HABITABILITY AND PLANETARY REDOX
Yasuhito Sekine1,2Earth-Life Science Institute (ELSI), Tokyo Institute of Technology, 2-12-1 Ookamaya, Meguro,
Tokyo 152-8520, Japan
2
Institute of Nature and Environmental Technology, Kanazawa University, Kakuma, Kanazawa,
Ishikawa 920-1164, Japane-mail: sekine@elsi.jp(Received: November 21, 2020 Accepted: December 9, 2020)

1

Abstract
This paper reviews results of Solar System
explorations
andtelescope
observationsofgeologically-active icy bodies (Europa, Enceladus, andCeres) and Mars, where liquid waters are, or were,
present. In particular, we focus on the availability ofboth reductants and oxidants as well as water chemistryon these planetary bodies. This paper also proposes anidea of “planetary redox”—a planetary-scale systemthat can generated a redox and pH gradient throughgeohydrological cycles. Based on the understandings ofplanetary redox on Europa, Enceladus, and Mars, we
discuss about bioavailable energy on these bodies andscience targets in future missions. Knowledge on thepresence or non-presence of life in these habitableworlds would shed light on the conditions where life
could emerge on early Earth.

water samples or sediment samples and to perform
in-situ chemical analyses remotely, which wastechnically challenging.
Since early 2010’s, however, recent space missions
have provided much observational data on chemistry,
which are essential to constrain the availability ofenergy and organic matter beyond Earth. NASA’s
Cassini spacecraft directly collected plume materials
erupted from the subsurface ocean of Enceladus during
the flybys [e.g., 12]. On Mars, NASA’s Mars Science
Laboratory Curiosity rover landed on paleolake
sediments within Gale Crater and drilled them forin-situ chemical, mineralogical, and organic analyses[e.g., 13].
In the present paper, we provide a review on resultsfrom these recent Solar System explorations andobservational data on chemistry of these planetary
bodies and discuss the available energy and organic
matter there. First, we describe a general view on aplanetary-scale system that can provide chemicalenergy to life in Sec. 2. Next, we discuss both of theoxidants and reductants available on the planetary
bodies in Sec. 3. Bases on the observational dataobtained by recent space missions, such as Cassini,
Curiosity, and Dawn, we review water chemistry andpossible redox reactions in icy bodies and on Mars in
Sec. 4. Finally, we summarize this paper in Sec. 5.

Key words: Habitability, Astrobiology, ocean worlds,
Mars, Solar System explorations, Europa, Enceladus,
Ceres
1. Introduction
Planetary habitability can be defined as aplanetary-scale system that develops and maintains
ingredients of life—liquid water, organic matter, andenergy—and provides them to life continuously
through hydrogeochemical cycles over geological time.
Owing to Solar System explorations, we have foundlines of geophysical (e.g., induced magnetic fields [1];gravity [2,3]; shape and libration [4–6]) and geologicalevidence (e.g., valley networks [e.g., 7,8], deltas [e.g.,9]; evaporates [e.g., 10,11]) for the long-term existenceof liquid water on multiple Solar System bodies. Thesebodies include Mars, Europa, Ganymede, Callisto,
Enceladus, Titan, Ceres, and Pluto. The number of suchplanetary bodies having liquid water would increase
when spacecrafts acquire detailed observational data oflarge icy bodies in the outer Solar System, such as
Triton and trans-Neptune objects.
Despite the findings of liquid water, availability ofenergy and organic matter on these Solar System bodies
had been poorly constrained until early 2010’s. This is
the case because both of reductants and oxidants, which
generate bioavailable chemical energy, and organic
matter are, or were, usually trace in aqueous
environments on these planetary bodies. Thereby, inorder to reveal availability of energy and organic matter,spacecrafts or rovers need to directly collect liquid

Viva Origino 2020, 48, 6© 2020 by SSOEL Japan

2. Planetary redox
Life is a dynamic-equilibrium system that iscomposed of complex organic matter having functions(e.g., protein and DNA for Earth’s life). Since thecomplex organic matter having functions isthermodynamically unstable, they are graduallymetamorphosed or decomposed, or oxidized on Earth,over time. This results in loss of functions and,eventually, a termination of a life system (i.e., death).
To avoid metamorphose and decomposition, life needs
to continuously recreate the complex organic matterthat consists themselves. The recreation processes
require continuous chemical energy and supply ofbuilding materials of the complex organic matter, suchas bioessential elements.
Either on terrestrial planets (e.g., Earth and earlyMars) or geologically-active icy bodies (e.g., Europa
and Enceladus), a planetary-scale system havinghydrogeochemical cycles can provide chemical energy

