The mysterious hot band of Venus

A heat map of Venus, obtained by the method of occultation, reserve two surprises: a layer mysteriously hotter than it should, and an unusually warm morning. The leader of the study is Arianna Piccialli at the French LATMOS laboratory.


Credit: ESA

Although Venus Express, the European probe that has scrutinized the “hot sister” of the Earth for eight years, has ceased working definitively last December, the data collected during its honorable service still surprises. The last in order of time is revealed in the pages of Planetary and Space Science by an international team of scientists, led by Italy’s Arianna Piccialli of LATMOS (Laboratoire Atmosphères, Milieux, Observations Spatiales) in Guyancourt, France.
During the compilation of a thermal map of the upper atmosphere in the “night” side of Venus, scientists have discovered a layer of warm air in the atmosphere of the planet, the nature of which is, at present, unknown.


Credit: Piccialli at al./ Planetary and Space Science

“We measured the temperatures at altitudes varying between 90 and 140 km,” says one of the authors of the study, Denis Belyaev Institute of Space Research of the Russian Academy of Sciences. “On the side not illuminated the planet temperatures normally tend to fall with altitude, but we have noticed a spike in the graph at the range between 90 and 100 km. Here the atmosphere was 20-40 ° C warmer than we expected. ”
“We do not know what is causing this warming,” continues Belyaev, “but the ozone layer of Venus is right at this altitude, so it’s possible that there is some connection. We cannot exclude that this phenomenon can be explained by the chemical reactions, namely the ozone decomposition, when they are in contact with substances containing chlorine. Reactions that would be able to release the heat. ”
“There is also another possible explanation for the presence of the layer of hot air,” adds Piccialli. “It may be the result of adiabatic heating due to the subsidence of the air – the downward movement of an air mass that causes heating by compression – on the night side of the planet.”
Piccialli and colleagues analyzed data obtained from the instrument SPICAV (Spectroscopy for Investigation of Characteristics of the Atmosphere of Venus), a spectrometer for the investigation of the characteristics of the atmosphere of Venus installed on board the Venus Express.


Credit: Denis Belyaev, MIPT

The atmospheric temperature is measured thanks to the channel sensitive to ultraviolet (UV) of which is equipped SPICAV, stellar occultation by the method, where the spectrometer captures the light emitted by a star just when it passes behind the planet. The light from the star passes through the atmosphere of Venus, carrying information about its features, read by experts in the spectral decomposition that SPICAV performs on the star’s light.
It’s a job that requires patience. Before scientists select the stars that shine more in the wavelengths of ultraviolet, the working ranges of the spectrometer. Then, during the occultation, which lasts a few minutes before the star disappears behind the horizon of the planet, the spectrometer performs a survey every second. Subsequently scientists determine the composition, the density and the temperature of the atmosphere at different altitudes. From June 2006 to February 2013 were obtained well 587 of these snapshots from the atmosphere, which provided an overview practically complete unilluminated hemisphere of Venus.


Credit: Piccialli at al./ Planetary and Space Science

“In almost all sessions of these seven years, we have detected a layer at 90-100 km height that is well 20-40 degrees warmer than it should be, with temperatures around 220-240 degrees Kelvin (between -53 and -33 degrees Celsius, ed), while they should be even lower than 200 ° K,” reiterates Belyaev.
The researchers also discovered another peculiarity of the upper atmosphere of Venus. Venus is a unique planet, which does not rotate in the direction of its movement along the orbit circumsolar, but in the opposite direction, because of its axis of rotation inclined by 177 degrees. In addition, rotates very slowly, taking 243 days land for one complete rotation. However, precisely because of the retrograde motion combined displacement orbit, between sunrise and the other spend “only” 117 days terrestrial.
During the long night Venusian, the upper part of the atmosphere cools, then the dawn Venusian should be colder than at sunset. Instead, the researchers found that the atmospheric temperature is 20 degrees warmer in the early morning than in the evening, just the opposite of what they expected.
“This is probably due to the global circulation of the atmosphere,” says Belyaev, “in which, at an altitude of about 100 km, there is a transition where you overlap two different circulation regimes: it goes from the super-rotation, observed in the lower mesosphere, movement solar-sun protection. In this zone, on the night side, the air mass decreases to 70 km, which can lead to adiabatic heating of the atmosphere. “

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