By Steven J. Grisafi, PhD.
I became quite puzzled by the fervor evoked by the discovery of Phospine in the atmosphere of Venus. The reason for my perplexity is that Phospine has also be found in the atmospheres of both Jupiter and Saturn. The discovery of Phospine on Venus has raised the possibility that the compound was formed biologically. Yet, I am at a loss to understand why Phospine would be produced abiotically on both Jupiter and Saturn, but not on Venus.
The gas giants Jupiter and Saturn are believed to have a small rocky core. We actually don’t know the true depths of their atmosphere. We arbitrarily set the surface of each planet to occur at the depth at which the atmospheric pressure is one Bar. The atmosphere of Jupiter is approximately 90% Hydrogen and 10% Helium on a mole basis. The atmosphere of Saturn is approximately the same as that of Jupiter. The atmosphere of Venus is approximately 96% Carbon Dioxide and about 3.5% Nitrogen with the remainder being several trace gases. Let us compare the discovery of Phospine on Venus with the previously known discovery of Phospine on Saturn since Saturn is much colder than Jupiter. For approximate comparison we take the atmosphere of Venus to be Carbon Dioxide and the atmosphere of Saturn to be Hydrogen. The critical point of Carbon Dioxide occurs at approximately 304 Kelvin and 74 Bars. The critical point of Hydrogen occurs at approximately 5 Kelvin and 13 Bars. We measure at the surface of Venus a temperature of approximately 740 Kelvin and a pressure of 93 Bars. Consequently, the atmosphere at the surface of Venus is a supercritical fluid. We don’t actually know the depth of Saturn’s atmosphere but we do know that its atmosphere has an average temperature of about 135 Kelvin. Since the critical pressure of Hydrogen is only 13 Bars we can assume that the atmosphere of Saturn is also a supercritical fluid.
Now, what would be the significance of having an supercritical fluid atmosphere? Above the critical point there is no gas-liquid equilibrium. We cannot distinguish a gas phase from a liquid phase. There is only the “fluid” phase. However, the only distinction that we actually make between a gas and a liquid is that a gas expands to fill its vessel, while a liquid does not. In the context of a planetary atmosphere this distinction is meaningless. The fluid molecules are all held by gravity and to each other by electromagnetism. So what we actually have on both Venus and Saturn is a single phase fluid ocean. In both cases we can assume inviscid fluid and ideal gas behavior. Phospine has an enthalpy of formation at STP (standard temperature and pressure) of 5470 joules per mole. If we divide the enthalpy by the Ideal Gas Constant we find a temperature of 658 Kelvin. The surface temperature on Venus exceeds this by 82 Kelvin suggesting that there is sufficient energy to form Phospine from its constituents. However, Phosphorus does not occur in its elemental form on Earth and presumably neither does on Venus. On Earth it occurs primarily in rocks as Calcium Phosphate. Two moles of Calcium Phosphate would react with six moles of Hydrochloric Acid to form three moles of Calcium Chloride and two moles of Phosphoric Acid. We would expect the same on Venus. On Saturn we are not even sure that rocks exist.
Regardless of however Phospine can be formed abiotically on Earth, the pertinent question is: Why would Phospine form abiotically on Saturn but not on Venus? If Saturn lacks a rocky core, whatever pressure that is achieved at its greatest depth, we can be certain no light penetrates to such depths at which we see that sunlight cannot penetrate the oceans here on Earth. There would be no photochemistry at such depths. Nor would temperatures be sufficient to provide thermal energy. So Phospine must form in the upper layers of Saturn’s atmosphere such that sunlight can penetrate and pressures are moderate. But those layers are exceedingly cold and can provide little thermal energy needed to overcome the activation energy of whatever reaction is causing the production of Phospine. So it seems apparent to me that we need not know the actual chemical reaction that creates Phospine on Saturn for us to conclude that, whatever it may be, it produces Phospine on Venus. I understand that astronomers love their work. We all think that our work is special. But there is nothing remarkable about finding Phospine on Venus because it is found on Saturn.