Little to no active faulting likely at Europa’s seafloor today

Domain: earth_science
Published: 2026-01-06
Editorial outcome: major_revision

Abstract
Many of the outer Solar System’s icy satellites feature known or suspected subsurface oceans, at least some of which are likely situated atop rocky interiors. Water–rock interactions at and beneath these seafloors might support active chemoautotrophic habitats, with sub-seafloor fluid flow facilitated by active faulting and hydrothermal systems. Absent such phenomena, however, any attainment of chemical equilibrium between the seafloor and ocean might limit the availability of chemical energy for life. Here, we characterise the stress state of the seafloor of Jupiter’s moon Europa, and thus the prospect for fracturing and associated sub-seafloor fluid flow there. We consider stresses from tidal forcing, global contraction, mantle convection, and serpentinisation. We find that none of these mechanisms is likely able to drive slip along even weak, pre-existing fractures in the present. Ocean water–rock reactions taking place today are therefore probably restricted to fluid flow through only the upper few hundred metres of the seafloor. Any processes able to sustain habitable conditions at the Europan seafloor today must therefore be independent of ongoing tectonic activity.

Introduction
On Earth, fluid-filled fracture networks can support subsurface habitable environments1,2; on the ocean floors, tectonic activity drives fracturing that continually exposes fresh rock to seawater3–5, allowing for the production of redox couples and the release of metabolites into the overlying water column6. Although the mechanical properties of the Europan seafloor are unknown, observations of the lithospheres of terrestrial planets imply that the brittle strength of Europa’s silicate interior is governed by the frictional properties of weaknesses inherent to large rock volumes such as pre-existing fractures at multiple scales and orientations. The magnitudes of stress will thus be limited at any given depth and pore fluid pressure by those fractures on which the resistance to frictional sliding is matched.

Here, we make a first-order characterisation of the stress state and level of geological activity of the seafloor of Europa, one of Jupiter’s largest moons and a major astrobiology target (e.g., refs. 7,8). Combining rock mechanics with spacecraft measurements, we estimate the tectonic strength of the moon’s seafloor and then establish whether available stresses today are sufficient to drive failure. We further calculate the brittle lithospheric thickness of Europa’s rocky interior and assess whether convection of the silicate mantle could overcome the strength of that lithosphere.

Results
Absent any direct observations of Europa’s seafloor, and in an effort to understand the minimum criteria required for seafloor faulting, we purposefully consider parameters that lead to the ocean floor being relatively mechanically weak—that it has been serpentinised, that it hosts pre-existing fractures, and that hydraulically connected pore spaces remain throughout the brittle portion of the moon’s silicate interior.

We envisage the seafloor already being chemically altered by passive interaction with seawater9 or perhaps by an earlier phase of heating of the interior arising from time-varying orbital resonances with Io or changes in orbital eccentricity or tidal dissipation10. As a result, seafloor rocks are thus more likely to undergo fault slip than unaltered rock (see Table 1 and Methods). Key to this scenario is the assumption that the rocks at the seafloor are already substantially fractured from, for instance, serpentinisation and/or thermal cooling stresses (e.g., ref. 11).

By analogy with Earth, the seafloor of Europa is likely permeable at the rock–water interface (e.g., ref. 12), decreasing in permeability as depth (and overburden stress) increases. Accurately estimating a porosity profile is challenging, however13, and so we assume that pore space (e.g., from fracture networks inherent to large rock volumes) remains constant throughout the brittle rock volume and that these pores are hydraulically connected to the ocean above—that is, that the sub-seafloor rock is permeable and pore fluid pressure is hydrostatic. This is a conservative approach, as deviation from any of these properties or assumptions would only serve to further decrease the likelihood of faulting at Europa’s ocean floor. We benchmark our modelling approach against known or proposed mechanical behaviour on Earth’s seafloor and within Saturn’s icy moon Enceladus (see Methods).

Our calculated limits for the brittle strength of tectonic regimes under which pre-existing and favourably oriented normal and thrust faults (i.e., pure Andersonian extension to compression, respectively) can slip are shown in Fig. 1. To demonstrate how brittle strength increases with depth, we report values for sub-seafloor depths of 100 m and 1000 m in Table 2; we illustrate the effect of pore fluid pressure on rock strength in Fig. 2. Although absolute values for lithospheric stresses at these depths are of order ~180 MPa (Fig. 1b, Table 2), we also give the tectonic stresses needed to cause faulting as differential stresses, i.e., the difference between the maximum and minimum principal compressive stresses (Table 2). At the rock–water interface itself, the stress required for failure must meet or exceed the unconfined compressive strength of the rock. For normal or thrust faulting to occur, the differential stresses at 100 m below the seafloor must reach 2.1 MPa and 2.9 MPa, respectively, increasing to 2.6 MPa and 3.5 MPa at a depth of 1000 m (Table 2).

Several mechanisms could plausibly act on pre-existing, favourably aligned fractures beneath the seafloor. For example, the 84-h diurnal tide raised by Jupiter likely drives deformation of Europa’s ice shell14 and may contribute to viscous heating within the moon15. However, the maximum tidal stress at the Europan seafloor is currently about 54 kPa (Table 2) (see Methods), only about 3% of the lithospheric strength for extension at 100 m depth (and about the same percentage at a depth of 1000 m). The eccentricity required to drive faulting at a depth of 1000 m beneath the seafloor is 0.441, far higher than Europa’s present eccentricity of 0.009.

It is possible that sub-critical crack growth and fatigue16,17 resulting from repeated tidal cyclic loading over geological time has weakened the seafloor rocks. Under the assumption that cycling reduces short-term, normal fault strength by about 75% (ref. 18), the modern diurnal tidal stress may come within a factor of ten of matching the strength of fatigued rocks at 100 m beneath the seafloor, but even then is twelve times too low to induce failure at a subseafloor depth of 1000 m.

Global contraction from secular interior cooling is accommodated in the brittle silicate portion of a body by the formation of thrust faults19. We find that the Europan silicate interior would need to contract radially by 1 km before the elastic strength of the seafloor is matched20 and thrust faulting can begin (see Methods). This value is greater than that found for the Moon (~several tens of metres21) but less than estimates for Mars (≤3.8 km, ref. 22) or Mercury (≤7 km, ref. 23). The timing of such contraction of Europa’s silicate interior, and when and for how long faults might have been actively accommodating contractional strain, remains to be quantified. Yet the record of global contraction on Mercury, for example, points to substantial radius decrease within the first billion years of the planet’s history24, and a reduction in silicate radius of 1 km over Solar System history gives a strain rate of only 5 × 10–21 s–1. Moreover, the sealing of faults by gouge and, in submarine settings especially, by rapid mineral precipitation will serve to substantially decrease fault permeability in the absence of sustained tectonic activity25. Therefore, even if thrusts developed in the past from secular cooling, it is far from clear whether such faults might be active today or be able to conduct seawater to unaltered rock.

Fluid and heat transport persist in very low-permeability regions of Earth’s seafloor, facilitated by microfracture channels in the rock matrix that form by thermal expansion anisotropy between mineral grains. In the absence of widespread slip on pre-existing fractures, microfracturing may be the primary enabler of aqueous diagenesis in water-rich bodies such as Ceres and Europa26,27. Microfractures develop along grain boundaries because of stress concentrations from flaws such as precipitates, and form when the stress intensity at the tip of the grain exceeds some critical value (see Methods). For a conservatively low cooling rate of 1 K/Gyr, the normal stress on a two-dimensional grain boundary28 can exceed 3 MPa. Assuming that microfracturing develops parallel to the intermediate principal compressive stress, this process serves to add to the local differential stress and so might help facilitate slip—but only at shallow crustal levels. We calculate that increasing the differential stress by 3 MPa may contribute to faulting only to a subseafloor depth of 2.5 km and 3 km for normal and thrust faults, respectively. (These values are for rocks with hydrostatic pore fluid pressure; without it, thermal cracking has basically no effect upon rock strength.)

On Earth, observations of triggered and induced seismicity suggest that many faults have elevated pore pressures, are critically stressed, and are sensitive to kPa-scale changes from hydrogeological loads, tides, and teleseismic waves (e.g., refs. 29,30). Such structures, aided by locally higher-porosity damage zones, represent pathways for fluid transport even in the absence of active slip. This flow is generally driven by large thermal gradients31, compaction of saturated materials, or dehydration of minerals32. Although the silicate interior could still be dehydrating33, Europa’s relatively low gravity has been proposed to substantially reduce the vigour of gravity-driven fluid advection34, and the relatively high overburden suggests a reduction of permeability with depth of about two orders of magnitude relative to smaller bodies such as Enceladus35. The fluid-flow-relevant pressure gradients produced by metamorphic reactions within Europa’s silicate portion remain largely unknown but, for a conservative hydraulic diffusivity of 10–2 m2/s (ref. 36), may take of order 104–106 years to diffuse through the interior.

Should the pore network be closed off from the ocean and pressurised above hydrostatic conditions, the seafloor rocks would be subject to a lower effective pressure and the stresses needed to drive faulting would be decreased. At a depth of 1 km below the ocean floor, the amount of pressurisation over hydrostatic required for normal faulting is 8 MPa. However, it is unclear what mechanism(s) would drive such overpressurisation on Europa in the absence of tectonic activity, major sediment sources, or present-day dehydration reactions at shallow depth—processes that serve to overpressure subseafloor rocks on Earth37.