1Viva Origino 2020, 48, 6

and building materials to life (Fig. 1) [14]. In theatmospheres or on the surfaces, solar UV/visible lightand high-energy particles (solar winds and cosmic rays)
dissociate H2O, which results in irreversible escape of
produced H and consequent production oxidants (suchas O2 and H2O2) (Fig. 1) [e.g., 15–17]. If the surface
contains sulfur or nitrogen, the surface oxidation
involves acidification through formation of acids, such
as SO42- and NO3-. On the other hand, water-rockreactions on seafloor or within the subsurface provide
both reductants, such as H2, and alkaline, such as Na+,
Mg2+, and Ca2+, to the hydrospheres [e.g., 18–21]. In
addition to these “secondary” oxidants/reductants thatare formed by in situ chemical reactions, planetary
bodies could possess “primordial” oxidants/reductants(see Sec. 3 for details), which were originally containedin building materials of the planetary bodies andavoided from dissociation during accretion andevolution. Both of the primordial and secondaryoxidants/reductants are transported and mixed throughhydrological cycles and atmospheric circulations on theplanetary bodies, which generate a redox and pH
gradient near the surface (Fig. 1).
We call this dynamic planetary-scale system that cancreate a redox gradient as “planetary redox”. Using aredox disequilibrium in the planetary redox,
chemoautotrophic life can acquire chemical energy(chemical affinity) to recreate the complex organic
matter by producing thermodynamically stablemolecules (metabolic products) [14]. In other words,
the thermodynamically stable molecules formed by life
through reactions of available reducing and oxidizing

molecules are predictable. These thermodynamicallystable molecules can be treated as potential biomarkers.
For instance, H2 (reductant) and CO2 (oxidant) areavailable in the subsurface ocean of Enceladus [22].
The measured abundances of ~1 % of H2 and 0.3–0.8 %of CO2 relative to H2O are in a redox disequilibrium[22]. The thermodynamic calculations show that CH4 isthermodynamically more stable than the mixture of H2and CO2 [22]; accordingly, CH4 can be a biomarker ofthis icy moon [22]. On Titan’s surface,photochemically-produced H2 (reductant) and C2H2(oxidant)
canberecombinedtothermodynamically-stable CH4, which is a potential
biomarker of Titan’s life [23]. Thereby, knowledge on
the planetary redox is critical to predict potential
biomarkers and quantitative biomass on planetary
bodies for future missions that aim to findextraterrestrial life.
3. Oxidants and reductants in Solar System bodies
Depending on the formation process of planetary
bodies, combinations of available oxidants andreductants are different. They are also changed overgeological time even on a single planetary bodydepending on the evolution of the planetary body. In
this section, we review the chemical compounds of theprimordial and secondary origins of both oxidants andreductants available on the planetary bodies (Fig. 2).
Primordial reductants: The primordial reductants
include organic matter, CH4, and NH3 contained
originally in icy planetesimals and proto-planets (Fig.
2). Organic matter in icy planetesimals is considered to

Figure 1. A conceptual image of “planetary redox” on icy bodies and terrestrial planets that possess liquid water. The
planetary redox is defined as a planetary-scale system that can generate oxidants and reductants continuously and can provide
them to life through hydrological cycles. On planetary surfaces, including atmospheres, oxidants and acids can be producedby photochemical/ionchemical reactions and subsequent hydrogen escape to the space. In subsurface or on seafloor,
reductants and alkaline can be provided from rocks through hydrothermal reactions. These chemical compounds aretransported by global-scale hydrological cycles and atmospheric circulations, generating redox and pH gradients inhydrospheres. Chemoautotrophic life on Earth uses these redox disequilibria generated in the planetary redox in order toacquire energy. See the main text for the details.

2Viva Origino 2020, 48, 6

have been generated from polymerization of simple
organic molecules, such as aldehyde and ammonia, inthe circumplanetary disk through radiogenic heating[24–26]. Organic synthesis would have also proceeded
through heating events induced by large-scale impacts[27]. These primordial reductants could survive during
the accretion if a planetary body did not experience
very high temperatures (> ~1000 °C).
The undifferentiated rocky interiors of Enceladus
suggest that they did not experience the hightemperatures (rock melting) [3], implying that
primordial reductants are available in these planetary
bodies. In fact, Enceladus’ plume erupting from its
subsurface ocean contains a significant level of CH4,
NH3, and other organic matter [12]. Since theirabundances are comparable to those in comets (i.e., icy
planetesimals), Enceladus’ CH4, NH3, and, at least,
some of organic matter may be primordial [12].
Primordial organic matter would also exist in an earlyocean of Ceres given its chemical and mineralogicalsimilarities to C-type asteroids [e.g., 28]. The
widespread occurrence of NH4-bearing phyllosilicate
on Ceres [29] suggests that its early ocean would havecontained primordial NH3 [28].
On Mars and Earth, the primordial reductants oforganic matter would have been decomposed in magma
oceans during the accretion (e.g., giant impacts) [30].
Although the primordial reductants would have lostfrom the terrestrial planets, decomposition of organic
matter in magma ocean results in formation of theirprimordial atmosphere [e.g., 31]. If the magma oceancontains metallic iron, the primordial atmosphereshould have contained high abundances of reductants,
H2 and CH4. In a giant impact, fragments of a metallic
Fe core of an impactor would have also reduced theatmosphere-surface system through re-entering to theterrestrial planet [32]. These atmospheric reductants
would have been eventually lost through H2 escape tothe space over 0.1–1 billion years [33].
It is unclear whether Europa possessed theprimordial reductants, such as organic matter.
Primordial CH4 and NH3 would not have been
contained in the planetesimals that formed Europa due
to relatively high temperatures (~150 K) around
Europa-forming regions in the circum-Jovian disk [34].
During the accretion stage of Europa, impact velocityof icy planetesimals onto proto-Europa in thecircum-Jovian disk would be too low (~2 km/s) to
decompose primordial organic matter (as well as otherprimordial NH3 and CH3OH) [36–38]. Long-lived
radiogenic heating and tidal heating may be able toincrease the interior temperatures sufficient todecompose the primordial organic matter on Europa[e.g., 39]. Detailed gravitational data and revealing theexistence of metallic Fe core on Europa are a key tounderstand whether Europa has experienced hightemperatures and whether primordial organic matter isavailable on this icy moon.
Secondary reductants: Hydrogen molecule (H2) is
the important secondary reductant available on the

