According to studies by the Spanish Javier Peralta, the 'Great Dark Cloud' of Venus, which covers a large part of its surface, is associated with the propagation of a superfast atmospheric wave with the appearance of a tsunami.
The study of this wave helps to understand phenomena related to the intense greenhouse effect of Venus, phenomena that may have a counterpart on Earth.
Super fast
Since ESA and NASA announced a couple of months ago that they would send three spacecraft to Venus in the 2030 horizon, this planet has become at the center of today. Astronomers turn their gaze to the Earth's twin, already thinking about preparing new observations that will reveal its many mysteries.
Among these mysteries is a large atmospheric cloud first observed in infrared images of the planet taken more than 30 years ago. In fact, infrared light makes it possible to explore the night side of Venus, thus revealing the characteristics of the different clouds that cover the planet. There are two types of clouds: bright and dark. The bright ones are very transparent clouds that let through the thermal emission that comes from the planet's surface. The dark ones are, due to their composition, more opaque and block this thermal emission.
At the equator, the bright and dark clouds are separated by a discontinuity that was characterized by an international team led by the Spanish Javier Peralta, who was then working at the Japanese Space Agency (JAXA) but has recently joined the University. of Sevilla. The work by Peralta and collaborators, published in 2020, studied the infrared images sent by the Akatsuki spacecraft in 2016 and found that after the passage of this discontinuity or "tsunami" a large dark cloud remained, a phenomenon already present in other images obtained, by other means, in the 1980s, but which went completely unnoticed then.
The cloud was designated as "Giant Dark Cloud": dark for appearing less bright than its surroundings and giant for extending up to 30 degrees above and below the Venusian equator. The discontinuity "sweeps" the clouds every 4.9 days, moving faster than the rest of the clouds, which take between 6 and 7 days to complete one revolution around the planet.
Several telescopes have taken high-quality images of Venus in the infrared.
The one that heads this article was obtained in July 2015 with a 3-meter telescope that NASA has installed in Hawaii (Infrared Telescope Facility, IRTF).
The part to the right of the image is the daytime (illuminated) side of the planet where we cannot see any detail as it saturates the infrared detectors of the telescope.
But in the left zone, the night side of the planet, the Giant Dark Cloud appears clearly visible covering the equatorial region.
However, neither with the IRTF nor with Akatsuki, it was possible to obtain spectroscopic data that allowed to study its chemical composition, something essential to find out what role the dark cloud plays in the powerful greenhouse effect that has turned Venus into hell.
In the files
A team led by Kevin McGouldrick, a scientist at the Laboratory for Atmospheric and Space Physics at the University of Colorado in Boulder, and again by Javier Peralta, has now conducted another study on this phenomenon.
To do this, the scientists investigated data taken by the European spacecraft Venus Express, which orbited Venus between 2006 and 2014.
Venus Express was indeed equipped with an optical and infrared spectrometer, called VIRTIS, which gave it the ability to study the chemical composition of the different atmospheric zones on the planet.
In this way, scientists have found that the abundances of sulfuric acid and water are peculiar in the Large Dark Cloud.
They have also been able to determine the height of the cloud base, which would be located around 50 kilometers.
With all these data, and taking into account the speed at which the discontinuity between clouds moves, McGouldrick, Peralta and collaborators conclude that, quite possibly, the dark cloud is formed by the propagation in the atmosphere of a type of waves called ' Kelvin waves'.
These waves are disturbances that propagate in a fluid, in a low dispersive way, on a very large scale.
Like a tsunami, this discontinuity sweeps up the lower atmosphere every few days.
These types of waves are very important on Earth, where they play an important role in the development of phenomena such as El Niño and La Niña.
But on Venus, the Kelvin waves are much larger and allow us to study them from another point of view.
A laboratory called Venus
Venus is very different from Earth.
But, in some respects, both planets also have great similarities, to the point that Venus is considered the most similar to Earth of all the planets in the solar system, a kind of twin brother.
Some climatic phenomena that occur on Earth, such as the greenhouse effect, also occur on Venus, but on a much more exaggerated scale.
Comparison of Venus and Earth
The greenhouse effect, which keeps the surface of Venus at almost 500 degrees Celsius, occurs in the lower layers of its atmosphere, where the proportion of carbon dioxide reaches 98%. Kelvin waves can serve to transport energy from inner to outer layers of the atmosphere. Hydrodynamic simulations show that these waves could be generated at an altitude of about 20 kilometers and that they would be capable of depositing much of their energy at much higher altitudes.
Studying all these phenomena in this exceptional laboratory represented by Venus is extremely useful to understand the greenhouse phenomenon in general terms and in great detail. In fact, many physical-chemical processes could go unnoticed on Earth, but can fully manifest themselves in the extreme conditions of Venus. For example, Kelvin waves seem capable of altering the energy balance of clouds and, therefore, the chemical processes that take place there, which can lead to important changes in the composition of the gas.
Venus can also be considered as the perfect laboratory to study the atmosphere of a planet similar to Earth but without the influence of biological processes.
Comparing the details of Venus with those of Earth, in a kind of game of differences, is extremely useful to ponder how life can determine the properties of the atmosphere of a rocky planet.
The article by McGouldrick, Peralta and collaborators, entitled "Using VIRTIS on Venus Express to Constrain the Properties of the Giant Dark Cloud Observed in Images of Venus by IR2 on Akatsuki" has been published in a recent issue of The Planetary Science Journal and may be consulted in this link.
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Rafael Bachiller is director of the National Astronomical Observatory (National Geographic Institute) and academic of the Royal Academy of Doctors of Spain.
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