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The Phosphine Debate: The Case for Incessant Study

By Allison Kubo Hutchison 
 
The scene is Venus, the second planet from the sun similar in mass and size to the Earth but with a dense, cloying atmosphere of 96% carbon dioxide. This thick atmosphere, like a thick wool blanket, leads to extremely high temperatures and pressures on Venus’ surface with a mean temperature of 867 F (464 C) and a pressure 92 times that of sea level.

From our understanding of life, it doesn’t seem at first that Venus could ever be hospitable. But scientists have speculated that life could exist floating in Venus’ atmosphere. Although the surface of Venus is extreme, there are layers in the cloud cover that actually host conditions similar to Earth. Approximately 51 km (31 mi) above the surface, temperatures are actually quite mild at 146 F (65C) or as low as 86 F (30C) and at approximately Earth surface pressures. On Earth, there is an extremely rich biosphere suspended in the air with thousands of different types of bacteria in each breath.

In September 2020, scientists announced they found phosphine, a possible sign of life on Venus. Phosphine, PH3, is a foul-smelling flammable gas. Phosphine is rare on Earth because it is unstable in environments with oxygen and the production on Earth is associated with biologic processes either human producing it for industrial purposes or, as was recently proved, bacteria. It has been found to be produced by anaerobic (no oxygen) bacteria in locations like sewage or landfills although the specific bacteria or process is not known. The Venusian atmosphere is completely oxidized so phosphine should not be produced on Venus. It has been proposed that phosphine would be a good biosignature since it is unlikely to be produced by non-biotic processes. 

However, it unlikely is not impossible. It is possible to produce phosphine by a variety of physical processes such as lightning strikes or in volcanic eruptions. However, in order to produce the proposed amounts of phosphine Venusian lightning would have to be significantly higher than theorized and current estimates of phosphine produced by lightning only explain a small fraction of that observed. Volcanic activity too would have to be 200 times more active than here on Earth. Despite the many volcanic structures on Venus’ surface, not much is known about current levels of activity. In addition, although we have a rough knowledge of atmospheric chemistry more study is necessary to see if there is a way to produce phosphine in the atmosphere although this would be surprising considering the amount of oxidizing species.

Before we declare it to be true, an important reminder that science works by questioning the methods. Authors of the paper originally published by Nature Astronomy used spectral analysis from the ground-based James Clerk Maxwell Telescope (JCMT) and the Atacama Large Millimeter Array (ALMA). These telescopes can be used to detect the presence of different molecules in the atmosphere by looking at how it absorbs different wavelengths of radio waves. However, the data is not as clear as looking at a picture. In addition, there is significant noise in the data due to Earth’s own atmosphere and the signal is difficult to detect in Venus’s own brightness. Like trying to see a flashlight on a very sunny day. So scientists must process the data and try to subtract out the sources of noise. This can be a major headache and a large source of errors.

Two additional groups of scientists reprocessed the data from ALMA and found no statistically significant sign of phosphine [1][2]. The new paper argued that the previous work over fitted their equation to reduce noise using a long twelfth-degree polynomial. Changing the polynomial reduces the strength of the signal of phosphine significantly but does not completely negate it. Additional work published on September 30, 2020, went back to 2015 and analyzed data from another telescope in a different spectrum and found no signal of phosphine during their observation. They also calculated the upper limit of what could be in the cloud top of Venus by looking at infrared data and found that at most 5 ppbv (parts per billion by volume) four times lower than the original study.

So we can’t yet confirm that phosphine exists in the Venusian atmosphere. However, if it did it would be a significant sign to get back to Venus. Currently, there is only one probe in orbit around Venus JAXA’s Akatsuki which arrived over ten years ago. Compared to Mars which has a working rover (two enroute) and six orbiters (two more enroute), Venus starts to look like the neglected middle child. Venus holds a wealth of information as it represents the other end of the spectrum from Mars. Mars, a planet with little to no atmosphere, and Venus a planet with an overly robust atmosphere may have both had oceans in the past.

As the saying goes, Venus is calling and I must go, and I will work on while I can, studying incessantly.

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