Figure 2. A comparison of the primordial and secondary reductants/oxidants available on the planetary bodies in the Solar
System. Possible metabolic products and types of life (e.g., heterotroph/autotroph) are also shown. Enceladus possesses theprimordial oxidant of CO2, contained in icy planetesimals, and secondary reductant of H2, formed by hydrothermal reactions(the blue broken square). On Europa, secondary oxidants of O2, H2O2, and SO4 are available on the surface; whereas, it isunconstrained whether primordial matter and/or secondary reductant of H2 are available (the green broken square). On earlyMars, both of primordial reductants and oxidants would have been decomposed in the early stage of its evolution (i.e., magma
ocean). The secondary reductants (H2 and H 2S) and oxidants (H2O2, O 2, SO4, CO2, ClO3, and ClO 4) are suggested to have been
available on early Mars (the pink broken square). See the main text for the details and references.

3

Viva Origino 2020, 48, 6

planetary bodies that possess liquid water (Fig. 2). H2 isgenerally formed through hydrothermal alterations ofFe(II)-bearing rocks, such as olivine and pyroxene, as a
product of formation magnetite in serpentinization [e.g.,40]. To produce H2 through water-rock reactions
efficiently, reaction temperatures need to be, at least,~100 °C [e.g., 41].
The Fe(II)-bearing minerals are considered to becommonly contained in rocky interiors of icy bodies,regardless of the degree of differentiation of rocks. If
the rocky interior is undifferentiated (i.e., no rockmelting), and if its composition is similar to unalteredchondritic rocks (composed mainly of Fe-Mg-richolivine and pyroxene), as suggested for Enceladus[21,42], hydrothermal alterations of Fe(II)-bearing
minerals, together with metallic iron, should result in
production of high levels of H2 [21,43]. Suchhydrothermally-generated H2 has been found on
Enceladus [22]. If the rocky interior is differentiatedthrough rock melting, the seafloor rocks would becomposed mainly of basaltic rocks that have less Mg
and Fe than undifferentiated chondritic rocks.
Hydrothermal alterations of basaltic rocks on theseafloor can also generate H2 [e.g., 18], although theabundance of H2 production would be lower than thoseof undifferentiated chondritic rocks.
Similar to icy bodies, H2 is an important secondary
reductant on Mars [e.g., 19,44]. Hydrothermal
alterations of mafic basaltic crusts would havegenerated H2 through serpentinization of Fe(II)-bearing
minerals and Fe(II) oxidation on early Mars [19]. Given
the collision-induced absorption of H2 in theatmosphere, its formation is also important to keep thesurface temperature above freezing point of water onearly Mars [45,46]. Mineralogical evidence forhydrothermally-altered crustal rocks (i.e., the existenceof serpentine and phyllosilicates) has been found on
Mars through remote sensing [e.g., 47,48]. Althoughthese remote-sensing data suggest the occurrence ofhydrothermal alterations and serpentinization on earlyMars, one possibility that may inhibit the efficient H2
production is the presence of abundance CO2 in thefluids [e.g., 49,50]. Effective formation of carbonate
mineral would reduce the number of available Fe2+ foroxidation and could inhibit efficient H2 production [49],which could have happened on early Mars [50].
Another key secondary reductant on early Mars is
Fe2+ ion. Fe2+ can be provided through water-rockreactions by surface water (i.e., chemical weathering).
Although hydrothermal alterations in the subsurface
can also provide Fe2+ into groundwater, itsconcentrations would be very low [51]. This is the casebecause pH of hydrothermal fluids that interact withmafic rocks tend to be alkaline, and because Fe2+ tend
to be incorporated into the solid phase (i.e., minerals)
under alkaline pH conditions [51]. Since pH of thesurface water is suggested to be circumneutral to acidic
owing to a CO2-rich atmosphere and volcanic SO4 onearly Mars, chemical weathering of surface rocks could
have provided Fe2+ to rivers and lakes more efficiently
than hydrothermal alterations [51–53].
Primordial oxidants: As for icy bodies, such as
Enceladus, the major primordial oxidant would be CO2,
originally contained in icy planetesimals as ice (Fig. 2).
Observations of comets suggest that CO2 contents inicy planetesimals would have reached as 1–5 % relative
to H2O [54]. In addition to CO2, primordial O2 formedin molecular clouds can be contained in icy
planetesimals [55]. However, due to the high volatilityof O2, primordial O2 would be available only in cold
regions of the protoplanetary disk (e.g., 20–40 K).
Secondary oxidants: Various secondary oxidants,
including O2, O3, H2O2, SO4, ClO3, and ClO4, havebeen found or suggested on icy bodies and Mars (Fig.
2). Spacecraft and telescope observations show thepresenceofphotochemicallyorionchemically-produced, high-Eh oxidants of O2, O3,
and H2O2 in the current atmosphere of Mars [17]. These
high-Eh oxidants are also found in the tenuous
atmosphere and surface of Europa [e.g., 56]. On Europa,
surface sulfate acid (H2SO4) is also found [57]. These
H2SO4 considered to be generated by high-energy
particle irradiations onto mixtures of H2O and S [e.g.,58]. On Mars, perchlorate salts, such as Ca(ClO4)2,
have been found through in-situ chemical analyses forthe surface soils [e.g., 59]. These perchlorates would be
generated through photochemical reactions of HCl andoxidative radicals, such OH and O, in the atmosphere[e.g., 17]. Since ClO/ClO2/ClO3 are intermediates toform ClO4 from Cl [60], the presence of reactive
chlorate is also suggested on early and current Mars
[61,62].
Figure 2 compares the above-mentioned primordialand secondary reductants/oxidants available on theplanetary bodies in the Solar System. In addition to thephotochemical and hydrothermal reactions that form
the secondary oxidants/reductants shown in Fig. 2,
volcanic activity and bolide impacts could be additionalsources of reductants and oxidants. Owing to the
reducing mantle of Mars compared with Earth, magma
formed by partial melting of Martian mantle would
contain an oxidant of CO2 and reductants of H2 and H2S[e.g., 45,63]. Cometary and asteroidal impacts would
also provide both of oxidants (such as CO and CO2)
and reductants (such as H2) through shock heating oforganic matter and evaporates in target rocks [e.g., 64–66]. Catalytic reactions in impact-induced plumes andin planetary atmosphere may have converted
impact-induced CO, CO2, and H2 into a reductant of
CH4 [67–69].
As for the combination of the primordial oxidant of
CO2 and primordial reductant of organic matter or CH4,
life would be unable to acquire sufficient energy
because the chemical reactions would not proceed
under the P-T conditions of aqueous environments oficy bodies and terrestrial planets (Fig. 2). In order for4Viva Origino 2020, 48, 6

