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Volcanic-Tectonic Modes and Planetary Life
Potential
A. Lenardic
Contents
Introduction.................................................................. 2
Potential Volcanic-Tectonic Modes ............................................... 4
Active Lid Volcanic-Tectonic Modes............................................ 6
Stagnant Lid Volcanic-Tectonic Modes.......................................... 8
Episodic and Sluggish Lid Volcanic-Tectonic Modes............................... 8
Life Potential Under Variable Volcanic-Tectonic Modes .............................. 10
Discussion................................................................... 16
References................................................................... 16
Abstract
Volcanic and tectonic activity affects the climate evolution of terrestrial planets
and, by association, the potential that a planet could maintain liquid water at
its surface over geological time scales. This connects the volcanic-tectonic state
of a planet to the potential that it could allow for life as we know it. The
Earth’s current volcanic-tectonic mode is plate tectonics. That mode may not
have prevailed over the Earth’s geologic history, and it is not presently observed
on other terrestrial planets in our solar system. As we have found more and
more planets orbiting stars beyond our own, questions regarding the range of
volcanic-tectonic modes that terrestrial planets can operate in and how these
modes connect to planetary habitability have generated increased interest. A key
issue for assessing life potential in our galaxy is the degree to which variable
volcanic-tectonic modes can or cannot regulate surface temperatures to allow for
persistent liquid water over geological time scales. This chapter reviews the range
of volcanic-tectonic modes that have been proposed for terrestrial planets to
A. Lenardic ()
Department of Earth Science, Rice University, Houston, TX, USA
e-mail: ajns@rice.edu
© Springer International Publishing AG 2018
H. J. Deeg, J. A. Belmonte (eds.), Handbook of Exoplanets,
https://doi.org/10.1007/978-3-319-30648-3_65- 1
1
2A. Lenardic
date. The degree to which each particular mode can or cannot regulate planetary
surface temperature remains uncertain but general considerations have been put
forward and they are discussed herein. In particular, the hypothesis that volcanic-
tectonic modes other than plate tectonics can allow a planet to maintain surface
water, over time scales that allow for biological evolution, remains viable.
Introduction
Life requires energy. The flow of free energy is critical to the origin, development,
and maintenance of a biosphere. The energy sources for a biosphere, on a terrestrial
planet or moon, are solar (energy from the star, a planet, or moon orbits) and/or
internal (geothermal energy). The internal energy of a terrestrial planet or moon
comes from the decay of radioactive isotopes within its rocky interior, heat
retained from planetary formation, and tidal heating (the latter is not a significant
contribution for the terrestrial planets in our solar system at present but is significant
for several moons of the giant planets and could be significant for exoplanets
(Barnes et al. 2009)).
Planetary life potential ties into energy sources in two ways (Fig. 1a). Energy
from a star or from a planet’s/moon’s interior can be used as a direct energy
source to power photosynthesis or chemosynthesis. The energy sources also drive
cycles that influence surface and near surface environmental conditions. Under
the assumption that there is a limited range of environmental conditions under
which a biosphere can exist, this implies that energy sources can affect planetary
life potential by maintaining livable conditions over extended time periods (for
inhabited planets, life has the ability to feedback and affect the cycles that maintain
environmental conditions suitable for life (Lovelock and Margulis 1974)). For
Earth-life (life as we know it), the existence of liquid water is considered crucial.
The classic idea of a “Habitable Zone” (more correctly, liquid water zone) ties
directly into mapping the environmental conditions that allow a terrestrial planet
to maintain liquid water (Kasting et al. 1993). This is, in large part, an issue of
climatology (Fig. 1b) – conditions cannot lead to a runaway greenhouse (all water
goes into the gas phase and is eventually lost to space) or a protracted hard snowball
state (all water goes into the solid phase).
It is through the chain of reasoning outlined above that the volcanic-tectonic
mode of a terrestrial planet or moon enters most prominently into discussions
of planetary life potential (Kasting 2010; Ward and Brownlee 2004; Lenardic et
al. 2016b). Volcanic and tectonic activity drives geophysical cycles that transfer
volatiles (CO2,H
2O) between a planet’s surface envelopes (atmosphere, hydro-
sphere, biosphere) and its rocky interior (crust, lithosphere, mantle). Volcanism
cycles greenhouse gasses into the atmosphere. Tectonics creates weatherable topog-
raphy, and weathering reactions draw greenhouse gasses out of the atmosphere.
