Sky & Telescope - January 2021 - 10

NEWS NOTES
tion; the theorists and observers joined
forces in December 2018.
Phosphine absorbs light at other
wavelengths, too, but the spectral transition that Greaves's team observed with
JCMT and ALMA is by far the easiest
absorption line to see from Earth.
Looking into Venus's atmosphere to
ﬁnd the subtle signature was not easy,
though, as the planet is bright and
almost ﬁlls ALMA's ﬁeld of view. Anita
Richards (Manchester University), the
team's ALMA liaison, helped Greaves's
team with the long and intricate process
of data reduction.
When the result ﬁrst came out,
David Grinspoon (Planetary Science
Institute), who was not involved in the
project, predicted it would be contentious. "People will look very critically at
this, as they should," he said. "We don't
want to jump to the wrong conclusion
about something this important." (Read
more from Grinspoon on page 15.)

Potential Biosignature
Found at Venus
AN INTERNATIONAL TEAM of research-

ers, led by Jane Greaves (Cardiff University, UK), has announced the detection
of phosphine - a gas potentially associated with life - in the relatively cool
cloud decks of Venus.
Astronomers have previously found
phosphine surfacing from the depths
of Jupiter and Saturn, where the highpressure, hydrogen-rich, and intensely
hot environments provide the right
conditions to make the molecule. But
anaerobic microbes produce it in less
extreme places on Earth, too.
As reported September 14th in
Nature Astronomy, Greaves and her
colleagues observed Venus ﬁrst in 2017,
with the James Clerk Maxwell Telescope
(JCMT), then again with the Atacama
Large Millimeter/submillimeter Array
(ALMA) in 2019. They found a chemical
ﬁngerprint of phosphine at a wavelength of 1.123 millimeters, at which
the molecule absorbs thermal radiation coming from clouds lower in the
planet's atmosphere.
The team investigated numerous
possible abiotic sources for phosphine,
including geological activity, meteorite
10

JANUARY 2 021 * SK Y & TELESCOPE

S Akatsuki captured ultraviolet light from Venus's dayside in this 2016 image.

impacts, and lightning. Although the
detection indicates only trace amounts
of the gas, around 20 or 30 parts per
billion, none of the scenarios explored
could account for it.
PHOSPHINE AS BIOSIGNATURE
In its pure form, phosphine is a colorless, odorless gas made up of one phosphorus and three hydrogen atoms. It
is ﬂammable at room temperature and
highly toxic, which is why it is produced
mainly as a fumigant and occasionally
used as a chemical weapon.
But in the 1990s, Dietmar Glindemann (University of Leipzig) showed
that it was also a naturally occurring
part of Earth's anaerobic biosphere. His
research led to the discovery of phosphine-producing microbes.
Quantum astrochemist Clara SousaSilva (MIT) and colleagues researched
the compound's potential as a biosignature gas on rocky exoplanets as part of
the All Small Molecules project. It was
this work that caught Greaves's atten-

CONFIRMING PHOSPHINE
The team is conﬁdent in the analysis
but agrees that additional observations
are needed. As the discovery is based on
the detection of a single absorption line,
additional spectral lines would verify
phosphine's presence.
However, the thickness of the
Venusian atmosphere and the general
propensity for molecular signals to get
tangled up with each other both hinder
remote detections - and make a good
case for a mission to our sister planet.
A direct observation made within the
atmosphere would be ideal because the

S Pioneer Venus hovers above the planet in this
illustration.

U V IM AG E OF V ENUS: JA X A; PIONEER V ENUS: N ASA

SOLAR SYSTEM

Sky & Telescope - January 2021

Table of Contents for the Digital Edition of Sky & Telescope - January 2021