life to acquire sufficient energy (e.g., ATP synthesis for
Earth’s life), a planetary system needs to provide
continuously the secondary oxidants, or secondary
reductants, or both, to life (Fig. 2).
If only primordial oxidants or primordial reductants
are available on a planetary system, this system would
lose habitability when the primordial oxidants or
reductants are consumed by life. As discussed below inSec. 4.1, this situation could occur on Enceladus in
future. Sustainable bioavailable chemical energy overgeological time would be supported by continuous
formation of both of the secondary oxidants andreductants. This can be achieved, for instance, on
Europa, early Mars, Titan, and Earth in the Solar
System (see Sec. 4 below).

early subsurface ocean of Ceres based on the global
appearance of serpentine, NH4-bearing phyllosilicate,and Mg carbonate minerals on the surface [28,29,42].
These secondary minerals are considered to have been
generated in an early subsurface ocean of Ceres, which
was initially formed due to short-lived radiogenic
heating [42,79,80]. The secondary minerals haveappeared on the surface upon freezing of the earlyocean and subsequent sublimation of the surface ice[42]. To form both NH4-bearing phyllosilicate and Mg
carbonate, oceanic pH of Ceres would have been 7–10[28]. The appearance of aliphatic hydrocarbons on
Ceres’ surface suggests that in-situ organic synthesis
proceeded within the reducing early ocean throughhydrothermal reactions [81].
On Enceladus and Ceres, water-rock reactions withthe chondritic rocks results in formation of reducing
and alkaline oceans [21,43]. Water-rock reactions withultramafic chondrite rocks lead to generation ofalkaline-pH fluids (pH ~9–13) [43] and formation ofdissolved H2 through oxidation of metallic Fe andFe(II)-bearing
mineralsinassociationofserpentinization [21,22].
On Enceladus, the major oxidant is the primordial
CO2 (Fig. 2). Using the secondary reductant of H2
formed by hydrothermal reactions, CH4-producing
autotrophic life (methanogenic life) could acquiresufficient energy to maintain biological activities, e.g.,synthesis of ATP, in Enceladus [22] (Fig. 2). Thismeans that CH4 is a potential biomarker on this icy
moon. As described in Sec. 3 above, one important
issue for continuous habitability on Enceladus is thatbioavailable energy would be lost when the primordial
CO2 is consumed by life. Solar UV irradiation on
Enceladus’ surface and Saturn’s E-ring particles could
generate secondary oxidants; however, theirconcentrations in the ocean are very low based on theplume compositions [12]. In addition, the secondaryoxidants need to be provided into the ocean
continuously for sustainable habitability; however, it isunclear whether prolonged and continuous resurfacing
has been promoted on Enceladus. Some theoreticalstudies suggest that Enceladus’ current geologicalactivity started in the recent past and could beshort-lived [e.g., 82]; whereas, others propose that
prolonged ocean and geological activity on Enceladus[e.g., 83,84].
Given the fact that Earth’s life uses available energy
in environments as much as possible, the fact that bothH2 and CO2 coexist in Enceladus’ ocean may raise afundamental question on the habitability; that is, why
the chemical energy is unused by life if it exists in
Enceladus? One possible idea to explain this is that
another factor, such as availability of bioessential metal,
limits the bioactivities within Enceladus. In particular,
Earth’s methanogen requires Ni for an active center ofenzyme [85]; however, the concentration of dissolved
Ni2+ in Enceladus’ ocean would be low given the high