Over geologic timescales, atmospheric CO2content is influenced by the balance
between volcanic degassing and weathering. Factors that act to increase volcanic
degassing will, all else being equal, warm the climate. Factors that act to increase the
Volcanic-Tectonic Modes and Planetary Life Potential 3
Fig. 1 (a) Schematic of how solar and planetary energy can affect life potential directly and
indirectly through effects on the surface environment. (b) Schematic of how volcanic-tectonic
activity has entered most prominently into discussion of planetary life potential
weathering rate will act to cool the climate by drawing CO2from the atmosphere.
Weathering depends on processes governed partly by surface temperature, which
allows for the potential that coupling between climate and the volcanic-tectonic
state of a planet can buffer/stabilize surface conditions in a manner that allows liquid
water to exist over extended time frames (Walker et al. 1981; Kasting et al. 1993).
The volcanic-tectonic mode of the Earth is plate tectonics. We know of no other
planet that operates in a plate tectonic mode and we know of no other planet
that is inhabited. It is no surprise that discussions of planetary life potential, as
4A. Lenardic
it relates to volcanism and tectonics, have been dominated by the idea that plate
tectonics may be crucial for life (Ward and Brownlee 2004; Kasting 2010). The
correlation between an inhabited planet and a plate tectonic planet is based on a
very small sample size relative to what is hoped for from future explorations of
exoplanets. There is also a potential temporal bias. Debate exists as to when plate
tectonics initiated on Earth with several proposed dates being more recent than
those from which we have evidence of life (Stern 2005; Piper 2013). What can
also be neglected, by focusing on plate tectonics, is the range of volcanic-tectonic
modes that have been proposed and explored via geodynamic modeling (driven by
the idea that the Earth may not always have operated in a plate tectonic mode and
by investigations of other terrestrial planets/moons which do not show signs of plate
tectonics but which do have volcanic constructs and signs of tectonic deformation).
The potential that volcanic-tectonic modes other than plate tectonics can main-
tain surface conditions that allow for life cannot be ruled out. The next section
reviews the range of modes that have been proposed and discusses how plate tec-
tonics, the most precise theory we have for solid Earth dynamics over geologically
recent times, fits into that range. The section that follows discusses what currently
can and cannot be said about planetary life potential under variable volcanic-tectonic
modes.
Potential Volcanic-Tectonic Modes
A critical aspect of plate tectonics is that the lithosphere, despite having high bulk
strength due to the temperature-dependence of mantle viscosity (Kohlstedt et al.
1995), is an active part of mantle convection. That is, it participates in convective
overturn due to the formation of weak zones that define plate boundaries (Bercovici
et al. 2015). This has led to the terminology “active lid convection” to describe any
terrestrial planet where the lithosphere not only participates in mantle convection but
also drives convective overturn – that is, the negative buoyancy of the lithosphere
is the principal driver of its subduction back into the mantle (Schubert et al. 2001).
Plate tectonics is an example of an active lid mode of mantle convection but not all
active lid modes need to be plate tectonics, as discussed below in subsection one.
The opposite of active lid convection is the case where convective stresses cannot
overcome lithosphere strength, and mantle convection proceeds in the portions
of the mantle below the strong, coherent lithosphere. The term “rigid lid” or
“stagnant lid convection” is used to describe this mode (Moresi and Solomatov
1995; Solomatov and Moresi 2000). Figure 2shows results from 3D numerical
experiments that highlight the active and stagnant lid end members (Hoink et al.
2012; Stamenkovic et al. 2016). Not only is the mode of surface deformation
different between these end-member modes but the efficiency of heat transfer from
a planet’s interior to its surface is also different, as reflected in the heat flux plots
of Fig. 2. As with the active lid mode, a stagnant lid mode of mantle convection
need not be associated with only a single mode of volcanic and tectonic activity
(subsection two).
