Unidentified ‘Absorbers’ Soak Up Solar Energy in Upper Cloud Layer of Venus


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Venus’ surface has never been seen by the human eye because of opaque, highly reflective clouds which permanently cover the planet. What is curious about Venus’ upper clouds is that in those clouds are mysterious dark patches, called ‘unknown absorbers’ as the tiny particles that make up them soak up most of the ultraviolet and some of the visible light from the Sun and thus affect the planet’s energy budget and reflective properties (albedo). Whatever their composition, Venus’ ‘unknown absorbers’ are having an effect on the planet’s weather, according to an analysis of images and spectral data from ESA’s Venus Express, JAXA’s Akatsuki, the NASA/ESA Hubble Space Telescope, and NASA’s MESSENGER spacecraft.

A composite image of Venus as seen by JASA’s Akatsuki spacecraft. Image credit: Institute of Space and Astronautical Science / Japan Aerospace Exploration Agency.

Venus’ mysterious dark patches were first observed by ground-based telescopes more than a century ago. They ebb and flow over time, changing their distributions and contrasts.

“The particles that make up the dark splotches, have been suggested to be ferric chloride, allotropes of sulfur, disulfur dioxide and so on, but none of these, so far, are able to satisfactorily explain their formation and absorption properties,” said Dr. Yeon Joo Lee, a planetary researcher with the Technische Universität Berlin.

“On the other hand, observations that the particles are about the same size and have the same light-absorbing properties as microorganisms found in Earth’s atmosphere, and scientists, beginning with the noted biophysicist Harold Morowitz and astronomer Carl Sagan, have long speculated about the possibility that the shadowy patches in the clouds of Venus are, in fact, microscopic life,” added University of Wisconsin-Madison planetary scientist Dr. Sanjay Limaye.

The team studied changes in Venus’ albedo using 11 years of ultraviolet observations of the planet from instruments aboard Venus Express, Akatsuki and MESSENGER probes as well as the Hubble Space Telescope.

Between 2006 and 2017, Venus’ albedo halved before beginning to rebound.

Those changes sparked big variations in the amount of solar energy absorbed by the clouds and, consequently, the circulation of Venus’ atmosphere.

In particular, the albedo changes help explain variations in the vigorous activity of the planet’s upper atmosphere, which exhibits what scientists call superrotation, a phenomenon driven by winds exceeding 200 mph (322 kmh).

“The new results of the changes in the planet’s albedo provide a link between solar heating and the powerful gusts that underpin the dynamics of the planet’s upper atmosphere,” said Hokkaido University researcher Dr. Takeshi Horinouchi.

“It suggests that something is changing. We can see the change in brightness. If the albedo is changing, something is driving those changes. The question is, what is the cause?” Dr. Limaye said.

“The ebb and flow of the mysterious dark splotches at the tops of Venus’ clouds, the ‘unknown absorbers,’ are near the top of the list of suspects and could, in fact, be playing a major role in those changes. Haze above the clouds and composed of smaller particles can make Venus appear even brighter,” the scientists said.

Weather and climate, be it on Earth or on Venus, are driven by solar radiation, including the ultraviolet radiation that we can’t see. And clouds and their changing ability to reflect light have a huge influence.

“Is solar ultraviolet light impacting Venus’ cloud cover? Are cosmic rays — subatomic particles from space that continuously rain down on all the planets — affecting cloud cover by triggering cloud nucleation? Would the planetary sulfuric dioxide affect the formation of sulfuric acid cloud?” Dr. Lee said.

The study helps flesh out the dynamics of Venus’ albedo and the potential effects of the enigmatic dark splotches that wander the tops of the planet’s cloud banks, if not their composition.

But the results are only likely to pique the interests of the planetary researchers who study Venus and its weather, which may impart new lessons about Earth and its climate and weather.

The study was published in the Astronomical Journal.


Yeon Joo Lee et al. 2019. Long-term Variations of Venus’s 365 nm Albedo Observed by Venus Express, Akatsuki, MESSENGER, and the Hubble Space Telescope. AJ 158, 126; doi: 10.3847/1538-3881/ab3120

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