Apollo seismic measurements on Earth’s Moon detected quakes at depths of 700–1200 km linked to tidal cycles38,39. Despite occurring within an expected plastic deformation regime, some of these deep quakes are consistent with slip on faults showing extremely low apparent values of cohesion and friction40, together with small differential stresses that are not predicted by our fault strength calculations. Yet most deep lunar quakes are M < 3 (ref. 41) and do not appear to be kinematically linked with larger, shallower seismic events, even if some of those more powerful quakes are also tidally induced42. Although shallow moonquakes show recurrence intervals of months to years42, the frequency at which resolvable (thrust) fault strain is accumulated at the lunar surface is of order tens of millions of years43. Faulting is thus an extremely inefficient means of exposing fresh lunar rock to the surface (see Methods). By direct analogy with the Moon, then, small, tidally-driven seismicity deep in Europa’s silicate interior40 is not incompatible with a tectonically quiescent ocean floor.

Fluid infiltration and rock reactions within the seafloor might also facilitate fracture propagation44. Laboratory experiments show that hydration and carbonation reactions increase permeability when rocks are essentially impermeable (<10–18 m2) and reaction rates greatly exceed fluid flow rates45. This finding challenges the prevailing understanding, based on flow-through experiments on peridotite hydration at 150–300 °C that commonly show permeability reductions by pore clogging46,47. All laboratory serpentinisation experiments demonstrating substantial (>50 vol.%) mineral conversions have been conducted at temperatures ≥200 °C (refs. 48,49). Whether such reactions could enhance permeability at Europa’s seafloor temperature (i.e., ~0 °C) remains unknown. If rapid reaction rates are needed to enhance permeability, then feedbacks that facilitate fluid flow would necessarily be short lived.

We do not consider stresses arising from non-synchronous rotation14 given that we focus only on the moon’s silicate interior. Further, we discount impact-induced stresses since it is not clear how often (if at all) an impact or that breaches Europa’s ice shell50 might plausibly retain sufficient energy to have any effect upon the ocean floor.

Brittle–plastic transition depth
With increasing temperature and pressure in rocks, brittle failure gives way to plastic deformation (thus dramatically decreasing permeability). Establishing the depth of the transition zone between fracturing and crystal plasticity within Europa’s silicate interior with brittle and plastic flow strength profiles therefore offers information on whether, for example, mantle flow is able to transmit stresses to the surface and drive failure there (see next Section). To estimate the depth and therefore strength of the brittle–plastic transition (BPT) zone, we consider a strain rate of 10–20 s–1, below the upper bound for intraplate strain rates on Earth (i.e., ~10–17 s–1: ref. 51), and a thermal gradient of 10 K/km (ref. 52) to reflect an elevated heat flow arising from, for instance, substantial tidal dissipation in the silicate mantle. For these calculations, we treat Europa’s bulk silicate interior as altered peridotite (i.e., wet olivine: see Methods).

The intersections of the brittle and plastic strength profiles occur at a sub-seafloor depth of about 56 km (Table 2). At that depth, the brittle lithospheric strength for normal and thrust faults peak at ~51 MPa and ~67 MPa, respectively (Fig. 3). The total lithospheric thicknesses, which we define here as where the ductile strength of the lower lithosphere drops below 10 kPa, is 120 km.

For this same strain rate and thermal gradient, the BPT zone depth for Mercury is ~47 km and for Mars is ~65 km, assuming a dry olivine rheology in both cases52. These values bracket Europa’s silicate interior, and thus the lithospheres of Mercury and Mars can be thought of as its inner Solar System mechanical cognates. Even so, the thermal gradient we use here is likely unrealistically high: a study of melt production in the Europan mantle featured a gradient of 3.3 K/km, corresponding to a “high radionuclide content” model (ref. 53), in which the satellite’s mantle radionuclide abundances correspond to that of LL chondrites54. Should Europa’s mantle be closer in radiogenic abundances to carbonaceous chondrites54, then the interior temperature will be lower, and thus the corresponding BPT zone depth even greater, than the results we report here.

Mantle convective stresses
Stresses in the lithosphere arising from mantle convection, if sufficiently high, are capable of driving slip along existing fractures and even generating new fractures in the brittle lithosphere. If stresses are great enough to exceed the yield strength of the entire lithosphere (as estimated from lithospheric strength profiles such as those we show above), then even plate tectonic-like behaviour could be initiated55,56. We therefore estimate the stresses resulting from mantle convection using scaling relationships developed by ref. 57 (see Methods). For this calculation, we assume that Europa’s silicate mantle convects, but under a stagnant-lid regime. This approach provides the strongest test as to whether modern Earth-like plate tectonics could operate, as this phenomenon is thought to be more difficult to initiate from a stagnant-lid state (e.g., ref. 58).

The mantle’s interior viscosity sets the vigour of convection and resultant stress scale. Europa’s mantle interior viscosity is unknown, and is therefore the largest uncertainty in the scaling law for stress. We explore a range of 1018–1020 Pa s (that used in ref. 53). Increasing mantle viscosity increases the convective stress, but even for a viscosity of 1020 Pa s the highest stress we calculate is only about 150 kPa (Fig. 4), consistent with earlier results53—over two orders of magnitude below the minimum lithospheric strength of 51 MPa for normal faults, and less than even the rock strengths we calculate at 100 m depth. The small size of Europa—its mantle volume13 is ~57% that of the Moon and 48% that of Mars59—mean that mantle convective vigour should be low and able to only weakly apply stresses to the lithosphere. It therefore seems unlikely that mantle convection induces brittle deformation in Europa’s lithosphere at all, much less drives a mobile-lid regime at the seafloor.

Implications of limited seafloor tectonics for Europan habitability
Our findings suggest that neither tidal stresses, nor global contraction, nor mantle convection, nor serpentinisation is able to drive frequent slip along pre-existing, favourably-oriented faults. These findings hold true even within mechanically fatigued rock, except perhaps at the very uppermost portion of Europa’s silicate lithosphere. Since such faulting enables sustained rock–water interactions on Earth25, a lack of tectonic activity implies that any seafloor rocks exposed along fault surfaces will come to attain chemical equilibrium with the overlying ocean. Given that chemotrophic metabolisms exploit chemical disequilibria associated with mixing fluids7,60, a tectonically inert Europan seafloor does not appear conducive to sustained habitability over geological time.

Moreover, although melt may be generated in Europa’s interior53, recent work has found that, because of Europa’s size and low melt-generation ability in the present, the maximum buoyancy stress likely to be generated by pooled melt is only ~1–3 MPa (ref. 61)—much lower than the confining strength at the BPT zone depth we calculate (and at the base of the thermal lithosphere calculated by ref. 61). Even if magma overpressures were capable of regularly overcoming the confining strength at such depths, a dyke’s ability to propagate upwards is limited to only a few kilometres because of low magmatic buoyancy stresses61, and thus the prospect of dyke-induced fractures reaching the seafloor seems remote.

The largest hydrothermal flows on Earth’s seafloor occur far from sites of volcanic or tectonic activity associated with spreading centres62,63. Instead, massive “ridge-flank” circulation systems discharge reacted fluids at temperatures generally <100 °C and recirculate ocean water in and out of the seafloor every ~105–106 yr, enhanced by low-permeability sediments, high-permeability aquifers, and interconnected outcrops64–66. A system of this kind at the Europan seafloor could facilitate fluid flow34 and promote chemical reactions able to support chemosynthetic ecosystems67. Such reactions might even extend beyond ultramafic mineral alteration associated with serpentinisation to include the radiolytic production of H2 (refs. 68,69), sulphate70, and low-molecular-weight carbon compounds71 driven by U-, Th-, and K-decay. Radiolysis may thus provide a basis for an alien seafloor biosphere even in the absence of substantial fracturing, thermal gradients, or large volumes of unaltered rock because of earlier eras of enhanced geological activity71—albeit at even lower levels than explored in this study68–73, and so remains to be quantified for Europa.

Whether by serpentinisation or radiolysis, the potential for chemosynthesis-driven habitability requires consideration of other signatures beyond the classic target of O2 detection74. Indeed, the Europa Clipper spacecraft will measure compositional abundances in any plumes that might be present, to test whether water–rock interactions take place today or did in the past75. Clipper will also provide unprecedented information on the interior of Europa, including the depth to the rock–water interface, the distribution and amount of tidal heating, and perhaps even whether there is topography on the seafloor76,77. Ultimately, however, the true test of our results here will require directly accessing the ocean and, perhaps one day, the ocean floor itself.

For now, we posit that fracture-moderated habitability at and within Europa’s seafloor today is unlikely. Future studies of Europa’s modern habitability should focus on the generation of energy for chemoautotrophic life by mechanisms that do not require ongoing seafloor tectonics in the current epoch.

Methods
Lithospheric strength
With recent interior structure models for Europa13, we calculate how overburden pressure, P, changes as a function of depth, i.e.,

dP/dr = –ρg, (1)

where r is body radius (1560.8 km in total), ρ is local material density, and g is acceleration due to gravity (1.315 m s–2 for the outer surface of Europa’s ice shell). We use values of 925 kg m–3 for ρice, 8000 kg m–3 for ρmetal (i.e., the moon’s metal core), 3429 kg m–3 for ρrock, 2530 kg m–3 for ρcrust, and ρwater of 1025 kg m–3, after refs. 12,13. We include in our interior structure model a 1-km-thick layer of “crust” at the seafloor that, together with adjusting the value of ρrock from ref. 13, serves to maintain the mass of the silicate interior. The presence of this layer does not meaningfully alter our findings and is taken to be lithologically and mechanically identical to the underlying silicate mantle, but helps ensure that our results are internally consistent.

Gravitational acceleration at a given depth is given by

dg/dr = 4πGρ – 2(g/r), (2)

where G is the gravitational constant78.