Volcanic-Tectonic Modes and Planetary Life Potential 5
0
160
80
120
80
40
0
0.0 0.4 0.8 1.2
0
0.4 0.5 0.6 0.7
0.0
Active Lid Regime
Episodic Regime
Stagnant Lid Regime
0.4 0.8
time (Gyr)
time (Gyr)
time (Gyr)
Surface and Basal Heat Flux
Surface & Basal Heat Flux (mW/m2)
1.2
40
80
120
Fig. 2 Results from numerical mantle convection experiments (shown as iso-temperature images,
temperature cross sections, and surface projection of low strain rate regions, gray, together with
regions of concentrated deformation shown in yellow). Three modes are shown: Active lid (top);
Episodic lid (middle); Stagnant lid (bottom). Surface (blue) and basal (green) heat flux time series
are plotted to the right of the images for each respective numerical experiment
Episodic and sluggish lid modes sit between active and stagnant lid end members.
The episodic mode characterizes a planetary state that transitions between end-
member behaviors over time (Fig. 2, middle panel). In a sluggish lid mode of
convection, lithosphere velocities are finite (not stagnant) but lower than that of
the mantle below. This differs from an active lid mode in that lithosphere motion
and associated surface deformation are not self-driven. Instead, convective tractions
on the base of the lithosphere drive surface deformation and potential lithosphere
recycling (lithosphere recycling in this mode is driven by basal drag on the
lithosphere from below which is different from subduction on present day Earth).
We will discuss the modes that sit between active and stagnant end members in
subsection three.
6A. Lenardic
Active Lid Volcanic-Tectonic Modes
Plate tectonics is defined by rigid plate interiors and narrow zones of active
deformation, i.e., plate boundaries. Even allowing for diffuse plate boundaries
(Gordon 1998), plate tectonics is associated with deformation zones that cover a
small portion of the Earth’s surface area. Plate interiors do not experience significant
levels of deformation. The lack of deformation is what defines “rigid” plate interiors
and allows Euler’s theorem for rigid body rotations on a sphere to be applied to the
relative motion of plates (Morgan 1968). The fact that a mathematical theorem sits at
its core highlights the high level of precision associated with plate tectonics theory.
It is that level of precision that makes the theory a highly testable one – it makes
specific predictions that can be compared to planetary observations (Cox 1973). It
is also that precision that distinguishes plate tectonics from other potential volcanic-
tectonic modes.
An example from planetary exploration can make it clear that focusing on
precision, and the quantitative defining features of plate tectonics, is not a purely
pedantic exercise (Kaula and Phillips 1981). When radar-mapping images of
Venus’ surface first became available, there was a debate on whether the surface
morphology of the planet was consistent with the planet operating in a plate tectonic
mode. It was the precise defining characteristics of what plate tectonics is and what
it is not that allowed that debate to be quantitatively resolved (Kaula and Phillips
1981). It was not only Euler’s theorem that came into play but also the prediction of
expected surface topography moving away from a relatively narrow, divergent plate
boundary (Parsons and Sclater 1977). The conclusion was that Venus, which shows
clear evidence of volcanism and tectonic deformation (e.g., Nimmo and McKenzie
1998), lacks plate tectonics (Kaula and Phillips 1981). Had the quantitative aspects
of what defines plate tectonics not been brought to bear the debate could not have
been resolved to the degree it was.
The creation and maintenance of a global network of narrow plate boundaries
is a critical condition for plate tectonics. This is what allows Euler’s theorem to be
applied to plate motions. It is also why continental reconstructions can be used to
determine continental configurations in the Earth’s past – continents are not being
deformed to the degree that the “pieces of the jigsaw puzzle” get too mangled
to put them back together. One can imagine active lid planets that are dominated
by broad zones of distributed deformation, as opposed to narrow boundaries that
effectively define tectonic plates. Beyond imagining, this type of mode has been
explored through a number of geodynamic models. Figure 3shows results from
two representative modeling studies (Richards et al. 2001; Hoink et al. 2012). The
surface velocity plots of Fig. 3show the distinction between concentrated and
distributed deformation – regions with lateral velocity gradients (i.e., deformation
zones) cover large portions of the surface in the distributed mode but are confined
to small, narrow areas in a plate tectonic mode.
Geodynamic models that are dominated by distributed surface deformation are
considered unsuccessful when it comes to addressing the dynamics of plate tectonic
Volcanic-Tectonic Modes and Planetary Life Potential 7
Fig. 3 (a) Results of 2-D numerical experiments of coupled mantle convection and tectonics
(Richards et al. 2001). Shown are plots of surface velocity from three experiments. One is in a
stagnant lid mode of convection with zero surface velocity. The other two are in active lid modes.
