Venus Express has made the first detection of the molecule hydroxyl in the upper atmosphere of Venus, giving scientists a vital tool to unlock the mechanics of our sister planet’s dense atmosphere.
Hydroxyl, which is made up of one hydrogen and one oxygen atom, is thought to be important for a planet’s atmosphere because it is highly reactive. On Earth it has a key role in purging pollutants from the atmosphere, and on Mars, although no actual detection of hydroxyl has been made, it is thought to help stabilise the carbon dioxide in the atmosphere, preventing it from turning into carbon monoxide. It is also thought to play a vital role in sterilising the martian soil, making the top layers inhospitable for microbial life.
Hydroxyl, an important but difficult-to-detect molecule, has been found in the upper atmosphere of Venus, by Venus Express's VIRTIS instrument. Image: ESA (C. Carreau).
Hydroxyl is difficult to detect, but was spotted on Venus by turning Venus Express away from the planet and squinting at the faintly visible layer of atmosphere surrounding the planet’s disc. The VIRTIS (Venus Express Visible and Infrared Thermal Imaging Spectrometer) instrument spied the hydroxyl molecules by measuring the amount of infrared light that they give off. The molecules were found in the upper reaches of the atmosphere, 100 kilometres above the surface, in a narrow band just 10 kilometres wide.
"Venus Express has already shown us that Venus is much more Earth-like than once thought," says Giuseppe Piccioni, one of the principal investigators of the VIRTIS instrument. "The detection of hydroxyl brings it a step closer."
On Earth, the glow of hydroxyl has been closely linked to the abundance of ozone, an extremely important molecule that protects the Earth’s biosphere from harmful ultraviolet rays. Moreover, the amount of radiation absorbed by ozone drives the heating and dynamics of a planet’s atmosphere. The hydroxyl glow on Venus is now also thought to be related to the presence of ozone, and scientists will be able to take stock of the amount of ozone in the Venus atmosphere. Already Venus Express has shown that the amount of hydroxyl can change by up to 50% from one orbit to the next; computer models will be able to tell how this change in ozone levels over short intervals affects the restless atmosphere of Venus. The detection of hydroxyl also provides new constraints on the chemistry and budgets of water vapour and carbon dioxide in the entire climate system of the planet.
This initial detection was based on just a few orbits of Venus Express; the VIRTIS team still have around 50 orbits to analyse and plan on making even more observations in the future.