Venus running hot and cold: Ultraviolet layer

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  • Published: Apr 1, 2015
  • Author: David Bradley
  • Channels: UV/Vis Spectroscopy
thumbnail image: Venus running hot and cold: Ultraviolet layer

Our Venus

Ultraviolet observations by a team at the Moscow Institute of Physics and Technology reveal the existence of a warm layer in the atmosphere of the planet Venus the origins of which are yet to be explained. The discovery was uncovered when the team built a temperature map of the upper atmosphere on the planet's night side using data collected by the Venus Express probe.

Ultraviolet observations by the Laboratoire Atmospheres, Milieux, Observations Spatiales (LATMOS), Guyancourt, France (Franck Montmessin, Emmanuel Marcq, J.L. Bertaux and Arianna Piccialli) in collaboration with colleagues from Russia and Belgium reveal the existence of a warm layer in the atmosphere of the planet Venus the origins of which are yet to be explained. The discovery was uncovered when the team built a temperature map of the upper atmosphere on the planet's night side using data collected by the Venus Express probe.

"We measured temperatures at altitudes of 90 to 140 kilometres," explains team member Denis Belyaev of MIPT and the Space Research Institute of the Russian Academy of Sciences. "On the night side of the planet, temperatures normally fall with altitude, but we noticed a peak in the chart in the 90 to 100 kilometre range. Here, the atmosphere was 20 to 40 degrees warmer than we expected. We don't yet understand what causes the warming, but Venus' ozone layer is at this altitude. There may be a connection."

"There is another possible explanation for the presence of the warm layer observed in the atmosphere of Venus," Piccialli told SpectroscopyNOW, "it could also be interpreted as the result of adiabatic warming due to air subsidence on the night side. This observation is indeed in agreement with a general circulation model (Brecht et al., 2012), which suggests that the day-to-night circulation from the warm day side region produces downwelling winds on the night side resulting in adiabatic heating at about 90 to 100 km."

Belyaev, and MIPT's Anna Fedorova and Oleg Korablev worked with colleagues at the French laboratory LATMOS, from Belgium, Germany and the USA, to analyse the data obtained by the SPICAV spectrometer on board Venus Express between June 2006 and February 2013. Principal Investigator of the SPICAV/SOIR instrument on board Venus Express is Jean-Loup Bertaux (LATMOS). The European mission Venus Express was launched from the Baikonur space centre in 2005 on board the Russian rocket Soyuz-FG. The scientific payload was developed by an international team of scientists and while the orbiter was decommissioned in February 2015, scientists are continuing to pore over the data it sent back to Earth during its active mission.

SPICAV

The SPICAV system, Spectroscopy for the Investigation of the Characteristics of the Atmosphere of Venus, comprised two spectrometers, an infrared instrument built by Russian specialists and an ultraviolet instrument made by the French. The current research used data obtained by the UV spectrometer during stellar occultation in which emissions from a distant star passing through the planet's atmosphere are observed and analysed. In this case light from fifty UV-bright stars, at 118 to 320 nanometres, the working range of the instrument, was recorded at the point of occultation. By subtracting the clean "atmospheric" spectrum of the star, they could then determine the gas composition and density of the atmosphere of Venus at different altitudes, as well as calculate temperatures. 587 spectra were recorded from June 2006 to February 2013 to covered almost the entire night hemisphere.

In almost every observation session during this seven year period, the team detected a layer at an altitude of 90 to 100 km that was 20 to 40 Kelvin warmer than it ought to be based on atmospheric models of Venus, explains Belyaev. The air temperatures at these altitudes are 220-240 Kelvin, while they should be lower than 200. The explanation may well lie in the fact that the planet's ozone layer lies at this altitude, the researchers suggest.

"We are carrying out correlation analysis to determine if these are connected or not," Belyaev adds. "We can't rule out that this phenomenon may be explained by chemical reactions, namely the decomposition of ozone when it comes in contact with chlorine-based substances - these reactions may result in the release of heat."

Paradoxical planet

There is another seeming paradox. The researchers also found that the upper atmosphere of Venus is warmer early in the morning than it is in the evening; the converse ought to be true. Venus is a unique planet in that rotates in the opposite sense to its direction of movement along its path around the Sun with its axis of rotation tilted at 177 degrees. Moreover, its day is very long, lasting the equivalent of 116 days on Earth. During the long Venutian night the upper atmosphere cools, so at night it should be warmer than in the morning. "We found that the atmospheric temperature is 20 degrees warmer in the morning than in the evening," says Belyaev. "This is probably due to the global circulation of the atmosphere." Piccialli adds that, "Tthe altitude region between 70 and 120 km acts as a transition zone in which the two major dynamic regimes are superimposed: the zonal super-rotation dominates the lower mesosphere while a solar-antisolar circulation driven by a day-to-night temperature gradient can be observed in the upper mesosphere/lower thermosphere."

Piccialli joined the EuroVenus project team at LESIA-Observatoire de Paris in October 2014 as a postdoctoral researcher. "I am characterising Venus upper mesosphere by monitoring minor species, winds and the thermal structure at submillimetre wavelengths using observations by the Atacama Large Millimeter Array (ALMA)," she told us. "With the end of Venus Express operations, ground-based observations provide a unique opportunity to continue studying Venus atmosphere."

Related Links

Planetary Space Sci 2015, online: "Thermal structure of Venus nightside upper atmosphere measured by stellar occultations with SPICAV/Venus Express"

Article by David Bradley

The views represented in this article are solely those of the author and do not necessarily represent those of John Wiley and Sons, Ltd.

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