The surface velocity plots show how an active lid mode need not be one that represents plate
tectonics as it operates on Earth at present. (b) Results from 3-D numerical experiments of coupled
mantle convection and tectonics (Hoink et al. 2012). The gray regions, in the surface projection
plots, show low strain rate regions (regions that behave as effectively rigid). The case to the left
captures essential aspects of plate tectonics – narrow regions of concentrated deformation (plate
margin analogues) that separate regions that move as effective rigid plates. The case on the right
is dominated by distributed deformation. It lacks well-defined plate boundaries. It also lacks large
sections of the surface that move relative to one another in a way that can be represented by Euler’s
theorem (Morgan 1968)
because they do not account for key aspects of the process they seek to model,
i.e., they do not produce plate boundaries nor do they predict surface velocities
consistent with observations (Richards et al. 2001; Bercovici et al. 2015). Although
not plate tectonics, and thus not representative of Earth at present, an active lid mode
8A. Lenardic
of convection dominated by distributed deformation is a possible volcanic-tectonic
mode for terrestrial planets and moons outside of our own solar system.
Stagnant Lid Volcanic-Tectonic Modes
Stagnant lid (single plate) planets can be associated with subsets depending on
internal temperatures. The subsets delineate single plate planets with different
degrees of volcanic activity. If the interior is so cold that no melting can occur, then
a planet will be volcanically inactive (a “cold stagnant lid” (Weller et al. 2015)).
This is a geologically dead planet.
Hot stagnant lid planets allow for volcanic and localized tectonic activities.
For increasing interior temperatures, volcanism becomes more important as a heat
transport process. A planet or moon enters a heat pipe mode when volcanism
dominates total heat transport from its interior to its surface. The heat pipe mode
characterizes the volcanic-tectonic state of Jupiter’s moon Io (O’Reilly and Davies
1981) and has been argued to provide a good explanation for many features of
Earth’s most ancient geologic record (Moore and Webb 2013). A heat pipe mode
allows for degassing of volatiles and the creation of new, potentially weatherable,
topography. In addition, volcanism is a mass transport process that can cause the
surface temperature to be advected downward as old, cool flows are buried by
newer flows. This allows for a vertical cycling of crust from the surface toward
the interior of a planet (Moore and Webb 2013). The vertical cycling can stress the
planet’s/moon’s lithosphere and drive localized tectonic deformation.
As volcanism becomes less dominant as a heat transport process, a stagnant lid
planet or moon can transition from a heat pipe mode to a single plate planet that
remains volcanically active. As an example, Mars is a stagnant lid planet that shows
signs of geologically recent volcanic activity not consistent with a heat pipe mode
of behavior. The most prominent Martian volcanic constructs are more consistent
with a mantle plume origin in morphology, gravity, and topography (Kiefer and Li
2016). Recycling of crust back into the mantle remains possible in this single plate
mode with several recycling mechanisms having been put forward (Lenardic et al.
1993; Zegers and van Keken 2001; Elkins-Tanton et al. 2007; Smrekar et al. 2007).
Episodic and Sluggish Lid Volcanic-Tectonic Modes
The center panel of Fig. 2shows results from an experiment that is in an episodic
mode (Moresi and Solomatov 1998). That mode is associated with periods of
lithosphere overturn (active lid behavior) interspersed with periods of stagnant lid
behavior. An episodic mode has been advocated for the volcanic-tectonic state of
Venus (Turcotte 1993) and for the early Earth (Moyen and van Hunen 2012; O’Neill
et al. 2013). The mode can recycle lithosphere globally during episodes of overturn.
Alternatively, regions of thick and chemically buoyant crust can resist recycling
Volcanic-Tectonic Modes and Planetary Life Potential 9
Fig. 4 Representative numerical experiments of coupled mantle convection and surface tectonics
illustrating the distinction between active lid and sluggish lid modes (Hoink and Lenardic 2010).
Vertical profiles of horizontal velocity (black lines) are plotted over thermal fields (blue is cold
upper boundary layer material – model analogue for a tectonic plate or a deforming lithosphere;
red is warm upwelling mantle)
during overturn events (Cooper et al. 2006). Lithosphere recycling would not be
global in such a case and zones of thickened crust could remain persistent over time.