4. Water chemistry, planetary redox, andhabitability
This section summarizes the recent results ofspacecraft and rover observations for water chemistry
and redox states of the hydrospheres on the icy bodies
and terrestrial planets in the Solar System. Based on theidea of planetary redox described in Secs. 2 and 3, we
also discuss the habitability in terms of bioavailable
chemical energy on these planetary bodies.
4.1. Ocean worlds
Several geologically-active icy bodies in the outer
Solar System are known to possess subsurface oceans
beneath the icy crusts [e.g., 1–4]. These icy bodies
having the oceans are called as “ocean worlds”.
Together with tidal heating for some of icy moons
around gas giants, short-/long-lived radioactive
nuclides and exothermic serpentinization are heatsources to form and maintain the subsurface oceans[e.g., 39,70]. The water chemistry in the subsurface
oceans have been investigated directly through in-situ
analyses of plume materials erupting from the oceans(e.g., Enceladus) and indirectly through non-ice
materials, such as salts, on the surface (e.g., Ceres andEuropa) by recent space explorations [22,42,57,71].
Enceladus and Ceres: The in-situ analyses forplume materials by the Cassini spacecraft have revealed
that Enceladus’ ocean contains sodium chloride [72],simple and complex organic molecules [73], nanosilica
and H2 formed by hydrothermal activity within the
rocky core [22,74], and primordial CO2 and NH3 [12].
To form nanosilica and H2 within Enceladus,hydrothermal reactions at temperatures ≥~100 °C are
required [21,74]. Complex organic matter found in
Enceladus’ plume has aromatic compounds and aminefunctions in its structure [73,75]. To explain a series of
the in-situ observations, geochemical models suggestthat Enceladus has a mildly-saline (~1% of NaCl),
reducing (~1–3% of dissolved H2), and alkaline (pH 9–11) ocean [21,72,74,76–78]. In Enceladus’ ocean,hydrothermally-produced H2 and primordial CO2 are in
redox disequilibrium [22].
Similar water chemistry has been proposed for an5Viva Origino 2020, 48, 6

pH. Another possibility is that Enceladus ishabitable—satisfying the availability of ingredients forlife—but it supports no life. This may imply that
Enceladus would not meet the conditions for the
emergence of life. If Enceladus is habitable and lifeless,it would provide unique insights into the conditions forthe emergence of life on planetary bodies, including
Earth.
Europa: The existence of sodium/magnesium
chloride and sulfuric acid (H2SO4) [e.g., 57,86,87] on
Europa’s surface implies that its ocean may be moreoxidizing and circumneutral pH than those of
Enceladus and Ceres [88,89]. Sulfuric acid on thesurface of Europa’s trailing hemisphere is considered tobe generated through UV and/or high-energy particle
irradiations onto mixtures of H2O ice and S [57].
Through resurfacing of the icy crust on Europa, thesurface sulfuric acid would be supplied into the ocean,
providing acidity [15,16]. The provided sulfate would
be consumed through hydrothermal sulfate reduction if
oceanic pH is low (< 7) and basaltic rocks appear on
Europa’s seafloor [89]. In contrast to the sulfate-richtrailing hemisphere, sodium/magnesium chloride
appears in the leading hemisphere, especially ingeologically-active chaotic terrains [57]. The chaotic
terrains are suggested to be formed by eruption of
subsurface liquid water reservoir and represent thesubsurface oceanic composition [e.g., 90]. The
existence of sodium/magnesium chloride in the chaotic
terrains suggests that the major dissolved species ofEuropa’s ocean would be Na+, Mg2+, and Cl-.
Why is the water chemistry different between Europa
and Enceladus? One important factor that determines
the water chemistry on an icy body is the rockcomposition of seafloor [89]. Water-rock reactions withdifferentiated basaltic rocks tend to result in lower pH,
whereas those with undifferentiated chondritic rocks
lead to alkaline pH [89]. In order to generate basaltic
rocks on the seafloor, the rocky mantle needs toexperience high temperatures sufficient to cause rockmelting (~1500 °C or greater), which might have beenachieved for Europa due to strong tidal heating with
Jupiter and radiogenic heating [39]. Another importantfactor is effective formation of acids and oxidants
through UV and high-energy irradiations onto the icy
surface. On Europa, Jupiter’s strong magnetosphereaccelerates electrons and protons of solar winds. These
high-energy particles are irradiated onto Europa’s
surface, forming a variety of oxidants and acid, H2SO4,
O2, O3, and H2O2 [15,16]. Lastly, H2 escape from thesubsurface ocean is another important factor todetermine the redox state of the ocean. Througheffective resurfacing and eruption of oceanic water
induced by strong tidal force of Jupiter, dissolved H2 inthe ocean would have been irreversibly lost to the space,resulting in oxidation and acidification of the ocean[88]. Owing to Europa’s size and massive Jupiter,
formation of basaltic seafloor rocks and high-energy

