Scientists, using data from a high-resolution spectrometer aboard ESA’s Venus Express spacecraft, have discovered a surprisingly cold layer high in Venusian atmosphere that may be frigid enough for carbon dioxide to freeze out as ice or snow.

The planet Venus is known for its thick, carbon dioxide atmosphere and oven-hot surface. But in a new analysis of data from the Venus Express, the scientists have uncovered a very chilly layer at temperatures of around – 283 ºF (– 175 ºC) in the atmosphere 125 km above the planet’s surface.

The curious cold layer is far frostier than any part of Earth’s atmosphere, for example, despite Venus being much closer to the Sun.

The discovery, published in the Journal of Geophysical Research – Planets, was made by watching as light from the Sun filtered through the atmosphere to reveal the concentration of carbon dioxide gas molecules at various altitudes along the terminator – the dividing line between the day and night sides of the planet.

Armed with information about the concentration of carbon dioxide and combined with data on atmospheric pressure at each height, the team could then calculate the corresponding temperatures.

“Since the temperature at some heights dips below the freezing temperature of carbon dioxide, we suspect that carbon dioxide ice might form there,” said study lead author Dr Arnaud Mahieux of the Belgian Institute for Space Aeronomy.

Clouds of small carbon dioxide ice or snow particles should be very reflective, perhaps leading to brighter than normal sunlight layers in the atmosphere. “However, although Venus Express indeed occasionally observes very bright regions in the Venusian atmosphere that could be explained by ice, they could also be caused by other atmospheric disturbances, so we need to be cautious,” Dr Mahieux said.

The study also found that the cold layer at the terminator is sandwiched between two comparatively warmer layers.

“The temperature profiles on the hot dayside and cool night side at altitudes above 120 km are extremely different, so at the terminator we are in a regime of transition with effects coming from both sides. The night side may be playing a greater role at one given altitude and the dayside might be playing a larger role at other altitudes.”

Similar temperature profiles along the terminator have been derived from other Venus Express datasets, including measurements taken during the transit of Venus earlier this year.

Models are able to predict the observed profiles, but further confirmation will be provided by examining the role played by other atmospheric species, such as carbon monoxide, nitrogen and oxygen, which are more dominant than carbon dioxide at high altitudes.

“The finding is very new and we still need to think about and understand what the implications will be,” says Dr Håkan Svedhem, ESA’s Venus Express project scientist. “But it is special, as we do not see a similar temperature profile along the terminator in the atmospheres of Earth or Mars, which have different chemical compositions and temperature conditions.”

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Bibliographic information: A. Mahieux et al. 2012. Densities and temperatures in the Venus mesosphere and lower thermosphere retrieved from SOIR on board Venus Express. Carbon dioxide measurements at the Venus terminator. Journal of Geophysical Research – Planets, vol. 117, E07001; doi: 10.1029/2012JE004058