Interesting. Is what Greg Cochran said here and below reasonable?
“A lot of ice moons seem to have interior oceans, warmed by tidal flexing and possibly radioactivity. But they’re lousy candidates for life, because you need free energy; and there’s very little in the interior oceans of such system.”
A primary candidate for free energy in icy moons is thermal venting at the bottom of the liquid oceans; they do have rocky cores, after all. If Jupiter’s tidal forces can cause the volcanism on Io, then it’s reasonable to assume that they can also cause the rocky interior of Europa to produce volcanoes that vent heat and interesting ions in to the liquid water.
There’s also a surprising amount of electrolysis going on in the ice of Europa, because Jupiter has such a terrifying electrical field. I doubt that’s enough to sustain an ecosystem, but it’s enough for me to fantasize about giant upside-down forests of filter-feeders digging their roots upwards to get at the free oxygen.
The preliminary results we’re seeing on Pluto should also adjust your expectations in favor of ice-moon habitability; there, we see active tectonics on a Kuiper Belt Object even without the tidal forcing of a nearby planet. It seems that a giant pile of silicates and water ice provide a great deal of dynamism all on their own.
Earth gets, on average, 340 watts per square meter of sunlight (more at the equator than the poles and more during the day than at night) for a total flux of 1.7 10^17 watts. Earth also has a geothermal heat flux (partially from primordial heat of formation, partially tidal heating, partially radioactive heating) of 4.7 10^13 watts for 0.087 watts per square meter (concentrated in hot spots of course). Our geothermal flux is thus about 1⁄4000 our solar flux. Only a fraction of the geothermal energy flux will be in a form available to living things, specifically that which causes geochemical gradients to form. Though geothermal effects can also bring deep substances into contact with surface substances and allow them to interact and produce more energy than is contained in the gradient—like serpentinization, by which Fe2+ in particular very deep rock types plus surface water become Fe3+ oxides and hydrogen gas for a net release of energy. I am unprepared to compare the energy of dredged up chemicals to the heat flux. I do know that living things on Earth can easily live off these fluxes at hot springs and vents.
Europa is estimated to receive 7 * 10^12 watts of tidal heating driving geothermalisms (a full seventh that of Earth for something only 0.008 times as massive and 1⁄16 as much surface area as Earth, though one or two other sources I found have estimates a factor of two or three higher). Its radioactive heating is negligible compared to that number. This gives it an average geothermal energy flux of 0.23 watts per square meter, about 1/1500 what we get from the sun, a fraction of which becomes geochemical gradients accessible to life. There will be more geothermal action going on in there than on Earth. Again these geothermal flows could also dig up already-reactive substances from deep below that could react with substances in the ice/ocean, increasing the available energy.
There is indeed a bunch of water cracking happening at its icy space-exposed surface via radiation, with hydrogen sputtering off into space and oxidized compounds winding up in the ice which is believed to circulate down on megayear timescales into the lower layers and potentially into the liquid layers. This would allow another energy source via the oxidation of minerals or hydrocarbons dissolved in the liquids, which life could insert itself into as a middleman.
All the forthcoming numbers I am using are from “Energy, Chemical Disequilibrium, and Geological
Constraints on Europa” by Hand et al. About 4 watts per square meter of sunlight is absorbed by the surface (of about 13 watts of total average incident light), only a tiny fraction of which would cause water-cracking. It gets about 0.125 watts per square meter of particle irradiation. If we assume a ridiculously unrealistically high far-upper-bound of 0.25 watts per square meter of water-cracking which generates oxygen at 237 kilojoules per mole, and that oxidizes iron from, say, a metallic state to rust (about 550 kilojoules per mole of oxygen gas) you get something like half a watt per square meter of oxygen-based energy flow.
That is DEFINITELY a drastic overestimate though. The paper mentioned above goes into some analysis I am utterly unprepared to comment on and suggests that given the surface age of Europa and its energetic environment, up to about 5 * 10^9 moles of ‘oxidants’ are delivered to the interior of Europa per year. Let’s be completely naive and just compare that to the total photosynthetic flux of the Earth, assuming it’s all oxygen. It comes to something like one millionth the photosynthetic productivity of Earth.
Of course, since these matter flows via geothermalisms or crust downwelling are very uneven compared to incident sunlight there will be localized hotspots like our own geothermal vents where things could be much more interesting than the above numbers would seem to indicate.
They’re great candidates for life, especially given mounting ideas on the origin of life possibly being tied up with geochemistry and geology.
They’re lousy candidates for big biospheres that utterly transform the geochemistry such that you could see it far away like ours has, or for complex life, since the total energy flux available in potentially clement environments is miniscule compared to here. But Earth’s biosphere had to start with something a lot like the energy sources you have in the icy moons, not photosynthesis.
Interesting. Is what Greg Cochran said here and below reasonable?