particle irradiations may have occurred on Europa
effectively than small Enceladus orbiting around lessmassive Saturn. Although no data on H2 escape from
Europa is available, its globally young surface implies
that efficient resurfacing could have induced loss of
substantial amounts of H2 from the subsurface ocean[88].
If an oxidizing ocean occurs on Europa, it is essential
for habitability in terms of the availability of energy inthe subsurface ocean (Fig. 2). When the primordial
reductant of organic matter is available, CO2 would bea potential metabolic product of heterotrophic life
through oxidation of primordial organic matter on
Europa (Fig. 2). If the secondary reductant of H2 areformed through hydrothermal reactions on the seafloor,
H2S could be a metabolic product of sulfate-reducing
autotrophic life using a secondary oxidant of SO4 (Fig.
2). An experimental study indicates that abiotic sulfate
reduction proceeds only at pH < 7 [89]. Thus, if thewater chemistry of Europa’s ocean is circumneutral toalkaline, H2S in the ocean would be a promisingbiomarker. These biogenic H2S could be supplied to thesurface through ice crust convection, and then, could berecycled back to SO4 through the oxidation on thesurface. One issue on habitability on Europa would bewhether continuous supply of surface oxidants occurs
over geological time. Active and continuous
resurfacing of the icy crust is crucial for sustainable
habitability on Europa, which could be science targets
in future missions.
4.2. Terrestrial planets
The oceanic chemistry and planetary climate on
terrestrial planets can be recorded in sedimentary rocks(e.g., evaporates, clays, and salts) and geologicalfeatures (e.g., valley networks, deltas, fans, and gullies).
Unlike the ocean worlds, we can reveal their time
evolutions based on geochemical analyses for
sedimentary rocks and geomorphic analyses forgeological features on both Earth and Mars in the Solar
System (Fig. 3).
Earth: On Earth, the redox state of theocean-atmosphere system has shifted from reducing tooxidizing at around 2.4–2.3 billion years ago (Ga),which is called the Great Oxidation Event or GOE [e.g.,
91–93] (Fig. 3). The GOE is evidenced by a positive
excursion of carbon isotope [e.g., 94], deposition of Fe
oxides [e.g., 95–97], mass independent fractionation of
sulfur [e.g., 98], and oxidative weathering of redox
sensitive elements [e.g., 99,100]. Although the trigger
for the GOE is still in debate, this may have beencaused by cyanobacterial blooming in response to adramatic climate change [e.g., 97,101–104] and/or adecline of a degassing flux of reducing gases [e.g.,
105,106]. The second oxidation in Earth’s history
occurred at around 0.7–0.6 Ga, which is called the
Neoproterozoic Oxidation Event or NOE [e.g., 92,93](Fig. 3). These two oxidation events in Earth’s history

6Viva Origino 2020, 48, 6

seems to coincide with Snowball Earth events [107](Fig. 3). The redox states of the ocean-atmospheresystem would have been also linked to biologicalevolution (i.e., the emergences of Eukaryotes andmulticellular life) [97] (Fig. 3), although themechanisms for the linkages remain poorly constrained
based on factual evidence.
Mars: Unlike Earth, Mars had possessed
abundance of liquid water on the surface only around
3.8–3.5 Ga, evidenced by the occurrence of valley
networks, deltas, lake sediments, Al phyllosilicates, andsulfate deposits [e.g., 47,48,108] (Fig. 3). These
geological features and secondary minerals can beformed through prolonged (e.g., > 103 years)hydrological cycles. Formation models of valley
networks, Al phyllosilicates, and hyposaline paleolakewater on early Mars suggest that they would have been
generated in relatively short-term (e.g., ~104–106 years)
warm and wet climate periods during 3.8–3.5 Ga[8,109,110]. This implies that in 3.8–3.5 Ga, Mars may
have been generally cold and dry, punctuated byepisodic warming periods driven by emission ofgreenhouse effect gas and/or changes in orbital forcing[108–110] (Fig. 3).
Although warm and wet climates were short-lived
compared to Earth, Mars would have experienced adramatic change in pH and redox states of thehydrosphere at around 3.5 Ga [48,111,112] (Fig. 3).

