Fora
given heat flow, it only takes a few million years for the ice sheet to
achieve thermal equilibrium, thus for computational efficiency, we run
the thermophysical evolution models for 5 million years. Given the
relatively short timescale of our models, we assume a steady-state heat
production rate and neglect the secularcooling of the planet. In some
special cases, wealso model the thermophysical evolution of icesheets
for a billion years to ascertain the stability of basal melt. In these
scenarios, we assume that the heat flow declines exponentially over a
specified period.
The goal is to solve for the physical (melting) and thermal (heat
transport) evolution of a thick, multilayered ice sheet containing Ice Ih,
high-pressure ices, and any potential melt layers. To do so we solve a
modified version of the 1-D thermal diffusion equation that accounts
for the potential existence of three distinct heat transport regimes
within the ice sheet. These regimes focus on representing the initial
production of meltfrom heat transfer over the specified timeintervals,
and do not account for additional factors such as salinity prevalence
and the buoyancy effects associated with viscosity contrasts at the
resulting phase boundaries. Regime I is characterized by classic ther-
mal diffusion in a solid and describes the heat transport in solid
regions of the ice sheet tha t are not undergoing solid state convection.
This includes ice layers that are too thin to undergo solid state con-
vection as well as the conductive boundary layers of ice layers that are
undergoing solid state convection (see below for the calculation of
these boundary layer thicknesses). Conservation of energy in these
regionsisdescribedby
ρcp
∂T
∂t=∂
∂zk∂T
∂z
ð2Þ
where ρis density, c
p
is specificheat,Tis temperature, tis time, zis the
vertical spatial coordinate, and kis thermal conductivity53.Thepres-
sure and temperature dependence of density and specificheatare
iteratively calculated using Sea-Freeze. The ice-phase dependency of
thermal conductivity is updated based on the values reported in
Supplementary Table 1.
Regime II is characterized by solid ice layers that are undergoing
solid state convection. If there exist significant temperature-
dependent density differences between the top and bottom of an ice
Fig. 9 | Radiogenic heat-production rate required for basal melting compared
with the age-dependent heat production rate of cosmochemically Earth-like
planets.a The black curve shows the radiogenic heat production rate asa function
of time forcosmochemically Earth-like planets. The vertical extent of theerror bars
shows the heat production rate required for basal melting of ice sheets 1–20km in
thickness (also see Supplementary Fig. 2) on Proxima Centauri B, GJ 1061 (d), and
TRAPPIST-1 (e). The dotted error bars show the range of heat production required
for basal melting assuming Earth-like core mass fraction. The horizontal error bar
shows theuncertainty inthe age of these star systems. b–dSame as (a)butshowing
the range inthe heat production rate and agesof various other exo-Earths listed in
the legend. dA boxplot graph showing the range of heat flow required for basal
melting on various exo-Earths as a function of the ice-thickness compared to the
heat flow on Earth’s continents and oceans. The two thin, horizontal black lines
show the range of heat flow on the Moon(lower line) andMars (upper line).Source
data are provided as a Source Data file.
Article https://doi.org/10.1038/s41467-022-35187-4
Nature Communications | (2022) 13:7521 9
Content courtesy of Springer Nature, terms of use apply. Rights reserved
layer heated from below the layer is susceptible to solid state
convection73,74,. To determine if any portion of an ice layer is under-
going solid state convection we check to see if the layer’s Rayleigh
number (Ra) exceeds a preset critical Rayleigh number (Ra
c
= 1000), a
common approach in solid state mantle convection models75,76,.The
Rayleigh number can be described as
Ra = αvρ2cpg4Th3
kηð3Þ
where ΔTis the temperature difference between the top and bottom of
the ice layer, his the thickness of the ice layer, and ηis the viscosity
of the top of the ice layer53. We additionally allow for the possibility of
stagnant lid convection by testing whether any regions within each ice
layer can undergo solid state convection. We assume ice viscosity
follows a temperature dependent Arrhenius law77
η=η0exp ATm
T1
ð4Þ
where η
0
is a characteristic viscosity (values for each ice type listed in
Supplementary Table 1), Ais a constant coefficient here taken to be 7.5
(in line with ref. 77)andT
m
is the melting temperature of the ice
(calculated by SeaFreeze).
When a layer of ice is undergoing solid state convection it will
retain transitional conductive boundary layers at both its upper and
lower interface, the thicknesses of which can be described by75,76,78
δupper =hRac
Ra
1=3
ð5aÞ
δlower =h0:28Ra0:79
1=3ð5bÞ
Ice within these boundary layer regions will not be convecting and heat
transport in these layers will begoverned by Eq. 2. Heat transport in ice
undergoing solid state convection is described by an amplified diffu-
sion equation
ρcp
∂T
∂t=∂
∂zNu × k ∂T
∂z
ð6Þ
where Nu is the Nusselt number for the top layer of the convecting
region (Nu = (Ra/Ra
c
)1/3)75,76.
