Marco Fulvio Barozzi: My Five Cents On Venusian Phosphine
Marco Fulvio Barozzi (b. 1955), a former Maths teacher and scientific blogger (https://keespopinga.blogspot.com), as well as a humorous poet, is a regular contributor to scientific and mathematics magazines. I read a very interesting piece he wrote on Phosphine, the substance recently discovered on the Venus atmosphere, and asked him if I could reproduce it here. He graciously agreed, so here it is below.
Phosphine (phosphane according to IUPAC, or hydrogen phosphide or phosphorus trihydride) is the chemical compound with the brute formula PH3. Its structure vaguely resembles that of ammonia (NH3) with the phosphorus atom (P) bonded to three hydrogen atoms (H) to form a skewed pyramid with a triangular base. It is a colorless and flammable gas, with the smell of rotten fish, and a boiling point of 185 K (-88° C) at atmospheric pressure. Phosphine is highly toxic and can kill easily even at relatively low concentrations by inhalation or skin contact. In nature, phosphines (each basic molecule generates a family) can be produced enzymatically by the microorganisms responsible for the putrefaction of phosphorous organic matter, and it is no coincidence that they are responsible for the will-o'-the-wisps, the elusive blue flames that they are sometimes seen in old cemeteries, swamps, and ponds.
It is news of these days that unusually high amounts of phosphine have been discovered in the temperate zone of the atmosphere of Venus (about 50 km of altitude), where temperature and pressure are similar to those we have on the surface of the Earth: about 20 molecules for each billion molecules of other gases, a concentration about a thousand times greater than that of our planet. As far as we know, the only way to obtain phosphine here on Earth is in a laboratory, otherwise it is formed (we do not yet know how) as a by-product of the metabolism of some anaerobic bacteria where molecular oxygen is very low.
Having ruled out the possibility of a Venusian civilization with an advanced chemical school, even the second hypothesis is quite surprising. To tell the truth, phosphine had already been found in Jupiter's stormy atmosphere and no one was surprised, because up there there is so much of that energy and so much of that hydrogen that an abiotic production cannot be excluded (the gaseous giants are high-energy chemical laboratories). But on Venus there are no such high temperatures and pressures, it is a different environment. Phosphine is not expected to persist in the Venusian atmosphere, as in an oxidizing atmosphere such as that of Venus and at less extreme temperatures than on Jupiter, it would be necessary to admit that thermodynamically improbable chemical reactions are possible in such environmental conditions, capable of overcome the barrier given by the low oxidation state of phosphorus. Furthermore, ultraviolet radiation would cause the phosphine to combine with water and carbon dioxide, eliminating it. The article that announced the discovery puts forward the hypothesis that it "could originate from unknown photochemical or geochemical, or, similarly to the biological production of PH3 on Earth, from the presence of life", because only living beings can sometimes carry out thermodynamically unfavorable processes.
But how was it possible to detect phosphine on Venus? - as long as it's not a false positive: it may happen. The phosphine brand is a radio wave with a length of 1.123 mm. This means that it was detected with a radio telescope, the James Clerk Maxwell Telescope of the Mauna Kea Observatory in Hawaii. Confirmation of the discovery required the use of 45 antennas from the Atacama Large Millimeter / submillimeter Array (ALMA) in Chile, a more sensitive telescope of which the European Southern Observatory (ESO) is a partner.
Apart from the construction techniques, a radio telescope works like a normal optical telescope, with the difference that it detects electromagnetic waves with greater lengths λ, and, consequently, shorter frequencies ν, since the two quantities are inversely proportional (c, the speed of the light, is the constant of proportionality). Radio waves cannot be seen because they have wavelengths greater than 1 mm, while visible light ranges between 380 nm of violet to 680 nm of red, however we are able to detect and study them.
Each chemical element or compound has its own electromagnetic fingerprint, which is given by the interference lines that appear on the electromagnetic spectrum of the waves it emits. They in turn are a function of (i.e., they depend on) the energy levels typical of that type of molecule and only of that. In short, each type of atom or molecule, when excited, produces electromagnetic waves that correspond to its discrete energy levels. Thus, if it is hit by electromagnetic waves of all frequencies, generated by the background radiation of the planet, the waves corresponding to those of its energy levels are absorbed, so that they appear as dark lines on the spectrum, precisely in correspondence with those levels. It is the study of the arrangement of those dark lines that allows us to identify the various chemical elements and compounds present in celestial bodies. And one of those dark lines on the spectrum, in the radio wave band, indicates the presence of phosphine and perhaps some kind of microorganism capable to produce it.
Signs of life? It is early to say. The research has just begun and it is not certain that, even if the presence of phosphine is confirmed, unknown chemical processes may be excluded.