Does low energy rule out life swimming in the oceans of Europa and Enceladus?

[G]

Jupiter's moon Europa and Saturn's moon Enceladus (and several other moons too) have large subsurface oceans of liquid water, an ingredient considered essential for life. These two moons occasionally spew up plumes of water vapor from their ocean into space, and some people have suggested orbiting spacecraft may be able to collect this ejected water to try to detect microbial life in the ocean.

But is there enough energy in these moons to support life, of the type which energetically swims around?

All life requires a source of power: to function life has to be able to tap into and harness a flux of energy. On Earth, via photosynthesis, life taps into radiant energy from the Sun, and this energy fuels the whole chain of life on Earth.

The Earth as a whole receives 173,000 terawatts of power from the Sun on a continuous basis. Ref: here. So there's lot of available energy to power life.

But the subsurface oceans of Europa and Enceladus are not heated by sunlight, but by tidal flexing from their planets' strong gravity.

In terms of the power in watts that Europa receives from tidal heating, that's actually simple to work out: my calculation (detailed below) indicates that the ocean of Europa receives only about 2.5 terawatts of power from tidal heating.

A 2.5 terawatt power supply is minuscule, 70,000 times less than the power supply to Earth.

So if there is life in Europa's ocean, those lifeforms may be desperately short of energy. It is thus unlikely Europa's ocean as a whole will contain the sort of energetic life we see in Earth's oceans — swimming lifeforms like jellyfish, octopuses or fish. There is just not enough power on Europa for all that energetic swimming.

And perhaps this shortage of energy might also impact microbial life too: the density of microbes in the ocean may be very low because of the energy shortage. Which means that even if there were some microbial life in Europa's ocean, it might be too sparse to detect in the plumes.

It is however possible that energetic swimming lifeforms as well as dense microbial life could live near hydrothermal vents that may exist at the bottom of the ocean of Europa, living off the hydrothermal energy.

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Calculation of the amount of power in watts received by Europa's ocean from tidal heating:

In equilibrium, the power the ocean receives from tidal heating will be equal to the power dissipated by thermal conductivity through the Europa ice sheet and out into space. So to calculate the power dissipated by thermal conductivity, we need the following data:

Europa surface area: A = 3 × 10^7 km2 = 3 x 10^13 m2. Ice cover thickness estimate: S = 10 km = 10,000 m. Europa average surface temp: -190ºC Europa subsurface ocean temp: assume around +50ºC Difference in temperature between ocean and surface: d = 240ºC. Ice thermal conductivity (at -100ºC): k = 3.48 W/mK. Ref: here

Total power dissipated through entire Europa ice sheet given by equation: Power = kAd / S Ref: here

So power dissipated = 3.48 x 3 x 10^13 x 240 / 10,000 = 5.1 x 10^12 = about 2.5 terawatts

So the power received by Europa from tidal heating is about 2.5 terawatts.

[redliox]

I don't know much about calculating energy, but we do have real examples of low energy and high energy environments right here on Earth under our oceans.  Here's some examples:

1) Hydrothermal vents: The black smokers discovered, fittingly around the time of Voyager's visit to Europa, that life can exist without sunlight underwater.  While not common, the fact organisms can send clouds of their eggs into hostile oceans and still luck out in finding these sporadic spots imply any deep sea organism can find and utilize them, be it on Earth or icy, ocean moons.

2) Abyssal plains: These make up nearly 60% of the Earth's surface, but equate to deserts of the deep sea.  These would definitely be the low energy environments, but these aren't completely void of life any more than terrestrial deserts are.  What organisms that are here basically hibernate a lot and improvise with whatever falls down or is in the mud.

3) Polar seafloor: Around both Antarctica and the Arctic Ocean are lifeforms that thrive in frigid temperatures, and in many cases the low temperatures enrich the water with oxygen and nutrients.

On icy bodies with internal oceans there will be regions of both low and high energy and of low and high nutrient counts.  An older body probably would lose geothermal energy over time, but considering Pluto, Miranda, and Ceres have signs of past oceans I would think Europa and Enceladus, with access to gravitational tides, likely had some heat over their eons.  Their oceans probably are more stable and long-lived than any Mars or Venus had for comparison, although obviously we don't know if life exists on any of them.

So no, low energy doesn't rule life out alone.

[Nomadd]

 Life can adapt to pretty extreme places. That's not saying that life can start there.

[Johnnyhinbos]

Rumor has it the Chinese have discovered tardigrades on the moon...

[baldusi]

What we don’t know is the depth of the oceans. An interesting part is that if the oceans are 600km or more deep, they can have a floor of solid ice VIII and then I do seriously doubt that any life can form.

[Orbiter]

Rumor has it the Chinese have discovered tardigrades on the moon...

Where did you hear that?

[ugordan]

What we don’t know is the depth of the oceans. An interesting part is that if the oceans are 600km or more deep, they can have a floor of solid ice VIII and then I do seriously doubt that any life can form.

I believe gravity science data and overall density of those two moons definitively rule out an ocean NOT in contact with the rocky layer. For Enceladus, additionally the composition of the plumes strongly suggests chemistry that can only occur with liquid water in contact with rock.

[daedalus1]

Rumor has it the Chinese have discovered tardigrades on the moon...

They were part of the Israeli lander.

[Torbjorn Larsson, OM]

"Does low energy rule out life swimming in the oceans of Europa and Enceladus?"

Yes, that is trivially likely, anything else would be a whale of a history.

All eukaryotes rely on a two orders of magnitude more energy dense oxygen metabolism, and there is little free oxygen in these oceans. That includes deep crust nematodes and hydrothermal ocean vent life that has been discovered kilometers down (as well as free ocean life). They all rely on oxygen supplied by oxygenating photosynthesis at the surface.

Putative life in ice moon oceans, or in general, does not tap into high entropy thermal energy, this is instead the entropy sink to space (IR photons). Metabolism is essentially redox reactions, and before life evolved photosynthesis at all (whether oxygenating or not), it used chemical free energy. The last paper I saw estimating the potential biosphere mass on Europa gave it a thin prokaryote cell layer between the ice (which provides oxidation potential, including small amounts of oxygen) and the ocean surface: pond scum is all we expect to see. Enceladus is better situated since the alkaline hydrothermal vents that the chondritic core makes are seen to produce biomolecules directly in the ocean environment - lots of cell material and metabolic energy for free.

[Torbjorn Larsson, OM]

I should add that the oxidation potential of Europa ice is higher than just the release of hydrogen into space. Seems from observations that Jupiter and Io cooperate with radiation and acidic compound transport hitting the top layer; convection is slow but not nil. Dunno how good the models of that are at the moment. But again, it will be pond scum on top of the ocean, if that. (Since IIRC the biomass paper hedged the population sizes; I did not compare with estimates of nutrient poor desert biomass production but the paper description gave that feeling.)