The term active lid means that lithosphere subduction is self-driven by the
negative buoyancy of the lithosphere. That is, lithosphere motion drives motion in
the mantle below. This is the classic view of plate tectonics connected to mantle
convection with the plates being self-driven and the bulk internal mantle responding
to plate motions (Turcotte and Oxburgh 1967; Forsyth and Uyeda 1975). If the
lithosphere is self-driven, then the mantle below plates resists plate motion. If
the velocity of the mantle below plates (or a lithosphere that is deforming in a
distributed mode) exceeds plate velocities, then mantle flow would provide a driving
force for lithosphere motion (Fig. 4). A mode of that type, where lithosphere
velocities are finite (not stagnant) but lower than that of the mantle below, has been
termed a sluggish lid mode (Solomatov 1995). Theoretical and numerical work
has shown that a sluggish lid mode of behavior can exist over a broad range of
planetary parameter space (Hoink and Lenardic 2010; Hoink et al. 2011;Crowley
and O’Connell 2012; Foley and Bercovici 2014).
A sluggish lid regime has been proposed for the preplate tectonic early Earth
(Davies 1992). It has also been proposed for Venus in submodes where the
lithosphere deforms globally (Bindschadler et al. 1992) or in localized regions
(Phillips 1990). More recently, another submode has been identified in which
lithosphere deformation, associated with sluggish lid convection, is not exclusively
in the form of distributed deformation. In zones of divergence, within the convecting
mantle, localized features, akin to mid-ocean ridges, can form while at zones of
convergence deformation remains distributed. That mode has been termed the ridge-
only mode (Rozel et al. 2015).
10 A. Lenardic
Fig. 5 Potential volcanic-tectonic modes, identified to date, for terrestrial planets
Life Potential Under Variable Volcanic-Tectonic Modes
Figure 5shows the range of volcanic-tectonic modes reviewed. All of them, except
for a cold stagnant lid, are associated with geothermal activity. In terms of a direct
energy source for life, geothermal heat flux is more critical than a particular tectonic
mode. Life on Earth may have initiated at hydrothermal vents (Baross and Hoffman
1985). In such a scenario, be it for Earth or life-bearing exoplanets, geothermal
energy, as opposed to solar, provides the direct energy source for life. The vents need
not be the same as those at mid-ocean ridges on present day Earth. Stated another
way, even if life does originate and/or is currently maintained at hydrothermal vents
on a planet or moon, it does not follow that the planet/moon operates in a plate
tectonic mode. Considerations of life potential associated with hydrothermal vents
at the base of a subsurface ocean on Jupiter’s moon Europa are not dependent on the
moon’s rocky interior operating in a plate tectonic mode. Furthermore, the ridge-
only mode should make us pause before we assume that a planet or moon with
ridges, and associated hydrothermal activity should it have an ocean, is necessarily
a plate tectonic planet. In terms of direct energy sources for life, the majority of
volcanic-tectonic modes proposed to date remain viable.
Coupling between volcanic-tectonic activity and climate can have an indirect
effect on life potential. For life as we know it, a key issue is the degree to which
volcanic-tectonic activity can buffer/stabilize climate in a way that maintains liquid
water at, or near, a planet’s surface. The term stable or buffered is not being used in
this context to imply that climate variations cannot occur. The Earth’s climate has
gone through significant changes over time (Bice et al. 2006; Zachos et al. 2008;
Hoffman et al. 1998; Schrag et al. 2002). Climatic conditions have varied and life
has survived over our planet’s history. A stable or buffered climate, in the context
of planetary life, is one that does not go into a runaway greenhouse or a protracted
hard snowball state (Fig. 2b).
Climate variability, over Myr timescales, is influenced by a greenhouse forcing
that is modulated by a balance between the rates at which CO2is expelled from
Volcanic-Tectonic Modes and Planetary Life Potential 11
volcanoes and drawn down from the atmosphere via chemical weathering processes
(Walker et al. 1981; Staudigel et al. 1989; Dessert et al. 2001; Coogan and Dosso
2015). The global rate of CO2outgassing is governed by the character and pace of a
planet’s volcanic activity. Chemical weathering is mechanically paced by the rates
at which new surfaces are created (Sleep and Zahnle 2001; Whipple and Meade
2004; Roe et al. 2008; Lee et al. 2013,2015). The protracted clement climate of our
own planet is, in part, a consequence of this long-term carbon cycle not having gone
so far out of balance as to initiate a transition to an alternate global climatic state.
