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Journal of Cosmology, 2010, Vol 5, 912-919. Cosmology, November 10, 2009 Dirk Schulze-Makuch, Ph.D., School of Earth and Environmental Sciences, Washington State University, Pullman, WA
Key Words: Io, microbial life, sulfur, lava, solvent 1. Introduction
Io, the innermost of Jupiter’s Galilean moons, is the most volcanically active planetary
body in the Solar System. With a diameter of 3630 km it is roughly the size of our Moon.
Io’s density is about 3.55 g/cm3, which implies that Io has a relatively large iron core.
This is also consistent with the presence of a magnetic field (e.g., Kivelson et al. 1996).
However, the enormous tidal forces to which Io is exposed to by Jupiter on one side, and
the other Galilean satellites on the other side, may have prevented a clear differentiation
into core, mantle, and crust. Io has a slightly eccentric orbit and this eccentricity causes
tidal amplitudes of 100 m or more.
 Fig 1: Io as viewed from space from different viewponts. Credit: NASA/JPL
Io is exotic for us in many ways: observations by the Galileo spacecraft indicate that it has an active lava lake, perhaps crusted over (e.g., Spencer et al. 2000b), an active lava curtain, active lava flows, calderas, mountains, plateaus, and plains (McEwen et al. 2000). The highest mountain is 16 +/- 2 km high (Schenk and Bulmer 1998). Io has no significant atmosphere and its mean surface temperature is about 143 K. However, near volcanic centers temperatures reach up to 1900 K or more (McEwen et al. 2000), high enough to keep the lava, which consists of sulfur compounds or mafic silicates, liquid. In addition, solidified liquid SO2 flows are thought to be present on the surface as well (Carlson et al. 2007). Some volcanic vents appear to eject gaseous SO2 into the atmosphere (Strom and Schneider 1982), which later condenses as frost on Io’s surface (Lopes-Gautier et al. 2000). Hot lava flows through lava tubes and pours over some of the SO2 snowfields. No impact craters were found on Io underlining its youthful surface which is estimated to be less than 1 million years old (Johnson and Soderblom 1982). Much of the colorful appearance of Io’s surface (Fig. 1-3) appears to be caused by polymerization of S2 gas (Spencer et al 2000a).
 
Figures 2 & 3. Io with circular deposit of Pele, 1200 km in diameter (bright red color). The plume was more than 400 km high and rich in S2 and SO2 gas. Image taken by Galileo during its sixth orbit. Credit: NASA/JPL.
2. A Chance for Life?
The plausibility for life on Io has been rated as low with a Plausibility Index of IV by
Irwin and Schulze-Makuch (2001) and also more recently by Schulze-Makuch and Irwin
(2008). Category IV is defined as a persistence of life being conceivable in isolated
habitats, or a reasonable inference of past conditions that could have been suitable for the
origin of life prior to the development of conditions so harsh as to make its perseverance
at present unlikely but conceivable in isolated habitats.
The negative ranking of Io is
mostly based on an extremely energetic plasma particle interaction with Jupiter (Geissler
et al. 1999), the lack of detected organics on Io’s surface, and the existence of only an
extremely thin atmosphere devoid of detectable water vapor. Temperature gradients are
extreme and temperatures on the surface are mostly very cold. However, hot spots in the
500-600 K range are common, and the median temperature of the Loki Patera caldera
floor is 273 K (Lopes-Gautier et al. 2000). The complex thermal profile is very dynamic
as evidenced by the movement of lava across snowfields of SO2 at about 5 m/day
(Kieffer et al. 2000).
Though Io certainly cannot be considered a benign habit for life by any definition,
models suggest that Io formed at an average temperature of 250 K in a region of the solar
system where water ice is plentiful (Consolmagno and Lewis 1976). This combination of
liquid water and geothermal heat could have made the origin of life plausible early in
Solar System history (Irwin and Schulze-Makuch 2001). As water was lost on the surface
from Jupiter’s radiation, life could have retreated to the subsurface. Water and carbon
dioxide may still be abundant in Io’s subsurface - though the driving force for the vents
on Io is more likely sulfur dioxide or other sulfur compounds (McEwen et al. 2000).
Salama et al. (1990) suggested that water is present on Io in ppm-levels based on a
suggestive band at 3.15 μm in the spectra of Io.
