As a lot of people will know by now, the human race is officially going back to Jupiter and its moons. It’s currently scheduled to get there by about 2026. I’m seriously looking forward to it, myself. There’s an awful lot about our friendly neighbourhood giant that we still don’t really understand. In honour of this, I decided to take a look at a classic paper written by Carl Sagan and Edwin Salpeter on the fascinating possibility of life and potential ecologies on a hulking gas giant like Jupiter.
Interestingly, this audacious concept actually dates back to 1975, before Voyager had even made it to Jupiter. An odd thing about Jupiter is that it has a very low reflectivity in the near ultraviolet. The idea Sagan and Salpeter put forward to explain this was the presence of chromophores (complex, light absorbing molecules) in the jovian atmosphere. On Earth, there’s one chromophore which is seen easily across the whole planet. Chlorophyll. It’s interesting to note at this point, that the cause of Jupiter’s striking red colour is still not really understood.
The habitat they went on to describe is not unlike that found in Earth’s oceans. Deep convection currents which stir the atmosphere, from the highest levels down to the deep hot interior. They refer to the lower levels as “pyrolytic depths” — in other words, regions where the temperature reaches over a thousand degrees and any complex organic molecules would literally burn up. The life forms they consider in this bizarre habitat fit into four categories: sinkers, floaters, hunters and scavengers.
The sinkers, they hypothesise, would be the primary autotrophs in the ecosystem, performing photosynthesis and creating the basis for a jovian food chain. These creatures could be responsible for those colourful chromophores. They compare them to plankton in Earth’s oceans, passively sinking through the water until they fall into the darkness and die. On Jupiter though, they’d fall until their environment was too hot, eventually being pyrolysed, burning up in the heat. They calculate that a single celled organism in the jovian clouds would take around 1-2 months to sink. During this time it could reproduce, as plankton do on Earth, continuing the cycle.
But such creatures wouldn’t necessarily have to sink. A creature with control over its internal pressure could learn how to float, in a similar way to fish using a swim bladder to control their depth. Once life evolved such a mechanism, it could take on a predatory lifestyle and evolve into the hunters. Being motile rather than passive, these heterotrophic hunters could survive by eating the sinkers, probably in a similar way to filter feeding creatures (such as krill) in Earth’s oceans.
Finally, the hunters could evolve into floaters, which could be either heterotrophic or autotrophic. With the ability to actively move and control their depth, Sagan and Salpeter explain how these creatures could grow to gargantuan sizes. If such a creature could indeed reach kilometre proportions it would be clearly visible from orbit. While we certainly haven’t seen any evidence for kilometre sized creatures amongst Jupiter’s clouds, the idea alone is a fascinating one.
The final type of creature they mention, which they don’t discuss at great length, are the scavengers. These creatures, presumably evolved from hunters, would inhabit the deep atmosphere, almost at pyrolytic depths. There, they’d feed off carrion drifting down, as deep ocean creatures do on Earth.
The whole paper is well presented, with some in-depth hydrodynamical calculations. While a few facts were unknown at the time (such as the absence of any mesosphere on Jupiter), the idea still looks quite sound to me. It’s certainly worth considering, given the number of Jupiter-sized planets we’re discovering around other stars. But what of Jupiter itself? Could something be lying under the clouds still to be discovered?
Well as I said before, a lot about Jupiter is still unknown. We don’t really understand the convection in the jovian atmosphere, and we can’t narrow down the source of the red colouration. As far as we know, it could be from phosphorus, sulfur compounds (I’d suggest, perhaps S3) or complex organics. Interested, and hoping to fill in a couple of the gaps, I decided to take a look at the chemistry of Jupiter’s clouds.
Jupiter’s atmosphere is enriched in carbon, nitrogen and sulfur. The clouds themselves are made of ammonia, NH4SH and water. In fact the water clouds (the lowest and densest of the three types) seem to hold the bulk of Jupiter’s water, and indeed Jupiter’s oxygen. Being mainly made of hydrogen, Jupiter’s environment is strongly reducing, and most chemicals are hydrides. H2O, NH3, PH3, CH4… The atmosphere above the clouds is also full of haze, thought to be created by the Sun’s UV striking the atmosphere and breaking apart any methane and ammonia molecules carried up above the clouds. This would create a mix of hydrazine, acetylene and PAHs.
The most interesting thing though, is the brief glimpse below the clouds which comet Shoemaker-Levy 9 afforded us. Interestingly, they didn’t find any oxygen-bearing molecules beneath the clouds, but they did find several sulfur compounds. Specifically, S2, CS2 and H2S. The interesting part here is that S2 and CS2 are directly chemically analogous to O2 and CO2 — S2 even has a triplet (diradical) ground state like O2. It’s a rare molecule, actually, having only ever been found in two astronomical objects.
To me, it seems like these chemicals are out of equillibrium with the rest of Jupiter’s atmosphere. They can be reduced further, but due to some process beneath those beautiful swirling clouds, they’re not. Now, I’m not saying that these compounds are evidence for life on Jupiter, but it does make you think…
The joint NASA-ESA mission to Jupiter is set to spend a lot of time exploring Jupiter’s icy moons like Europa, and just maybe we might find evidence of life there. In the meantime, perhaps we shouldn’t write off the possibility that something might be lurking beneath Jupiter’s clouds waiting for us to find it!
Top: Jupiter’s Great Red Spot (noted by Sagan and Salpeter as potentially a favourable habitat on Jupiter) and the swirling clouds which surround it.
Middle: Benthocodon jellyfish, photographed over 3km below the surface of Earth’s oceans, and a potential morphology for a jovian sinker or floater. ©2002 Monterey Bay Aquarium Research Institute (MBARI).
Bottom: Glass squid, also from Earth’s deep ocean. A jovian hunter might resemble this organism.
C. Sagan, E. E. Salpeter (1976). Particles, environments, and possible ecologies in the Jovian atmosphere The Astrophysical Journal Supplement Series, 32 DOI: 10.1086/190414