Tikalon Blog by Dev Gualtieri

Phosphine on Venus

March 8, 2021

Phosphorus is a chemical element of low atomic weight (atomic number = 15). As a consequence, phosphorus is quite abundant in Earth's crust, existing at about the 0.1% level. Since it sits below nitrogen in the periodic table, it has an expected valence of three. Phosphorus combines with oxygen to form the phosphate anion, PO₄^-1, that combines with other elements to form many phosphate minerals.

Pandemonium phosphate, a parody of the many phosphates known to chemists.

(Image © 2015 by the author, but it can be used for personal and educational use. License required for commercial use.

Click for larger image. Other STEM images are here.)

Life on Earth evolved to make use of the available materials, one of which is phosphorous. Adenosine triphosphate (ATP, C₁₀H₁₆N₅O₁₃P₃), a molecule found in all forms of life, is the energy source and means of energy storage in cells. ATP is created by glucose in a process called glycolysis. The 1997 Nobel Prize in Chemistry was awarded to Paul D. Boyer (1918-2018), John E. Walker (b. 1941), and Jens C. Skou (1918-2018) for discoveries related to ATP.

One phosphorous compound, phosphine (PH₃) is uncommon in nature, since phosphorus is more stable as an oxide in oxidizing environments. However, phosphine is a byproduct of chemical reactions of life, so its existence has been proposed as a biosignature for astrobiology. Natural non-biological processes for creation of even trace amounts of phosphine are unknown, so detection of phosphine would be indicative of life.

Molecular structure of phosphine (PH₃).

(Modified Wikimedia Commons image by NEUROtiker.)

Early in astronomy, the planet, Venus, was viewed as Earth's twin, since its radius is nearly identical to that of Earth, smaller by just 5%, and it's orbital period around the Sun is 224.7 Earth days (0.615 Earth years). For such reasons and because it's closer to the Sun, Venus was depicted in early science fiction as a warmer Earth that hosted tropical forests and often dinosaurs. As scientific instrumentation progressed, Venus was found to be quite unlike the Earth.

A day on Venus lasts 243 Earth days. Its atmosphere is mainly carbon dioxide (CO₂, 96.5%) with nitrogen (3.5%) and some sulfur dioxide (SO₂, 0.015%), and the atmospheric pressure is a crushing 91 atmospheres. The surface temperature is 462 °C, 863 °F). Such conditions make the existence of life on Venus, as we understand life, quite unlikely at its surface. However, it might be possible for thermoacidophilic extremophile microorganisms, which are known on Earth, to exist in the temperate upper layers of the Venusian atmosphere.

Photograph of the surface of Venus by Venera 9.

Image of the surface of Venus by the Soviet Venera 9 lander, October 22, 1975. This lander was the first to successfully image the surface of another planet, and it revealed the surface to be composed of non-eroded rocks about 12-16 inches (30 to 40 centimeters) in size. The Soviet Mars 3 lander was the first spacecraft to attain a soft landing on Mars, December 2, 1971. That lander failed after less than two minutes and transmitted no images. The first US Mars lander was the 1997 Mars Pathfinder. (Wikimedia Commons image. Click for larger image.)

In September 2020, a huge international team of astronomers reported the presence of phosphine in the atmosphere of Venus, sparking a debate on whether this observation was indicative of microbial life on Venus.[1-4] Research team members were from the Massachusetts Institute of Technology (Cambridge, Massachusetts), Cardiff University (Cardiff, UK), the University of Cambridge (Cambridge, UK), The University of Manchester (Manchester, UK), the MRC Laboratory of Molecular Biology (Cambridge, UK), Kyoto Sangyo University (Kyoto, Japan), Imperial College London (London, UK), the Royal Observatory Greenwich (London, UK), The Open University (Milton Keynes, UK), and the East Asian Observatory (Hilo, Hawaii).[1]

Spectral detection of a 1.123 millimeter wavelength absorption by a PH₃ rotational transition was done against the bright Venus atmosphere over five mornings in June 2017.[1] The initial estimate of the phosphine concentration was 20 parts per billion by volume (ppbv), but further analysis based on criticism from another research group forced a revised estimate of 1-4 ppb as a whole planet average, with a peak of 5 to 10 ppb and a 4.8-sigma confidence level.[1-4] For reference, there is just a global average of a few parts per trillion phosphine in Earth's atmosphere.[4] Such a high level of phosphine cannot be explained by steady-state chemical and photochemical processes. There are presently no known ways to produce phosphine in the atmosphere of Venus, at its surface and subsurface, or from lightning, volcanoes or by meteor impacts.[1]

Phosphine spectral curve for Venus (ALMA)

Venus whole-planet absorption spectrum from re-processed Atacama Large Millimeter Array data.

The upper curve is the phosphine rotational transition, and the lower curve is for an HDO transition observed simultaneously and offset for clarity..

(Fig. 3 of Ref. 3, released under a Creative Commons License. Click for larger image.)

Another research group observes that this phosphine feature can be explained by mesospheric SO₂ at the 100 ppb level[2], and it had further issues with the data analysis.[2] The original research team counters that this level of SO₂ is not realistic.[3] However, the measured spectral line is hidden in a complicated background signal, and the research team had to fit the data with a 12th-order polynomial, an extremely messy business.[4]

Composite image of Venus using sata from the NASA Magellan spacecraft and NASA Pioneer Venus Orbiter.

Composite image of Venus using data from the NASA Magellan spacecraft and NASA Pioneer Venus Orbiter.

Since the surface of Venus is hidden by its dense atmosphere, topography is detected by radar altimetry.

(NASA/JPL-Caltech image. Click for larger image.)

Phosphine on Venus

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