May 29, 2010 7 Comments
Jupiter has so many moons which may possibly harbor exotic life including major candidates for exotic life Europa and Ganymede. Although Jupiter itself is suggested as to be capable of harboring jovian life. But today I read a article published on cosmology magazine which suggested that life could be possible on Jupiter’s moon Io.
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 . 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.
Unlike the Earth and the Moon, Io’s main source of internal heat comes from tidal dissipation rather than radioactive isotope decay, the result of Io’s orbital resonance with Europa and Ganymede. Such heating is dependent on Io’s distance from Jupiter, its orbital eccentricity, the composition of its interior, and its physical state. Its Laplace resonance with Europa and Ganymede maintains Io’s eccentricity and prevents tidal dissipation within Io from circularizing its orbit. The resonant orbit also helps to maintain Io’s distance from Jupiter; otherwise tides raised on Jupiter would cause Io to slowly spiral outward from its parent planet. The vertical differences in Io’s tidal bulge, between the times Io is at periapsis andapoapsis in its orbit, could be as much as 100 m (330 ft). The friction or tidal dissipation produced in Io’s interior due to this varying tidal pull, which, without the resonant orbit, would have gone into circularizing Io’s orbit instead, creates significant tidal heating within Io’s interior, melting a significant amount of the moon’s mantle and core. The amount of energy produced is up to 200 times greater than that produced solely from radioactive decay. This heat is released in the form of volcanic activity, generating its observed high heat flow (global total: 0.6 to 1.6×1014 W). Models of its orbit suggest that the amount of tidal heating within Io changes with time, and that the current heat flow is not representative of the long-term average.[ref:wikipedia]
Io has 100 to 150 mountains. These structures average 6 km (4 mi) in height and reach a maximum of 17.5 ± 1.5 km (10.9 ± 0.9 mi) at SouthBoösaule Montes.
Io has an extremely thin atmosphere consisting mainly of sulfur dioxide (SO2), with minor constituents including sulfur monoxide (SO),sodium chloride (NaCl), and atomic sulfur and oxygen. The atmosphere has significant variations in density and temperature with time of day, latitude, volcanic activity, and surface frost abundance. The maximum atmospheric pressure on Io ranges from 0.33 × 10-4 to 3 × 10-4 Pascals (Pa) or 0.3 to 3 nbar, spatially seen on Io’s anti-Jupiter hemisphere and along the equator, and temporally in the early afternoon when the temperature of surface frost peaks. Localized peaks at volcanic plumes have also been seen, with pressures of 5 × 10-4to 40 × 10-4 Pa (5 to 40 nbar). Io’s atmospheric pressure is lowest on the moon’s night-side, where the pressure dips to 0.1 × 10-7 to 1 × 10-7 Pa (0.0001 to 0.001 nbar). Io’s atmospheric temperature ranges from the temperature of the surface at low altitudes, where sulfur dioxide is in vapor pressure equilibrium with frost on the surface, to 1800 K at higher altitudes where the thinner atmospheric density permits heating from plasma in the Io plasma torus and from Joule heating from the Io flux tube. The low pressure limits the atmosphere’s effect on the surface, except for temporarily redistributing sulfur dioxide from frost-rich to frost-poor areas, and to expand the size of plume deposit rings when plume material re-enters the thicker dayside atmosphere. The thin Ionian atmosphere also means any future landing probes sent to investigate Io will not need to be encased in an aeroshell-style heatshield, but instead will requireretrorockets for a soft landing. The thin atmosphere also necessitates a rugged lander capable of enduring the strong Jovian radiation, which a thicker atmosphere would attenuate.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.protection from radiation
Gas in Io’s atmosphere is stripped by Jupiter’s magnetosphere, escaping to either the neutral cloud that surrounds Io, or the Io plasma torus, a ring of ionized particles that shares Io’s orbit but co-rotates with the magnetosphere of Jupiter. Approximately one ton of material is removed from the atmosphere every second through this process so that it must be constantly replenished. The most dramatic source ofSO2 are volcanic plumes, which pump 104 kg of sulfur dioxide per second into Io’s atmosphere on average, though most of this condenses back onto the surface. Much of the sulfur dioxide in Io’s atmosphere sustained by sunlight-driven sublimation of SO2 frozen on the surface. The day-side atmosphere is largely confined to within 40° of the equator, where the surface is warmest and most active volcanic plumes reside. A sublimation-driven atmosphere is also consistent with observations that Io’s atmosphere is densest over the anti-Jupiter hemisphere, where SO2 frost is most abundant, and is densest when Io is closer to the sun. However, some contribution from volcanic plume are required as the highest observed densities have been seen near volcanic vents. Because the density of sulfur dioxide in the atmosphere is tied directly to surface temperature, Io’s atmosphere partially collapses at night or when the satellite is in the shadow of Jupiter. The collapse during eclipse is limited somewhat by the formation of a diffusion layer of sulfur monoxide in the lowest portion of the atmosphere, but the atmosphere pressure of Io’s nightside atmosphere is two to four orders of magnitude less than at its peak just past noon. The minor constituents of Io’s atmosphere, such as NaCl, SO, S, and S derive either from: direct volcanic outgassing;photodissociation, or chemical breakdown caused by solar ultraviolet radiation, from SO2; or the sputtering of surface deposits by charged particles from Jupiter’s magnetosphere..[ref:wikipedia]
Possibility Of Life
It is suggested that possibility of life on Io is low and consideration are mostly based on an extremely energetic plasma particle interaction with Jupiter, 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. The complex thermal profile is very dynamic as evidenced by the movement of lava across snowfields of SO2 at about 5 m/day.
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. This combination of liquid water and geothermal heat could have made the origin of life plausible early in Solar System history. 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 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 , through this is controversial. 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. 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.
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 and caves in general are a great model for potential subsurface ecosystems.
Lava tubes on Io may be an ideal habitat for any remaining microbial life, because they provide:
- Protection from radiation
- Insulation to keep temperatures sufficient high and constant
- Trap moisture
- Provide nutrients such as sulfide and H2S that could be oxidized to sulfur dioxide or sulfates.
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 which may affect alien life are shown in diagram below
[image credit: cosmology magazine]
The another basic prospect for life is solvents that could be available on that planet. A solvent is required because it fascilates:
- An environment that allows for the stability of some chemical bonds to maintain macromolecular structure
- Promoting the dissolution of other chemical bonds with sufficient ease to enable frequent chemical interchange and energy transformations from one molecular state to another
- The ability to dissolve many solutes while enabling some macromolecules to resist dissolution, thereby providing boundaries, surfaces, interfaces, and stereochemical stability
- Density sufficient to maintain critical concentrations of reactants and constrain their dispersal
- The 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
- A buffer against environmental fluctuations. 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 since
- 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.
Possibility is that 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. A study suggested that 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. 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.
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 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 liquids on Io. However a little chance of life still chance exhibit for life on Io, considering the major gradients necessary for life on our own basis. What a exotic life needs, who knows?
[ref: A research work by Durk Schulze -Makuch, Cosmology Magazine]