The maintenance of Earth’s clement climate, over geologic time, is most
generally attributed to a silicate weathering-climate feedback (Walker et al. 1981;
Berner et al. 1983; Kasting et al. 1993; Kasting 2010). Weathering depends on
processes governed partly by surface temperature. If surface temperatures increased,
weathering rates could also increase thereby drawing greenhouse gasses out of the
atmosphere and driving climate cooling that could offset the initial temperature
rise. If surface temperatures dropped, weathering rates could decrease such that
the drawdown of greenhouse gasses from the atmosphere would not keep pace
with injection into the atmosphere from volcanic activity. This could drive climate
warming that could offset the initial temperature drop. The silicate weathering-
climate feedback cycle requires that a planet or moon allows for the injection of
greenhouse gasses into the atmosphere, via volcanically activity, and for the creation
of weatherable topography, via tectonic activity and/or the creation of volcanic
constructs.
A weathering-climate feedback relies on the cycling of volatiles (CO2,H
2O)
between a planet’s atmosphere and its rocky interior. The phrase “deep volatile
cycling” is used to distinguish volatile cycling between a planet’s outer envelopes
and its interior, from volatile cycling between surface envelopes (atmosphere,
hydrosphere, biosphere). Deep volatile cycle models initially focused on water
cycling linked to the thermal evolution of the Earth (McGovern and Schubert
1989). They were soon extended to include deep carbon cycling with the intent of
using them to address climatic conditions over Earth’s thermal and geologic history
(Tajika and Matsui 1990,1992; Franck et al. 1999). The most recent extensions of
this modeling trajectory have focused squarely on the ability of a planet to maintain
conditions that allow for surface water over geologic time scales (Foley 2015;Foley
and Driscoll 2016).
The studies cited above employ 1-D parameterized models of mantle convection,
thermal history, and associated volcanism and tectonics over time (Davies 1980;
Schubert et al. 2001). Interior planetary temperature and surface velocity are
scaled with the level of a planets’ internal energy over time. Different scaling
exponents apply for different mantle convection modes (e.g., active vs. stagnant lid).
Temperature history, together with a melting model, can be used to track volcanic
activity and greenhouse gas injection into the atmosphere. Surface velocity can be
used to track creation of mountain topography associated with tectonic deformation
(Whipple and Meade 2004; Roe et al. 2008). Topography generation can be linked
with a weathering parameterization to track greenhouse gas withdrawal from the
atmosphere (Walker et al. 1981; Dessert et al. 2001; Riebe et al. 2004). The volcanic-
12 A. Lenardic
tectonic and thermal history model can then be linked to a radiative-convective
climate model to track surface temperature over the thermal history of a model
planet (Foley 2015; Foley and Driscoll 2016; Lenardic et al. 2016b).
Research that has followed the methodology above has dominantly focused on
the Earth and used parameterizations appropriate for an active lid planet. Often
one will hear such studies described as “exploring the role of plate tectonics in
planetary life potential.” This is slightly misleading. Parameterized models make no
predictions about whether surface deformation will follow the defining features of
plate tectonics (rigid plates that move relative to each other, in accordance with
Euler’s theorem, separated by narrow zones of deformation). Active lid scaling
relations do apply for a plate tectonic mode, but they also apply to a planet
dominated by distributed deformation (the approach does not discriminate between
the two). What the studies to date have shown is that an active lid planet can
potentially provide a level of climate buffering that allows water to exist at its
surface over geologic time (Foley 2015; Foley and Driscoll 2016). They have also
shown that this is not guaranteed. In particular, if the area of exposed land becomes
small, weathering on land can become supply limited. The supply of fresh rock
then provides the limit on weathering rate as opposed to reactions between CO2
and rock. Stated another way, weathering rates become insensitive to climate and
the weathering-climate negative feedback breaks down. Volcanic injection of CO2
into the atmosphere can then overwhelm CO2drawdown leading to hot climatic
conditions that cannot maintain liquid water at a planet’s surface (Foley 2015).
Parameterized models lend themselves to probabilistic approaches. In principal, a
large range of conditions that might apply to terrestrial planets, both in and beyond
our own solar system, could be mapped out to formalize the probability that an active
lid planet could maintain conditions favorable for life as we know it. In practice, this
has yet to happen although it is on the horizon.