Geothermal activity and reduced sulfur compounds could still provide microbial life with
sufficient energy sources. Particularly, hydrogen sulfide is probably a common
compound in Io’s subsurface. Hydrogen sulfide has been claimed to be present in trace
amounts in Io’s ionosphere (Zolotov and Fegley 1999), through this is controversial
(McGrath et al. 2004). In the atmosphere it would rapidly oxidize by the ionic radiation
from Jupiter. H2S, however, has often been suggested as a surface component based on
features at 3.9 μm in the spectra of Io (Nash and Nelson, 1979; Nash and Howell, 1989;
Salama et al. 1990, Carlson et al. 2007). Aside from various sulfur polymers (e.g., S2, S4,
S8), NaCl has been identified as minor plume species, and probably KCl, atomic chlorine
that may produce ClSO2 and related compounds (Carlson et al. 2007).
Volcanic activity is prevalent on Io and lava tubes resulting from that activity could present a favorable habitable environment.
Microbial growth is common in lava tubes on Earth, independent of location and climate,
from ice-volcano interactions in Iceland to hot sand-floored lava tubes in Saudi Arabia.
Lava tubes also are the most plausible cave environment for life on Mars (Boston 2003;
Boston et al. 2006) and caves in general are a great model for potential subsurface
ecosystems.
 Fig 4. Vocanic eruption on Io with a plume expanding into space. Credit: NASA/JPL. Lava tubes on Io may be an ideal habitat for any remaining microbial life, because they provide (1) protection from radiation, (2) insulation to keep temperatures sufficient high and constant, (3) trap moisture, and (4) provide nutrients such as sulfide and H2S that could be oxidized to sulfur dioxide or sulfates (Schulze-Makuch et al. 2004a). Any deep enough subsurface environment on Io would provide a radically different environment for life than found at the surface by protecting it from the hazardous surface conditions of both radiation and desiccation. The major environmental factors in regard to possible life on Io are summarized in Fig. 5.
 Fig. 5. Major environmental conditions on Io with relevance to possible life.
3. Possible Solvents
Any life needs a suitable solvent because of the complex chemical interactions that are
required for life processes. These include (1) an environment that allows for the stability
of some chemical bonds to maintain macromolecular structure, while (2) promoting the
dissolution of other chemical bonds with sufficient ease to enable frequent chemical
interchange and energy transformations from one molecular state to another; (3) the
ability to dissolve many solutes while enabling some macromolecules to resist
dissolution, thereby providing boundaries, surfaces, interfaces, and stereochemical
stability; (4) a density sufficient to maintain critical concentrations of reactants and
constrain their dispersal; (5) a medium that provides both an upper and lower limit to the
temperatures and pressures at which biochemical reactions operate, thereby funneling the
evolution of metabolic pathways into a narrower range optimized for multiple
interactions; and (6) a buffer against environmental fluctuations (Schulze-Makuch and
Irwin 2008). Though water was likely the solvent of choice for any early putative life on
Io, this might have drastically changed later on. Given the current environmental
conditions on Io, hydrogen sulfide, sulfur dioxide, and sulfuric acid provide possible
options.
Hydrogen sulfide remains a liquid at temperatures from 187 to 213 K (at 1 bar) and thus
falls within the environmental conditions that would prevail in the shallow subsurface of
Io. However, its temperature range as a liquid is only 26 degrees. Hydrogen sulfide does
not moderate temperatures very well, given its low heat of fusion (2.4 kJ/mol), heat of
vaporization (18.7 kJ/mol) and dielectric constant (5.9). It is not particularly efficient as
an ionic solvent, given its low dipole moment (0.98), but it does dissolve many
substances, including many organic compounds. Similarly to water, hydrogen sulfide
dissociates into H+ and SH-. In a biochemical scheme with H2S as solvent, the SH- anion
could simply replace the hydroxyl group in organic compounds. Hydrogen sulfide
appears to be reasonably abundant in the subsurface of Io and a subsurface layer of
hydrogen sulfide could turn liquid when overhead lava warms the subsurface layer up to
its range of liquidity. If any putative life on Io would have developed dormant forms such
as spores, then these spores could become activated, reproduce, and form an exotic
subsurface microbial ecosystem (Schulze-Makuch and Irwin 2008).