Our approach assumes that the upper, brittle portion of Europa’s silicate interior features pre-existing fractures that, if oriented in favourably to stress, will slip. Were such planar weaknesses not present, then the stress required to fracture Europa’s silicate portion would be substantially higher than that needed solely for frictional sliding. We calculate the stresses required to enable slip on those pre-existing fractures by the following relations for thrust and normal faults, respectively, using the linearized Mohr–Coulomb failure criterion37:

SH = ((fμ2+1g 0.5 + μ)2 × σ*) + Pp + σc (3)

Sh = (1/(fμ2+1g 0.5 + μ)2) × (σ* – σc) + Pp, (4)

where SH and Sh are the maximum and minimum horizontal stresses, μ is the coefficient of static (or internal) friction, Pp is the pore fluid pressure, σ* is the effective pressure, and σc is the unconfined compressive strength of the rock37.

Rather than calculating a specific porosity–depth profile, we regard pore fluid pressure at a given depth to correspond to the effective pressure, σ*, at that depth, i.e.,

σ* = σ1 – Pp, (5)

where σ1 is the maximum principal compressive stress (i.e., SH for thrust faults and SV for normal faults). As a result, our model intrinsically assumes that pore space remains throughout the brittle rock volume and that these pores are hydraulically connected to the overlying ocean. Without this assumption, the rock strength would dramatically strengthen as permeability decreases with depth.

Because we assume the effective pressure at the seafloor to be equal to pore fluid pressure, lithospheric shear strength at the rock–water boundary is only that of the rock’s unconfined compressive strength, which is related to cohesion, C0, by the relation79

σc = 2C0((fμ2+1g0.5) + μ), (6)

where C0 = 1.2 MPa (after ref. 80). Finally, we employ a value of μ for an altered (0.15, e.g., chrysotile80) lithology, representing rock that, having been mineralogically hydrated by interaction with seawater (perhaps during an earlier phase of heating of the entire interior10), is mechanically weaker and so more likely to undergo faulting.

To validate our lithospheric strength model, we also apply this approach to Enceladus and Earth. For Enceladus, we use an interior structure model based on ref. 13, featuring 50 km ice and 13 km of water. Calculated rock strengths at a sub-seafloor depth of 1 km require 2.9 MPa for thrust faults and 2.1 MPa for normal faults. Earth is modelled with an ocean depth of 3.68 km, altered seafloor rock, and a 20 K/km temperature gradient. Brittle lithosphere strengths 1 km below the seafloor are determined to be 15 MPa and 11 MPa for thrust and normal faults, respectively.

Stress from the diurnal tide
Tidal extensional stresses develop at two nodes at equatorial latitudes81, leading to the prospect of extensional structures developing there—optimal for exposing fresh rock to the ocean. The maximum stress from a tidal bulge is14

σ = 3eEH/r, (7)

where e is the eccentricity of the satellite’s orbit, E is the Young’s modulus, in turn given by

E = 2S(1 + v), (8)

in which S is the shear modulus and v is Poisson’s ratio, and where H is the height of the tidal bulge and r is the radius of the satellite. We use values of 70 GPa for S (ref. 82) and 0.28 for the Poisson’s ratio83, appropriate for peridotite. The height of the tidal bulge is given by

H = h2 r(Mp/Ms)(r/as)3, (9)

where h2 is the solid body tidal Love number, Mp and Ms are the masses of the planet and satellite, respectively, and as is the average orbital radius of the satellite, with a value of 671,100 km (ref. 14). The value of h2 for the interior is approximated to first order as

h2 ≈ 5/19 gρr / S, (10)

where g is the gravitational acceleration at the seafloor, ρ is the bulk density of the solid interior, and rint is the radius of the interior84.

Stresses from global contraction
The stress state arising from global contraction leads to horizontal shortening of the surface, forming thrust faults if that stress matches the corresponding yield strength of the upper, brittle lithosphere19. Although thrusts would not be as efficient in exposing fresh rock to the ocean as joints or normal faults, such structures could still plausibly allow the cycling of seawater through the upper seafloor if critically stressed (cf. ref. 24). The stress from contraction has a value of

σ = 2S(f1+v)/(1–v))(Δr/r), (11)

where Δr is the change in radius of the satellite85. For a 980-km-thick silicate mantle and a 475-km-thick metallic core for Europa13, the required decrease in radius of the solid interior to produce the stresses needed for thrust faults to slip (178 MPa at a depth of 100 m beneath the seafloor) is 1 km (~0.07%). Future thermal evolution models should seek to establish whether such a change in radius is plausible for the Europan interior, over what time frame global contraction might operate, and if and when the rate of radius change and thrust fault formation is permissive of near-surface fluid flow.

Lunar tectonic resurfacing rates
To first order, we estimate the rate at which shallow quakes could tectonically “resurface” the Moon via the exposure of new crustal material along fault normal planes. Most such quakes are Mw < 3 (ref. 41), but for this purpose we presume shallow lunar quakes are regularly Mw 6. This quake magnitude is typically associated with a fault rupture length of ~10 km and slip of ~50 cm. Under the assumption that all shallow moonquakes break the surface, a conservative upper bound then of new rock exposed through faulting is ~10–3 to 10–2 km2/yr. Such an exposure rate, even though substantially overestimating lunar fault slip, would require billions of years to resurface a major portion of the Moon given current seismicity, and is qualitatively consistent with today’s largely tectonically quiet lunar surface.

Brittle–plastic transition depth
To establish the depth of the transition zone between fracturing and crystal plasticity within Europa’s silicate interior, we calculate ductile stress by the relation

ε̇ = Aσn COHr exp((–Ev + PV) / RT), (12)

where ε̇ is strain rate, A is a constant, σ is deviatoric stress, n is the stress exponent, COH is water fugacity, r the water fugacity exponent, E is activation energy, V is activation volume, P is pressure (which we take to be lithostatic), R is the universal gas constant, and T is temperature86. This method of calculating the BPT zone depth is widely used in Earth and planetary science (e.g., refs. 52,87,88).

We use values appropriate to wet olivine deforming by dislocation creep, where A = 89 MPa–3.5 s–1, n = 3.5, COH = 100, r = 1.2, E = 480 kJ/mol, and V = 1.1 × 10–5 m3/mol (ref. 89). Temperature gradient and strain rate are 10 K/km (after ref. 52) and 10–20 s–1, respectively. (These values are listed in Table 1).

Stresses from mantle convection
Reference 57 developed scaling laws for lithospheric stress resulting from stagnant-lid convection. Two scaling laws were presented in that study, one for a regime with large lid slopes and another for a flat lid. We use the latter model, which results from time-dependent convection as expected for planetary bodies. The scaling law for stress, τ, is

τ = c1 ραgΔTrh δrh, (13)

where c1 ≈ 0.4 is an empirical constant, ρ is the density of the mantle, α is the thermal expansion coefficient, g is gravity, ΔTrh is the temperature difference across the rheological sublayer (the layer at the base of the stagnant lid that participates in convection), and δrh is the thickness of that rheological sublayer. Values for density, the thermal expansion coefficient, and gravity are all taken from those provided in ref. 53. The rheological temperature difference ΔTrh and rheological sublayer thickness δrh are given by90

ΔTrh = (Ti – Ts) / θ, (14)

where Ti is the interior temperature of the mantle, Ts is the surface temperature at the top of the silicate layer, and θ is the Frank–Kamenetskii parameter, which defines the viscosity structure across the layer91 and is defined as

θ = (Ev (Ti – Ts)) / RTi2, (15)

where, in turn, R is again the universal gas constant, and Ev the activation energy for viscosity. The rheological sublayer thickness is found to follow the scaling relationship90

δrh = c2 d (Rai / θ)–1/3, (16)

where c2 ≈ 2 is an empirical constant, d is thickness of the mantle, and Rai is the internal Rayleigh number, itself given by

Rai = (ραg(Ti – Ts)d3) / (κμi), (17)

with α as the thermal expansion coefficient again, κ is thermal diffusivity, and μi is the viscosity of the mantle at the interior temperature Ti. Combining the above equations gives a final scaling relationship for stress,

τ = c1 c2 (κμi / d) (Rai / θ)2/3, (18)

which we use to calculate convective stress for Europa. The Frank–Kamenetskii parameter is estimated as θ = 18.7 assuming an activation energy of 300 kJ/mol, a mantle temperature of Ti = 1600 K, and a (seafloor) surface temperature of Ts = 273 K (ref. 53). Other assumed parameters are ρ = 3500 kg/m3, a = 3 × 10–5 1/K, g = 1.3 m/s2, d = 800 km, and κ = 10–6 m2/s. Mantle interior viscosity, μi, is varied over the range 1018–1020 Pa s as explained in the main text.

Seafloor volcanism
To evaluate the plausibility of volcanism at Europa’s seafloor, we constructed for a separate, related study61 a model of Europa’s mantle in the geodynamic simulation code StagYY92, within which we designed and implemented a new melt-extraction routine. The mantle model is a 2D spherical annulus 821 km in thickness with temperature ranging from 273 to 1600 K. Heat is generated within the model by radioactive decay93 and tidal dissipation94. We benchmark internal heating to conditions in the models of ref. 53 to ensure consistency with previous results. Melting occurs along a Herzberg–Bohler solidus curve95 and is then transported upwards through the viscous portion of the mantle via Darcy pore-space flow along grain boundaries. When melt reaches the base of the thermal lithosphere (which we define as 1400 K), it is then evaluated by the new dike–melt extraction treatment based on ref. 96. To erupt, the magma must then satisfy two criteria: 1) magma must exert sufficient buoyancy stress at the base of the lithosphere to match its yield strength and so initiate a dyke; and 2) that dyke must propagate upward through the lithosphere faster than conductive cooling solidifies and halts the intrusion. Full details of this model are described in ref. 61.