A sluggish lid mode allows for lithosphere recycling (and associated deep volatile
cycling), volcanism, and topography generation. However, the way volcanism and
surface velocities scale with internal energy sources is different for sluggish lid
behavior relative to an active lid mode. Coupled climate-tectonic modeling studies
that consider a sluggish lid mode are, to date, small in number. The limited
studies undertaken have shown that climate stabilization is possible (Foley 2015;
Jellinek and Jackson 2015). Those studies used scalings, for volcanic degassing and
topography generation as functions of a planet’s internal energy, that are appropriate
for the global distributed deformation submode (Fig. 5). That submode allows
for topography generation not only via volcanic constructs but also via tectonic
thickening of the crust driven by convective tractions at its base (Bindschadler et
al. 1992;Davies1992; Lenardic et al. 1991). From what is known of the ridge-only
mode (Rozel et al. 2015), it also allows for this potential. A regional distributed
deformation submode (Phillips 1990) would be associated with a different degree
of topography generation potential and it is not clear as this stage whether that could
preclude climate buffering.
An episodic mode allows for lithosphere recycling (and associated deep volatile
cycling), volcanism, and topography generation. However, it also allows for signif-
Volcanic-Tectonic Modes and Planetary Life Potential 13
Fig. 6 Modeling methodology used to explore the effects of episodic volcanic-tectonic activity on
planetary climate. Solid planet dynamic models of coupled mantle convection and surface tectonics
are used to map out variations in volcanic and tectonic activity over time for a range of planetary
parameter values (left image). Results from the solid dynamics models are then used to generate
volcanic-tectonic forcing functions for zonal energy balance climate models (Budyko 1969)that
include volcanic degassing, topography generation, and CO2 drawdown from the atmosphere due
to surface weathering (right image)
icant temporal gaps between injections of greenhouse gasses into the atmosphere.
The magnitude of mantle degassing during episodic bursts can also be large relative
to what would be associated with active lid convection. Whether an episodic mode
could allow for stabilization of surface temperatures is a question that has only
recently come to be addressed (Lenardic et al. 2016b). Figure 6shows a schematic
of the modeling methodology employed by Lenardic et al. (2016b) and Fig. 7shows
results from selected model cases.
The model results of Fig. 7highlight several tradeoffs and complexities.
Volcanic-tectonic forcing and solar forcing can both vary over the geologic lifetime
of a planet. As such, life potential, as connected to climatic conditions that allow
for liquid water at a planet’s surface, should also be treated as a time variable metric
(Fig. 7a highlights the potential that climate conditions that allow for liquid water
at a planet’s surface can emerge over a planet’s temporal evolution). In terms of
an episodic mode providing climate buffering, the preliminary models that exist
to date indicate that it is possible (Lenardic et al. 2016b). However, the range of
potential conditions that are applicable for terrestrial planets in an episodic mode,
at various times in their evolution paths, has only barely been scratched. At present,
the likelihood of climate stabilization under this mode and/or how it may compare
to an active lid mode cannot be assessed – climate buffering, at a level that increases
the potential of life as we know it, is possible under an episodic mode of tectonics
but no probability measure has, as yet, been determined.
Figure 7indirectly highlights added tradeoffs that can enter from considerations
of planets at variable stellar distances (or planets orbiting stars with different solar
luminosities than our own). The model of Fig. 7a is predicted to be in a hard
snowball state at 3.5 Gyr. All the models of Fig. 7used Earth-like parameters
14 A. Lenardic
Fig. 7 (Continued)
Volcanic-Tectonic Modes and Planetary Life Potential 15
as a starting point. The cool conditions that allowed for the hard snowball state
could be offset if the planet was assumed to be at a closer stellar distance. That
assumption would be more consistent with exploring the evolution of Venus (it
has been argued that Venus operated in an episodic mode of tectonics (Turcotte
1993; Moresi and Solomatov 1998)). The preliminary results that explore tradeoffs
in solar luminosity and volcanic-tectonic forcing are consistent with the idea that
the window of increased life potential may have opened earlier in Venus’ evolution
as compared to Earth (Lenardic et al. 2016b). The idea that Venus may have been
habitable in its past has also been argued for based on general circulation models
of Venus’ atmosphere (Way et al. 2016). The full range of model tradeoffs needs
to be more fully explored but, at this stage, the potential that an episodic mode of
tectonics could allow for life as we know it is a viable one.
