The researchers also perform an intellectual flyby of Jupiter's moon, Europa. The icy surface of this body, smaller than our Moon, may conceal a subsurface ocean. Tidal disruption by two of Jupiter's other moons, Io and Ganymede, injects considerable energy into Europa. Even so, the amount of available energy at Europa is likely to have been even smaller than that of Mars. If there are any Europans, they will be scarce and shy indeed.
Photosynthesis provides the final contrast between Earth and Mars.
Jakosky and Shock calculate that the total density of biomass that Mars could have produced in its entire history would have been 20 grams per square centimetre. Therefore, the 'lifetime' productivity of the Earth is about four million times as great as Mars. Much of this difference can be attributed to photosynthesis. For life to really take off, photosynthesis provides an enormous boost.