Thermal expansion anisotropy
The normal stress, σyy, on a two-dimensional grain boundary can be derived28 via the following relation:

σyy(x) = (EΔα(T'–T) / 2π(1–v2)) * (4L2 / (4L2 + x2) – 4L2 / (4L2 + (2L–x)2) + ln(4L2 + (2L–x)2 / 4L2 + x2) * (2L–x) / x), (19)

where E is Young’s modulus, α is flaw size, T' is the viscous–elastic transition temperature, L is half grain size, and x is distance44. As microfractures accumulate along grain boundaries, cracking occurs when the stress intensity at the tip of the grain exceeds a critical value (K > KIC) that is in the range 0.2–3.0 MPa m1/2. For instance, the KIC value for olivine is 0.6 ± 0.3 MPa m1/2 (ref. 97). The critical stress intensity for a flaw of size α is

KIC = 2 * σyy(x) * sqrt(α) / sqrt(π) – Pp * sqrt(πα), (20)

Model comparison with Earth and Enceladus
As a check of our modelling approach for Europa, we consider two worlds for which the seafloor structure and mechanical properties are either well known (Earth) or have been proposed on the basis of comprehensive spacecraft observations (Saturn’s diminutive icy moon Enceladus). We list the parameters and associated values for each body in Tables 3 and 4. The question of whether fault slip can occur in Earth’s seafloors is a complex one, since laboratory tests suggest that oceanic crust is mechanically stronger than the driving stresses from mantle flow and thus faulting (and, in particular, the initiation of subduction) ought not to occur (e.g., ref. 51). Our calculated results for Earth are given in Table 5. Of note, the absolute differential lithospheric strength we calculate at 100 m depth is 3 MPa in a normal fault regime, and 4 MPa in a thrust fault regime. These values are qualitatively similar to numerical results for subduction initiation of ~3–7 MPa (refs. 57,98). At a depth of 1000 m below the seafloor, these values rise by around a factor of four to 11 MPa and 15 MPa, respectively. Of note, these calculations are for a friction coefficient, μ, of 0.15, corresponding to a basaltic seafloor altered to chrysotile. We revisit the effect of a substantially lower value for μ below.

For Enceladus, given its low mass and surface gravity, its seafloor strength is much lower than that of Europa. The absolute differential lithospheric strength in normal and thrust fault regimes at 100 m depth is 2 MPa and 3 MPa, respectively; the corresponding values at a depth of 1,000 m are functionally identical (Table 6). Even so, Enceladus is unlikely to experience seafloor faulting at all in the present since the maximum tidal stress is only 2 kPa. Although some elastic strain may be expected from Enceladus’ tidal deformation and from compressive stresses arising from secular cooling, no modelled stress is high enough to expose by faulting fresh rock surfaces for reaction with seawater.

In calculating the brittle–plastic transition depth for Earth, we use the values given in Table 3. With a strain rate of 10–15 s–1 (ref. 86) for a dry olivine dislocation rheology, the BPT zone depth is 30 km below the seafloor surface, a value qualitatively consistent with the view that most intraplate earthquakes in oceanic lithosphere occur at temperatures ≲ 873 K, i.e., the 600 °C isotherm99. Our model for Enceladus employs the same flow law and component values as for Europa (Table 4), for which we find the Enceladan BPT zone to be at a depth of 65 km beneath the seafloor. This finding, as well as that of relatively low fault strengths, is consistent with a substantial portion (~72%) of Enceladus’ silicate interior (i.e., that above the BPT zone) being mechanically unconsolidated, porous, permeable, and hydrated100.

One solution for Earth to the apparent paradox of a lithosphere substantially stronger than mantle convective stresses is to use much (much) lower friction coefficients57,87,101, possibly from fault weakening by seawater. For our calculated convective stress for Earth (Table 5), these seafloor rocks require a value of μ of 3.5 × 10–5 to undergo failure at a depth of 100 m below the ocean. This is functionally, within our models, the mechanical threshold needed for plate tectonics on Earth. Importantly, even with this friction coefficient applied to Europa, we find that the lithosphere is about 1.3 times stronger than the mantle convective stress (for any of the viscosities we consider). This result suggests that, even with a dramatically lower coefficient of friction, faults at Europa’s seafloor are probably too strong to undergo deformation from mantle convection sufficient to initiate subduction, such that the silicate lithosphere is in a stagnant lid regime.

For the Europan seafloor to be in a mobile-lid regime today, our modelling indicates that the rocks there must proportionally weaker than seafloor rocks on Earth. Additional interior heating (e.g., from tidal dissipation) will not account for this weakening, since such heating serves to lower mantle viscosity (Fig. 4). It is not clear to us what geological process(es) might have weakened Europa’s seafloor enough to allow for mantle-driven faulting there at the present.

