Clouds and hazes on Venus

Venus Express reveals the complex cloud patterns of our neighbouring planet

For earthbound sky watchers Venus is the bright star that chases the Sun below the horizon in the western evening sky. When it comes to scientific knowledge of our neighbouring planet, popular accounts often paint a Dantean picture of a fiery cauldron with a crushingly dense atmosphere.

Venus Express VMC image of southern hemisphere clouds
Ultraviolet image of the southern hemisphere of Venus, taken by the VMC instrument onboard Venus Express. The picture was taken from a distance of 30,000 km from the surface, and shows how the planet’s clouds change from spotty and fragmented at low-latitudes, to streaky at mid-latitudes, and hazy around the poles.

That hellish vision is part of the story, but only with the European Space Agency’s Venus Express mission have we come to appreciate the complexity of a world which like Earth is meteorologically active. On 11 April 2010 Venus Express will have been four years in orbit around the planet, and during that time the mission has provided much scientific insight into the Venusian atmosphere.

Orbiting close to the Sun, Venus is subject to intense heating, and this leads to a dynamic atmosphere. We have long known that Venus is covered with dense clouds rich in sulphuric acid. Now, with observations down to a few hundred metres’ resolution from the Venus Monitoring Camera (VMC) and VIRTIS thermal imaging spectrometer onboard Venus Express, we have learned that the clouds range from fragmented patches around the equator, covered with a bright lace of sulphuric acid droplets, to an almost featureless haze high above the poles.

The equatorial patchwork of clouds is due to vigorous convective heating of discrete parcels of air in the region where most of the incoming solar energy is deposited. At mid-latitudes this mottled cloud pattern gives way to streaky, banded shapes, providing evidence of more laminar atmospheric flow. The transition between these two regimes is one of the outstanding questions in our understanding of the atmosphere of Venus.

Perhaps the most spectacular cloud feature observed by Venus Express is an ‘S’-shaped polar vortex around 1500 km across, which appears to be part of a global vortex-like organisation of the planet’s atmosphere. The feature rotates around the south pole at latitudes above 70 degrees, with a period of around two Earth days. On a planetary scale, clouds on Venus extend upwards from around 45 km altitude; the cloud top at low- and mid-latitudes is located at about 75 km altitude, descending to 65 km in the eye of the polar vortex.

By analysing light from stars as they set through the atmosphere of Venus, the SPICAV spectrometer on Venus Express has revealed that the night-side cloud layer extends up to 90 km altitude in the form of an opaque haze, and another 15 km on top of that as a transparent haze. Ozone is present at higher altitudes.

In October of last year the European Space Agency extended the Venus Express mission to the end of 2012, allowing for some overlap in time with the Japanese Planet-C spacecraft. It is expected that the complementary approaches of the two missions will lead to further major discoveries about the atmosphere of Earth’s sister planet.

Further reading

Titov et al., “Venus express: Highlights of the nominal mission”, Solar System Research 43, 185 (2009)