High-resolution remote sensing data suggest that pH of
surface water on early Mars would have been
circumneutral at 3.8–3.6 Ga based on widespreadoccurrence of phyllosilicates [48,111]. At around 3.5
Ga, pH of the surface water would have changed tohighly acidic, based on the occurrence of sulfate-silica
mineral assemblages [52,111]. After 3.4 Ga, theoccurrence of surface water becomes only transient (Fig.
3). The surface was covered with thin, oxidizing soil
layers [48].
At Gale Crater, NASA’s Curiosity rover has
investigated lacustrine sediments to reveal the
paleo-environment of closed-basin lakes developed inearly Hesperian (~3.7–3.5 Ga) [13,113,114]. According
to the mineralogical and chemical compositions of thelake sediments [e.g., 113,114], pH of early Gale lakes issuggested to have been circumneutral to weakly
alkaline [110,115]. The water chemistry of the earlylake is suggested to have been Na-Cl type and mild
salinity (~0.3–1 %) [110]. This composition is similar
to those of closed-basin lakes developed in semiarid
climate zones on Earth [110,116].
The Curiosity rover has found that Gale’s lacustrine
sediments preserve redox disequilibria in the secondary
minerals (e.g., existence of Fe (hydro)oxides and Fe
smectite) [110,113,117]. One suggested idea for thedisequilibria is a redox stratification within the earlyGale lake [117]. In this scenario, shallow-surface lake

Figure 3. A comparison of evolution of aqueous environments on Earth and Mars. Earth’s climate has been generally warm tosupport oceans on the surface. Earth has experienced two-step oxidations of the atmosphere-surface system; that is, the Great
Oxidation Event (GOE) at around 2.4 Ga and the Neoproterozoic Oxidation Event (NEO) at around 6 Ga. In the GOE, theatmospheric O2 levels rose to ~0.1–1 % of the present atmospheric level; whereas, the deep oceans remained as reducing. Afterthe NOE, the entire of the oceans has been oxidized. This redox shift seems to link to dramatic climate changes (i.e., Snowball
Earth: white triangles) and evolution of life (the three arrows). On Mars, abundant surface water was available only at around
3.8–3.5 Ga. During this period, climate is suggested to have been generally cold, punctuated by episodic warming. The redox
states and pH of surface environments have evolved dramatically from 3.8 to 3.4 Ga, characterized as large-scale acidificationand oxidation at around 3.5 Ga. See the main text for the details and references.

7

Viva Origino 2020, 48, 6

water contained high levels of oxidants, such as O2 andoxychlorides, formed by atmospheric and surface
reactions induced by Solar UV light [61,62,117–119].
Photo-oxidation of Fe2+ in the shallow lake water isanother source of oxidants [112]. From the bottom of
the lakes, reducing and alkaline groundwater could
have upwelled, providing secondary reductants (such as
H2 and Fe2+) into the lakes [19,50,51]. Authigenic
deposition of green rust—a precursor of magnetite in
the lake sediments—would have also generated H2upon its formation within the early lakes [120]. If theredox stratified lake formed at Gale on early Mars, bothof the oxidants from the atmosphere and reductantsfrom the subsurface would have been available at theredox boundary within the lake.
Alternative interpretation of the redox disequilibria
inGale’s sediments is a post-depositional intrusionof oxidizing and acidic fluids into the reducing
lacustrine sediments [110,113]. In this scenario, theoriginal lake environments were reducing [110,113]. A
later transient rewetting event would have supplied
oxidizing and acidic fluids to the subsurface sediments
and would have partly oxidized the sediments byforming Fe(III) (hydro)oxides [113]. In fact, the lake
sediments contain Ca sulfate veins [13]. The in-situ
K-Ar dating for the Ca sulfate veins suggests thatoxidative and acidic post-depositional alteration may
have occurred in a much later period (e.g., ~2 Ga) [121],
although the formation mechanisms of oxidizing andacidic water are unclear. If the latter scenario of
intrusion of oxidizing and acidic fluids is the case,secondary oxidants could have been accumulated
within the surface ice and soils [17] A transientwarming on early Mars could have melted oxidizing ice
and have driven redox interactions in aqueous
environments [110].
Through pyrolysis analyses for Gale’s lacustrine
sediments, the Curiosity rover also found organic
matter [e.g., 122–124]. The organic contents in Gale’s
sediments are generally very low (in order of 100–300
ppb.) [122]; however, organic matter with order of ppm
levels were found in ~3.5-Ga mudstone of Gale’s lake
sediments [123]. The organic matter in the mudstone
contains high levels of sulfur as thiophenic and thiol
compounds, and 5% carbon as organic sulfur molecules[123]. This characteristic of the organic matter isdistinct from those in carbonaceous chondrites,
Enceladus’ plume, or Earth’s sediments [123]. The
stable isotope analysis for pyrolyzed organic mattershows that there were multiple sources of organic
matter in Gale’s sediments, including exogenic inputs
from meteorites as well as in situ organic synthesis on
early Mars [124]. NASA and ESA’s Mars Sample
Return (MSR) missions may provide the detailed
structure and chemical compositions of organic matterpreserved in the ancient lake sediments at Jezero Crater
deposited in ~3.8 Ga.
After the periods of circumneutral pH and relatively