Peer review rounds
[{"round": 1, "reviewer_comments": {"reviewer_1": "Review of ms # NCOMMS-25-11067-T “Little to no active faulting likely at Europa’s modern day seafloor” by P. K. Byrne et al. This manuscript has been written by a high profile group of authors with backgrounds in planetary science and astrobiology. There is broad consensus that life in Europa’s undisputed subsurface ocean would be chemoautotrophic. Nutrients would have to be derived from Europa’s rocky component, most likely from a rocky sphere underlying the ocean. Ongoing tectonic and/or volcanic activity would replenish fresh rock and thus nutrients to keep the biosphere alive. In the absence of tectonic and/or volcanic activity, the chemoautotrophic biosphere would have to rely on the supply from hydrothermal convection in the first few hundred meters below the putative seafloor. This is stated and briefly discussed by the authors in the concluding paragraph, although one wonders where the radiation to support the suggested radiolysis will come from. This points to a general problem with the paper: A complicated multifaceted topic is squeezed into an eight-page Letter to Nature, unfortunately sacrificing much-needed depth and detail of treatment. Even the supplementary methods section is very brief and only touches on issues that would benefit from a more careful treatment. While the conclusions are not unreasonable they leave many open questions some of which we will address below: The authors begin by discussing the brittle strength of Europa’s elastic lithosphere which is calculated in the methods section. The calculation there uses parameter values which are neither discussed nor are their uncertainties discussed and considered. The tidal stresses and compressional stresses from contraction are then compared with the strength and found to be insufficient for faulting. This is reasonable given the present orbital eccentricity and the assumption of little present tidal dissipation. It would probably not work if Europa were in, or had geologically recently passed through, one of the phases of strong tidal dissipation described in the literature (e.g., the Husmann et al papers cited in the paper). The required 1 km radius change could have been caused by a cooling by 200 ( 100) K according to my own estimate. Not impossible given the possible peaks in tidal dissipation in Europa’s history. The paper goes on to discuss seismic deformation using the Moon as a reference. The Moon is similar in radius but (probably) different in other parameters such as water content, mantle thickness, level of tidal stress (by a factor of about 6) and the estimation of seismic activity would require a more thorough treatment such as has been published for the Moon and Mars in the literature. Next, the thickness of the elastic lithosphere is discussed and whether stresses from mantle convection could rupture it. The paragraph begins by discussing the depth to the brittle-ductile transition. The parameters chosen are for dry olivine (not peridotite as is stated in the main body) without much effort to justify this choice. One would have thought that if olivine, then wet olivine would be a better choice. Moreover, is dislocation creep the most appropriate choice or should diffusion creep be considered given the strain rates expected? In any case, what are the uncertainties in the parameter choices and how would these effect the conclusions? The convective stress is calculated using a Frank-Kamenetsky rheology which is known to give too weak a temperature dependence, let that there is no dependence on stress or strain rate that is assumed for characterizing the lithosphere. In any case, the reasoning seems questionable when compared to Earth. It is one of the mysteries of geophysics why the Earth undergoes plate tectonics when the strength of its lithosphere generally outweighs convective stresses. One answer lies in regional weakness. As for volcanic activity, there are claims in the literature that this would be possible which are rejected based on another Nature letter apparently accepted for publication but not provided. In any case, it seems that simple global considerations do question tectonic activity for Europa but I would not bet on it given the evidence provided. More specific comments: Line 50 …with remotely sensed geophysical data we estimate the tectonic strength of the moon’s seafloor… (I cannot quite see which “remotely sensed geophysical data” are used here?) Line 403 ff and why are Enceladus and Earth referred to here? Line 504 ff: The paper cited here is not published and not available unfortunately.", "reviewer_2": "The manuscript “Little to No Active Faulting Likely at Europa’s Modern-Day Seafloor” convincingly argues that fault slip, if possible, does not extend deep into the interior of Europa’s silicate core. This insight is important because it contradicts the assertion often made in the literature that this kind of activity might sustain hydrothermal activity in this satellite of Jupiter. The argument is supported by a detailed analysis of the conditions at which rocks might fail in Europa’s silicate core and a comparison with reasonable stress sources. I believe this contribution is highly significant and will lead to numerous discussions and inspire rethinking throughout the scientific community involved in Europa research. Despite a generally positive opinion of this paper and great confidence in its results, I have two major concerns about its presentation. 1) The objectives of the manuscript are vague at times. The title and most of the work focus on faulting, which is appropriate. However, the motivation and the closing statements of both the abstract and the main text discuss instead habitability, which is separated from faulting by several insufficiently described steps: How the faulting (or its absence) enables hydrothermal circulation, how this circulation facilitates reactions, and how these reactions can support life. It may be appropriate to include a discussion of “Implications for Europan Habitability” (lines 179-207), but the paper structure does not make it clear that this section is indeed a discussion. It appears at the same level as the step in the tectonic analyses. It also consists of a sort of 180° turn: the text up to now makes a convincing case that tectonic activity, which, as explained in the first paragraph, supports subsurface environments on Earth, is impossible, and now we are discussing how these environments may be supported anyway. It’s almost as if you finished by telling us that all these analyses don’t matter anyway. Well, maybe they don’t, but as the title reminds us, the paper is about faulting, not habitability. At least finish the paper with a summary of what we learned about faulting instead of poorly linked comments about Europa Clipper and hydrothermal signatures. By the way, it would be good to have a quantitative estimate of how much fault activity you find necessary to maintain hydrothermal circulation and biological activity. 2) The analyses presented here are all based on specific choices of parameter values. This is disappointing as 1) none of these parameters is well constrained, and 2) the equations in “Methods” show more general relations. This almost reminds me of students scared of using general algebraic relations and needing to replace every variable with a number. I know the authors are more comfortable with algebra than I describe here. It’s clear that the conditions for fault slip are far from being met on Europa, but the more interesting question is, what would it take for fault slip to occur? Would a 40 km ice shell be OK? Would you need twice the eccentricity? These questions can be answered and would make this work more generally useful. 3) In relation to this last point, the methods describe sets of parameters for applying the model to Enceladus and the Earth. However, I couldn’t find the results of these analyses in the main text (the word “Enceladus” appears in the methods). I suggest that these applications be included in the text and discussed. Is fault slip possible on Enceladus? More importantly, is fault slip possible on the Earth’s seafloor? If your model answers the latter question in the negative, something is clearly missing. This kind of sanity check is important to gain confidence in your results. 4) Pore fluid pressure in the calculations is hydrostatic (e.g., Line 70). However, Note that pore fluid pressure on Earth can exceed these hydrostatic values. Overpressure is common in many tectonic environments and can reach almost lithostatic values (DOI 10.1029/96JB02485, 10.1130/G31649.1, 10.1029/2022GL098862, 10.1029/2023GL105127 and many others). If possible on Europa, overpressure could solve all the issues here and allow fault slip. You should argue why you do not consider higher pore fluid pressures. In addition to these major points, I have a few minor comments. 5) Hydration and carbonization reactions can increase porosity and be self-sustaining (DOIs: https://doi.org/10.1016/j.epsl.2017.06.045, https://doi.org/10.1073/pnas.2110776118). This could avoid the need to produce new surfaces by continuing fault slip. I doubt this process can continue for millions of years (the resulting accelerated dissolution rate doesn’t help), but it may be worth mentioning. 6) Line 61: Are you modeling the possibility of fault slip (as I thought from the earlier paragraphs) of “brittle failure” in general? 7) The rock density used in the calculation looks very high. However, the seafloor density in your Ref.10 is listed as 2530 ± 120 kg/m3, only ~3/4 of the value you used. 8) Line 109: The grain boundary stress you mention is a normal stress, not a pressure. It would add to the differential stress instead of the overburden stress. 9) Please expand on the idea that substantial water circulation would be unlikely today (lines 115-117). I imagine that seawater circulation depends on lateral water pressure gradients (head). Many sources of pressurization depend on local sources (e.g., dehydration) and so may generate large pressure gradients regardless of the low gravity and background pressure. 10) Line 136: Actually, there is evidence for void nucleation in the plastic regime (DOI 10.1038/nature08051) 11) Line 139: I didn’t see in Ref. 38 a strong justification for the 10^-18 to 10^-16 strain rate value. If the silicate core is mostly inactive, as you argue here, a lower strain is likely appropriate, e.g., 10^-21 or less, below the detection limit on Earth’s plate interiors. The stagnant lid mantle convection assumption would imply a 0 strain rate at the surface. Either way, I don’t find 10^-18 s^-1 to be justified. Lower values would lead to a shallower BDT. 12) Line 143: Considering model assumptions, I doubt the precision of “59 km” is warranted. I’d report 60 km. To be honest, I don’t see why these BDT determinations are important for the paper’s focus on fault at the seafloor. 13) I am confused with the shading in Figure 1a. I guess one of the horizontal boundaries is the seafloor, but what are these two triangles fanning out of the seafloor? Why is the ice shell thickness at 30 km (Table 1) not indicated in a similar manner? What is the thick line near 0? I also think the labels a) and b) are too small. 14) Line 345: specify “hydrostatic pore fluid pressure.” 15) More mysterious shading. The interior of the strength envelope may be brighter to highlight the envelope (guess here), but this kind of artistic expression interacts with the more rigorous scientific of the figure (seafloor), and I am left confused. 16) Table 1: Please justify the “model parameters”; the BDT cannot be exactly at 59 km in both extension and compression. In fact, it seems to be 60 km in a normal faulting regime, so the precision of the 59 km value is not justified. Please include the stress estimate from convection and thermal expansion anisotropy for comparison with the other stress estimates in the table. Laurent Montesi"}, "author_response": "Response to Reviewer Comments on NCOMMS-25-11067-T | Little to No Active Faulting Likely at Europa’s Modern-Day Seafloor Text in black: Reviewer comment/reference Text in green: Reviewer suggestion/question Text in blue: Author response Line numbers and references in red correspond to the clean, revised NCOMMS-25-11067A manuscript. Note that we have renumbered the respective Reviewer’s comments in this document. Responses to Individual Reviewer Comments Reviewer #1: This manuscript has been written by a high profile group of authors with backgrounds in planetary science and astrobiology. There is broad consensus that life in Europa’s undisputed subsurface ocean would be chemoautotrophic. Nutrients would have to be derived from Europa’s rocky component, most likely from a rocky sphere underlying the ocean. Ongoing tectonic and/or volcanic activity would replenish fresh rock and thus nutrients to keep the biosphere alive. In the absence of tectonic and/or volcanic activity, the chemoautotrophic biosphere would have to rely on the supply from hydrothermal convection in the first few hundred meters below the putative seafloor. 1) This is stated and briefly discussed by the authors in the concluding paragraph, although one wonders where the radiation to support the suggested radiolysis will come from. We appreciate the Reviewer’s encouragement to expand on the basis and literature on radiolysis- driven habitability, and we now elaborate somewhat on this note at the end of the main text (Lines 241–247). In short, even in the absence of tectonic and/or volcanic activity, the ubiquitous presence of U, Th, and K in all likely lithologies present provides an additional mechanism to sustain habitability. Radioactive decay of these elements drives radiolytic chemistry shown in Earth analogs, experiments, and modelling to produce H (Lin et al., 2005; Tarnas et al., 2018), SO (Li et al., 2016; 2 4 Altair et al, 2018), and other low-molecular-weight organic compounds such as acetate (Sherwood Lollar et al., 2021). All of these studies are now referenced in the main text (citations 68 and 70–73). To sum, in the absence of more active hydrothermal circulation, radiolysis may sustain habitability— although at levels lower than we explore here. The true capacity of Europa to sustain a habitable environment via radiolysis is beyond the scope of the present study, although our finding that tectonic activity cannot be looked to do so should motivate future work to focus more on radiolysis. 