“A lot of ice moons seem to have interior oceans, warmed by tidal flexing and possibly radioactivity. But they’re lousy candidates for life, because you need free energy; and there’s very little in the interior oceans of such system.”
A primary candidate for free energy in icy moons is thermal venting at the bottom of the liquid oceans; they do have rocky cores, after all. If Jupiter’s tidal forces can cause the volcanism on Io, then it’s reasonable to assume that they can also cause the rocky interior of Europa to produce volcanoes that vent heat and interesting ions in to the liquid water.
There’s also a surprising amount of electrolysis going on in the ice of Europa, because Jupiter has such a terrifying electrical field. I doubt that’s enough to sustain an ecosystem, but it’s enough for me to fantasize about giant upside-down forests of filter-feeders digging their roots upwards to get at the free oxygen.
The preliminary results we’re seeing on Pluto should also adjust your expectations in favor of ice-moon habitability; there, we see active tectonics on a Kuiper Belt Object even without the tidal forcing of a nearby planet. It seems that a giant pile of silicates and water ice provide a great deal of dynamism all on their own.
How does the free energy on an ice moon compare to the amount the sun shoots down at earth?
Earth gets, on average, 340 watts per square meter of sunlight (more at the equator than the poles and more during the day than at night) for a total flux of 1.7 10^17 watts. Earth also has a geothermal heat flux (partially from primordial heat of formation, partially tidal heating, partially radioactive heating) of 4.7 10^13 watts for 0.087 watts per square meter (concentrated in hot spots of course). Our geothermal flux is thus about 1⁄4000 our solar flux. Only a fraction of the geothermal energy flux will be in a form available to living things, specifically that which causes geochemical gradients to form. Though geothermal effects can also bring deep substances into contact with surface substances and allow them to interact and produce more energy than is contained in the gradient—like serpentinization, by which Fe2+ in particular very deep rock types plus surface water become Fe3+ oxides and hydrogen gas for a net release of energy. I am unprepared to compare the energy of dredged up chemicals to the heat flux. I do know that living things on Earth can easily live off these fluxes at hot springs and vents.
Europa is estimated to receive 7 * 10^12 watts of tidal heating driving geothermalisms (a full seventh that of Earth for something only 0.008 times as massive and 1⁄16 as much surface area as Earth, though one or two other sources I found have estimates a factor of two or three higher). Its radioactive heating is negligible compared to that number. This gives it an average geothermal energy flux of 0.23 watts per square meter, about 1/1500 what we get from the sun, a fraction of which becomes geochemical gradients accessible to life. There will be more geothermal action going on in there than on Earth. Again these geothermal flows could also dig up already-reactive substances from deep below that could react with substances in the ice/ocean, increasing the available energy.
There is indeed a bunch of water cracking happening at its icy space-exposed surface via radiation, with hydrogen sputtering off into space and oxidized compounds winding up in the ice which is believed to circulate down on megayear timescales into the lower layers and potentially into the liquid layers. This would allow another energy source via the oxidation of minerals or hydrocarbons dissolved in the liquids, which life could insert itself into as a middleman.
All the forthcoming numbers I am using are from “Energy, Chemical Disequilibrium, and Geological Constraints on Europa” by Hand et al. About 4 watts per square meter of sunlight is absorbed by the surface (of about 13 watts of total average incident light), only a tiny fraction of which would cause water-cracking. It gets about 0.125 watts per square meter of particle irradiation. If we assume a ridiculously unrealistically high far-upper-bound of 0.25 watts per square meter of water-cracking which generates oxygen at 237 kilojoules per mole, and that oxidizes iron from, say, a metallic state to rust (about 550 kilojoules per mole of oxygen gas) you get something like half a watt per square meter of oxygen-based energy flow.
That is DEFINITELY a drastic overestimate though. The paper mentioned above goes into some analysis I am utterly unprepared to comment on and suggests that given the surface age of Europa and its energetic environment, up to about 5 * 10^9 moles of ‘oxidants’ are delivered to the interior of Europa per year. Let’s be completely naive and just compare that to the total photosynthetic flux of the Earth, assuming it’s all oxygen. It comes to something like one millionth the photosynthetic productivity of Earth.
Of course, since these matter flows via geothermalisms or crust downwelling are very uneven compared to incident sunlight there will be localized hotspots like our own geothermal vents where things could be much more interesting than the above numbers would seem to indicate.
They’re great candidates for life, especially given mounting ideas on the origin of life possibly being tied up with geochemistry and geology.
They’re lousy candidates for big biospheres that utterly transform the geochemistry such that you could see it far away like ours has, or for complex life, since the total energy flux available in potentially clement environments is miniscule compared to here. But Earth’s biosphere had to start with something a lot like the energy sources you have in the icy moons, not photosynthesis.