In this type of environment another sulfur solvent, sulfur dioxide, may compete with
hydrogen sulfide. Sulfur dioxide has a dipole moment of 1.6 and remains a liquid at
temperatures from 198 to 263 K (at 1 bar). SO2 is clearly abundant on Io. However, due
to the double bond in sulfur dioxide the development of a biochemical scheme would be
more complicated for SO2 than for H2S, because rearrangements would be needed. Also,
SO2 is not a proton-based solvent. Proton-based solvents have the advantage that organic
macromolecules such as nucleic acids are constructed via hydrogen bonds and are able to
exchange materials with the solvent or change their formation for biological purposes
without having to overcome a high-energy barrier (Schulze-Makuch and Irwin 2008).
Interestingly, sulfates including sulfuric acid (H2SO4) are insoluble in sulfur dioxide, and
would thus be rock material in a pool of sulfur dioxide.
Sulfuric acid is another potential solvent for microbial life. Benner (2002) suggested
sulfuric acid (H2SO4) as a possibility for Venus and indeed the Venusian atmosphere is
rich in sulfuric acid. Sulfuric acid has a huge liquidity range from 283 to 610 K (at 1
bar), a dielectric constant (101) and dipole moment (2.7) larger than water, and extremely
high viscosity (0.26 P). The prospect of pure sulfuric acid as life-sustaining solvent
would require a biochemistry very different to the one which we are familiar with.
However, sulfuric acid mixes well with water and if life exists in the atmosphere of
Venus (e.g, Schulze-Makuch et al. 2004b), it would be expected to be adapted to a
sulfuric-acid water mixture. This option may also be a possibility for Io, though only in
the highly heated areas near volcanic vents. Also, the scarcity of water on Io would make
the presence of large amounts of sulfuric acid less likely.
Schulze-Makuch and Irwin (2008) performed a qualitative assessment on possible
solvents on Io (among other planets and moons in our Solar System). They concluded that
the special circumstances of Io make it difficult for any solvent to function here, but that
a combination of water and H2S might work beneath Io’s surface. One possible microbial
survival strategy in this type of environment would be that microorganisms remain in a
dormant-type of state most of the time and are reverting back to a vegetative state only
when heated by nutrient rich lava flows.
4. Possible Chemical Building Blocks and Available Energy
Not much is known about possible building blocks of life on Io or even the subsurface
chemistry of this volcanic moon. All what can be said is that the organic macromolecules
that would have to be there to support life-sustaining metabolic reactions would have to
be consistent with the liquidity requirements of the fitting solvent, probably H2O, H2S,
SO2, or H2SO4, with each of these potential solvents requiring a different temperature
range.
There is no evidence so far of any organic molecules on Io and little hint of carbon
of any kind other than trace amounts of CO and CO2. In case of Io perhaps sulfur might
play a larger role as a potential building block of life. Sulfur occurs in a large number of
oxidation states, even fractional ones, forms a variety of polymers and ring structures,
and could conceivably form polymers with carbon, nitrogen, oxygen, and phosphorus on
this Galilean moon (Irwin and Schulze-Makuch, 2010). Though a large amount of sulfur
has been detected in a variety of chemical compounds present on Io, there is no evidence
for any type of complex compounds that would come close to biology. We have to be
cautious, however, not to dismiss the case for organic molecules out of hand, because
even if they would be there, it would be very challenging to detect them based on their
extremely low residence time in Io’s atmosphere, which is due to the harsh radiation
environment.
Energy, on the other hand, is plentiful on Io, from electromagnetic radiation to heat, and
also gravitational energy, which is responsible for the tidal forces. Life on Earth is based
on chemical energy, redox-reactions in particular, and light energy. Chemical energy
would be an option for life on Io as well as the presence of both reducing and oxidizing
compounds attests (for example H2S, S2-8, and SO2). Thus, metabolic reactions would be
feasible, in principle. Light from the Sun is dim in the Jovian system and ionic radiation
appears unsuitable as an energy source for life, but Jupiter’s strong magnetic field opens
up the possibility that perhaps magnetic energy could be an alternative energy source for
Io’s subsurface.
5. Conclusions
Based on a consideration of possible life-sustaining solvents, organic building blocks,
and energy sources, the plausibility of life on Io has to be considered low. Certainly,
Europa and also Ganymede are the higher priority targets for astrobiology in the Jovian
system. Nevertheless, there could conceivably be a habitable niche in the shallow
subsurface, particularly in lava tube caves on Io, an idea which we can not dismiss
without further investigation. Thus, when launching the next mission to the Galilean
satellites (e.g., the Europa Jupiter System Mission), Io should not be neglected as a
worthwhile target. Much insight could be gained by sending a radiation-resistant robotic
probe capable of detecting the chemistry and physical state of subsurface and surface
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