2) This points to a general problem with the paper: A complicated multifaceted topic is squeezed into an eight-page Letter to Nature, unfortunately sacrificing much-needed depth and detail of treatment. Even the supplementary methods section is very brief and only touches on issues that would benefit from a more careful treatment. We appreciate the Reviewer’s assessment of the paper, and recognise that the manuscript could be improved to better support its findings. Through numerous edits to the text (which we enumerate in our later responses in this letter) we have worked to streamline the paper to focus on its key message—that Europa’s seafloor today is stronger than any likely stress mechanism capable of driving faulting at the rock–water interface, even if that seafloor is mechanically weak. Importantly, we have considerably expanded the Methods section to more fully justify our assumptions and model parameters. (Note that, although we have added text to the main manuscript, and notwithstanding Nature Communication’s 5,000-word limit, we have elected to keep the main text and its message lean—the manuscript itself is being only about 20% longer than before.) While the conclusions are not unreasonable they leave many open questions some of which we will address below: 3) The authors begin by discussing the brittle strength of Europa’s elastic lithosphere which is calculated in the methods section. The calculation there uses parameter values which are neither discussed nor are their uncertainties discussed and considered. As we review in detail in our responses to Comments 8 and 14, we intentionally choose fixed values that collectively render the Europan seafloor mechanically weak, to investigate what conditions are required at a minimum for faulting to take place in the present. By definition, there are no uncertainties associated with these selected values; the systematic uncertainties in the physical condition of the moon’s seafloor are discussed throughout the text (e.g., Lines 58–62, 70–74, 96–97, 178–181, and 204–205), and how our choice of parameters is predicated upon the seafloor being stronger if we are wrong. Again, we discuss and justify this philosophy more fully in response to Comments 8 and 14, below. 4) The tidal stresses and compressional stresses from contraction are then compared with the strength and found to be insufficient for faulting. This is reasonable given the present orbital eccentricity and the assumption of little present tidal dissipation. It would probably not work if Europa were in, or had geologically recently passed through, one of the phases of strong tidal dissipation described in the literature (e.g., the Husmann et al papers cited in the paper) The sole focus of this study is on the geological and tectonic state of Europa’s seafloor today. As the Reviewer notes, (an) earlier era(s) of enhanced geological activity driven by greater tidal dissipation than today are plausible (e.g., Lines 64–65). Indeed, we assume such (a) phase(s) took place for to have hydrated the silicate portion of the moon. To emphasise our focus on modern Europa, however, we have made a minor change to the paper’s title (from “…Europa’s Modern-Day Seafloor” to “…Europa’s Seafloor Today”). 5) The required 1 km radius change could have been caused by a cooling by 200 ( 100) K according to my own estimate. Not impossible given the possible peaks in tidal dissipation in Europa’s history. We do not in this first study perform detailed numerical modelling of Europa’s thermal evolution, which would necessitate considerably more assumptions than we already make. Nonetheless, the Reviewer is correct: a reduction in silicate radius of 1 km over Solar System history is entirely plausible. The question, then, isn’t whether Europa’s rocky mantle has contracted—it’s whether, in the process of contraction, the resulting thrust faults remain able to conduct seawater to sufficient depths where unreacted rock still remains (assuming such rock is still present at all). By analogy with Earth, such thrusts likely generated fault gouge materials (e.g., clays) that have very low permeability and thus (highly) limited fluid flow, a point we now make on Lines 110–112. Moreover, should Europa’s silicate interior have contracted by 1 km, that radius decrease corresponds to a radial shortening strain of 0.07%. Over, for the sake of argument, 4 billion years, that strain rate is 0.25 m/Myr. Whether as a few major, deeply penetrating structures or as many distributed but shallower faults, the circulation of seawater through the silicate interior at rates able to sustain rock–water interactions over geological time but under such a low strain rate appears to us unlikely. 6) The paper goes on to discuss seismic deformation using the Moon as a reference. The Moon is similar in radius but (probably) different in other parameters such as water content, mantle thickness, level of tidal stress (by a factor of about 6) and the estimation of seismic activity would require a more thorough treatment such as has been published for the Moon and Mars in the literature. The point of our including a discussion about the Moon’s seismicity is to establish the important observation that, despite measured, deep-seated lunar seismic activity there is no commensurate active tectonic deformation at the Moon’s surface. This point, in turn, means that even if there were also deep-seated seismicity within Europa’s silicate mantle there may not be, by analogy, much or any associated tectonism at the seafloor. We include this discussion in the manuscript to address the view that “well, faulting happens deep in the Moon somehow, so surely it must also operate within Europa”. That may be true, but deep seismicity clearly does not necessarily translate to tectonic activity at the surface (Lines 156–157). The goal of this manuscript is not to address levels of seismic activity within Europa, which we agree would necessitate a fuller treatment. Indeed, such work already exists (Pou et al., 2024, which is our citation 40). But estimating seismic activity within Europa is outside the scope of our present study. For the purposes of this study—the prospect for fracturing taking place at the Europan seafloor today—we offer that the discussion presently in the paper is sufficient for the purposes of this analogy. Accordingly, we have made no change to the text in response to this comment. 7) Next, the thickness of the elastic lithosphere is discussed and whether stresses from mantle convection could rupture it. The paragraph begins by discussing the depth to the brittle-ductile transition. The parameters chosen are for dry olivine (not peridotite as is stated in the main body) without much effort to justify this choice. One would have thought that if olivine, then wet olivine would be a better choice. The Reviewer makes a fair point, and so we have recalculated our brittle–plastic transition zone depth analysis with flow laws for wet olivine (Lines 181–183). This change results in a slightly weaker plastic domain, and we have updated the text and our results table (now Table 2) accordingly. Importantly, the change in switching from dry to wet olivine does not meaningfully alter our conclusions (but, again, presumes that the mantle is already saturated to a depth where its strength drops to zero, which in turn implies a weaker interior that is thus more readily deformable than may actually be the case in the first place). 8) Moreover, is dislocation creep the most appropriate choice or should diffusion creep be considered given the strain rates expected? In any case, what are the uncertainties in the parameter choices and how would these effect the conclusions? The convective stress is calculated using a Frank-Kamenetsky rheology which is known to give too weak a temperature dependence, let that there is no dependence on stress or strain rate that is assumed for characterizing the lithosphere. In any case, the reasoning seems questionable when compared to Earth. It is one of the mysteries of geophysics why the Earth undergoes plate tectonics when the strength of its lithosphere generally outweighs convective stresses. One answer lies in regional weakness. For the depths in the Europan silicate mantle lithosphere on which we focus for the brittle–plastic transition zone calculation (within the uppermost ~200 km, and thus relatively high stress but low temperature), dislocation creep is the most appropriate mechanism. A simple comparison of both mechanisms at this temperature and pressure range (prepared in response to this review, and considering a grain size of 1 μm) indicates that dislocation creep is far more relevant than diffusion creep. (Of note, and relevant to later in this reply, our mantle scaling analysis indicates that the sub- lithospheric mantle should be convecting in a low stress state, which means diffusion creep is probably the dominant mechanism there. This is the deformation mechanism that is applied to the lithosphere, the strength of which is described by higher-stress deformation processes such as dislocation creep. It is this relationship that supports the low likelihood of mantle stresses driving brittle failure of the seafloor. See below for more.) In terms of our model parameters, we intentionally select specific values that will mean the seafloor is mechanically weak. The goal of this study is not to explore a broad parameter space, but to establish the minimum conditions under which fault slip is possible at the Europan seafloor. We have made edits to the text on Lines 58–62 to clarify this rationale, which in turn justifies the parameter values we use. Should those values be incorrect, by design they will only serve to make the seafloor stronger. Indeed, we note throughout the paper where this may be the case, e.g., regarding the brittle strength of the seafloor (Lines 65–66), the consideration of fatigue weakening (Lines 96–97), and the use of an arguably unrealistically high thermal gradient and low strain rate for calculating strength in the plastic domain (Lines 179–180 and 191–196). We do not use a Frank–Kamenetskii rheology in a numerical model for convective stress, where it does give a weaker temperature dependence. Instead, we are using scaling laws that employ this parameter as a means to describe viscosity variation with temperature at the base of the stagnant lid (i.e., as a means to define the viscosity structure across the silicate layer, as we describe in the Methods). There, temperatures are warm and the difference between a linear exponential and standard Arrhenius rheology is small. As for why Earth has plate tectonics—not the focus of this study, but of course highly relevant—the Reviewer is absolutely correct that the lithosphere appears too strong to be deformed by mantle convective stresses, and yet plate tectonics takes place. The answer, as the Reviewer notes, may lie in dynamic weakening of the lithosphere (at least once) by mechanisms not fully understood. We elaborate further on this point in our response to Comment 15 and in new text in the Methods (Lines 650–664), but it’s worth noting here that we have precedent for a planetary body with a weakly convecting mantle to not drive tectonics at its surface: the thick lithospheres of the Moon, Mercury, and Mars show very little deformation in the present, despite at least one (Mars) having a convecting mantle. Of course, the ocean atop Europa’s seafloor may serve as a means to weaken the rock in a way not applicable to the inner Solar System worlds, but here again we already consider hydrated, mechanically weaker rock in our strength calculations. 9) As for volcanic activity, there are claims in the literature that this would be possible which are rejected based on another Nature letter apparently accepted for publication but not provided. In any case, it seems that simple global considerations do question tectonic activity for Europa but I would not bet on it given the evidence provided. In the time since this manuscript was submitted, the related paper (now our citation 61) on seafloor volcanism has been published. We have updated our manuscript’s bibliography accordingly. Regarding our proposed finding that tectonic activity for the silicate portion of Europa, it is our hope that with these revisions and responses the Reviewer is satisfied that we have better made our case for limited to no tectonic activity on the Europan seafloor today. More specific comments: 10) Line 50 …with remotely sensed geophysical data we estimate the tectonic strength of the moon’s seafloor… (I cannot quite see which “remotely sensed geophysical data” are used here?) Arguably, measurements used to parameterise interior structure models (e.g., polar moments of inertia) qualify as remotely sensed geophysical data, but to improve the clarity of the manuscript we have changed this wording to “spacecraft measurements” (Line 52). 11) Line 403 ff and why are Enceladus and Earth referred to here? As the opening sentence to this paragraph read, “To validate our lithospheric strength model, we also apply this approach to Enceladus and Earth”. However, we have expanded this section considerably in the revised manuscript, reviewing not only what values our models predict for these two worlds but discussing how our results compare with known or proposed behaviour within Earth and Enceladus. This new material in the Methods section is on Lines 611–664, and we now explicitly refer to it in the text on Lines 76–77. 