A hot stagnant lid mode allows for volcanism and the creation of topography
via volcanic constructs. A heat pipe single plate planet allows for vertical crustal
cycling, which has the potential to move volatiles between surface reservoirs and the
outer most portions of the solid planet (Moore and Webb 2013; Kankanamge and
Moore 2016). A mantle plume single plate planet allows for localized volcanism and
topography generation. Recycling of the lithosphere and crust, into the convecting
mantle, is possible in this submode (Elkins-Tanton et al. 2007; Smrekar et al. 2007).
Qualitatively, the ingredients needed for climate buffering can operate on a single-
plate planet. Quantitatively, can the ingredients, as they operate in a stagnant lid
mode, stabilize climate to the degree that free liquid water could exist for geologic
time periods? Early generation modeling suggested that it is possible (Pollack et al.
1987). Since that study, there has been little work focused on this question relative to
the amount of work focused on the coupling between climate and active lid tectonic
modes. There are signs that this is changing (Halevy and Head 2014). Studies into
the dynamics of a volcanically active single-plate planet are coming to be linked to
climate models to address the question of whether a single-plate planet can provide
for climate buffering (Tosi et al. 2017). No conclusive statements can be made at
this time other than to say the potential that a single plate planet could maintain
water at its surface for geologic time scales cannot be ruled out.
J
Fig. 7 (a) Model response to an episodic lid-driven oscillation in CO2 partial pressure, assuming
that the unperturbed greenhouse forcing is fixed at the present-day Earth value (large-filled circles).
For current day insolation, the episodic volcanic-tectonic forcing does not drive the planet to a
pan-glacial or an ice-free state (small red-filled circles and dashed arrows). At 1 billion years
ago, accounting for changes in solar luminosity (Bahcall et al. 2001), the oscillation does drive
this model Earth climate to vary between variably warm partial ice solutions and a pan-glacial
state (light blue-filled circles and dashed arrows). At 3.6 billion years ago, the oscillations send
the model Earth into a hard, and potentially protracted, freeze (dark blue dashed arrows). (b)
Model response to episodic volcanic-tectonic forcing as in (a) but now assuming an unperturbed
greenhouse forcing corresponding to the Jellinek and Jackson (2015) solution at 3.5 Ga. Oscillation
in CO2, in response to episodic volcanic-tectonic forcing, drives this model Earth climate to
oscillate, but it does not enter a pan-glacial solution as it does for the model of (a) at an equivalent
geologic time
16 A. Lenardic
Discussion
Considerations of how volcanic-tectonic activity affects the surface of a terrestrial
planet, in ways that may favor life, start with our own planet within our own solar
system. The Earths’ current volcanic-tectonic mode is plate tectonics. The Earth
is inhabited. Mercury, Mars, and Venus do not show evidence of currently active
plate tectonics and are not inhabited. Those are the agreed upon observations we
have from terrestrial planets at this point in our explorations. Is this also where the
considerations end? That is, do we take these observations to provide support of the
hypothesis that plate tectonics is required to maintain conditions suitable for life as
we know it on a terrestrial planet and do we use that as a guide for future exoplanet
exploration? There is no observational data available to answer that question (even
for Earth, the planet with the most observational data, it is still debated as to when
plate tectonics initiated and whether it was before or after life took hold on our
planet). Any arguments for or against using the observations we have from our solar
system related to potential connections between tectonics and life, as guides for
expected behavior in other solar systems are arguments of theory and/or modeling
at this stage (conceptual as well as quantitative modeling (Lenardic et al. 2016a)).
Multiple volcanic-tectonic modes have been proposed and their dynamics
explored from a modeling perspective (Fig. 5). Considerations of how volcanic-
tectonic modes, distinct from plate tectonics, could interact with planetary climate,
so as to maintain surface conditions suitable for life as we know it, have only
just begun (motivated in no small part by the potential of new data coming
from exoplanet exploration). It remains too early in this research line to make
probabilistic statements as to how the modes compare, relative to one another, in
terms of increasing or decreasing life potential. What can be said is that several of
the modes remain viable for maintaining surface conditions that allow liquid water
to exist on a terrestrial planet or moon over geologic time scales. Stated another
way, the alternative working hypothesis, relative to the one noted in the paragraph
above, remains viable – that is, the hypothesis that plate tectonics is not required for
a planet to have life as we know remains a physically plausible one.
Acknowledgments This work was supported by NSF Frontiers of Earth Systems Dynamics grant
OCE-1338842.
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