Purported phosphine on Venus more likely to be ordinary sulfur dioxide


Date:
January 27, 2021
Source:
University of Washington
Summary:
Astronomers revisited and comprehensively reinterpreted the radio
telescope observations underlying a widely reported 2019 claim that
phosphine gas was present in the atmosphere of Venus. In a paper
accepted to the Astrophysical Journal, they report that sulfur
dioxide, a common gas in the atmosphere of Venus, is likely what
was detected instead of phosphine.



FULL STORY ==========================================================================
In September, a team led by astronomers in the United Kingdom announced
that they had detected the chemical phosphine in the thick clouds of
Venus. The team's reported detection, based on observations by two
Earth-based radio telescopes, surprised many Venus experts. Earth's
atmosphere contains small amounts of phosphine, which may be produced
by life. Phosphine on Venus generated buzz that the planet, often
succinctly touted as a "hellscape," could somehow harbor life within
its acidic clouds.


========================================================================== Since that initial claim, other science teams have cast doubt on the reliability of the phosphine detection. Now, a team led by researchers
at the University of Washington has used a robust model of the conditions within the atmosphere of Venus to revisit and comprehensively reinterpret
the radio telescope observations underlying the initial phosphine
claim. As they report in a paper accepted to the Astrophysical Journal
and posted Jan. 25 to the preprint site arXiv, the U.K.-led group likely
wasn't detecting phosphine at all.

"Instead of phosphine in the clouds of Venus, the data are consistent
with an alternative hypothesis: They were detecting sulfur dioxide," said co-author Victoria Meadows, a UW professor of astronomy. "Sulfur dioxide
is the third- most-common chemical compound in Venus' atmosphere, and it
is not considered a sign of life." The team behind the new study also
includes scientists at NASA's Caltech-based Jet Propulsion Laboratory,
the NASA Goddard Space Flight Center, the Georgia Institute of Technology,
the NASA Ames Research Center and the University of California, Riverside.

The UW-led team shows that sulfur dioxide, at levels plausible for Venus,
can not only explain the observations but is also more consistent with
what astronomers know of the planet's atmosphere and its punishing
chemical environment, which includes clouds of sulfuric acid. In
addition, the researchers show that the initial signal originated not
in the planet's cloud layer, but far above it, in an upper layer of
Venus' atmosphere where phosphine molecules would be destroyed within
seconds. This lends more support to the hypothesis that sulfur dioxide
produced the signal.

Both the purported phosphine signal and this new interpretation of the
data center on radio astronomy. Every chemical compound absorbs unique wavelengths of the electromagnetic spectrum, which includes radio waves,
X-rays and visible light. Astronomers use radio waves, light and other emissions from planets to learn about their chemical composition, among
other properties.



==========================================================================
In 2017 using the James Clerk Maxwell Telescope, or JCMT, the U.K.-led
team discovered a feature in the radio emissions from Venus at 266.94 gigahertz.

Both phosphine and sulfur dioxide absorb radio waves near that
frequency. To differentiate between the two, in 2019 the same team
obtained follow-up observations of Venus using the Atacama Large Millimeter/submillimeter Array, or ALMA. Their analysis of ALMA
observations at frequencies where only sulfur dioxide absorbs led the
team to conclude that sulfur dioxide levels in Venus were too low to
account for the signal at 266.94 gigahertz, and that it must instead be
coming from phosphine.

In this new study by the UW-led group, the researchers started by
modeling conditions within Venus' atmosphere, and using that as a basis
to comprehensively interpret the features that were seen -- and not seen
-- in the JCMT and ALMA datasets.

"This is what's known as a radiative transfer model, and it incorporates
data from several decades' worth of observations of Venus from multiple sources, including observatories here on Earth and spacecraft missions
like Venus Express," said lead author Andrew Lincowski, a researcher
with the UW Department of Astronomy.

The team used that model to simulate signals from phosphine and sulfur
dioxide for different levels of Venus' atmosphere, and how those
signals would be picked up by the JCMT and ALMA in their 2017 and 2019 configurations. Based on the shape of the 266.94-gigahertz signal picked
up by the JCMT, the absorption was not coming from Venus' cloud layer,
the team reports. Instead, most of the observed signal originated some 50
or more miles above the surface, in Venus' mesosphere. At that altitude,
harsh chemicals and ultraviolet radiation would shred phosphine molecules within seconds.

"Phosphine in the mesosphere is even more fragile than phosphine in
Venus' clouds," said Meadows. "If the JCMT signal were from phosphine in
the mesosphere, then to account for the strength of the signal and the compound's sub-second lifetime at that altitude, phosphine would have to
be delivered to the mesosphere at about 100 times the rate that oxygen is pumped into Earth's atmosphere by photosynthesis." The researchers also discovered that the ALMA data likely significantly underestimated the
amount of sulfur dioxide in Venus' atmosphere, an observation that the
U.K.-led team had used to assert that the bulk of the 266.94-gigahertz
signal was from phosphine.



==========================================================================
"The antenna configuration of ALMA at the time of the 2019 observations
has an undesirable side effect: The signals from gases that can be
found nearly everywhere in Venus' atmosphere -- like sulfur dioxide --
give off weaker signals than gases distributed over a smaller scale,"
said co-author Alex Akins, a researcher at the Jet Propulsion Laboratory.

This phenomenon, known as spectral line dilution, would not have affected
the JCMT observations, leading to an underestimate of how much sulfur
dioxide was being seen by JCMT.

"They inferred a low detection of sulfur dioxide because of that
artificially weak signal from ALMA," said Lincowski. "But our modeling
suggests that the line-diluted ALMA data would have still been consistent
with typical or even large amounts of Venus sulfur dioxide, which could
fully explain the observed JCMT signal." "When this new discovery was announced, the reported low sulfur dioxide abundance was at odds with
what we already know about Venus and its clouds," said Meadows. "Our
new work provides a complete framework that shows how typical amounts
of sulfur dioxide in the Venus mesosphere can explain both the signal detections, and non-detections, in the JCMT and ALMA data, without the
need for phosphine." With science teams around the world following up
with fresh observations of Earth's cloud-shrouded neighbor, this new
study provides an alternative explanation to the claim that something geologically, chemically or biologically must be generating phosphine in
the clouds. But though this signal appears to have a more straightforward explanation -- with a toxic atmosphere, bone-crushing pressure and some
of our solar system's hottest temperatures outside of the sun -- Venus
remains a world of mysteries, with much left for us to explore.


========================================================================== Story Source: Materials provided by University_of_Washington. Original
written by James Urton. Note: Content may be edited for style and length.


========================================================================== Journal Reference:
1. Andrew P. Lincowski, Victoria S. Meadows, David Crisp, Alex
B. Akins,
Edward W. Schwieterman, Giada N. Arney, Michael L. Wong, Paul
G. Steffes, M. Niki Parenteau, Shawn Domagal-Goldman. Claimed
detection of PH3 in the clouds of Venus is consistent with
mesospheric SO2. Astrophysical Journal, 2021 [abstract] ==========================================================================

Link to news story: https://www.sciencedaily.com/releases/2021/01/210127140147.htm

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