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?
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.
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.