12) Line 504 ff: The paper cited here is not published and not available unfortunately. As we note in reply to Comment 9, this paper is now publicly available and is cited in the revised version of our manuscript. Reviewer #2 (Laurent Montesi): The manuscript “Little to No Active Faulting Likely at Europa’s Modern-Day Seafloor” convincingly argues that fault slip, if possible, does not extend deep into the interior of Europa’s silicate core. This insight is important because it contradicts the assertion often made in the literature that this kind of activity might sustain hydrothermal activity in this satellite of Jupiter. The argument is supported by a detailed analysis of the conditions at which rocks might fail in Europa’s silicate core and a comparison with reasonable stress sources. I believe this contribution is highly significant and will lead to numerous discussions and inspire rethinking throughout the scientific community involved in Europa research. We thank the Reviewer for this positive assessment of the paper’s significance. Despite a generally positive opinion of this paper and great confidence in its results, I have two major concerns about its presentation. 13) The objectives of the manuscript are vague at times. The title and most of the work focus on faulting, which is appropriate. However, the motivation and the closing statements of both the abstract and the main text discuss instead habitability, which is separated from faulting by several insufficiently described steps: How the faulting (or its absence) enables hydrothermal circulation, how this circulation facilitates reactions, and how these reactions can support life. It may be appropriate to include a discussion of “Implications for Europan Habitability” (lines 179-207), but the paper structure does not make it clear that this section is indeed a discussion. It appears at the same level as the step in the tectonic analyses. It also consists of a sort of 180° turn: the text up to now makes a convincing case that tectonic activity, which, as explained in the first paragraph, supports subsurface environments on Earth, is impossible, and now we are discussing how these environments may be supported anyway. It’s almost as if you finished by telling us that all these analyses don’t matter anyway. Well, maybe they don’t, but as the title reminds us, the paper is about faulting, not habitability. At least finish the paper with a summary of what we learned about faulting instead of poorly linked comments about Europa Clipper and hydrothermal signatures. By the way, it would be good to have a quantitative estimate of how much fault activity you find necessary to maintain hydrothermal circulation and biological activity. In response to this feedback, we have made numerous changes to the main text to better clarify the core message of this paper—that Europa, today, has little prospect of tectonic activity on its seafloor, which means that any concept of habitability there cannot rely on active fracturing as a means to sustain chemoautotrophic life via rock–water reactions. These edits are described in this response letter (and other, minor changes tracked throughout the revised manuscript). To a major point raised by the Reviewer, rather than moving away from the habitability angle we have instead emphasised it! Europa is best known in the planetary science community and, arguably, the public as a prospective abode for extant life. To that end, we have added text regarding the moon’s suspected habitability (Line 51), renamed the final section of the paper “Implications of Limited Seafloor Tectonics for Europan Habitability” (to better link the focus of the paper with its broader impact on Europa science), and added a brief summary of the tectonic analyses to that section (Lines 220–223). Regarding the part of the final discussion, through deletions and additions to the text we have worked to better distinguish the faulting-related analysis we present with the work yet to be done to understand how non-fracture-related fluid circulation and hydrothermal alteration or radiolysis might persist through geological time in Europa. (Note especially the new text on Lines 246–247, and the edits to the closing sentences on Lines 251–254). The key take-away of this work is that fault- moderated habitability at Europa’s seafloor is likely not possible today. But work investigating Earth’s seafloor tells us that life may endure in the absence of faulting, at least in some locales for some amount of time. There is very much more work yet to be done to broaden these studies to encompass Europa and other icy moons, which we hope this paper will stimulate. On this note, however, and to the last point the Reviewer makes here: we have worked to show what conditions are required to enable faulting (e.g., in terms of eccentricity, or fluid overpressure); we more fully discuss these points in our responses to Comments 14 and 16. But efforts to develop “quantitative estimate[s] of how much fault activity… necessary to maintain hydrothermal circulation and biological activity” are substantially beyond the present study. They should be done! But they require foundational studies such as this one to serve as a basis for exploring the enormous parameter space for past and present habitability within icy satellites. (In following the formatting for Nature Communications papers, we have not added section numbers or subsections; we take the view that the final Section is as important as those preceding it, although we welcome editorial guidance on this matter. We also retain a brief mention of Europa Clipper and hydrothermal systems; it’s our view that such mentions are warranted and contribute to the timeliness of this study.) 14) The analyses presented here are all based on specific choices of parameter values. This is disappointing as 1) none of these parameters is well constrained, and 2) the equations in “Methods” show more general relations. This almost reminds me of students scared of using general algebraic relations and needing to replace every variable with a number. I know the authors are more comfortable with algebra than I describe here. It’s clear that the conditions for fault slip are far from being met on Europa, but the more interesting question is, what would it take for fault slip to occur? Would a 40 km ice shell be OK? Would you need twice the eccentricity? These questions can be answered and would make this work more generally useful. As we note in our response to Comment 8, the analyses underpinning this study intentionally use the weakest values we can reasonably defend, because we want fault slip to occur (cf. the minor changes to the text on Lines 58–60 and 71–74). The values we consider are, admittedly, not well constrained; how could they be with present technology and limited observational data? But the science question as to the tectonic state of Europa’s seafloor is important, and so we start with a first-order assessment of this question. To wit, we employ specific numbers, not ranges, because the rationale behind our approach is that if we have erred in the use of a given value we will have underestimated the strength of Europa’s seafloor, such that faulting is even less likely than we find it to be. The equations we use have established precedent in the rock mechanical and geophysical literature, and are given in their generic forms. However, we acknowledge that it would be easier for the reader to have all of the values we assume in one place rather than having to find them individually in the Methods section, so we now list them in a new Table 1. (The model results we formerly gave in the latter section of Table 1 are now in a standalone Table 2.) The Reviewer’s underlying point here in well taken, though: we should articulate what conditions would enable faulting as well as demonstrating that modern conditions do not. To that end, we now report that the orbital eccentricity needed to drive faulting at the seafloor is 32 times that of the present value (Lines 94–95). Pore fluid overpressure is conducive to fault slip, with an overpressure value of 8 MPa needed to drive normal faulting at a depth of 1 km below the seafloor (Lines 142– 143). We address this point more fully in our response to Comment 16. (Although we do not say as much in the manuscript, to the Reviewer’s query the difference in overburden pressure between a 30 km ice shell as modelled here and a 40 km ice shell is 1 MPa, a negligible change when compared with the absolute pressure at the seafloor (175 MPa).) 15) In relation to this last point, the methods describe sets of parameters for applying the model to Enceladus and the Earth. However, I couldn’t find the results of these analyses in the main text (the word “Enceladus” appears in the methods). I suggest that these applications be included in the text and discussed. Is fault slip possible on Enceladus? More importantly, is fault slip possible on the Earth’s seafloor? If your model answers the latter question in the negative, something is clearly missing. This kind of sanity check is important to gain confidence in your results. We take the Reviewer’s point that the basis by which we benchmark our model against Earth and Enceladus should be easier for the reader to access. As we wish to keep the message of the manuscript focused on Europa, we have elected not to move the Earth and Enceladus analyses into the main text, although we now more directly refer the reader to that part of the Methods (Lines 76–77). More importantly, we have considerably expanded the Methods section to more fully explain our modelling approach for Earth and Enceladus, in particular by the addition of four new tables—one each for the model parameters and model results for both bodies (akin to Tables 1 and 2 for Europa in the main text). Briefly, we find that for Enceladus, fault slip is not possible because the stresses to which its silicate interior are subject are too low to drive faulting along even pre-existing structures. In fact, Enceladus is brittle throughout much of its rocky interior, a finding consistent with earlier work proposing that that interior is porous and deformable in bulk. For Earth, our calculated brittle–plastic transition zone depth is comparable to seismic analysis. As is the case for myriad other geodynamic studies, we find fault strength values too high for mantle convection to drive plate motion on Earth. One potential solution to this conundrum, as is widely used in the literature, is to reduce the friction coefficient of our faults; in doing so, we find that mantle motion can drive mobility at the seafloor. Although this seeming paradox is not the focus of our study, we do note that even if Europa’s faults had such a low coefficient of friction, the stress from its mantle undergoing convection are still too low to place the interior into a mobile lid. In short, we contend that—especially with our lengthened Methods section—we demonstrate the validity of our modelling approach even in the face of substantial uncertainty for Europa and for the processes that drive plate motion on Earth. We refer the Reviewer to the new text in the Methods on Lines 619–664, and especially to the discussion on Lines 650–664. 16) Pore fluid pressure in the calculations is hydrostatic (e.g., Line 70). However, Note that pore fluid pressure on Earth can exceed these hydrostatic values. Overpressure is common in many tectonic environments and can reach almost lithostatic values (DOI 10.1029/96JB02485, 10.1130/G31649.1, 10.1029/2022GL098862, 10.1029/2023GL105127 and many others). If possible on Europa, overpressure could solve all the issues here and allow fault slip. You should argue why you do not consider higher pore fluid pressures. The Reviewer is correct. We now report that 8 MPa of overpressure would allow for normal faults to slip at 1 km depth below the seafloor (Lines 142–143). However, on Earth, these pressures are typically due to tectonic motion, sediment compaction, or the dehydration of minerals during subduction. As Europa likely does not have subduction zones or major tectonic activity—per our findings—nor continental river systems dumping sediment loads into the ocean, nor active dehydration of minerals occurring at the depths that we analyse, it is unclear what mechanism(s) might be present within Europa to drive such over pressurization. A brief discussion of this point has been added to the text (Lines 140–146). In addition to these major points, I have a few minor comments. 17) Hydration and carbonization reactions can increase porosity and be self-sustaining (DOIs: https://doi.org/10.1016/j.epsl.2017.06.045, https://doi.org/10.1073/pnas.2110776118). This could avoid the need to produce new surfaces by continuing fault slip. I doubt this process can continue for millions of years (the resulting accelerated dissolution rate doesn’t help), but it may be worth mentioning. We thank the reviewer for suggesting the “chemical volume expansion” mechanism. We now address this point on Lines 158–167, noting that, although such reactions have been studied at moderately high fluid temperatures (e.g., 150–300°C), they have not been replicated at temperatures more appropriate to Europa’s seafloor (i.e., around 0°C). Moreover, if rapid reaction rates are required to enhance permeability then, as the Reviewer suggests, this process likely doesn’t operate over geological time. 18) Line 61: Are you modeling the possibility of fault slip (as I thought from the earlier paragraphs) of “brittle failure” in general? The Reviewer is correct: we specifically mean fault slip, and have changed the wording on Line 66 to clarify this point. 