Regime III describes the heat transport in melt (liquid water)
layers. The amplified thermal diffusion approach of Eq. 6is applicable
in regions where the convective timescale is on the order of or less
than the temporal discretization timescale (O(10,000 years)), and thus
is appropriate in high viscosity ice layers undergoing solid state con-
vection. However, in low viscosity liquid water layers where the con-
vective time scale is much less than 10,000 years an alternate
approach is needed. Given the significant length of our temporal dis-
cretization (O(10,000 years)), we assume that all liquid water layers
will be well mixed throughout the duration of our simulations. To
accomplish this, during each model time step we simulate thermal
diffusion into and out of the liquid water layer governed by Eq. 2.That
is, thermal diffusion is simulated throughout the entire domain gov-
erned by either Eq. 2or Eq. 6, where the only regions described by Eq. 6
are regions of ice layers undergoing solid state convection that are not
within their conductive boundary layers. Liquid water layers are
simulated as purely c onductive (Eq. 2). We implement an implicit Euler
finite discretization scheme to solve for the thermal diffusion
throughout the ice sheet, subject to a constant Dirichlet surface tem-
perature boundary at the top of the ice sheet and a Neumann geo-
thermal heat flux boundary condition at the base of the ice sheet23,26.
After the solution has stabilized, we calculate the enthalpy of each
discretized layer within the liquid water layers (typically enthalpies at
the base ofthe layers are higher because heat has been conducted into
the liquid layer base and out of the top of the layer)
Hj=cjTjð7Þ
where H
j
and c
j
are the enthalpy and specificheatofthejth discretized
layer. The enthalpies of a liquid layer are summed and then evenly
redistributed throughout the liquid layer to simulate mixing. Enthalpy
was employed, as opposed to temperature, to ensure conservation of
energy during mixing given depth dependent specificheats.The
temperature profile within the melt layer can easily be recovered by
solving Eq. 7for T.
The result is an energy conserving vertical temperature profile
within the ice governed by Regimes I-III; thermal diffusion in solid ice
layers not undergoing solid state convection (thin ice layers and con-
ductive boundary layers), Nu-Ra parameterized convection within ice
layers undergoing solid state convection, and thorough mixing of
liquid water layers. After each temporaliteration of the thermal model
the resulting temperature profile is fed to the SeaFreeze code to
determine the new phase stratigraphy of the ice sheet (i.e., if any phase
transformations between ice types or melting has occurred), and the
material properties of the ice sheet are updated prior to the next
model time step accordingly. This process is then iterated for the
duration of the five-million-year simulations.
Heat production rate
The total heat flow from the interi or of a planet (Q) can be estimatedby
multiplying the surface area of the planet (A) by the mean heat flow (q)
required for basal melting.The mean heat generation per unit mass (H)
is then given by
H=Q
Mð8Þ
where Mis the mass ofthe planet. Due tothe lithophilic nature of heat-
producing elements,they are assumed to be absent in the metallic core
of a planet; thus, ideally, Hshould be calculated only for the silicate
portionofaplanet.However,sincethesizeandmassofthecoreof
most exo-Earths are unknown, we compute Hfor exo-Earths, assuming
Earth-like coremass fractionof 30%. The Hrequired for basalmelting is
compared with the age-dependent radiogenic heat production model
for cosmochemically Earth-like planets54.
Reporting summary
Further information on research design is available in the Nature
Portfolio Reporting Summary linked to this article.
Data availability
The mass, radius, and the surface gravity estimate of the exo-Earths
considered here can be found at (https://exoplanetarchive.ipac.
caltech.edu/) and also in the Source Data files. The terrestrial heat
flow data used to create Fig. 4ecanbefoundat(http://heatflow.org/).
The time-dependent radiogenic heat production rate shown in Fig. 9is
included as supplementary information of the following publication
(https://doi.org/10.1016/j.icarus.2014.08.031) and can also be found in
the Source Data files. The datasets generated during and/or analysed
during the current study are available from the corresponding author
on reasonable request. Source data are provided with this paper.
Code availability
SeaFreeze was used to estimate the thermodynamic and elastic
properties of water and ice and can be found at the following location
(https://github.com/Bjournaux/SeaFreeze). The codes used to model
Article https://doi.org/10.1038/s41467-022-35187-4
Nature Communications | (2022) 13:7521 10
Content courtesy of Springer Nature, terms of use apply. Rights reserved
the temporal and depth-dependent evolution of the ice sheets can be
found here (https://github.com/lujuojha/ExoPlanetsBasalMelting)and
(https://zenodo.org/record/7331987#.Y3ud4uzMJJF).
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Acknowledgements
L.O. was supported by a startup grant from Rutgers University. BJ was
supported by the Habitability of Hydrocarbon Worlds: Titan and Beyond
nodes of NASA Astrobiology Institute (08-NAI5- 0021 and 17-NAI82-17)
and the NASA Solar System Workings grant 80NSSC17K0775.
Author contributions
L.O. conceived the project and performed the modeling with significant
aid from B.T. and J.B. L.O., J.B., B.T., and B.J. designed the heat flow
models and aided in the interpretation of the results. B.J. aided in the
thermophysical evolution, SeaFreeze models, and interpretation of the
results. G.M.D. aided in the interpretation of the results. L.O. wrote the
paper with significant feedback from all other authors.
Competing interests
The authors declare no competing interests.
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