reducing aqueous environments, Mars would haveexperienced acidification and oxidation [111]. At
Meridiani Planum, Mars Exploration Rover,Opportunity, has found large amounts of Fe sulfate,jarosite, in the middle to late Hesperian sediments,
possibly at around 3.5 Ga [125]. Jarosite isthermodynamically stable only at low pH 2–4 [52],strongly suggesting the occurrence of highly acidic
surface water at ~3.5 Ga [52]. Such highly acidic water
may be generated by photo-oxidation of Fe2+ ions
induced by solar UV light [112]. Massive depositionsof sulfate minerals, together with silica deposits, arefound widespread on Martian surface in the age of 3.5–
3.0 Ga through remote sensing [111]. The sulfate-silica
mineral assemblages are usually formed via chemical
weathering by highly acidic water [126].
After the oxidation and acidification at around 3.5
Ga, the redox boundary would have moved downward
in deep subsurface on Mars [e.g., 19,44]. Secondaryoxidants on the surface could be transported to reducing
subsurface through infiltration and movement of
surface fluids [127]. Although liquid water became
thermodynamically unstable on the surface of Mars
after the middle Hesperian (~3.5 Ga), it still can exist as
liquid in the subsurface owing to high pressures andtemperatures due to geothermal heating [19,47]. Thesesuggest that deep subsurface hydrosphere would becentral of biosphere, if exists, after ~3.5 Ga on Mars.
If life exists on the oxidizing surface of today’s Mars,
they could live only in localized “hot spots”, where
reductants are available. These hot spots include thelocations where upwelling of deep reducing
groundwater could occur. Previous studies proposedthat upwelling of deep groundwater might occur
seasonally along with fractures of bedrocks within
impact craters, where recurring slope lineae [e.g.,128]—dark streak features appeared seasonally on
steep slopes—can be found on current Mars [129].
Another possibility of supply of reductants to thesurface is destabilization of CH4 clathrate hydrate inpermafrost [e.g., 130,131]. Spikes of ppb levels of CH4
are observed at Gale Crater by the Curiosity rover [e.g.,132]. According to the atmospheric circulation model,
CH4 that generated the spikes is likely to originate froma permafrost area near Gale Crater [131]. A comparisonof a measured duration of a CH4 spike and results ofphotochemical models suggests that atmospheric CH4needs to be consumed very rapidly through unknown
processes near the surface [131]. These two localized
hot spots on current Martian surface—recurring slope
lineae and permafrost areas—could be science targets
for future landing missions.
5. Conclusions
The present paper proposed an idea of planetary
redox—a planetary-scale system that can provide
chemical energy to life through geohydrologicalcycles—developed in the icy bodies and on the8Viva Origino 2020, 48, 6

terrestrial planets in the Solar System. Based on the
results of recent explorations by spacecrafts and rovers,
we discussed the availability of primordial andsecondary reductants/oxidants on these planetary
bodies.
In the subsurface oceans of Enceladus and early
Ceres, both of the primordial and secondary reductants(i.e., organic matter and H2) would have been abundant;
whereas, primordial CO2 would be the only majoroxidant. Given the existence of reducing andorganic-rich aqueous environments, Enceladus andCeres might be suitable for chemical evolution oforganic matter in hydrothermal environments overgeological time. Because of low production capacity of
secondary oxidants, these icy bodies would have lost
habitability when primordial CO2 is consumed by
autotrophic life.
On the other hand, Europa and early Mars would becapable of generating both of the secondary reductant
and oxidants. If the primordial organic matter isavailable on Europa, heterotrophic life could emerge inthe subsurface ocean. One major issue for Europa’s
habitability is the occurrence of continuous resurfacingof icy crust that guarantees to provide the surface
oxidants to the subsurface ocean. On current Mars, thebiosphere, if exists, could have been located in deep
subsurface, because the redox boundary would havebeen moved downward after the surface oxidation andacidification, and because liquid water would be alsoavailable in subsurface. If we seek for living life onoxidizing Martian surface, we need to access tolocalized “hot spots”, where subsurface reductants
could be upwelled seasonally (e.g., upwelling of deep
groundwater or permafrost melting).
The origin-of-life problem may be unsolvable on
Earth because of lack of the geological records ofHadean Earth. On the other hand, geological records ataround 4 Ga are abundant on Mars [e.g., 44]. On
Europa and Enceladus, direct sampling of subsurface
oceanic water would be possible through capturing
plume materials by spacecrafts and in-situ analyses bylanders [e.g., 133–136]. Findings of the presence or
non-presence of life on these Solar System bodies
would provide unique insights into the conditions
required for the origin of life on Earth.

2.
3.
4.
5.
6.

7.

8.

9.
10.
11.
12.
13.
14.

15.

16.

Acknowledgements: This paper is financially
supported by Grant-in-Aid KAKENHI from the
Ministry of Education, Culture, Sports, Science, andTechnology (MEXT) (Grant Nos. JP17H06454,
JP17H06456) and from Japan Society of Promotion of
Science (JSPS) (Grant No. JP20H00195). YS thanks to
Y. Kebukawa for providing this opportunity to write
this paper.

17.
18.

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