19) The rock density used in the calculation looks very high. However, the seafloor density in your Ref.10 is listed as 2530 ± 120 kg/m3, only ~3/4 of the value you used. We had previously used a value of rock density taken from reference 11 that represents the average density of the silicate portion of the interior. To ensure our interior model for Europa is self- consistent, we have now included a 1 km-thick crustal layer with a density of 2,530 kg/m3 at the seafloor. This layer is qualitatively consistent with, say, an earlier era of seafloor volcanism or mineral precipitates from the ocean, but its presence does not have a meaningful impact on our results or conclusions. We list the thickness and density of this layer in Table 1, and describe it in the Methods (Lines 463–467). 20) Line 109: The grain boundary stress you mention is a normal stress, not a pressure. It would add to the differential stress instead of the overburden stress. The Reviewer is correct, and we have modified the text on Lines 122–127 to reflect that this stress may in fact enable fault slip—but only to shallow depths below the seafloor. Further, this scenario is possible only if the strike of microfractures is dominantly parallel to the intermediate principal compressive stress (and for fully reacted rock with hydrostatic pore fluid pressure). Absent these conditions, the relevance to fault slip of this grain boundary stress is reduced or essentially irrelevant. 21) Please expand on the idea that substantial water circulation would be unlikely today (lines 115-117). I imagine that seawater circulation depends on lateral water pressure gradients (head). Many sources of pressurization depend on local sources (e.g., dehydration) and so may generate large pressure gradients regardless of the low gravity and background pressure. The Reviewer raises an important point: that although gravity driven flows dominate settings on Earth, chemically driven flows have been shown to be important, particularly in mineral deposits. Importantly, however, the relatively low permeability (because of the high confining pressure) would lead to much slower pore pressure diffusion and the potential for prolonged pressures at depth. That said, we are unaware of quantitative estimates of what these pressure gradients might be and how long they are likely to exist, simply because this issue may not have been looked at yet. We have tried to capture this nuance in the revised text (Lines 133–139). 22) Line 136: Actually, there is evidence for void nucleation in the plastic regime (DOI 10.1038/nature08051) Is void nucleation possible within the Europan silicate mantle? Perhaps, but if so then voids cannot close and plastic deformation cannot operate. The specific deformation mechanisms of Europa’s mantle motivate future work, but for this study we take the view that, at depths where lithostatic pressure exceeds yield stress, voids close and cavitation creep is not possible. We have made a minor change to the text on Line 175 to acknowledge this uncertainty, whilst continuing to indicate that permeability drops substantially with increasing depth. 23) Line 139: I didn’t see in Ref. 38 a strong justification for the 10^-18 to 10^-16 strain rate value. If the silicate core is mostly inactive, as you argue here, a lower strain is likely appropriate, e.g., 10^-21 or less, below the detection limit on Earth’s plate interiors. The stagnant lid mantle convection assumption would imply a 0 strain rate at the surface. Either way, I don’t find 10^-18 s^-1 to be justified. Lower values would lead to a shallower BDT. The Reviewer is correct that the possible strain rate within Europa’s modern-day mantle is unknown. Since our guiding principle is to make the Europa silicate interior as weak as reasonably possible, we have adjusted the strain rate to 10–20 s–1, which we now better justify as being a few orders of magnitude below intraplate strain rates on Earth (Lines 179–180). Assuming an elastic strain limit of 0.1% strain, a strain rate any lower would not allow any strain to accumulate over the age of the Solar System, and therefore we view 10–20 s–1 as the lowest meaningful value we could choose. Note that this assumption holds that Europa’s silicate mantle is convecting in the present, which is required for our assessment later in the manuscript as to whether mantle flow is able to drive faulting at the seafloor. In any case, this change in strain rate, along with the usage of a weaker, wet olivine flow law (see reply to Comment 7, has reduced our calculated BPT zone depth to ~55–56 km (Line 145). (This reduction from ~59–60 km, or only about 5%, illustrates the comparatively small influence strain rate has on BPT zone depth. Of note, and we do not include this value in the manuscript, changing the strain rate further to 10–21 s–1 reduces the BPT to 52 km, a change that does not meaningfully alter any of our conclusions.) 24) Line 143: Considering model assumptions, I doubt the precision of “59 km” is warranted. I’d report 60 km. To be honest, I don’t see why these BDT determinations are important for the paper’s focus on fault at the seafloor. The Reviewer’s point is taken, but rather than round to the nearest multiple of ten we now preface “56 km” with “about”, and add tildes to the calculated peak lithospheric stress values that follow. These edits should make it clear to the reader that the specific numeric values matter less than the relative quantities. More broadly, the reason we calculate the brittle–plastic depth for Europa’s silicate interior is to make a first-order attempt at characterising whether mantle convection could drive faulting at the seafloor—which necessitates knowledge of the brittle lithosphere’s thickness. To better make this point clear to the reader, we have added corresponding text to Lines 175–178. 25) I am confused with the shading in Figure 1a. I guess one of the horizontal boundaries is the seafloor, but what are these two triangles fanning out of the seafloor? Why is the ice shell thickness at 30 km (Table 1) not indicated in a similar manner? What is the thick line near 0? I also think the labels a) and b) are too small. The Reviewer reasonably points out several issues with this figure panel, which we have now addressed. The shading was intended to indicate that the zoomed-in area in Figure 1b comes from the small inset box in Figure 1a. But in the interest of clarity, we have removed the shading, and darkened the inset box instead. We have also correctly sized the 30 km-thick ice shell, and increased the font size of labels across all figures. 26) Line 345: specify “hydrostatic pore fluid pressure.” We have made this edit (now on Line 433). 27) More mysterious shading. The interior of the strength envelope may be brighter to highlight the envelope (guess here), but this kind of artistic expression interacts with the more rigorous scientific of the figure (seafloor), and I am left confused. We have retained the interior shading of the strength envelope, but now explain the shading in the caption. Moreover, we have corrected the thickness of the overlying ice layer and increased the font size of the labels. 28) Table 1: Please justify the “model parameters”; the BDT cannot be exactly at 59 km in both extension and compression. In fact, it seems to be 60 km in a normal faulting regime, so the precision of the 59 km value is not justified. Please include the stress estimate from convection and thermal expansion anisotropy for comparison with the other stress estimates in the table. As we note in reply to Comment 24, we give values to the nearest km, but importantly the normal and thrust fault regimes are sufficiently similar that they are almost identical. With rounding, they are now 56 km and 55 km, respectively. We now give these separate, rounded-to-the-nearest-whole-kilometre values in the paper. We have also added to the results table (what is now Table 2) the stresses calculated from mantle convection (for the two end-member mantle viscosities) and from thermal expansion anisotropy."}, {"round": 2, "reviewer_comments": {"reviewer_1": "I have reviewed the revised version of Byrne et al. \"Little to No Active Faulting at Europa's Seafloor Today\". After reading the rebuttal letter and the revised manuscript, I have no objections against publication. The paper remains highly speculative, and the arguments would have deserved a more in-depth treatment than allowed by the limited space provided by Nature Communications.", "reviewer_2": "I thank the authors for modifying parts of their manuscript in response to my comments and those of Reviewer 1. I particularly appreciate the added mention of what would be needed to enable fault motion on the Europan seafloor, which strengthens the case that this phenomenon is highly unlikely today (or even in Europa’s past). The discussion of habitability is now more clearly separated from the presentation of tectonic results. I have two minor follow-up comments. First, it isn’t quite right to say that intraplate strain rates on Earth are ~10^-17 s^-1. These values are achieved in intraplate settings only in active seismic zones or in the presence of Glacial Isostatic Rebound (unlikely on Europa). In a more recent publication than the one cited in the manuscript, Mazzotti and Gueydan (https://doi.org/10.1016/j.tecto.2017.12.014) make it clear that the measurements are barely upper bounds, and that the true intraplate strain rate is lower below detection limits: “Most recent studies provide upper bounds on velocities and regional strain rates, indicating that actual strain rates are below the limit of GPS resolution.” I don’t think it is necessary to lower the strain further than done in the paper (at some points, it should simply be considered 0, in which case there is clearly no activity, and the BDT is immaterial), but the text might be slightly modified. Second, I think the focus on the BDT depth (Point 24) is a bit misleading. This is motivated on Line 177 by the question of whether “mantle flow is able to transmit stresses to the surface and drive failure there”. The point, then, is to compare the maximum strength of the lithosphere with mantle convective stresses. So, what the authors are after is the strength, not the depth, of the brittle-ductile transition. I understand the two are linked, but the section in Lines 174-196 may fit better with the rest of the paper if it focused on strength rather than depth."}, "author_response": "Response to Reviewer Comments on NCOMMS-25-11067A | Little to No Active Faulting Likely at Europa’s Seafloor Today Text in black: Reviewer comment/reference Text in green: Reviewer suggestion/question Text in blue: Author response Line numbers and references in red correspond to the clean, revised NCOMMS-25-11067B manuscript. Responses to Individual Reviewer Comments Reviewer #1: I have reviewed the revised version of Byrne et al. “Little to No Active Faulting at Europa's Seafloor Today”. After reading the rebuttal letter and the revised manuscript, I have no objections against publication. The paper remains highly speculative, and the arguments would have deserved a more in-depth treatment than allowed by the limited space provided by Nature Communications. We continue to appreciate the Reviewer’s feedback on our paper, and welcome their view that they see no further issues to prevent publication. Their review has helped to substantially improve the manuscript. Reviewer #2 (Laurent Montesi): I thank the authors for modifying parts of their manuscript in response to my comments and those of Reviewer 1. I particularly appreciate the added mention of what would be needed to enable fault motion on the Europan seafloor, which strengthens the case that this phenomenon is highly unlikely today (or even in Europa’s past). The discussion of habitability is now more clearly separated from the presentation of tectonic results. We are pleased that the Reviewer is satisfied with the revised version of the manuscript. I have two minor follow-up comments. 1) First, it isn’t quite right to say that intraplate strain rates on Earth are ~10^-17 s^-1. These values are achieved in intraplate settings only in active seismic zones or in the presence of Glacial Isostatic Rebound (unlikely on Europa). In a more recent publication than the one cited in the manuscript, Mazzotti and Gueydan (https://doi.org/10.1016/j.tecto.2017.12.014) make it clear that the measurements are barely upper bounds, and that the true intraplate strain rate is lower below detection limits: “Most recent studies provide upper bounds on velocities and regional strain rates, indicating that actual strain rates are below the limit of GPS resolution.” I don’t think it is necessary to lower the strain further than done in the paper (at some points, it should simply be considered 0, in which case there is clearly no activity, and the BDT is immaterial), but the text might be slightly modified. We accept the Reviewer’s point here, and have made a corresponding, minor change to the text on Lines 179–180. Moreover, we have updated ref. 51 to that noted by the Reviewer (Mazzotti and Gueydan, 2018). 2) Second, I think the focus on the BDT depth (Point 24) is a bit misleading. This is motivated on Line 177 by the question of whether “mantle flow is able to transmit stresses to the surface and drive failure there”. The point, then, is to compare the maximum strength of the lithosphere with mantle convective stresses. So, what the authors are after is the strength, not the depth, of the brittle-ductile transition. I understand the two are linked, but the section in Lines 174-196 may fit better with the rest of the paper if it focused on strength rather than depth. This is a fair point. We have made minor changes to wording on Line 178 to more clearly place the emphasis on brittle–plastic transition zone depth and strength."}]

Decision letter