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Watching Venus glow in the dark



Posted: 25 February, 2009

An eerie glow has been observed in the night time atmosphere of Venus that shows Earth’s neighbouring planet as a temperamental place of high winds and turbulence.

ESA’s Venus Express spacecraft observed the infrared glow using the Visible and Infrared Thermal Imaging Spectrometer (VIRTIS) instrument, which has made two unambiguous detections of the phenomena associated with nitric oxide. This is the first time such infrared detections have been made for any planet and provide a new insight into Venus’s atmosphere.

“The nightglow can give us a lot of information,” says Antonio Garcia Munoz of the Instituto de Astrofisica de Canarias, Tenerife, and formerly of the Australian National University. “It can provide details about the temperature, wind direction, composition and chemistry of an atmosphere.”

Artist impression of Venus Express examining the planet’s atmosphere.

Image: ESA/AOES Medialab.


The effect results from the interaction of the Sun’s ultraviolet light with the planet’s atmosphere, breaking up the atmospheric molecules into atoms and other simple molecules. If the free atoms recombine again, in some cases, excess energy is then emitted as light. On the dayside the emission is outshone by sunlight, but on the night-side, atoms are transported by a vigorous diurnal circulation and so the glow can be picked out by instruments such as VIRTIS.

Although the nightglow on Venus has been observed in the infrared before now, revealing the presence of oxygen molecules and of the hydroxyl radical, this is the first detection of nitric oxide at these wavelengths. Similarly, a nitric oxide nightglow has never been observed in the infrared for Mars or Earth, either, although nitric oxide is known to be present because it has been observed in the ultraviolet.

The new data offers valuable information on the Venusian atmosphere that lies above the cloud tops at around 70 kilometres. The oxygen and hydroxyl emissions originate from 90-100 kilometres, and the nitric oxide comes from an altitude of 110-120 kilometres and varies in intensity.

The oxygen nightglow (top panel) seen at a wavelength of 1.27 microns and an altitude of approximately 96 km over the surface of the planet. The bottom panel shows the same portion of the atmosphere observed at the same time, but at a different wavelength (around 1.22 microns), revealing the nightglow of nitric oxide, which is much weaker than that of oxygen and comes from 110 km above the surface.


“Luckily for us, Venus has a temperamental atmosphere,” says Munoz. “Packets of oxygen and nitrogen atoms are blown around.” Sometimes these become dense enough to boost the brightness of the nightglow, making it visible to VIRTIS.

Venus Express can observe the three nightglow emissions simultaneously, and has demonstrated that they do not necessarily happen together. “Perhaps when we have more observations, we will understand the correlation between them,” says Munoz. The team are therefore continuing their observations to build up a database of this curious phenomenon.

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